Download - Ground Investigation Parit Baru

SUMMARY OF GROUND INVESTIGATION

SITE INVESTIGATION ON PARIT BARU THERMAL PLANT PROJECT

(255 MW)

TABLE OF CONTENTS

I. LAPORAN AKHIR: PENYELIDIKAN TANAH

II. FINAL REPORT: GROUND INVESTIGATION

III. LAYOUT OF CORE DRILLING AND CPT POINTS

IV. SUMMARY OF LABORATORY TESTS RESULTS

V. BORLOG (B1 B8)

VI. DUTCH CONE PENETRATION TESTS (S1 - S6)

Bab 6 PENYELIDIKAN TANAH

6.1 Pengantar Penyelidikan tanah tambahan ini dimaksudkan untuk meyakinkan kondisi geoteknis tanah dasar di lokasi rencana PLTU Parit Baru, sehingga dapat ditentukan secara baik kedalaman dan jenis fondasi yang cocok untuk dasar perencanaan konstruksi PLTU Parit Baru. Penyelidikan tanah tambahan terdiri dari dua pekerjaan utama, yaitu pekerjaan lapangan dan pekerjaan laboratorium. Pekerjaan lapangan terdiri dari pemboran, SPT, serta pengambilan sampel tanah dan air tanah.

Pemboran tanah dilakukan di dua lokasi, yaitu di titik B7 dan B8 (lihat Gambar 6.1). Pemboran di setiap titik bor dilaksanakan sampai kedalaman 60 m. Pada pemboran, dilakukan pengujian perlawanan/kekuatan relatif tanah yang diukur melalui uji penetrasi (SPT) dan pengambilan sampel tanah, baik yang tak-terganggu (undisturbed samples) maupun yang terganggu (disturbed samples) untuk keperluan uji laboratorium.

6.2 Pekerjaan Lapangan

6.2.1 Pekerjaan Pengeboran Pekerjaan pengeboran dilakukan sebanyak 2 titik dengan metoda pemboran inti (core drilling) dengan menggunakan single core barrel. Selama pengeboran, digunakan casing dan/atau drilling mud untuk mencegah kelongsoran dalam lubang bor. Tanah hasil pengeboran (core) disusun dalam kotak-kotak sampel sepanjang 1 m dengan lebar dibagi 5 kompartemen sehingga dalam 1 kotak dapat tersimpan 5 m core. Identitas dan keterangan lengkap mengenai nomor titik dan kedalaman dicantumkan pada kotak tersebut. Pada kondisi tertentu, tanah dibungkus dalam kantong plastik memanjang sehingga kadar air tidak terlalu berubah sehingga memungkinkan bila diperlukan untuk uji laboratorium tambahan.

Pengeboran dilaksanakan sampai salah satu kriteria berikut dicapai:

1. kedalaman 60 meter, atau

Site Investigation Tambahan PLTU Parit Baru (255 MW) Kalimantan Barat 6-1

2. dihentikan sampai menjumpai lapisan batuan keras dan kompak, dimana mata bor biasa tak mampu lagi menembusnya.

Posisi titik-titik pengujian lapangan (bor dan sondir) dapat dilihat pada Gambar 6.1 di bawah ini.

Gambar 6.1 Perletakan titik-titik penyelidikan lapangan (bor dan sondir). B7 pada Steam

Turbin Block sedangkan B8 pada Chimney.

6.2.2 SPT (Standard Penetration Test) Pengujian SPT dilaksanakan menurut ASTM D 1586-84 dimana sebuah tabung belah standar ditumbuk dengan penumbuk seberat 63.5 kg yang dijatuh-bebaskan setinggi 760 mm. Pengujian dilakukan dalam 3 tahapan penetrasi, masing-masing 150 mm, dan nilai SPT (N) adalah jumlah tumbukan yang dihasilkan pada penumbukan ke-2 dan ke-3. Dengan demikian, nilai N adalah jumlah tumbukan pada tabung belah standar tersebut untuk penetrasi 30 cm. Pengujian dilakukan dalam lubang bor pada interval kedalaman sekitar 2 m.


6.2.3 Pengambilan Sampel Tanah dan Air Tanah Pengambilan contoh tanah yang relatif tak terganggu dilakukan dengan tabung tipis yang ditekan ke tanah. Metoda sampling mengikuti cara yang diuraikan dalam ASTM D1587-83. Sampel yang diambil diberi identitas (nomor titik dan kedalaman), kemudian bagian atas dan bawah ditutup dengan lilin (wax) untuk mencegah kehilangan kandungan air. Sampel dibawa ke laboratorium mekanika tanah untuk diuji sesuai dengan jenis tanah dan kepentingannya.

Pada akhir pemboran, setelah bor mencapai kedalaman 60 m, sampel air tanah di dalam lubang bor diambil untuk keperluan uji laboratorium kualitas air.

6.3 Pekerjaan Laboratorium

6.3.1 Kadar Air Kadar air tanah diuji sesuai dengan ASTM D2216-90 yang berlaku untuk tanah dan batuan. Kadar air didefinisikan sebagai rasio (dinyatakan dalam persen) antara massa air pori terhadap massa material padatnya. Pengujian dilakukan dengan cara mengeringkan tanah basah (M) di dalam oven pada suhu 110oC selama 24 jam sehingga seluruh air pori/bebas dalam tanah habis menguap dan hanya material padatnya (Ms atau massa kering) yang tertinggal. Massa air adalah selisih dari massa basah dan massa kering tanah.

6.3.2 Berat Jenis Berat jenis tanah diuji sesuai ASTM D 854-91, menggunakan piknometer kapasitas minimum 50 ml. Tanah yang diuji adalah tanah yang lolos ayakan No. 4. Berat jenis merupakan rasio dari massa volume partikel padat tanah pada suhu tertentu terhadap massa volume air destilasi bebas udara pada suhu yang sama.

6.3.3 Batas-batas Atterberg Batas-batas Atterberg yang diuji meliputi batas cair dan batas plastis tanah. Pengujian mengikuti ASTM D 4318-84 dan dilakukan untuk tanah yang bersifat plastis. Batas cair tanah adalah kadar air tanah, dalam persen, dimana tanah pada kondisi batas antara cair dan plastis. Kondisi ini didefinisikan sebagai kadar air tanah dimana jika tanah tersebut dimasukkan pada mangkok standar dengan ketebalan tertentu, dipisahkan dengan pembarut/colet berukuran standar, akan mengalir di dasar barutan sepanjang 13 mm jika diketukkan sebanyak 25 kali dengan tinggi jatuh 10 mm pada alat uji batas cair standar dengan kecepatan jatuh 2 kali per detik. Dalam pelaksanaan, biasanya pengujian dilakukan dengan coba-coba terhadap 2 sampel pada kadar air di atas batas cair (jumlah ketukan kurang dari 25) dan 2 sampel lagi pada kadar air di bawah batas cair (jumlah ketukan lebih dari 25). Batas cair dicari dengan interpolasi 4 data tersebut yang digambarkan pada sumbu-y berupa kadar air skala linier dan sumbu-x berupa jumlah ketukan dalam skala logaritma.


Batas plastis adalah kadar air tanah pada batas antara kondisi plastis dan semi padat, ditentukan dengan cara menggulung-gulung tanah basah membentuk batang berdiameter 3.2 mm. Kondisi batas cair dicapai jika pada diameter tersebut tanah mulai retak-retak.

6.3.4 Distribusi Ukuran Butir Penentuan distribusi ukuran butir tanah dilakukan sesuai ASTM D 422-63 (1990). Dalam standar ini, partikel kasar (diameter lebih dari 0.075 mm) diuji dengan ayakan dari diameter 75 mm sampai yang paling halus No. 200 (0.075 mm). Untuk fraksi tanah yang lolos ayakan No. 200 (diameter kurang dari 0.075 mm) dilarutkan dalam air dan dianalisis dengan teknik sedimentasi menggunakan alat hidrometer. Distribusi ukuran butir tanah disajikan dalam kurva hubungan antara diameter (mm, skala log) terhadap persentasi lolos diameter yang bersangkutan.

6.3.5 Uji Triaksial Uji triaksial dengan kondisi tak-terkonsolidasi tak-terdrainasi dilakukan pada tanah kohesif berdasarkan ASTM D 2850-87. Pengujian dilaksanakan pada tanah tak terganggu berbentuk silinder yang dibungkus dengan membran karet, diberi tekanan kekangan dalam alat triaksial, dan kemudian digeser sampai runtuh. Pada pengujian ini, sampel tidak diberi kesempatan untuk drainasi yang diatur dengan menutup semua katup yang berhubungan dengan sampel serta mengatur kecepatan penggeseran yang cukup tinggi. Pengujian ini akan menghasil parameter kuat geser tak-terdrainasi dan perilaku tegangan-regangan tanah.

6.3.6 Uji Tekan Bebas Uji tekan bebas dilakukan pada tanah kohesif yang cukup kaku dan dilaksanakan berdasarkan ASTM D 2166-91. Pengujian dilakukan pada tanah tak terganggu yang berbentuk silinder dengan tinggi sebesar 2 kali diameter. Sampel diberi beban aksial berangsur-angsur sampai runtuh. Tegangan yang terjadi dikoreksi terhadap luasan yang berubah akibat pemendekan dengan asumsi volume konstan. Hasil pengujian disajikan dalam bentuk grafik hubungan tegangan dan regangan. Tegangan maksimum yang dihasilkan disebut sebagai kuat tekan bebas tanah (unconfined compressive strength) dan sering dikaitkan dengan kohesi tak-terdrainasi tanah kohesif. Pengujian ini dilakukan pada sampel lempung yang tidak diuji triaksial.

