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The 17th Southeast Asian Geotechnical Conference Taipei, Taiwan, May 10~13, 2010

ATC-03-03

Bored Pile Solution for Embankment Failure on Clay Shale: Design and Analyses of Static and Earthquake Conditions

of the KM 97+500 Cipularang Toll Road in Indonesia

Masyhur Irsyam1, Agus Himawan2, Endra Susila1, and Hendriyawan1

1 Department of Civil Engineering, Bandung Institute of Technology, Bandung, Indonesia 2Geotechnical Engineering Division, PT. LAPI ITB, Bandung, Indonesia

E-mail: masyhur.irsyam@yahoo.co.id

ABSTRACT: The Cipularang Toll Road was built in 2004 to 2005 to connect two main Indonesian cities: the capital city of Jakarta and the capital city of the West Java province Bandung. The toll road passes through hills and valleys on clay shale of the Miocene Djatiluhur Marl formation. In early February 2006, slope failure occurred on a road embankment at Km 97+500. The embankment is on clay shale. This paper presents causes and mechanism of the slope failure, the selected slope reinforcement system and analyses for both static andearthquake conditions. The strength degradation of the clay shale due to exposure and records of slope movement monitoring wereconsidered in the back calculation to simulate the failure mechanism by the finite element method. The elastic plastic constitutive model and the Mohr-Coulomb failure criteria were selected to model soils. The dynamic finite element analysis was also performed to checkdeformation on bored pile under earthquake loading. A synthetic acceleration time-histories at bedrock was generated to represent ground motion of the 500 year return period of earthquake. The performed analyses and records of field slope monitoring showed that a group of 1.0 m diameter of bored piles is effective to stabilize the slope.

Based on the document of original design, the original top soil consists of weathered soft clay shale with an approximate thickness of 0.3 to 2.0 m. Even though the top soft soil has been stripped off prior to fill work, soil investigation performed after slope failure revealed that a layer of the original soft clay shale was still found directly below fill embankment, especially below the toe of slope. Back calculation by slope stability analysis confirmed that the slope failure had occurred on the soft clay shale. Irsyam et al. (2007) concluded that shale loses its shear strength due to soil stripping/excavation which next has resulted in quick weathering.

1. INTRODUCTION

The Cipularang Toll Road was constructed in 2004 to 2005 to connect two major cities: the capital city Jakarta and the capital city in west Java province Bandung and the surrounding area. Since being opened for public transportation in March 2005 the toll road has become its major role for the economic growth for Bandung and its surrounding area. Due to its topographical and geological conditions, the highway have to pass hills and valleys on clay shale of the Miocene Djatiluhur Marl formation (Irsyam et al., 2006).

After approximately one year operation, slope failure occurred on a road embankment on clay shale at Km 97+500 in early February 2006 (Fig. 1). Fig. 2 shows slope monitoring result and crack on road pavement along the median curb. The figure indicated that failure plane started from toe of embankment to the top of embankment at the median of the highway.

2. CLAY SHALE AND ITS BEHAVIORS

Shale is a fine grained sedimentary rock formed from clays compacted by pressure. Shale is generally characterized by thin laminae breaking with an irregular curve fracture, often splintery and usually parallel to the often – indistinguishable bedding plane (Wikipedia, 2007). Shales are typically deposited in very slow moving water and are often found in lake and lagoonal deposits, in river deltas, on floodplains and offshore of beach sands (Wikipedia, 2007). The main engineering behavior of shale is that it is very hard, however, once it is exposed to sunrays, air, and water within a relatively short time it will become soft clays (muds). Its strength and volume stability are time dependent.

Stark and Duncan (1991) concluded that the shear strength of the desiccated clay decreases very rapidly to the fully softened strength when the clay is soaked. Skempton (1977) concluded that heavily overconsolidated clay is usually firm and stable and has comparatively high shear strength at its original condition. When the clay is subjected to a cyclic loading, the strength decreases gradually from the fully softened to its residual value. Skempton (1977) found the peak strength parameters of the clay are: c’ = 14 kN/m2 and = 20o. Gartung (1986) observes that water absorption during unloading process of a clay - that is originally dry and hard with high shear strength - rapidly turns into stiff or even to soft clay with an extremely low shear strength. As the weathering process continues, the shear strength distribution of the soil profile changes with time (Irsyam et al., 2007). For design, Gartung (1986) suggested to use the reduced parameters for the long term condition of = 20o and c = 20 kN/m2. Based on triaxial tests of a London Clay with the largest triaxial samples (diameter of 250 mm) to include a representative assemblage of fissures, Sandroni (1977) found that the cohesion of the larger diameter samples is smaller, c’ = 7 kN/m2 and = 20o.

