IMPROVEMENT OF BEARING CAPACITY OF BORED PILES BY GROUTING METHOD Andrzej Tejchman and Kazimierz Gwizdala, Geotechnical Department, Technical University of Gdansk, Poland

A method of improvement of a subsoil under bored pile base by injection is presented. The method employs a special chamber attached to the reinforcement at the bottom of the drilled borehole. The chamber is made of semi-permeable textile bag. The injection entered under high pressure into the bag, causes prestressing of subsoil under the pile base. The results of load tests of piles improved in this manner have shown a substantial increase of their capacity together with simultaneous reduction of settlements. This technology has been implemented for four constructions such as trestle bridges in Gdynia and Warszawa and the bridges in Szczecin and Gdansk. The injection effects are shown based on the load test results.

INTRODUCTION

The loads applied on a single large diameter bored pile reach currently the values of the order of 5 to 12 MN. Piles of this type are commonly used as the supports for bridges and viaducts, therefore for the safety of these structures the achievement of the designed bearing capacities and permissible settlements has to be assured. It is generally well known that during installation of bored piles the phenomena of soil loosening connected with unstressing of a subsoil along the shaft and under the base of the pile occur. The degree of this process depends on the type and state of the soil, technology applied and the degree of the compliance of technology requirements. In order to eliminate the phenomenon of the soil loosening and to simultaneously increase the bearing capacity together with the decrease of the pile settlements the injection under the pile base is applied. The comparison of the load-settlement relationship for the pile with injection and without it is shown in Fig. 1. It can be seen clearly quicker process of the mobilisation of the soil resistance under the pile base which takes place already at small settlements of the pile.

Fig. 1. Generalized load-settlement curves

for large diameter bored piles

However, commonly applied injection methods do not protect against uncontrolled inject breakdown and do not assure appropriate soil improvement under the entire pile base. Additionally, they are strongly dependent on the local soil conditions.

DESCRIPTION OF THE METHOD

In order to reduce the phenomena of soil loosening and unstressing under the bases of bored piles together with the improvement of the pile work conditions the injection is applied using special chamber. The chamber made of semi-permeable geotextile in the form of folded bag is fixed to the main reinforcement and lowered on the bottom of the borehole prior the concretting of the pile. Flat injection nozzle consisting of two steel plates is placed inside the chamber. The entire injection set is pressured down to the subsoil by the weight of the fresh concrete during concretting of the pile. The injection is carried out after several days when the setting process of the concrete is advanced. The inject is introduced to the chamber through two steel pipes entered to the pile head. Practical pressure of the inject applied is between 2.0 to 4.0 MPa. The flat chamber construction assures uniform injection under the entire area of the pile base and prevents the uncontrolled breakdown and potential leakage of the inject along privileged flow ways. This method appears to be effective in every soil conditions. It can be applied in non-cohesive as well cohesive soil as well as coarse-graded soils. The control of the injection parameters allows the assessment of the installation quality of each pile. The basic injection parameters are the following: pressure and the volume of the inject entered, the lifting of the pile head during injection process and injection time (Fig. 2). The example of the injection chamber for the pile of the diameter of 1.8 m is presented in Fig. 3.

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Injection time [min]

Cap

acity

[dm

3]

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emen

t [m

m]

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ssur

e [M

pa]

Capacity Pressure Displacement

Fig. 2. Measured parameters of injection, example

Fig. 3. Mounting of injection chamber to the cage

SOME EXAMPLES

Trestle bridge in Gdynia Road trestle connecting the Container Terminal in Gdynia harbour with the inland has been designed as multispan reinforcement concrete construction founded onto bored piles with the diameter D = 1500 mm and the length varying from 11.0 to 19.0 m. In order to safely transmit high construction loads of the order of 6000 ÷ 7000 kN it has been planned to extend the pile base from 2.8 to 3.7 m. The general soil conditions are shown in Fig. 4. Originally, due to some technical problems the Contractor was not able to carry out the designed extension of the pile base. In order to safely transmit calculated loads from construction onto the subsoil it has been proposed to improve the pile bearing capacity by application of the injection method described above. The method employed allowed also shortening of the pile length for some supports of the trestle bridge. In order to examine the efficiency of the method applied and to determine actual bearing capacity of the improved piles two load tests were carried out for piles with decreased length from L = 17 m to 15 m. Tested piles were installed in relatively worst soil conditions. The testing loads were applied by ballast method up to the value of 10100 kN and 9370 kN for piles No. 2 and No. 7, respectively. The testing result in the form of load-settlement curve for pile No. 2 is presented in Fig. 5.