6.3.7 Uji Konsolidasi Uji konsolidasi dilaksanakan sesuai ASTM D 2435-90. Pengujian ini bertujuan untuk mendapatkan parameter besaran dan kecepatan konsolidasi dari tanah tak terganggu yang terkekang arah lateral dan terdrainasi arah aksial sambil dibebani secara bertahap dengan beban/tegangan terkontrol. Metoda yang digunakan adalah dengan memberi beban konstan selama 24 jam. Beban kemudian dinaikkan menjadi 2 kali beban sebelumnya dan


masing-masing tahap beban diberikan selama 24 jam. Pada masing-masing beban, dihasilkan data hubungan antara waktu dan penurunan dimana parameter kecepatan konsolidasi bisa ditentukan. Hasil pembebanan secara keseluruhan menghasilkan data hubungan tegangan dan penurunan atau angka pori yang digunakan untuk menghitung besarnya penurunan

6.3.8 Uji Geser Langsung Metoda yang digunakan adalah ASTM D3080-90. Pengujian dilakukan pada tanah pasir atau jika uji triaksial dan tekan bebas tidak bisa dilaksanakan karena tanah berupa pasir atau lempung lunak yang tidak bisa berdiri bebas. Pengujian dilaksanakan dengan menggeser sampel pada satu bidang geser dengan kecepatan yang dikontrol oleh alat. Pada tanah pasir, pengujian cenderung pada kondisi terkonsolidasi dan terdrainasi. Pengujian ini menghasilkan parameter kuat geser tanah.

6.4 Hasil Penyelidikan

6.4.1 Kondisi Lahan Secara umum lahan yang diselidiki relatif datar, sedikit miring ke arah sungai Kapuas dan sungai Jungkat (arah selatan dan barat) dengan elevasi muka tanah relatif rendah. Posisi lahan yang terletak di dekat sungai besar yang menghadap ke laut menjadikan lahan selalu basah atau terendam air saat laut pasang. Kondisi ini akan sangat menyulitkan untuk membangun PLTU di atasnya tanpa peninggian lahan atau mengurug lahan sampai elevasi yang cukup aman terhadap pengaruh genangan air.

Pada saat penyelidikan berlangsung, kondisi lahan masih tetap seperti saat penyelidikan sebelumnya dimana sebagian besar lahan berupa lahan kosong yang ditumbuhi semak dan di sebagian lahan telah ada bangunan gardu induk (pembangkit tegangan tinggi) 150 kV, ruang panel, jalan aspal dan beberapa perumahan.

6.4.2 Lapisan-lapisan Tanah Susunan lapisan tanah dari hasil pemboran disajikan pada Gambar 6.2. Dari hasil pemboran, tampak bahwa lapisan tanah bagian atas umumnya tersusun atas lempung berlanau lunak dengan kandungan organik yang cukup besar. Di titik B7, lapisan dengan humus/gambut dibagian atas mencapai ketebalan sekitar 3.50 meter sedangkan di titik B8 hanya setebal sekitar 1 meter. Lapisan di bawahnya berupa lempung berlanau atau berpasir dengan pecahan kulit kerang. Lapisan ini mempunyai ketebalan bervariasi antara 4.50 m sampai sekitar 8.50 m dengan kondisi lunak atau sangat lunak dengan nilai N (SPT) kurang dari 2.


B8 B70.00

10.00

20.00

30.00

40.00

50.00

60.00

Silty CLAY

Silty CLAY

Sandy SILT

SAND

Very Softto Soft

Medium

Dense tovery dense

0.00

- 23.50

- 31.50

- 34.50- 33.50

- 60.00

- 26.50

Gambar 6.2 Lapisan-lapisan tanah dari hasil pemboran.

Lapisan di bawahnya masih berupa lanau lempung sampai kedalaman 31.55 sampai 34.40 meter. Dalam lapisan ini terdapat beberapa variasi, diantaranya lapisan dengan kandungan organik yang cukup signifikan terdapat pada B7 di kedalaman antara 24.00 sampai 26.00 meter dan pada B8 di kedalaman 20.00 sampai 21.00 dan 22.00 sampai 23.55 meter. Kondisi lapisan ini umumnya


lunak sampai sedang di bagian atas sampai kedalaman sekitar 27.50 meter, selanjutnya di bagian bawah kondisinya cukup kaku.

Di titik B7, lapisan selanjutnya berupa lapisan pasir dengan kondisi padat sampai sangat padat sampai kedalaman akhir pemboran 60.00 meter. Variasi dijumpai pada titik B8, dimana lapisan lanau berpasir dengan kondisi kaku setebal sekitar 2 meter, sebelum lapisan pasir padat sebagaimana dijumpai pada titik B7. Di titik B7, lapisan lanau berpasir di bagian atas lapisan pasir tidak dijumpai, sehingga diperkirakan penyebarannya terbatas. Namun di titik B7 terdapat lapisan lempung lanau padat pada kedalaman 54.00 sampai 55.00 meter.

Rangkuman hasil pengeboran masing-masing titik disajikan pada Tabel 6.1 dan Tabel 6.2.

Tabel 6.1 Kedalaman dan diskripsi hasil pemboran dan uji penetrasi test B7.

Depth Soil type N value

0.00-3.50 Silty CLAY, few peat, dark grey to black 2

3.50-6.00 Fine sandy CLAY, few shell fragments, grey 1 or less

6.00-11.00 Silty CLAY, few shell fragments, grey 1

11.00-16.00 Silty CLAY, light grey 2

16.00-19.55 Silty CLAY, few sand, grey 3

19.55-24.00 Silty CLAY, grey 2-4

24.00-26.00 Silty CLAY, few organic matter, dark grey 3

26.00-29.55 Silty CLAY, grey to yellow 6


34.40-35.55 Silty medium SAND, light grey to white -

35.55-54.00 Slightly silty medium SAND, white 50

54.00-55.00 Silty clay, brown -

55.00-60.00 Slightly silty fine SAND, light grey to white 50

Tabel 6.2 Kedalaman dan diskripsi hasil pemboran dan uji penetrasi test B8.


0.00-1.00 Silty CLAY, few peat, dark grey

1.00-5.50 Silty CLAY, few sand, shell fragments, grey < 2

5.50-14.00 Silty CLAY, dark grey 2

14.00-17.55 Silty CLAY, few fine sand, dark grey 2

17.55-20.00 Silty CLAY, grey 2

20.00-21.00 Silty CLAY, organic matter, dark grey to black -



21.00-22.00 Slty CLAY, grey 3


23.55-25.00 Silty CLAY, brown 4

25.00-25.55 Silty CLAY, reddish grey -

25.55-31.55 Silty CLAY, light grey 4-6

31.55-32.00 Fine sandy SILT, grey 17

32.00-33.55 Fine sandy SILT, brownish white -

33.55-35.55 Slightly silty SAND, white 34

35.55-60.00 Slightly silty SAND, white > 50

6.4.3 Hasil Uji Laboratorium Hasil uji laboratorium menunjukkan bahwa di beberapa lapisan, tanah mengandung bahan organik yang cukup yang ditunjukkan dengan nilai berat jenis tanah yang lebih rendah dibandingkan dengan berat jenis mineral tanah umumnya. Nilai terendah berat jenis yang diuji sebesar 2.16. Selain itu terdapat sampel tanah organik dengan nilai kadar air yang tinggi mencapai 237%. Lapisan dengan kandungan organik yang cukup signifikan terdapat di bagian permukaan dan di kedalaman antara 20 sampai 25 meter. Pada lapisan tanah berbutir halus (sampai kedalaman sekitar 34 meter), fraksi halus tanah (lolos ayakan no. 200) cukup besar, umumnya lebih dari 65% bahkan ada yang mencapai 98%. Batas cair tanah bervariasi pada nilai antara 42% sampai 94% yang termasuk tanah berplastisitas rendah dan sangat tinggi. Indeks plastis tanah berkisar antara 15% sampai 63%. Berdasarkan klasifikasi ASTM, plastisitas tanah dari lokasi penyelidikan cukup bervariasi dan mayoritas masuk pada kelompok lempung/lanau berplastisitas tinggi (CH-MH) dan sebagian lagi masuk kelompok lempung/lanau berplastisitas rendah (CL-ML). Sebagian tanah kemungkinan masuk kelompok lanau/lempung organik dengan plastisitas rendah sampai tinggi (OL-OH).

Untuk sample yang berupa pasir, fraksi halus umumnya kurang dari 3%. Koefisien keseragaman umumnya tidak lebih dari 4 dengan koefisien kelengkungan antara 1-3. Dengan kondisi tersebut pasir masuk kelompok simbol SP atau pasir yang relatif bersih bergradasi seragam.

Uji kuat geser tanah dengan triaksial terhadap tanah yang tak terusik menunjukkan bahwa tanah mempunyai parameter kuat geser yang bervariasi dari kondisi sangat lunak sampai kondisi sedang yang ditunjukkan dengan nilai kohesi (cu) antara 3 sampai 38 kN/m2. Sudut gesek internal tanah dari uji desak triaksial ini memberikan hasil tidak lebih dari 10 derajat.

Untuk lapisan pasir yang cukup padat atau yang lebih dalam, sampel tak terusik tidak dapat diambil sehingga pengujian dilakukan pada sampel yang diambil menggunakan tabung SPT. Hasil uji geser langsung menunjukkan


bahwa nilai kohesi tanah mendekati nol dan nilai sudut gesek internal tanah sekitar 40o.

Kompresibilitas tanah tak terusik yang diuji dengan konsolidasi, umumnya menunjukkan tingkat kompresibilitas tanah yang cukup tinggi dengan nilai indeks kompresi antara 0.4 sampai 2. Nilai tersebut menunjukkan bahwa jika tanah dibebani akan terjadi penurunan yang cukup besar. Sedangkan parameter waktu konsolidasi yang diwujudkan sebagai koefisien konsolidasi (cv) mempunyai rentang nilai antara 0.610-4 sampai 0.7510-3 cm2/detik atau sekitar 0.2 sampai 2.4 m2/tahun.