Figure 1 The slope failure at road embankment on clay shale at KM 97+500 in February 2006

Dt

= 3.16 m= 0.76 m

Dt

= 2.94 m= 0.74 m

Dt

= 1.33 m= 0.74 m

Dt

= 9.0 m= 0.1 m

?? ? ?

Crack monitored at the highway median

ROW

Average Depth, DFailure Plane Thickness, t

= 4.30 m= 0.17 m

Figure 2 Predicted failure plane based on field slope monitoring records Referring to the soil investigation results prior to construction

and deep coring after construction, the failure at road embankment

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ATC-03-03

slope at KM 97+500 of the Cipularang Toll Road was predictably caused by strength degradation of the top clay shale due to stripping works (Irsyam et al., 2007). Unfortunately, laboratory test result on the exposed clay shale was not available. The shear strength parameters of clay shale at failure condition were evaluated by performing back calculation based on predicted failure condition as indicated on the monitoring result of slope indicators (Fig.2). Soil profile (Fig. 3) and shear strength parameters of other layers were developed based on as-built drawing and the result of deep coring after slope failure. Parametric study was performed through the slope stability analysis to examine several predicted shear strength parameters of clay shale. The inputted shear strength parameters of clay shale were selected as the parameters when slope stability analysis resulted in the closest slope failure plane with the interpreted failure plane based on field slope indicators. Slope failure is indicated by a safety factor value of 1.0. (Irsyam et al., 2007).

Figure 3 Soil profile (Irsyam et al., 2007)

The back analysis results showed that the calculated shear strength parameters of the degraded soil layer at failure were c=5.0 kPa and =13o. These values are comparable with the parameters of soaked residual and remoded residual clays suggested by Stark and Duncan (1991) and residual soils recommended by Skempton (1977) but smaller than the design shear strength and residual parameters suggested by Gartung (1986). Results of back analysis by a professional finite element software, PLAXIS (Brinkgreve and Vermeer, 1998) are presented in Figures 4. As shown in the figures, the slope failure plane occurs at Layer D which is the soft silty clays and weathered clay shale.

Figure 4 The predicted slope failure mechanism based on slope stability analysis (Irsyam et al., 2007)

4. BORED PILES FOR REINFORCEMENT OF SLOPE

Due to time and space constrains and topographic condition at the site a group of bored piles was considered as the most suitable solution for the failure slope, therefore the reinforcement system was selected. With the critical function of the highways, the selected system was still considered cost effective (Irsyam, et al., 2006). This type of reinforcement had successfully overcome slope failure in a valve chamber of a power plant (Irsyam e et al., 1999). The length of bored piles was selected to be able to cut failure plane. The passive resistance of soil to bored piles below the failure plane had to be large enough to resist failure. Fig. 4 presents the selected arrangement of group of bored piles for this project. As shown in the figure the group of bored piles consisted of 2 rows of 18 m length of bored piles with a diameter of 1.0 m and pile spacing (center to center) of 2.0 m, arranged in a zigzag pattern (Fig. 5).

Another finite element analysis was performed to confirm the slope stability of the reinforced slope (by bored piles), the

sufficiency number of bored piles to resist bending moment, and adequacy of bore pile length. The bored piles were modelled as elastic plastic beam elements. Bending and axial stiffness parameters are summarized in Table 1. Fig. 6 presents result of finite element analysis after installation of the group of bored piles. The figure shows that failure plane is on top (left side) of bored piles. The analysis result also shows that bored pile has increased the factor of safety to a value of more than 1.3 and the capacity of bending moment of group of bored piles is more than the minimum required capacity.

