Grout-Soil Interaction During Base Grouting And Its Effects On The Behavior Of Grouted Piles

Pressure grouting below the pile tip after its construction has been an effective method to improve the bearing capacity and decreas the settlement of the pile. It has become a routine construction process in many parts of the world. Based on the analysis of soil groutability, the different grout-soil interaction modes are analyzed. A series of problems, including permeation, filtration, compaction and dehydration of grout, for sufficiently gourtable, unsufficiently groutable and nongroutable soil are investigated, respectively. On this basis, the influences of pressure grouting on the behavior of the pile are analyzed. The main work and conclusions are summarized as follows:(1) The influences of soil groutability on the grout-soil interaction are examed and the effects of filtration on the grout dispersion are studied. A linear filtration law is introduced into the mass balance relationship of the water, cement and soil to develop a cement grout dispersion model in spherical coordinates with account for filtration. The variations of grout concentration, soil porosity and pressure gradient during injection are obtained. The analysis results show that: The cement particles are filtered by the solid skeleton and the porosity of the soil reduce gradually during the injection process, resulting in the increase of pressure gradient. The closer it gets to the grouting hole, the more obvious filtration is achieved. The cement mass filtered by the soil increases with the increasing of filtration coefficient, injection rate, initial radius and injection duration while it decreases with increase in water-cement ratio of the grout. Moreover, the results of dynamic penetration test after post grouting show that great effectiveness has been achieved for the soil close to the pile.(2) The effects of seepage pressure on the soil under spherical and cylindrical condition are investigated, respectively. The strss and deformation of the soil under the seepage pressure during injection are obtained. The analysis results show that:the radial stress in the soil increases with decreasing water-cement ratio and permeability coefficient. The deformation of the soil decreases with the increase in soil modulus. Spherical and cylindrical compaction grouting model with account for seepage are presented respectively. The grouting pressure p0can be divided into two fractions αp0, used in expanding the borehole, and (1-α)p0, used in inducing seepage flow. The changes in stess during injection are obtained by considering the overall fractions of grouting pressure. The results show that the radial stress and displacement decreases with decreasing a for the main reinforcement region. The traditional cavity expansion theories give less conservative results. (3) The dehydration process of the grout can be divided into two stages:unrestricted dehydration and restricted dehydration according to the critical bleeding water-cement ratio. The dehydration model for the unrestricted stage is obtained based on the conservation of the cement. For the restricted stage, a spherical consolidation model considering the flow velocity continuity at the grout-soil interface is derived. Parametric studies show that the excess pore pressure in the filter cake dissipates with time and the dehydration efficiency reduces gradually. For the filter cake, an increasing dehydration coefficient corresponds to a better drainage capacity. In addition, laboratory dehydration tests are conducted to obtain the ranges of the dehydration coefficient for different grouting pressure and water-cement ratio.(4) The influences of preloading effect during grout injection on the side friction and tip resistance of the grouted pile are studied. The conditions for grout penetration upward along the sides of the pile are analyzed. A calculation method for penetration height of grout is proposed and a case history is presented to demonstrate validity of the method. The mechanism whereby base grouting affects the end bearing and side friction is illustrated. Combining the existing empirical equation, the analytical methods for end beraing and side friction of grouted piles are presented. Moreover, the results of standard penetration tests after post grouting show that good treatment results can be obtained for the sandy soil. Great effectiveness has been achieved for the soil close to the pile.(5) Two calculation methods for the settlement of grouted pile are presented. An iteration method based on the hyperbolic load transfer function is developed to determine the load settlement relationship of the grouted pile. Statistical analysis is conducted to give the distribution of the capacity improvement factor at different displacements for the grouted pile rested on gravel. The distribution curves are approximated by normal distribution function fitted to the statistical data and a method of predicting the bearing capacity of the grouted pile based on settlement criterion is presented. Case histories are cited to indicate the validity of the presented methods.