6.4.4 Komparasi dengan Hasil Penyelidikan Sebelumnya Secara umum hasil penyelidikan sebelumnya mirip dengan hasil penyelidikan tambahan ini. Jenis tanah bagian atas sampai kedalaman sekitar 32 meter didominasi oleh lempung dengan variasi kandungan lanau, pasir, pecahan kulit kerang dan kandungan material organik. Pada hasil penelitian tambahan ini, kandungan organik lebih banyak teramati dan dijumpai pada bagian atas dan beberapa pada kedalaman antara 20 sampai 26 meter. Hasil penyelidikan terdahulu memberikan nilai lebih tinggi yang disebabkan oleh penggunaan hammer manual sedangkan penyelidikan tambahan menggunakan hammer otomatis sehingga kualitas lebih dapat dijamin. Nilai parameter kuat geser dari hasil uji triaksial tampaknya mempunyai nilai yang hampir sama. Parameter besar penurunan menunjukkan hasil yang hampir sama, namun pada parameter waktu konsolidasi, hasil yang baru lebih kecil dibandingkan hasil sebelumnya.

Untuk lapisan tanah pasir di bagian bawah, hasil uji SPT sulit untuk dibandingkan karena nilai N yang dihasilkan lebih besar dari 50 dan pengujian dihentikan sebelum mencapai penetrasi 30 cm. Pada penyelidikan sebelumnya, uji laboratorium pada tanah pasir sangat terbatas sehingga secara langsung agak sulit diperbandingkan.

6.5 Pemilihan Fondasi Bangunan Sebagaimana telah diuraikan di depan bahwa lahan yang akan dibangun PLTU ini mempunyai kondisi tanah bagian atas lunak atau sangat lunak dengan ketebalan sampai 23 m. Lapisan di bawahnya dengan kondisi sedang sampai padat mempunyai ketebalan sampai 10 m. Sedangkan lapisan tanah keras atau sangat padat, dijumpai mulai kedalaman 35 m.

Dengan kondisi lapisan tanah tersebut, untuk mendukung beban yang cukup besar, tidak bisa menggunaan fondasi dangkal, karena disamping kekuatannya yang kecil, penurunan yang akan terjadi sangat besar sehingga sangat membahayakan. Fondasi yang sesuai untuk beban yang cukup besar adalah fondasi tiang yang menembus lapisan lunak dan meneruskan beban bangunan atas ke lapisan tanah yang cukup dalam. Dengan kondisi tanah bagian atas lunak atau sangat lunak, maka fondasi tiang perlu ditanam masuk ke lapisan padat atau sangat padat sedemikian sehingga stabilitas dapat dijamin.


Jenis fondasi tiang bisa berupa tiang bor (bored pile) atau tiang pracetak/pancang. Tiang bor mempunyai beberapa keuntungan diantaranya: ukuran tiang lebih mudah disesuaikan dengan beban dan keperluan, namun tiang ini perlu dukungan sarana-prasarana untuk pengecoran sehingga perlu sarana tranportasi darat yang lebih besar. Disamping itu, dengan kondisi tanah yang sangat lunak, kualitas tiang bor akan lebih sulit dijamin.

Tiang pracetak lebih sederhana karena tiang dibuat ditempat lain, dibawa ke lokasi proyek dalam jumlah yang cukup banyak, apalagi jika lewat angkutan air, kemudian dipancang sampai kedalaman rencana. Kelemahan tiang pracetak terutama pada penetapan panjang tiang yang sering perlu penyambungan. Di sisi lain, jika terlalu panjang akan ada sisa yang harus dipotong. Namun dengan mempertimbangkan efisiensi, penggunaan tiang pracetak lebih baik. Saat ini tiang pancang pracetak banyak diproduksi oleh pabrik pembuat khusus yang panjangnya disesuaikan dengan permintaan. Untuk ukuran panjang tiang yang melebihi kemampuan pembuatan pabrik, tiang dibuat menjadi beberapa segmen yang kemudian disambung saat pemancangan. Jenis tiang yang banyak diproduksi pabrik berupa tiang pancang beton bertulang prategang berpenampang bulat dengan bagian tengahnya berongga.

6.5.1 Analisis Kapasitas Tiang Panjang dari tiang pancang dirancang masuk ke lapisan padat untuk menjamin transfer beban ke tanah dapat berlangsung dengan baik. Dari titik-titik penyelidikan, disarankan panjang tiang mencapai kedalaman antara 37 sampai 40 meter di bawah muka tanah sekarang. Berdasarkan kedalaman tersebut kapasitas dukung tiang dianalisis dengan mempertimbangkan dukungan di ujung tiang dan dukungan gesek/lekatan.

1 Dukungan ujung

Mengingat kondisi tanah disekitar ujung tiang cukup padat sehingga sampel tanah yang didapatkan kurang representatif, maka analisis dukungan ujung dilakukan dengan menggunakan data uji penetrasi standar (SPT). Dengan anggapan tiang masuk cukup dalam ke lapisan yang padat, maka kapasitas dukung ultimit (Qu) di ujung tiang dihitung dengan rumus:

Qu (kN) = Ap400N (6.1)

Dengan Ap = luas penampang tiang (m2) dan N = nilai SPT rerata dari 10D di atas ujung tiang sampai 4D di bawah ujung tiang (D = diameter tiang).

Nilai N diambil dari hasil uji, namun untuk nilai N lebih dari 50 digunakan nilai N maksimum 50. Untuk mendapatkan kapasitas dukung tiang yang diperbolehkan, digunakan faktor aman antara 3 sampai 5. Sebagai gambaran data N yang digunakan dan hasil hitungan untuk tiang pancang berdiameter 400 mm disajikan pada Tabel 6.3, sedangkan rangkuman kapasitas dukung ijin untuk berbagai ukuran tiang disajikan pada Tabel 6.4.


Tabel 6.3 Kapasitas dukung tiang diameter 400 mm.

Titik bor Dalam (m) N Nrerata Qu (kN) Qa (kN) F=4

B7 37.00 7,50,50 35 1759 439

B8 37.00 17,34,50 33 1658 414

B7 40.00 50,50,50 50 2513 628

B8 40.00 50,50,50 50 2513 628 Tabel 6.4 Kapasitas dukung tiang untuk berbagai diameter tiang.

Kapasitas dukung ijin, Qa (kN) untuk diameter tiang Titik bor Dalam (m)

400 mm 500 mm 600 mm

B7 37.00 439 687 989

B7 40.00 628 981 1413

B8 37.00 414 647 933

B8 40.00 628 981 1413

2 Dukungan gesek/lekatan

Sebagaimana telah disampaikan di muka, lahan yang diselidiki mempunyai kondisi lapisan tanah bagian atas yang lunak yang tidak memungkinkan untuk diperhitungkan sebagai pendukung beban, bahkan kemungkinan bisa memberikan kontribusi negatif pada dukungan tiang. Untuk itu, kapasitas dukung gesek/lekatan dari tiang akan diperhitungkan pada segmen tiang yang berada pada lapisan yang cukup padat. Panjang tiang yang akan diperhitungkan untuk masing-masing titik penyelidikan disajikan pada Tabel 6.5.

Tabel 6.5 Kedalaman lapisan tanah yang diperhitungkan gesek.

Titik bor Kedalaman yang diperhitungkan

B7 mulai 27.00 m ke bawah

B8 mulai 23.00 m ke bawah

Pada kedalaman yang diperhitungkan tersebut, kondisi tanah dominan adalah lempung walaupun ada variasi pasir. Kapasitas dukung tiang berdasarkan gesekan/lekatan (Qs) pada lempung dihitung dengan rumus:

Qs (kN) = (pLf) (6.2) Dengan p = keliling penampang tiang, L panjang tiang yang menahan gesek/lekat, f = gesekan atau lekatan. Untuk lempung f = cu dengan =


faktor adhesi dan cu = kohesi tanah yang didekati dengan dari nilai SPT sebagaimana ditunjukkan pada Tabel 6.6, sedangkan untuk tiang pancang pada pasir nilai didekati dari nilai SPT, f = 2N (kN/m2).

Tabel 6.6 Korelasi nilai SPT (N) dengan kohesi tanah (cu).

Konsistensi Kohesi, cu (kN/m2) N (SPT)

Sangat lunak 0 12.5 0 2

Lunak 12.5 25 2 4

Sedang 25 50 4 8

Kaku (stiff) 50 100 8 16

Sangat kaku 100 200 16 32

Keras > 200 > 32

Untuk mendapatkan kapasitas dukung gesek/lekat yang diperbolehkan dari tiang (Qa) digunakan faktor aman sebesar 4. Hasil perhitungan dirangkum dan disajikan pada Tabel 6.7 sedangkan sketsa kedalaman tiang pancang yang disarankan dapat dilihat pada Gambar 6.3.

Tabel 6.7 Kapasitas dukung gesek/lekat tiang yang diperbolehkan

Kapasitas dukung gesek yang diperbolehkan Qa (kN) Titik Bor Kedalaman

400 mm 500 mm 600 mm

B7 37.00 171 214 257

B7 40.00 265 332 398

B8 37.00 188 235 282

B8 40.00 300 375 450

Kapasitas dukung tiang dapat berupa gabungan dari dukungan ujung dan gesekan/lekatan. Kapasitas dukung ijin rencana harus memperhitungkan juga kekuatan struktural bahan tiang, dan diambil nilai yang aman/rendah dari keduanya. Disamping itu, adanya lapisan lunak di bagian atas, bisa menyebabkan adanya lekatan negatif pada tiang (negative skin friction).

Dengan adanya rencana pematangan tanah, dimungkinkan terjadi perubahan sifat tanah. Daerah yang lahannya dimatangkan dengan prakonsolidasi, akan mempunyai kekuatan lapisan-lapisan tanah yang lebih tinggi. Untuk itu, pada daerah yang mengalami prakonsolidasi perlu dilakukan penyelidikan tanah lagi untuk mendapatkan data pada kondisi pasca prakonsolidasi yang mungkin akan menghasilkan rancangan fondasi yang lebih efisien.


B8 B70.00

10.00

20.00

30.00

40.00

50.00

60.00

Silty CLAY

Silty CLAY

Sandy SILT

SAND

Very Softto Soft

Medium

Dense tovery dense

Pile-1 37.00

Pile-2 40.00

0.00

- 23.50

- 31.50

- 34.50- 33.50

- 60.00

- 26.50

Gambar 6.3 Kedalaman tiang pancang yang disarankan pada lokasi titik bor B7 dan B8.