Figure 6 Cross section of slope reinforcement and bored pile arrangement

Table 1 The stiffness parameters of bored pile as beam element

EA EI w[kN/m2] [kN/m2] [kN/m2] [ - ]

1 Bored Pile D=1000 Elastic 3937500 257709 4.7 0.15

TypeNumber Identification

Figure 6 Result of slope stability analysis after installation of bored piles (SF>1.3)

4. SEISMIC SLOPE STABIITY ANALYSIS

The seismic hazard effects were also considered in the slope reinforcement analysis. Generally, there are four steps involved in assessing seismic load due to earthquake events: (1) collecting and analyzing earthquake data; (2) developing and characterizing seismic source models; (3) developing and selecting appropriate attenuation relationships; and (4) calculating seismic hazard using total probability theory.

In this study, historical earthquake events that influenced toll road regions were compiled from national and international institutions such as National Earthquake Information Service U.S. Geological Survey (NEIS-USGS), the Indonesian Bureau of Meteorology, Climatology and Geophysics (BMKG), The Advanced

AB

CDE

FG

AB

C

B

CD CDE D

A. Fill material (road bed) B. Fill material (side slope)

D. Silty clay and weathered Clay shale

E. Hard clay shale

C. UncompactedMaterial

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National Seismic System (ANSS), and the EHB catalog (Engdahl, and Buland, 1998). A composite seismicity catalog for those institutions covering period between 1908 A.D. to December 2007 A.D. and area between 90oE to 145oE longitudes and 15oS to 15oNlatitude was compiled. After catalog was compiled, events of magnitude less than 5.0 and duplicate events were deleted. As the result of merging the catalog, several events with conflicting magnitude were encountered, therefore both automatic and manual procedures that incorporate engineering judgment about source-catalog reliability and priority are used to cull duplicate entries from the combined catalog.

The source models were developed using the earthquake catalogs, tectonic boundaries, and fault information. Source zones were defined on the basis of the distribution and focal mechanisms of the cataloged earthquakes, and on the locations of the earthquakes with respect to the boundaries of major tectonic plates.

Firmansjah and Irsyam (1999) classified the seismic source zones of Java Island into three classifications: subduction zone, transform zone, and diffuse seismicity (Fig. 7). In the subduction zone south of Java, the Java segment of the Sunda Arc extends from Sunda Strait on the west to Bali Basin on the east. Java transform zones occurs on clearly defined shallow crustal faults on Java Island such as Sukabumi, Baribis, Lasem, and Semarang Faults. Diffuse seismic zones include all earthquakes that occur in areas where seismicity is not associated with a single fault or fault type. Most of this diffuse seismicity is found in back arc areas of collision zones, like Flores back-arc faulting behind the eastern end of Sunda Arc and western end of Banda Arc. In this study, seismotectonic model, source mechanism, slip-rate, magnitude maximum and other parameters refer to the published National and International Journals.

Figure 7 The source model for representing of faults surrounding West Java in the Probabilistic Seismic Hazard Analysis

(LAPI ITB, 2007)

In this study, the analysis utilized the attenuation relationships for subduction zone at rock sites developed by Youngs (1997) and that for shallow crustal developed by Boore, Joyner, Fumal (1997) and Sadigh (1997) for extension tectonic region. The selection was based on previous study by Firmansjah and Irsyam (2001) for the development of the Indonesia seismic zone map, which indicated that these attenuation functions have a low variability compared to others.

The probabilistic seismic hazard assessment (PSHA) was performed for a 10% probability of exceedance (PE) in a design time period of 50 years or corresponding to the return period of approximately 500 years. The analysis has implemented seismotectonic model based on 3-D earthquake sources. Similar approaches are used in this study to develop the newly spectral hazard maps for Indonesia. The result of PSHA shows that the peak ground acceleration (PGA) at the bedrock for 500 year return period of earthquake is 0.38g.

The seismic hazard study was continued for developing synthetic time histories at bedrock. The acceleration time-histories are required in the analysis of shear wave propagation in soil

deposits. Selection of time-histories appropriate for specific geological and seismological conditions plays an important role for obtaining accurate results. The time histories were generated by using spectral matching method. In this method, synthetic time histories were generated by modifying the existing time history from worldwide earthquake events. The actual ground motions from worldwide earthquakes are selected based on their similarity of their characteristics such as magnitude, distance and site conditions and then the spectrums are scaled for matching them with the spectrums from probabilistic analyses. The generated synthetic acceleration time-histories is shown in Fig. 8.

