Pressure Grouting

DR. C.T. TOH CONSULTANT, MALAYSIA

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PRESSURE GROUTING TO RECTIFY PILE INTEGRITY PROBLEMS 1. Introduction It is an on-going project located at Petaling Jaya of Malaysia. The project consists of three blocks of offices and one block of 39-strory service suites. The buildings are founded on big diameter bored piles ranging from 600mm to 1800mm diameters. The bored lengths of piles vary from 23m to 53m. A total of about 500 bored piles were installed, in which 136 of them are piles of 1500 to 1800mm diameters. The working loads of 1500mm and 1800mm diameter piles are 15,400kN and 22,000kN, respectively. The concrete used is of Grade 35. The subsoil generally consists of firm to hard sandy SILT. A typical subsoil profile is given in Figure 1. All piles were bored in soils using water as the support fluid. Concrete was placed under water by tremie method. No rock was encountered at the depths of pile termination. 2. Sonic Logging Sonic logging tests were performed on 21 of the big diameter bored piles (1500mm and 1800mm dia.) in order to check the pile integrity. The installation of five to six sonic logging tubes into a bored hole often took more than two hours before placement of concrete. Because of this prolonged installation time of sonic logging tubes, sedimentation or perhaps, soil collapse, of up to about 1 to 2m happened. The piling subcontractor believed that the sedimentation can be displaced by concrete using the tremie method. Sonic logging showed deficiencies in integrity in all the 21 piles tested. The problem of integrity can be categorized as follows: a) Piles where deficiencies in integrity are observed at different levels. There

were 11 such piles; b) Piles where deficiencies in integrity are confined to near the toe of the pile.

There were 10 such piles. Remedial works inclusive of replacement piles and pressure grouting were proposed. The latter option appears to be much more economical if it is effective.


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3. Piles Where Deficiencies In Integrity Are Observed At Different Levels

BS 8004 requires that the allowable compressive stress in pile shall be limited to 0.25 x the 28 day concrete strength. The method of assessing whether the concrete is of adequate strength is to core through the pile, test the cores under unconfined compression tests, and compare with the estimated vertical load in the pile at the corresponding depth. Photos 1 and 3 show some concrete cores obtained from the proof coring on the piles with integrity problems. Honeycombs, contaminated concrete or discontinuities were observed at the depths where loss of FAT signals or delay in FAT signals was observed from sonic logging. Pressure grouting was proposed to rectify the integrity problems of the piles. The coring holes were first flushed with high water pressure until they are clean. Then, cement grout was pumped in until clean grout was discharged from the coring hole. Finally, high pressure was applied in order to ensure that all voids in the concrete were filled with cement grout. Sonic logging tests were performed on the piles seven days after the pressure grouting. The results are encouraging in which significant improvements in integrity can be observed from the sonic logging. Appendices A and B present the sonic logging results for one of the treated pile, Pile No. 35 (1800mm dia., 6 sonic logging tubes), before and after the pressure grouting. 4. Piles Where Deficiencies In Integrity Are Confined To Near The Toe

Of The Pile In this case, pressure grouting was performed via the existing sonic logging tubes. The drilling rod was inserted through the existing sonic logging tube to the bottom of the tube. The end cap of the tube was broken through and the drilling was continued to the pile toe. After drilling through all the tubes in a pile, the pile toe was flushed with high water pressure until it was clean. Then, cement grout was pumped into the pile toe with high pressure via the sonic logging tubes. Three tubes were then washed with low water pressure for sonic logging. Photos 4 and 5 show the cleaning and pressure grouting works. Sonic logging after seven days shows encouraging results in which improvements in integrity were observed at the treated pile toes. Appendices C and D present the sonic logging results for one of the treated pile, Pile No. 145 (1800mm dia., 6 sonic logging tubes), before and after the toe grouting.


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5. Conclusions Pressure grouting can be effective in rectifying the pile integrity problems. It is a much cheaper solution compared to replacement piles.


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Figure 1. Typical Subsoil Profile


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Photo 1. Honeycombs were observed in concrete cores from proof coring

Photo 2. Contaminated concrete was also observed


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Photo 3. Discontinuities were observed in concrete cores

Photo 4. Mud was displaced out from sonic logging tube in the toe cleaning

process


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Photo 5. Pressure grouting in process

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Pressure Grouting