6.6 Pematangan Lahan Kondisi asli lahan untuk PLTU Parit Baru relatif rendah dan dapat dikatakan akan selalu terendam air jika air laut/sungai pasang. Dengan kondisi tersebut, perlu adanya pengurugan untuk memanfaatkan lahan ini. Berdasarkan data


tanah yang didapatkan, lahan mempunyai lapisan tanah yang sangat lunak di bagian atas, sehingga penambahan beban akan mengakibatkan tanah mengalami penurunan. Namun, dan jika teknik pengurugan tidak baik, maka tanah lunak akan tergeser. Dengan tanah lunak berupa lanau/lempung, maka penurunan yang dominasi adalah penurunan konsolidasi. Proses penurunan konsolidasi akan memerlukan waktu yang cukup lama sehingga apabila pembangunan PLTU tidak mendesak, penimbunan bisa dilakukan terlebih dahulu kemudian dibiarkan sampai penurunan selesai yang bisa memakan waktu beberapa tahun. Jika PLTU perlu dibangun segera, maka pematangan lahan bisa dipercepat dengan mempercepat proses konsolidasi.

Usaha mempercepat konsolidasi sering dilakukan dengan memberikan beban lebih besar dari beban rencana (preloading) sehingga proses konsolidasi sampai level penurunan yang direncanakan cepat selesai. Jika penurunan yang dikehendaki telah tercapai, maka kelebihan beban diambil dan lahan bisa dibangun. Teknik percepatan konsolidasi dengan preloading hanya bisa dilaksanakan dengan efektif jika lapisan tanah yang kompresibel relatif tipis. Untuk lahan rencana PLTU Parit Baru, lapisan tanah lunak dan kompresibel cukup tebal sehingga percepatan konsolidasi dengan preloading saja tidak mampu menghasilkan percepatan yang memadai. Untuk itu, jika diperlukan percepatan konsolidasi yang baik, drainasi vertikal (vertical drains) perlu digunakan untuk memperpendek lintasan air keluar sehingga proses konsolidasi bisa jauh lebih cepat. Jarak antar drainasi vertikal perlu diatur sedemikian rupa sehingga target penurunan konsolidasi untuk waktu yang dikehendaki bisa tercapai.

6.6.1 Analisis penurunan Dari hasil pengukuran, dilaporkan bahwa muka tanah rerata mempunyai elevasi sekitar +2.25 m dan diharapkan elevasi lahan jadi sekitar +5.15 m, sehingga diperlukan peninggian netto sekitar 2.90 m. Tinggi timbunan tersebut masih harus ditambah lagi dengan cadangan untuk penurunan konsolidasi. Untuk tanah lunak, biasanya timbunan akan mengakibatkan penurunan sekitar 40% sehingga untuk mencapai tinggi timbunan bersih 2.90 m diperkirakan akan diperlukan urugan dengan tinggi sekitar 5.0 m. Dengan material urug yang mempunyai berat volume sekitar 18 kN/m3, maka lahan akan menerima tambahan beban minimum 90 kN/m2. Gambar 6.4 menyajikan sketsa proses pematangan lahan untuk mencapai elevasi tanah seperti yang direncanakan.

1 Hitungan besaran konsolidasi

Besarnya penurunan konsolidasi (sc) dihitung dengan rumus:

Sc = mvpH (6.3)

Dengan mv = koefisien kompresibilitas volume, p = tambahan beban dan H = tebal lapisan yang ditinjau.

Ketebalan lapisan yang diperhitungkan tehadap penurunan konsolidasi adalah lapisan lempung dengan konsistensi sedang (medium) atau yang lebih lunak.


Dalam analisis ini, kedalaman penurunan konsolidasi dihitung sampai lapisan dengan kedalaman 27.50 m. Rangkuman besarnya penurunan konsolidasi pada masing-masing titik disajikan pada Tabel 6.8.

Tabel 6.8 Rangkuman hasil hitungan penurunan konsolidasi.

Point Tinggi Timbunan (m) Penurunan Tambahan tinggi

5.00 2.254 2.746

5.20 2.344 2.856

B7

5.30 2.389 2.911

5.00 1.551 3.449

5.20 1.613 3.587

B8

5.30 1.644 3.656

Dari hitungan di atas, tampak bahwa pada tinggi timbunan sekitar 5.30 m penurunan konsolidasi terbesar sekitar 2.389 meter di titik B7, sehingga perkiraan tinggi timbunan total sebesar 5.30 meter untuk mendapatkan peninggian netto 2.91 m.

Mengingat kondisi lapisan tanah bagian atas sangat lunak, maka tanah tidak akan mampu menopang tambahan beban tersebut jika diberikan secara serentak (satu tahap). Penyelesaian bisa dilakukan dengan memberikan beban secara bertahap, ditunggu proses penurunannya, jika tanah dasar telah turun/memadat dan kekuatan telah bertambah maka bisa ditambahkan beban lagi. Kondisi tanah yang sangat lunak juga bisa mengakibatkan tanah urug menggeser tanah asli atau masuk menyatu dengan tanah asli. Untuk mencegah masalah tersebut, sering digunakan lembaran pemisah antara tanah urug dan tanah asli, misalnya: geotekstil, anyaman bambu atau yang lain.


+ 2.90

0.00

10.00

20.00

30.00

40.00

50.00

60.00

Silty CLAY

Silty CLAY

Sandy SILT

SAND

Very Softto Soft

Medium

Dense tovery dense

B8 B7Fill material

Fill elevation

Final elevation

Original Ground level

Settlement

Gambar 6.4 Sketsa pematangan lahan untuk mencapai elevasi tanah rencana +2.90 m di

atas muka tanah asli.

2 Waktu konsolidasi

Waktu berlangsungnya proses konsolidasi bisa berlangsung sangat lama, tergantung terutama oleh koefisien permeabilitas tanah dan ketebalan lapisan tanah. Pengaruh koefisien permeabilitas tanah, diwujudkan pada parameter koefisien konsolidasi (cv). Proses konsolidasi tidak dipengaruhi langsung oleh


besarnya penurunan konsolidasi, namun lebih dikaitkan pada derajat konsolidasi (U) yang dikaitkan dengan faktor waktu (Tv). Waktu (t) untuk mencapai suatu kemajuan proses konsolidasi U, dihitung dengan rumus:

2dtcT vv

=

Dengan d adalah jarak lintasan drainasi terjauh, yang untuk kondisi lahan yang diselidiki besarnya sama dengan ketebalan lapisan yang ditinjau. Dalam analisis ini digunakan nilai cv rerata bagian atas. Gambar 6.5 menunjukkan hubungan antara derajat konsolidasi dengan waktu untuk nilai cv rerata bagian atas.

0

25

50

75

100

0 250 500 750 1000

Time (year)

Deg

ree

of C

onso

lidat

ion

(%)

Data B7

Data B8

Gambar 6.5 Hubungan antara waktu dan derajat konsolidasi untuk cv rerata.

Untuk suatu pembangunan, biasanya cukup aman jika penurunan bangunan kurang dari 25 mm atau 30 mm atau sekitar 1% sampai 2%. Apabila persyaratan tersebut yang harus dipenuhi, maka pembangunan bisa dimulai saat kemajuan konsolidasi sekitar 98% sudah tercapai. Berdasarkan hasil hitungan di atas, untuk mencapai kemajuan konsolidasi sebagaimana tersebut di atas akan diperlukan waktu lebih dari 500 tahun untuk nilai cv rerata bagian atas.

Hasil di atas didasarkan dari data hasil uji laboratorium. Menurut berbagai laporan disebutkan bahwa nilai cv lapangan dapat jauh lebih besar dibandingkan dengan nilai hasil uji laboratorium. Dengan demikian, tidak menutup kemungkinan waktu proses konsolidasi yang dari hitungan hasilnya lebih dari 500 tahun kenyataannya dapat jauh lebih cepat. Namun karena lapisan tanah cukup tebal maka waktu proses konsolidasi tentunya masih relatif sangat lama.

3 Percepatan konsolidasi dengan vertical drains

Dengan hasil sebagaimana telah disampaikan di atas, maka proses penurunan konsolidasi akan berlangsung sangat lama. Untuk keperluan pekerjaan yang


mendesak, harus ada usaha untuk mempercepat proses konsolidasi dengan cara khusus. Salah satu cara yang sudah sudah banyak digunakan dan berhasil dengan baik adalah dengan penggunaan drainasi vertikal (vertical drains) untuk mempercepat proses mengalirnya air pori keluar. Media penghantar drainasi yang saat ini cukup populer adalah dengan menggunakan material sintetik sering dikenal dengan strip/band/wick drains, dengan ukuran lebar tidak lebih dari 100 mm, tebal kurang dari 4 mm dan panjang bisa puluhan meter tergantung keperluan dan alat yang tersedia.

Analisis penggunaan band drains dilakukan dengan menggunakan rumus yang diusulkan oleh Hansbo (1982). Dengan penyederhanaan seperlunya, diberikan rumus hubungan antara waktu konsolidasi (t) setelah menggunakan band drains untuk derajat konsolidasi tertentu (U) dengan diameter pengaruh (D) atau jarak band drains yang juga tergantung dari diameter ekivalen dari band drains (d) dan koefisien konsolidasi arah horisontal (ch), sebagai berikut.

UdD

cDt

h

=1

1ln75.0ln8

2

(6.4)

Besarnya nilai ch dianggap dua kali nilai cv, dan karena drainasi arah horisontal yang dominan, maka digunakan nilai ch = 2cv rerata bagian atas. Jarak pemasangan antar band drains (S) sebesar D/1.05 untuk pola pemasangan segitiga dan sebesar D/1.13 untuk pemasangan pola bujur sangkar.

