Study On Size Effect, Bearing Capacity And Residual Pressures Of Large Diameter Grouted Pile

With the increase in the construction of high-rise buildings and large-span bridges, large diameter bored cast-in-situ piles have been widely used in practice because of its technical advantages. However, due to concerns of relating to the decrease of side friction and end resistance arising from the construction, post grouting has been employed to improve the bearing capacity as well decrease the settlement of the piles.On the basis of works of previous researchers, comparative tests between grouted and ungrouted large diameter piles and some field tests of size effect were conducted. The compressive capacity, tensile capacity and posting grouting residual pressures were studied preliminary by tests results analysis and theoretical derivation in this paper. Research achievement as bellow:(1) The compressive capacity of the large diameter bored cast-in-situ piles with post-grouting was analyzed based on the results of field tests in Hangzhou. The results show that the influence of size effect on skin friction decreases due to the solidification and compaction of the shaft side mud after grouting for the layer affected by the grouting. Whereas the size effect of skin friction can be found obviously for the higher layers unaffected by the grouting. It is less conservative to calculate the size effect coefficient with the method used in the current code. It is also found that the size effect coefficient for the sand or gravel layers is smaller than that for clay or silt soil layers. In addition, the size effect of tip resistance of the post-grouting piles can be eliminated.(2) Based on the studies of the size effect of large diameter grouted piles, a calculation method of ultimate compressive capacity for single pile incorporate the size effect and strength effect of the side friction and end resistance is presented in this paper.(3) The results of the tensile load tests on grouted and ungrouted piles show that pile-end post grouting technique can significantly reduce the pile tip settlement and increase the ultimate bearing capacity of pile. The ultimate bearing capacity of the pile with post-grouted is 1.25 times that of the pile without post-grouted; the tension compression of pile shaft is about 91.5% of the displacement at the pile head; the axial force decreases with depth but increase with the applied load at the pile head, however, the measured tip resistance is near zero during the whole loading cycle.(4) A new simple formulation for calculation of the ultimate bearing capacity of the uplift piles is proposed by considering the dead weight of pile and the climb height of grout; and the back-analysis shows that the ratio of the unit skin friction of the uplift pile to that of the compression pile is 0.65 to 0.80, whereas the ratio of the total skin friction in the uplift case to that in the compression case is 1.33 to 1.83, these values can be used in preliminary design of the pile.(5) Residual pressures locked in at the pile base during pressure grouting are studied theoretically based on the solutions of expansion of spherical cavity and one-dimensional consolidation. By analysis the dissipation process of the residual pressures, the analytical solutions of residual pressures dissipation with time and spread radius are obtained for assumed initial exponential pore pressure distribution and the influence of some key parameters are analyzed. The validity of the proposed model is also tested by comparing the results from field measurements.(6) A simplified solution discarding the grout spread radius is compared with the present method for different relative parameters. Parametric studies show that rheological index, consistency index, compression modulus and consolidation time are the key issues affecting pile residual pressures. The residual pressures decrease with the increase of the rheological index, consistency index, compression modulus and consolidation time.