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0 5 10 15 20 25 30 35 40

Time (sec)

Acc

eler

atio

n (g

)

Figure 8 The generated synthetic acceleration time histories at bedrock (PGA=0.38 g for return period, T=500 yr)

Application of the above synthetic time-histories in seismic slope stability using dynamic finite element calculation showed that the total displacement of the bored pile after earthquake loading was predicted about 11 cm. The developed bending moment can still be accommodated by bored piles. As shown in Fig. 9, the finite element mesh boundaries were enlarged to model free field condition during earthquake.

Total displacementsExtreme total displacement 651.02*10-3 m

Total displacements

Figure 9 The result of seismic slope stability

5. SLOPE CONDITION AFTER BORED PILE CONSTRUCTION

Bored piles were constructed around October in 2006. Figure 14 illustrates site condition during and after construction of bored piles.

Extreme total displacement 111.85*10-3 m

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During a rainy season around November 2006 to January 2007, with several times of hard rains, records of slope indicators showed that there was no significant soil movement.

(a)

(b)

Figure 10 Slope conditions: (a) during construction of bored piles and (b) after construction completion of soldier-bored piles

6. CONCLUSION

a. The slope failure at KM 91+500 of the Cipularang Toll Road was mostly led by the existence of low shear strength layer of silty clay and weathered clay shale. The back-calculated shear strength parameters at failure condition were c=5 kPa and =13o.

b. A group of bored piles was selected as the most suitable solution to overcome the slope failure due to time and space constrains and topographic condition at the site while the cost was still considered effective.

c. Application of the synthetic acceleration time-histories, with PGA=0.38 g for a 500 years return period, in seismic slope stability using dynamic finite element calculation showed that the total displacement of the bored pile after earthquake loading was predicted to be 11 cm. The developed bending moment can still be accommodated by bored piles.

d. Slope stability analysis result and records of slope indicators during the following several heavy rains have showed that the group of bored pile is effective for stabilizing the slope.

7. REFERENCES

[1] Brinkgreve, R.B.J. and Vermeer, P.A., PLAXIS: Finite Element Code for Soil and Rock Analyses, A. A. Balkema, Netherlands, 1998.

[2] Firmansjah, J. dan Irsyam, M. Development of Seismic Hazard Map for Indonesia. Prosiding Konferensi Nasional Rekayasa Kegempaan di Indonesia, ITB. Indonesia, 1999.

[3] Gartung, E., Excavation in Hard Clays of the Keuper Formation, Proceeding of Symposium, Geotechnical Engineering Division, Seattle, Washington, 1986.

[4] Irsyam, M., Surono, A., Himawan, A., and Nugroho, A. Laporan Disain Penanganan Kelongsoran Timbunan Badan

Jalan KM 97+500 Tol Cipularang, LAPI ITB-PT. JASA MARGA (persero), 2006.

[5] Irsyam, M., Tami, D., Sadisun, I.A., Karyasuparta, S.R., and Tatang, A.H., Solving Landslide Problem in Shale Cut Slope in The Construction of The Valve Chamber of The Tulis Hydro Electric Power, Journal of ’99 Japan-Korea Joint Symposium on Rock Engineering, ISSN 0917-2580, Fukuoka, Japan, 1995.

[6] LAPI ITB, Penelitian dan Penyelidikan STA. 97+500 Jalur B pada Proyek Pembangunan Jalan Tol Cipularang Tahap II Paket 3.1 Ruas Plered - Cikalong Wetan Kabupaten Purwakarta, Jawa Barat, 2006.

[7] Lembaga Afiliasi Penelitian Indonesia. Seismic Hazard Assessment for PLTU 3 Banten-Teluk Naga 3x (300-400) MW. Final Report for Rekadaya Elektrika. Jakarta, Indonesia, 2007.

[8] Sandroni, S.S., The Strength of London Clay in Total and Effective Stress Term, Ph.D. Thesis, University of London, 1977.

[9] Skempton, A.W., Slope Stability of Cuttings in Brown Clay, Tokyo, 1997.

[10] Stark, T.D. and Duncan, J.M., Mechanism of Strength Loss in Stiff Clays, Journal of Geotechnical Eng., Vol. 117, No. 1, 1991.