Dengan menggunakan salah satu produk dengan ukuran lebar 100 mm dan tebal 3 mm, dihitung hubungan antara waktu dan jarak pemasangan vertical drains. Gambar 6.7 merupakan hasil hitungan penggunaan vertical drains untuk derajat konsolidasi sekitar 98%. Dari hasil tersebut tampak bahwa di titik B7 kondisinya lebih kritis, dimana dengan jarak band drains 1.0 m untuk mencapai proses konsolidasi 98% masih diperlukan waktu sekitar 11 bulan.

Data Point B7

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5S (meter)

t (ye

ars)

2

using average cvusing max cv

Gambar 6.6 Hubungan waktu dan jarak vertical drains (untuk data point B7, U = 98%).


Data Point B8

00.5

11.5

22.5

0 0.5 1 1.5 2S (meter)

t (ye

ars)

using average cvusing max. cv

Gambar 6.7 Hubungan waktu dan jarak vertical drains (untuk data point B8, U = 98%).

4 Percepatan konsolidasi dengan ekstra beban (surcharge) dan vertical drains

Pemberian beban lebih (surcharge) akan menambah besarnya penurunan konsolidasi lapisan tanah. Kecepatan penurunan yang terjadi akan tetap dipengaruhi oleh persen atau derajat konsolidasi, sehingga untuk lapisan tanah yang sama pada waktu yang sama akan dicapai derajat konsolidasi yang sama. Namun untuk beban yang lebih besar, dengan derajat konsolidasi yang sama, besarnya penurunan dalam satuan panjang akan lebih besar. Kejadian ini sering dimanfaatkan untuk usaha mempercepat penurunan konsolidasi pada beban rencana (tertentu) dengan memberikan tambahan beban lebih agar penurunan untuk beban rencana lebih cepat tercapai kemudian beban lebih diambil. Diharapkan setelah beban lebih diambil, penurunan akibat beban rencana sudah tereliminasi.

Perlu ditegaskan lagi bahwa penimbunan harus memperhitungkan kapasitas dukung tanah sehingga kegagalan kapasitas dukung tanah tidak terjadi. Walaupun telah menggunakan vertical drains, proses pengurugan masih harus selalu dimonitor terhadap deformasi tanah arah vertikal maupun horisontal.

6.7 Kesimpulan Kondisi lapisan tanah di lokasi rencana PLTU Parit Baru tersusun dari lapisan lunak setebal sekitar 27.50 m di bagian atas dan lapisan keras terdapat mulai kedalaman sekitar 35 m. Fondasi bangunan yang dianggap paling sesuai adalah dengan tiang pancang beton pracetak yang dipancang sampai lapisan padat/keras.

Dengan peninggian rencana sekitar 2.90 m, maka diperlukan pengurugan setinggi 5.30 m. Akibat urugan dan beban yang akan bekerja, lahan akan turun. Penurunan diperkirakan cukup besar dan memakan waktu yang sangat lama. Untuk percepatan konsolidasi disarankan menggunakan vertical drains, dengan atau tanpa surcharge disesuaikan dengan jadwal proyek.


Mengingat kondisi tanah bagian atas sangat lunak maka penimbunan perlu dilakukan dengan hati-hati sehingga keruntuhan akibat kegagalan kapasitas dukung tanah tidak terjadi.

6.8 Saran Kedalaman tanah keras yang bervariasi, diperlukan cobaan pemancangan di beberapa bagian sehingga panjang tiang lebih mudah untuk diperkirakan. Teknik pengurugan perlu mempertimbangkan kondisi tanah dasar yang lunak, sehingga tidak terjadi masalah.

Perlu dirancang lebih detail program penimbunan bertahap dan sistem evaluasi kekuatan tanah dasar sebelum tahapan penimbunan selanjutnya dilaksanakan.


BAB 6 PENYELIDIKAN TANAH.................................................................... 6-1

6.1 Pengantar ..........................................................................................................................................6-1

6.2 Pekerjaan Lapangan ........................................................................................................................6-1 6.2.1 Pekerjaan Pengeboran...............................................................................................................6-1 6.2.2 SPT (Standar Penetration Test).................................................................................................6-2 6.2.3 Pengambilan Sampel Tanah dan Air Tanah..............................................................................6-3

6.3 Pekerjaan Laboratorium .................................................................................................................6-3 6.3.1 Kadar Air ..................................................................................................................................6-3 6.3.2 Berat Jenis.................................................................................................................................6-3 6.3.3 Batas-batas Atterberg................................................................................................................6-3 6.3.4 Distribusi Ukuran Butir ............................................................................................................6-4 6.3.5 Uji Triaksial ..............................................................................................................................6-4 6.3.6 Uji Tekan Bebas........................................................................................................................6-4 6.3.7 Uji Konsolidasi .........................................................................................................................6-4 6.3.8 Uji Geser Langsung ..................................................................................................................6-5

6.4 Hasil Penyelidikan............................................................................................................................6-5 6.4.1 Kondisi Lahan...........................................................................................................................6-5 6.4.2 Lapisan-lapisan Tanah ..............................................................................................................6-5 6.4.3 Hasil Uji Laboratorium.............................................................................................................6-8

6.5 Pemilihan Fondasi Bangunan..........................................................................................................6-9 6.5.1 Analisis Kapasitas Tiang.........................................................................................................6-10

6.6 Pematangan Lahan.........................................................................................................................6-13 6.6.1 Analisis penurunan .................................................................................................................6-14

6.7 Kesimpulan .....................................................................................................................................6-19

6.8 Saran................................................................................................................................................6-20

Gambar 6.1 Perletakan titik-titik penyelidikan lapangan (bor dan sondir). B7 pada Steam Turbin Block sedangkan B8 pada Chimney............................................................. 6-2

Gambar 6.2 Lapisan-lapisan tanah dari hasil pemboran. ............................................... 6-6

Gambar 6.3 Kedalaman tiang pancang yang disarankan pada lokasi titik bor B7 dan B8................................................................................................................................ 6-13

Gambar 6.4 Sketsa pematangan lahan untuk mencapai elevasi tanah rencana +2.90 m.. 6-16

Gambar 6.5 Hubungan antara waktu dan derajat konsolidasi untuk cv rerata. ............. 6-17

Gambar 6.6 Hubungan waktu dan jarak vertical drains (untuk data point B7, U = 98%)................................................................................................................................ 6-18

Gambar 6.7 Hubungan waktu dan jarak vertical drains (untuk data point B8, U = 98%)................................................................................................................................ 6-19


Tabel 6.1 Kedalaman dan diskripsi hasil pemboran dan uji penetrasi test B7. .............. 6-7

Tabel 6.2 Kedalaman dan diskripsi hasil pemboran dan uji penetrasi test B8. .............. 6-7

Tabel 6.3 Kapasitas dukung tiang diameter 400 mm. .................................................. 6-11

Tabel 6.4 Kapasitas dukung tiang untuk berbagai diameter tiang................................ 6-11

Tabel 6.5 Kedalaman lapisan tanah yang diperhitungkan gesek.................................. 6-11

Tabel 6.6 Korelasi nilai SPT (N) dengan kohesi tanah (cu). ........................................ 6-12

Tabel 6.7 Kapasitas dukung gesek/lekat tiang yang diperbolehkan............................. 6-12

Tabel 6.8 Rangkuman hasil hitungan penurunan konsolidasi. ..................................... 6-15


Bab 6 GROUND INVESTIGATION

6.1 Introduction/General This additional ground investigation was intended to provide a more accurate geotechnical data at the proposed power plan PLTU Parit Baru. The results will be used for designing foundation of the structures including selection of the type, depth and capacity of the foundation. The investigation consists of two main works i.e. field work which covers soil boring, standard penetration test, retrieving disturbed and undisturbed soil samples as well the water sample; and laboratory tests on soil samples.

Borings were conducted at two position, B7 and B8 (see Figure 4.1). The depths of both points were 60 meter. During boring work, soil resistance was measure using standard penetration test and collecting both disturbed and undisturbed sample for laboratory tests.

6.2 Field Works

6.2.1 Boring Two boring points performed in this site were carried out using a rotary core drilling machine that employed a single core barrel with tungsten bit. During drilling work, casing and drilling mud were used to prevent collapsing the soil into the hole. Samples obtained from coring were placed in core boxes, in which each box accommodated 5 m long continuous core. Soil characteristics were identified visually and measurement of the ground water table was made. In some cases, soil from split barrel was placed in plastic bag to prevent loosing its water content for additional test if required.

Boring were terminated whenever it reached one of the following condition

the depth as mentioned in the TOR (60 meter), or stopped at hard rock layer where the tungsten bit could not penetrate

properly.

The layout of site investigation points (borings and cone penetration tests) is shown in Figure 6.1.

Additional Site Investigation on Parit Baru Thermal Plant (255 MW) West Borneo 6-1

Figure 6.1 Site Investigation plan (boring and CPT).

6.2.2 SPT (Standard Penetration Test) These tests were conducted in the borehole at intervals of 1.50 meters. The test was carried out in accordance with ASTM D 1586-84 in which a split-barrel sampler was driven using 63.5 kg hammer dropped freely at 760 mm height. Testing was performed in three successive 150 mm increments, and number of blows at the first 150 mm was not taken into account. Hence, the N value is counted as the total number of blows for the following penetration of 300 mm. The SPTs were performed in borehole at 2 meter deep intervals.

6.2.3 Soil and Water Samplings Undisturbed samples of soft soil were taken from the boreholes. Sampling was conducted according to the ASTM D 1587-83. This method describes a sampling procedure that employs a thin-walled tube to recover relatively undisturbed soil samples. The thin-walled tubes are made from steel and have outside diameter of 76 mm and diameter of 500 mm long, and are made from steel. A relatively undisturbed sample was obtained by pushing the tube into the in-situ soil and removing the soil-filled tube. The ends of the sample in the tube were sealed to prevent the soil losing its moisture. The sample is recorded and the identity of the sample is written down at on the tube prior to sending them to soil mechanics laboratory for further tests.


