Laboratory Studies on Pressure Filtration in Post-Grouting of Drilled Shaft Tips in Clay

Abstract Practice has proved that significant tip capacity improvement can be realized through pressure grouting for drilled shaft tips in clay. A comprehensive series of laboratory model experiments were conducted in order to study the influence of pressure filtration on tip improvement and provide an insight into the influence factors of pressure filtration. The fundamental variables that could affect pressure filtration behavior, including filtration pressure P, water cement ratio wi, and clay thickness H were studied in these experiments. Results indicate that the velocity of pressure filtration is determined by the velocity of water through clay. The final water cement ratio of grout is maintained at about 0.3 after the pressure filtration process regardless of the initial water content under the pressures of 75–225 kPa. Atomic absorption spectrometry and scanning electron microscopy were used to study the mechanism of exchange of cations. The analyses reveal that the Ca2+ ions from cement grout exchange with Na+ and K+ adsorbed in clay particles. The separated clay particles become more close to each other after the exchange of cations. In addition, the results of laboratory direct shear tests show that the shear strength of the clay after pressure filtration is also increased due to the compression of the clay and the exchange of cations.