Fig. 4. CPT results for pile No. 2

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Load Q [kN]

Set

tlem

ent

s [m

m]

Fig. 5. Load-settlement curves for bored pile No. 2

in Gdynia

According to the interpretation of the curves based on Polish Piling Code the values of bearing capacity were assumed 7700 kN. It should be noted that these values take into account negative friction along pile shaft. The results of load test have shown that the improvement of pile base using injection method assured safe transmission of designed loads onto subsoil, even for shortened piles. Bridge in Szczecin The bridge crossing the Regalica River has been designed to be supported onto seven supports. The piers have been founded onto caps rested on bored piles of the diameter D = 1200 mm and the lengths L = 22 ÷ 23 m. Number of piles under the piers was ranging from dozen till several dozen. Soil conditions in this area were the following. Under the layer of 2.0 m

thick fill there were highly plastic mud and peat forming the layers of differentiated thickness up to 8.0 m. Below the weak soils there were fine sands laying on the deposits of medium sands and gravel in medium dense and dense state (ID ≅ 0.35 ÷ 0.80). The pile bases were embedded into dense soils. The primary load tests of piles performed during the first stage of bridge construction revealed that the design load varying from Qr = 2800 ÷ 3400 kN might not be safely transmitted onto a subsoil and additionally the possibility of nonuniform settlements of individual supports could occur. The field conditions encountered created a need to strengthen the pile base. Technical constrains eliminated either the possibility of elongating the piles or the increase of its number under individual piers, so the method described in previous section has been successfully applied. After installing the improved piles the load tests have been carried out. Totally 6 piles of four different supports were tested. Additionally, for the comparison purposes one load test was performed on the pile of the same length without base improvement. The typical results of load test in the form of load-settlement curves for two bridge supports are shown in Fig. 6. The interpretation of Q – s curve for a determination of pile bearing capacity according to Polish Piling Code PN-83/B-2482 yielded the following results:

– for pile No. 5A (without injection): Q ≅ 4000 kN, s ≅ 12.0 mm,

– for piles with injection: Q ≅ 5200 kN, smean ≅ 6 ÷ 7.0 mm.

After injection performed considerably better behaviour of piles in subsoil can be observed. The permissible settlement of piles s < 10 mm have been also reached.

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Load Q [kN]

Set

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m]

Pile No 5A (without injection)

Pile No 29A (with injection)

Pile No 45F (with injection)

Pile No 61G (with injection)

Fig. 6. Load-settlement curves for bored piles with injection and comparison without injection

Viaduct in Warszawa The access roads to the cable bridge across Wisla river in Warszawa being a part of Siekierkowska route have been designed as viaducts founded on 1000 mm bored piles and the lengths of 12 ÷ 15 m. In the subsoil, below

backfill and loose sandy soils there are graded, medium dense sands. It was the layer in which the pile bases have been embedded. Preliminary load tests shown insufficient bearing capacity of the piles, which was assessed to be 2500 kN, only at significant settlements of the order of 11 mm. It was a reason to apply injection under the pile base using chamber injection method what resulted in the apparent increase of pile bearing capacity with simultaneous reduction of the settlements. The bearing capacity of the improved piles was 3400 kN at the settlement equal to 5 ÷ 6 mm. The results of two load tests have been shown in Fig. 7 (pile with and without injection). For the same construction project but for other viaduct the direct injection was also performed (without the chamber), however the improvement effects were much worse in this case.

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Load Q [kN]

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m]

Support No. 4B - Pile No.18 (with injection)

Support No. 2B - Pile No.18 (without injection)

Fig. 7. Results of piles load tests

Bridge in Gdansk The bridge connecting two sides of Martwa Wisla river is a part of Sucharski route. It has been designed and performed as cable bridge with single pylon. The pylon’s support has been founded on 50 piles of the diameter of 1800 mm and the length of 30 m. The piles were installed by Bauer BG 36 G drilling rig with a help of casing pipe drilled up to the level of pile base. Such piles, with regard to its dimensions, have been installed in Poland for the first time. In order to increase the bearing capacity of the piles and simultaneously decrease its settlements the chamber injection method was applied. The cross-section of the support together with respective geotechnical profile is shown in Fig. 8. In general, within the area of whole bridge construction site (7 supports as a total) the subsoil is stratified. Under the surface fills there are alternatively sublayers of mud and fine and medium sands, mostly in medium dense state. The bases of piles have been embedded in dense medium sands with admixtures of gravel, ID = 0.73. Tested pile was to be loaded by vertical pushing force in terms of a set of twelve jacks of the total force equal to 14500 kN. The load test stand has been designed as

anchoring-ballasting system. Total weight of the ballast consisted of pavement slabs and steel construction was approximately 2000 kN. Based on load tests, the pull out bearing capacity of four piles has been estimated to be approximately 12000 kN. General view of the load test stand is presented in Fig. 9.