After completion of boring at 60 m deep, some water samples were taken from the hole. The water samples were tested in a laboratory to obtain data related to the quality of the onsite water

6.3 Laboratory works

6.3.1 Moisture/water Content Water content is determined according to the ASTM D2216-90, which covers soils, rock and similar materials by mass. Water content is the ratio (in percent) of the mass of pore or free water to the mass of solids in the soil.

The test is carried out by drying wet soil in an electric oven at constant temperature of 110oC for 24 hours to allow the pore water evaporating completely. Mass of the water is the difference between the mass of wet soil and the mass of solids (dry soil).

6.3.2 Specific Gravity Specific gravity of soil is tested according to the ASTM D 854-91, using pycnometers of 50 mL capacity. The specimen is taken from soil fraction passing sieve no. 4. Specific gravity is the ratio of the mass of a volume of solid soil particles at stated temperature to the mass in air of the same volume of gas-free distilled water at stated temperature.

6.3.3 Atterberg Limits Atterberg limits for this purpose are liquid limit and plastic limit. The test is conducted according to the ASTM D 4318-84. Liquid limit is the water content (in percent) of a soil at the arbitrary defined boundary between the liquid and plastic states. This water content is defined as the water content at which a pat of soil placed in a standard cup and cut by a groove of standard dimensions will flow together at the base of the groove for a distance of 13 mm (1/2 in.) when subjected to 25 shocks from the cup being dropped 10 mm in a standard liquid limit apparatus operated at a rate of 2 shocks per second. In this work, the test method is the multipoint liquid limit in which the water-contents of the specimen are adjusted to bring them to consistencies that give 2 points (blows) between 25 to 35 and 2 other points between 15 and 25 of the liquid limit device to close the groove. The data are plotted as a graph of the water-contents as ordinates (arithmetical scale) and corresponding number of blows as abscissas (logarithmic scale), and then, a best straight line through the three or more plotted points is drawn. The liquid limit of soil is the water content at the intersection of the line with the 25-blow abscissa.

Plastic limit is the water content (in percent) of a soil at boundary between the plastic and brittle states. The water content at this boundary is the water content at which a soil no longer be deformed by rolling into 3.2 mm in diameter threads without crumbling.


6.3.4 Particle Size Distribution Particle size distribution is determined in accordance to the ASTM D 422-63(1990). In this test method, the distribution of particle sizes larger than 0.075 mm is determined by sieving, while the distribution of particle sizes smaller than 0.075 mm is determined by a sedimentation process using a hydrometer to secure the necessary data. The result is presented as a curve on semi logarithmic plot, the ordinates being the percentage by mass of particles smaller than the size given by the abscissa.

6.3.5 Triaxial Compression Test Unconsolidated undrained triaxial compression test is conducted on cohesive soils according to ASTM D 2850-87. The tests were performed on undisturbed samples having cylindrical shape enclosed in a rubber membrane. The specimen was subjected to a confining pressure in a triaxial chamber, and an axial compression load is applied to failure under constant rate of strain. In this test method, no drainage of the specimen is permitted and the rate of strain is adjusted to be relatively high, providing a quick test condition. Undrained strength of soil is expected from this test and some stress-strain properties could be obtained.

6.3.6 Unconfined Compression Test Unconfined compression test is intended for cohesive soils of medium or stiffer according to the ASTM D 2166-91. The test will be conducted on cylindrical undisturbed sample having height twice of its diameter. Load is applied axially at strain rate of 0.50 to 2% per minute. Loading is continued until the load values decrease with increasing strain or until 15% strain is reached.

The compressive stress is computed by dividing the axial force with the corresponding corrected cross section area. The maximum value of compressive stress or compressive stress at 15% axial strain, whichever is secured first, is reported as the unconfined compressive strength. This value is often related to the soil cohesion.

6.3.7 Consolidation Test Consolidation tests were conducted according to the ASTM D 2435-90. The purpose of this test is to determine magnitude and consolidation rate of undisturbed specimens that is restrained laterally and drained axially while subjected to incrementally applied (controlled-stress) loading. In this work, the test was performed with constant load increment duration of 24 hours. The load increment was set doubling the previous loading intensity. At each stage of loading, a relationship between time and settlement is obtained from which consolidation parameters could be determined. In a full test, a relationship between load and settlement or void ratio could be produced, leading to parameter for calculation of consolidation settlement.


6.3.8 Direct Shear Test Method employed for this test is the ASTM D3080-90. This test will be conducted on sands or soils in which the unconfined compression test could not be performed. The test is performed by shearing a soil sample under controlled rate of shearing at an adjusted shear plane. For sandy soils, the test tends to be under consolidated drained condition. This test produces data of soil internal friction angle and possible cohesion.

6.4 Investigation Results

6.4.1 Site Conditions In general, the site being investigated is relatively flat, sloping slightly to the Kapuas and Jungkat rivers (to the south and west directions). The land has low level resulted in ponding during seawater tide. This condition may cause problems for constructing a power plan (PLTU) on this site without land reclamation to the safe level that is free from the water ponding effect.

At the additional ground investigation, the site condition was still same as the condition during the previous site investigation where most of the site was uncultivated land with various bushes. Some buildings and structures of power station facilities 150 kV have occupied a small part of this site and a housing complex.

6.4.2 Soil Conditions Boring results show that the upper soil layer in general consisted of very soft to soft silty clay with significant organic content. At point B7, soil with organic matter was found from ground surface to about 3.50 m deep but at point B8, it was recorded 1 meter at the top only. The layer underneath was silty or sandy clay containing shell fragments with thickness varied between 4.50 m to 8.50 m. This layer had very soft to soft consistencies with N values less than 2.

The following layer was still silty clay extending to depth of 31.55 m to 34.40 m. Some variations were recorded in which layers of soil containing significant amount of organic matter were found at depth of 24.00 m to 26.00 in B7 whereas in B8 they were at depths of 20.00 m to 21.00 m and 22.00 m to 23.55 m. In general, soil condition was soft to medium at the upper part to about 27.50 m deep and becoming medium to stiff at the lower part.

At B7, the soil underlay was dominated by sand at dense to very dense state. At B8, a layer of stiff sandy silt was found about 2 m thick (from 31.50 m to 33.50 m deep) then followed by sand layer as found in B7. The above sandy silt layer was not recorded in B7, and it was considered extending to a limited area. At B7 however, a very stiff silty clay layer was found at depth between 54.00 m to 55.00 m. The boring results are summarized and presented in Table 6.1 and Table 6.2 while Figure 6.2 illustrates the soil condition in both boreholes.


Table 6.1 Depth and soil description from boring and SPT at B7.


0.00-3.50 Silty CLAY, few peat, dark grey to black 2

3.50-6.00 Fine sandy CLAY, few shell fragments, grey 1 or less

6.00-11.00 Silty CLAY, few shell fragments, grey 1

11.00-16.00 Silty CLAY, light grey 2

16.00-19.55 Silty CLAY, few sand, grey 3


24.00-26.00 Silty CLAY, few organic matter, dark grey 3

26.00-29.55 Silty CLAY, grey to yellow 6


34.40-35.55 Silty medium SAND, light grey to white -

35.55-54.00 Slightly silty medium SAND, white 50

54.00-55.00 Silty clay, brown -

55.00-60.00 Slightly silty fine SAND, light grey to white 50

Table 6.2 Depth and soil description from boring and SPT at B8.


0.00-1.00 Silty CLAY, few peat, dark grey

1.00-5.50 Silty CLAY, few sand, shell fragments, grey < 2

5.50-14.00 Silty CLAY, dark grey 2

14.00-17.55 Silty CLAY, few fine sand, dark grey 2





23.55-25.00 Silty CLAY, brown 4

25.00-25.55 Silty CLAY, reddish grey -

25.55-31.55 Silty CLAY, light grey 4-6

31.55-32.00 Fine sandy SILT, grey 17

32.00-33.55 Fine sandy SILT, brownish white -

33.55-35.55 Slightly silty SAND, white 34

35.55-60.00 Slightly silty SAND, white > 50


B8 B70.00

10.00

20.00

30.00

40.00

50.00

60.00

Silty CLAY

Silty CLAY

Sandy SILT

SAND

Very Softto Soft

Medium

Dense tovery dense

0.00

- 23.50

- 31.50

- 34.50- 33.50

- 60.00

- 26.50

Figure 6.2 Estimated soil condition between borehole B7 and B8.

6.4.3 Laboratory Test Results Laboratory test results indicated that in some layers, soil contained significant amount of organic matter that could be identified using the values of their specific gravity. The lowest specific gravity value was recorded at 2.16. In a


peaty soil, the water content could reach 237%. The soil layers with organic matter were found near the ground surface and some others at depth between 20 m to 25 m. Soil samples at depth from 0 to about 34 m which were predominantly clay, mostly contained fine fraction (passing sieve no 200) more than 65%, and even it reached 98%. Liquid limits of the soil notably varied from about 42% to 94% or from low plasticity to very high plasticity. With plasticity indices ranging from 15% to 63%, using ASTM soil classification system, the majority of clay soils from this site could be classified into clay or silt of high plasticity (CH-MH). Some others were in the group of low plasticity clay or silt (CL-ML) and some others may be in group of clay/silt containing high organic content with various plasticities (OL-OH).

In sand samples, the fine fractions were usually less than 3%, with coefficients of uniformity were less than 4 and coefficients of curvature were in between 1 and 3. This data bring the sand samples into poorly graded sand group that was relatively clean and uniform.

Shear strength of undisturbed soils measured using triaxial compression tests indicated that soils have various strength parameters, ranging from very soft to medium stiff with undrained cohesion (cu) ranging from 3 to 38 kN/m2. The internal friction angles from triaxial compression test were recorded to be less than 10 degree.

For the sand layers that were relatively dense at deep position, undisturbed sample could not be retrieved, and laboratory tests were conducted on samples obtained by split spoon sampler (SPT tube). Direct shear tests on the sand samples showed that the soil cohesions were very low and negligible, whereas, the internal friction angles were around 40o.

