Study On Grout Diffusion Theory And Residual Stress On Base Grouted Drilled Shaft

Model test, field test, scanning electron microscopy, and theoretical analysis were carried out to study the spreading mechanism of grout and the bearing behavior of post grouted drilled shaft. The study has provided some key findings summarized as below.(1) Compaction grouting and fracture grouting were researched using a grouting simulation test device which was developed by the author. The results show that the diameter of grout ball and the width of grout fracture decrease with increasing grout water cement ratio, whereas the quantity and length of grout fractures increase with increasing grout water cement ratio. Under certain grouting pressure, the diffusion of grout will gradually turn from compaction grouting to fracture grouting as the water cement ratio increases. Fracture grouting in clay can be divided into three stages:(i) grout ball stage, (ii) first fracture surface stage, and (iii) following fracture surface stage.(2) Basd on the theoretical model of pressure filtration of cement grout in clay, the theoretical formulaof pressurefiltration was deduced, and then it was verified by 20 model tests. The test results show that cement grout can not penetrate into the clay without fractures. The flow of squeezed water has positive correlations with clay thickness and filtration pressure. The final water cement ratio is nearly a constant in these tests, it do not have correlations with initial water cement ratio, grout thickness, clay thickness and filtration pressure. The final squeezed water is linearly correlated with the grout thickness and the initial water cement ratio. The finish time is linearly correlated with grout thickness, clay thickness, and initial water cement ratio, and is in inverse proportion to filtration pressure. Atomic Absorption Spectrometry test shows that the Ca2+ ions from cement grout spread with the squeezed water into the clay, then the Ca2+ ions exchange with Na+ and K+ adsorbed in clay particles. This process reduces the thickness of the electri c I ayer so soi I particles become ti ghter.(3) Scanning electron microscope was used to analyze the effect of grouting and pressure filtration on clay and mudcake. The results show that the natural Xiaoshan clay is of flocculent structure. After consolidation, the porosity between the clay particles is significantly reduced. After pressure filtration, the separated clay particles become more close to each other and form clay agglomerates of diameter about 30μm. Pressure grouting will further improve the soil strength by making the clay agglomerates into a unit. The original mudcake is of flocculent structure, and after grouting, the porosity of the mudcake is significantly reduced. (4) The expression of permeation radius was derived from the spherical/cylindrical diffusion model theory and the rheological property of cement grout, and then it was simplified by utilizing the rheological property of cement grout.(5) The control equation of cavity expansion theory of compaction grouting in saturated clay was derived by introducing an effective stress ratio. In this control equation, the pressure filtration can be taken into account. Furthermore, equations of radial stress and radial deformation were derived. The comparisons between this present theory and the traditional cavity expansion theory show that traditional cavity expansion theory can not consider the impact of effective stress ratio and pressure filtration, so lower effective stress ratio will lead to larger error.(6) Basd on the assumption of exponential fluid and narrow plate model, formulas for calculating the penetration radius and the climb height in fracture grouting were deduced. It shows that the penetration radius and the climb height increase with increasing water cement ratio of grout, grouting pressure, and gap width (mudcake thickness).(7) Theoretical study and case study both indicate that thicker mudcake, shorter curing age, lower groutability of bearing layer, and thicker debris will lead to larger climb height and higher risk of overflow. The overflow of grout can be prevented by increasing the groutability of bearing layer, and reducing the water cement ratio of grout, the mudcake thickness, and the debris thickness. The overflow of grout can also be prevented by intermission grouting. In the same grouting quantity, the overflow shaft has higher bearing capacity than the ordinary shaft.(8) Theoretical analysis and field tests were carried out to study the generation and dissipation of residual stress on base grouted drilled shaft and their effects on ultimate bearing capacity. After grouting, the dissipation of shaft tip additional stress makes the bearing layer over consolidated, resulting in an increase in tip resistance at a given tip displacement. At the same time, by preloading the shaft tip soil, the residual stress improves the end bearing capacity, and thereby enhances the single shaft bearing capacity as well. If the residual stress is neglected, the skin friction will be underrated and the end bearing resistance will be overrated.