Fig. 8. Cross-section of the support

with geotechnical profile

Fig. 9. General view of the load test stand

According to the design, the calculated and characteristic maximum pile loads for the support under the pylon are Qr = 12600 kN and Qn = 9600 kN, respectively. Due to some difficulties in achieving the value equal to 1.5 x calculated load during the load test, the maximum test load applied on the pile No. 13 has been lowered to the value Qnr = 1.5Qn = 14400 kN what was equivalent to the largest test load applied whenever in Poland.

However, during the load test a lifting of the anchoring piles occurred The lifting exceeded the maximum permissible value of 10.0 mm what forced the decision to stop the further loading at the level of Qmax = 13540 kN. The load-settlement curve from the load test has been shown in Fig. 10. Based on the interpretation according to the Polish Piling Code PN-83/B-02482 it has been found that the designed load of the value of Qr = 12600 kN will be safely transmitted into the subsoil.

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m]

Fig. 10. Result of load test of pile φ 1800 mm with injection

For the tested pile the following injection parameters have been achieved: injection pressure – 3.10 MPa, the inject volume – 420 dm3 and lifting of the pile head – 1.05 mm. Comparison of these values with mean values for entire support (pressure – 3.3 MPa, volume – 425 dm3, head lifting – 0.60 mm) and small deviations of the injection parameters for a rest of injected piles caused that the tested pile No. 13 was treated as representative for whole support. Due to that the second planned load test has not been carried out. Additionally, for the support under the pylon the analysis of settlements was performed. Based on the load tests the settlement corresponding to the calculated load Qr = 12600 kN was s = 25 mm, whereas for characteristic load Qn = 9600 kN, s = 12.0 mm. It is well known that the settlement of the group of piles is usually several times higher than a single pile. The settlement of the group of piles can be determined either from empirical formulae (e.g. using Skempton’s, Vesic’s, Berezancew’s or Fleming’s proposals), from equivalent foundation method (e.g. acc. to PN-83/B-02482, Van Impe) or from theoretical relationships (e.g. Poulos’s, Randolph’s and Chow’s formulae). The experience and analyses show that results of settlements of pile groups based on theoretical considerations are too high with respect to actual settlements (Tejchman et al., 2001a). The calculation settlements of the pylon’s support have been carried out by several chosen methods. The some results of calculations have been collated in Table 1. Based on the analyses made by Tejchman et al. (2001b) more reliable results of settlements are obtained in terms of equivalent foundation method connected with the application of the Fox reduction coefficient. The analysis for this bridge has shown that the total average settlement of the pylon’s support should not exceed the values of 40 ÷ 50 mm, than twice more in comparison to the settlement of single pile.

Table 1. The results of the support settlements

Equivalent foundation method

Foundation settlements [mm]

Calculation method Options

constant modulus

Gibson’s model varying

modulus without Fox’s

coefficient 71.8 53.2 Equivalent foundation acc. to PN-83/B-02482 with Fox’s

coefficient 52.4 38.8

without Fox’s coefficient 78.7 60.4 OF90 numerical

code for pile foundation with Fox’s

coefficient 57.5 44.1

Theoretical methods

Calculation method Foundation settlement

[mm] moduli according to the

documentation 133.0 Poulos’s method based on the load test results 118.0

Randolph’s method 93.5

Chow’s method 110.0

Immediately, after the installation of piling foundations under the pylon the measurements of the settlements were started. After finishing of the construction of the bridge the average settlements of the pylon’s support

were equal about 11.0 mm. It is also proof that the used described injection method caused significant reduction of piles settlement. SUMMARY

The engineering applications of the original method of improving the base for large diameter bored piles has been presented. The method, which was originally de-signed in Geotechnical Department of Gdansk Technical University (Bolt A., Byczkowski M., Gwizdala K., Przew-locki Z., Tejchman A.), has been successfully adopted for many bridge supports in Poland. The test made have confirmed the increase of bearing capacity of piles (30% on the average) and substantial reduction of its settlements. The proposed injection into the semi-permeable bags made of geotextile and being placed at the bottom of the borehole can be applied in almost every soil conditions, assuring the soil improvement under the whole area of pile base. This method can be also employed for the analysis of the pile total bearing capacity distribution onto its shaft and the base. REFERENCES

Tejchman, A., 2001, Method of improvement of piled foundations, (in Polish), Inzynieria i Budownictwo, No. 12/2001.

Tejchman, A., et al., 2001a, Bearing capacity and settlements of piles, (in Polish), Monography, Technical University of Gdansk, March 2001.

Tejchman, A., Gwizdala K., Dyka I., 2001b, Settlement analysis of pile foundations, Proc. XV International Conference on Soil Mech. and Geotech. Eng., vol.2, Istambul, August, 2001.