Compressibility of undisturbed soil measured using odometer test showed that most of soils have relatively high compressibility with compression index values in between 0.4 to 2. Those values indicate that loading on the site will cause significantly large settlement. The parameter of consolidation time represented as consolidation coefficients (cv) were found in a range of 0.6 10-4 to 0.7510-3 cm2/sec or about 0.2 to 2.4 m2/yr.

6.4.4 Comparison to the previous data In general, most of this additional investigation results were close to the previous results. Some variations, however, were observed. Both results showed that the upper soil layer to depth about 32 m comprised of predominantly clay with variation contents on silt, sand, shell fragments and organic matter. In this additional investigation, the contents of organic matter were found higher than the previous results. They were recorded at upper part near the surface and some layers were recorded in depth between 20 m to 26 m. Standard penetration tests in the previous investigation gave higher values of N. This may be caused by the equipment type in which the previous tests used manually dropping hammer whereas in this additional investigation, an automatic dropping hammer was employed. It is expected that the result will be more reliable. Other similarities were observed in the triaxial test


results in clay soils. In compressibility tests, the parameters of consolidation magnitude were found to be similar but in the rate of consolidation parameters, the recent results were lower than the previous results.

For the sand samples taken from lower layers, N values from standard penetration tests could not be compared because most results had values greater than 50 and the tests were stopped before reaching penetration of 30 cm. Laboratory tests on sand sample of previous investigation were limited, hence comparison could not be established.

6.5 Selection of Foundation As mentioned before, soil layers at the proposed power plan PLTU Parit Baru consisted of very soft to soft soil at the upper layer. This soft layer extended to depth of about 23 meters. The following layer had medium to stiff condition with layer thickness of about 10 meters. The hard or very dense layer was found from depth of 35 meters downward.

Based on the soil condition, a shallow foundation is not suitable to support high intensity load because of the low bearing capacity. The settlement due the load could be significantly large and the stability of the structure built on this site is then questioned. The most suitable type of foundation for structures on this site is considered to be pile foundation, which penetrates through the soft soil and supported by hard layer at reasonably depth. The piles need to penetrate into hard layer to certain depth to guarantee the stability of the foundation because the upper layer of soft soil tends to flow.

Type of pile could be cast in situ pile (bored pile) or pre cast pile. Bored pile has some advantages: the pile length and size could be easily adjusted to meet requirement. However, this method needs heavier equipments and transportation especially to support concreting process. Because the upper layer soil is very soft; the quality of bored pile could not be assessed properly.

Precast pile is considered to be simple and more applicable for this site. Pile could be fabricated in a factory and transported to the site at large number, which is also possible, by shipping. The piles are installed by driving them into the soil to desired depth. The disadvantage of pre cast pile is in defining the exact pile length related to the available length product. The use of segmental piles requires splices, which may become the weak part of the pile. On the other hand, the uses of longer pile need some additional works to cut the unused part at the top and to dispose them. However, the use of pre cast pile is considered to be most efficient and the construction may be quicker. Many precast pile factories are available recently. The pile length could be ordered as required and for designed length that longer than product capacity, it could be accommodated by splicing several segmental piles. The most common pre fabricated pile is prestressed concrete pile of round cross section with hollow at the middle.


6.5.1 Pile Capacity Analysis Lengths of pile were designed to penetrate into hard layer to certain depth to ensure that the load could be transferred properly into the stable soil. Based on the investigation results, it is suggested to use pile lengths in between 37 m to 40 m below the existing ground level. Based on this pile length, the capacity of the pile was analyzed on both end bearing and fiction resistance.

1 End bearing

Due to the soil condition around the pile tip, which is dense to very dense, results the undisturbed samples could not be retrieved. The end bearing capacity of pile was calculated based on the SPT results. By assuming that the pile penetrated to enough depth into the dense layer, the ultimate end bearing capacity of pile (Qu) could be determined using a formula:

Qu (kN) = Ap 400 N (6.1)

where Ap = pile cross section area (m2) and N = average SPT value from 10D above to 4D below pile tip (D = pile diameter).

The N values were taken from test results, however, for N values greater than 50, the analysis considered only the maximum N value of 50. The allowable bearing capacity of a pile was obtained by applying safety factor (F) of 3 to 5. Table 6.3 shows an illustration of the analysis, N values for calculation and the results using precast pile of 400 mm diameter.

Table 6.3 Pile capacity (400 mm diameter).

Boring Depth (m) N Naverage Qu (kN) Qa (kN) F=4

B7 37.00 7,50,50 35 1759 439

B8 37.00 17,34,50 33 1658 414

B7 40.00 50,50,50 50 2513 628

B8 40.00 50,50,50 50 2513 628

For other pile diameters that may be used, the analyses are summarized in Table 6.4.


Table 6.4 Pile capacity (various pile diameters).

Allowable pile capacity, Qa (kN) related to pile diameter Boring Depth (m)

400 mm 500 mm 600 mm

B7 37.00 439 687 989

B7 40.00 628 981 1413

B8 37.00 414 647 933

B8 40.00 628 981 1413

2 Friction resistance

As mentioned before, soil condition of the site consists of soft soil at the upper layer where it cannot be considered in supporting load. In contrary, it may give negative friction on the pile. Due to these reasons, in analyzing the friction pile capacity, it considers only the pile segment that penetrated in relatively hard layer. The length pile taken into account for friction at each boring point is presented in Table 6.5.

Table 6.5 The depth of soil layer for friction resistance

Boring Depth for friction analysis

B7 From 27.00 m downward

B8 from 23.00 m downward

For the pile segment above, soil layers mostly consist of clay soils although some thin sand layers exist. The friction pile capacity in clay (Qs) is calculated according to formula:

Qs (kN) = pLf (6.2) where p = perimeter of pile cross section, L length of pile segment for friction, f = friction or cohesion.

For clays: f = cu, where = adhesion factor and cu = soil cohesion obtained from correlating to N value of SPT (Table 6.6). For precast pile in sand, the soil friction is taken also from correlation to the SPT results i.e. f = 2N (kN/m2).


Table 6.6 Correlation of N vs. undrained strength (cu).

Consistency Cohesion, cu (kN/m2) N (SPT)

Very soft 0 12.5 0 2

Soft 12.5 25 2 4

Medium 25 50 4 8

Stiff 50 100 8 16

Very stiff 100 200 16 32

Hard > 200 > 32

The allowable capacity on friction is obtained by applying a safety factor of 4 as summarized in Table 6.7. Figure 6.3 illustrated the suggested pile depth at borehole B7 and B8.

Table 6.7 Pile capacity based on friction resistance.

Allowable Capacity on Friction Qa (kN) Borehole

Number Depth 400 mm 500 mm 600 mm

B7 37.00 171 214 257

B7 40.00 265 332 398

B8 37.00 188 235 282

B8 40.00 300 375 450

The total pile capacity may be taken as combination of end bearing and friction capacity. The allowable pile capacity has to be checked to the structural pile capacity and the lower value should be taken. Due to the condition of soil that is soft at the upper part, negative skin friction need to be considered into account.

Considering that a ground treatment will be carried out, it would be possible to expect some improvement of soil properties. After treatment, soil may have higher resistance resulting higher friction capacity and reducing the negative friction. It would be suggested to have some additional investigation on the site after treatment to find out the final soil condition and possible new design that is more efficient.


B8 B70.00

10.00

20.00

30.00

40.00

50.00

60.00

Silty CLAY

Silty CLAY

Sandy SILT

SAND

Very Softto Soft

Medium

Dense tovery dense

Pile-1 37.00

Pile-2 40.00

0.00

- 23.50

- 31.50

- 34.50- 33.50

- 60.00

- 26.50

Figure 6.3 Suggested pile depth at borehole B7 and B8.

6.6 Ground improvement The site of the proposed power plan PLTU Parit Baru was relatively low and submerged most of the time under water especially during water tide. For the purpose of the project, the ground level needs to be increased to a safe level.


Based on the boring results, the upper layer of soil consists of very soft to soft clay. Fill material on this soil will cause a significant settlement and incorrect method of fill placement may result displacement of the original soil. The settlement of soft soil, which is dominated by clay, will be mainly consolidation settlement. The consolidation process will take for long period, and this becomes a special consideration of the project time schedule. When the project needs to be completed soon, the site could be treated to accelerate the consolidation process.

Accelerating the consolidation process could employed surcharge or extra load greater than design load in which the consolidation to the level of design load occurs more rapidly. When the magnitude of consolidation has reached the predicted settlement of design load, the extra load is removed and the construction could be started. This method is suitable only when the compressible layer was relatively thin. Unfortunately, the site of power plan PLTU Parit Baru has very thick layer of soft soil, hence, the use of preloading only to accelerate the consolidation process may not be effective. For this purpose, when the consolidation process has to be shortened, the use of vertical drains could not be avoided in which the excess pore water pressure could be dissipated by allowing the water to flow horizontally under significantly shorter drainage path. The distance of vertical drains could be adjusted to meet the schedule of the project construction.

6.6.1 Settlement analysis From topographical survey, it was reported that the average ground level was at +2.25 m and the final elevation of ground surface was expected to be at +5.15 m, hence, it is required a net elevation increase of 2.90 m. In addition, the height of fill needs some allowance to anticipate the settlement or consolidation of soft soil beneath the fill material. Based on information from previous works on soft soil reclamation, fill material will cause settlement of about 40% of the fill height. For a net fill height of 2.90 m, it is predicted the need of fill at least 5.0 m thick. Using a fill material of 18 kN/m3 unit weight, the consolidation settlement analysis will employ a uniform design load of about 90 kN/m2.

1 Magnitude of consolidation

The magnitude of consolidation settlement (sc) is calculated:

sc = mvpH (6.3)

where, mv = coefficient of volume compressibility, p = load increment and H = soil layer thickness.

The depths of soil layer for consolidation analysis involved the clay layers having medium consistency and softer. In the analysis, consolidation settlement was calculated to a depth of 27.50 m.


The summary of consolidation settlements at each point with several heights of fill is shown in Table 6.8. Figure illustrates the placement of backfill to obtain designed ground elevation at 2.9 m above the existing ground level.

+ 2.90

0.00

10.00

20.00

30.00

40.00

50.00

60.00

Silty CLAY

Silty CLAY

Sandy SILT

SAND

Very Softto Soft

Medium

Dense tovery dense

B8 B7Fill material

Fill elevation

Final elevation

Original Ground level

Settlement

Figure 6.4 Ground improvement to obtain designed ground elevation at 2.90 m above the

existing ground level.


Table 6.8 The depth of soil layer for consolidation analysis

Point Backfill Height (m) Settlement Additional Height

5.00 2.254 2.746

5.20 2.344 2.856

B7

5.30 2.389 2.911

5.00 1.551 3.449

5.20 1.613 3.587

B8

5.30 1.644 3.656

Based on the in above results, when the fill height is 5.30 m, the consolidation settlement will be about 2.389 m at point B7, thus using a fill height of 5.30 m, a net increase in surface elevation of 2.91 m will be expected.

However, due to the condition of soil that is very soft, the existing soil deposit will not be able to support the fill without any risk in bearing capacity failure. Reclamation needs to be constructed in stages to allow some consolidation settlement to occur which will increase the soil bearing capacity. Uncontrolled filling may cause fill material displacing the existing soil or mixing each other. To prevent this problem, in some cases, a mat was installed beneath the fill to separate the original soil and fill material. The mat could be natural material such as bamboo sheet, or synthetic material (geotextile).

2 Time for consolidation

Consolidation process may occur for very long time depending mainly on the soil permeability coefficient and layer thickness. The permeability coefficient parameter is express in the form of consolidation coefficient (cv). The consolidation process is not affected by the magnitude of consolidation, but it depends on the degree of consolidation (U) which related to the time factor (Tv). Time to reach a consolidation progress of U is calculated using formula:

2dtcT vv = (6.4)

where d is the longest drainage path, for this site, it is equal to the layer thickness. In this analysis, the value of cv was taken as the average value of soil layer from ground surface to the depth of 27.5 m. Figure 6.5 shows the correlation between degree of consolidation and time based on the average cv.


025

50

75

100

0 250 500 750 1000

Time (year)

Deg

ree

of C

onso

lidat

ion

(%)

Data B7

Data B8

Figure 6.5 Relationship between time and degree of consolidation.

For a project of development, it is considered that an acceptable settlement is about 25 mm to 30 mm; in this case, it will be 1% to 2% of the total consolidation settlement. To meet the above requirement, consolidation progress of 98% should be achieved before power plan construction could be started. From the calculation previously, to achieve that progress it needs 500 years for the average cv value.

The above results were based merely on the laboratory test data. Many report mentioned that onsite cv values were far higher than those measured in laboratory. Thus, it is possible that time for consolidation settlement become much lower compared to the analysis result, which would occur more than 500 years. However, the thickness of the clay layer is considered to be very thick, hence, the time for soil to consolidate will be still very long.

3 Acceleration of consolidation settlement using vertical drains

The results shown above indicate that the consolidation process occurs at relatively long period. For a limited project period, a special treatment is required to accelerate the consolidation process. One of the widely used methods is employing vertical drains to facilitate drainage of soil pore water. Various materials of vertical drains have been developed. The most successful material is made of synthetic that is known as strip/band/wick drains. These drains have width not more than 100 mm, about 4 mm thick and unlimited length depending on requirement and available installing equipment.

The analysis of the band drain application could be done either using method of ordinary vertical sand drain or a formula proposed by Hansbo (1982). Using some simplification, a formula of relationship between consolidation time (t) after employing the band drains for certain degree of consolidation (U) and the diameter of influence or band drain spacing which also depending on equivalent diameter of band drains (d) and horizontal consolidation coefficient (ch), is presented as follows.


UdD

chDt

=

11ln75.0ln

8

2

(6.5)

The ch value is assumed to be twice of the cv and because the radial drainage is dominant, the analysis employs the value of ch (in this case related to cv) of soil at the upper layer. Band drain spacing (S) is equal to D/1.05 for triangular installation pattern and D/1.13 for square pattern.

In this analysis, a band drain product of 100 mm wide and 3 mm thick was used for calculating the relationship between the time and spacing of the band drains. Figure 6.6 and Figure 6.7 show the results of band drain application for 98% degree of consolidation. The result shows that at point B7 the condition is more critical, where using 1.0 m band drain spacing the 98% degree of consolidation still requires about 11 months.

Data Point B7

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2S (meter)

t (ye

ars)

using average cvusing max cv

Figure 6.6 Relationship of time and band drain spacing at B7 for U = 98%.

Data Point B8

00.5

11.5

22.5

0 0.5 1 1.5 2S (meter)

t (ye

ars)

using average cvusing max. cv

Figure 6.7 Relationship of time and band drain spacing at B8 for U = 98%.


4 Consolidation acceleration using surcharge and vertical drains

Application of surcharge will increase the magnitude of consolidation settlement of soil layer. The rate of settlement is still affected by degree of consolidation, in which at a same time the consolidation progress reaches the same degree. However, for a larger loading intensity, at a same degree of consolidation, the magnitude of consolidation settlement will be greater. This phenomenon is used to accelerate consolidation settlement at certain designed load by giving some additional load in order to reach settlement of designed load quicker, and then the surcharge is removed afterward. It is expected that after the surcharge being removed, the settlement caused by designed load will be minimum.

It should be noted that filling work has to consider the soil bearing capacity to avoid failure of the soil soft soil deposit. Even though, the vertical drains have been installed, filling process requires a careful monitoring on soil deformations both at vertical and horizontal directions.

6.7 Conclusion Soil condition at the proposed site of power plan PLTU Parit Baru consists of about 27.50 m thick soft soil at the upper layer and the hard layer was found at 35 m deep downward.

Foundation of the structures is suggested to use precast concrete pile driven through soft soil and supported in hard layer.

Development of the site requires increasing the ground level of about 2.90 m. This needs about 5.30 m fill to accommodate the consolidation settlement.

Due to the filling, consolidation settlement takes place and this will occur for long period. To accelerate the consolidation process, it is suggested to employ vertical drains with or without additional surcharge depending on the project schedule.

Reclaiming this site requires a special care to avoid soil bearing capacity failure of the soft soil in this site.

6.8 Suggestion Due to some variation of the hard layer depths, it is suggested to carry out some trial pile driving in order to figure out a more definitive pile length.

The method of filling or reclamation needs to be studied properly to avoid serious problem of the soft soil.

Considering that the fill will be higher than 5 meter, it is suggested to design the stage fillings and evaluation system for the soft soil, which was subjected to stages of load increment.


BAB 6 GROUND INVESTIGATION ............................................................... 6-1

6.1 Introduction/General .......................................................................................................................6-1

6.2 Field Works.......................................................................................................................................6-1 6.2.1 Boring .......................................................................................................................................6-1 6.2.2 SPT (Standard Penetration Test)...............................................................................................6-2 6.2.3 Soil and Water Samplings.........................................................................................................6-2

6.3 Laboratory works.............................................................................................................................6-3 6.3.1 Moisture/water Content ............................................................................................................6-3 6.3.2 Specific Gravity ........................................................................................................................6-3 6.3.3 Atterberg Limits........................................................................................................................6-3 6.3.4 Particle Size Distribution ..........................................................................................................6-4 6.3.5 Triaxial Compression Test........................................................................................................6-4 6.3.6 Unconfined Compression Test..................................................................................................6-4 6.3.7 Consolidation Test ....................................................................................................................6-4 6.3.8 Direct Shear Test ......................................................................................................................6-5

6.4 Investigation Results ........................................................................................................................6-5 6.4.1 Site Conditions..........................................................................................................................6-5 6.4.2 Soil Conditions .........................................................................................................................6-5 6.4.3 Laboratory Test Results ............................................................................................................6-7 6.4.4 Comparison to the previous data...............................................................................................6-8

6.5 Selection of Foundation....................................................................................................................6-9 6.5.1 Pile Capacity Analysis ............................................................................................................6-10

6.6 Ground improvement.....................................................................................................................6-13 6.6.1 Settlement analysis .................................................................................................................6-14

6.7 Conclusion.......................................................................................................................................6-19

6.8 Suggestion .......................................................................................................................................6-19

Figure 6.1 Site Investigation plan (boring and CPT). .................................................... 6-2

Figure 6.2 Estimated soil condition between borehole B7 and B8. ............................... 6-7

Figure 6.3 Suggested pile depth at borehole B7 and B8. ............................................. 6-13

Figure 6.4 Ground improvement to obtain designed ground elevation at 2.90 m above the existing ground level. ............................................................................................ 6-15

Figure 6.5 Relationship between time and degree of consolidation............................. 6-17

Figure 6.6 Relationship of time and band drain spacing at B7 for U = 98%. .............. 6-18

Figure 6.7 Relationship of time and band drain spacing at B8 for U = 98%. .............. 6-18


Table 6.1 Depth and soil description from boring and SPT at B7. ................................ 6-6

Table 6.2 Depth and soil description from boring and SPT at B8. ................................ 6-6

Table 6.3 Pile capacity (400 mm diameter).................................................................. 6-10

Table 6.4 Pile capacity (various pile diameters). ......................................................... 6-11

Table 6.5 The depth of soil layer for friction resistance............................................... 6-11

Table 6.6 Correlation of N vs. undrained strength (cu). ............................................... 6-12

Table 6.7 Pile capacity based on friction resistance..................................................... 6-12

Table 6.8 The depth of soil layer for consolidation analysis........................................ 6-16


LAYOUT OF CORE DRILLING AND CPT POINTS There are 8 core drilling points (B1 B6 are of Site Investigation 2003 and the rest are of Site Investigation 2006) and 6 stations (of Site Investigation 2003) of Dutch Cone Penetration Test (CPT). Layout of these stations is presented in the figure below, while their coordinates can be found in the table.

Layout of Ground Investigation Points

Download - Ground Investigation Parit Baru

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