Influence Of Spatial Variability On Bearing Characteristics Of Cement-Soil Mixed Pile Composite Foundation

Strength is the core index of cement-soil mixing pile design and construction control.Different soil properties have a great impact on the strength of cement-soil.In practical engineering,cement-soil cannot be fully stirred evenly,and there are often non-uniform bodies inside the pile.Compared with other soil types,the mixing uniformity of silty soft soil is worse,and the strength of cement-soil decreases significantly.At present,researchers in China and abroad have conducted in-depth studies on the bearing capacity and deformation mechanism of cement-soil and cement-soil mixing pile composite foundation by using theoretical,experimental and numerical simulation methods.However,cement-soil mixing pile is basically regarded as homogeneous material.In fact,cement-soil is a kind of heterogeneous material containing heterogeneous body,which has obvious spatial variability.Based on the project of Shanshan high-speed railway train station,aiming at the situation that the cement-soil mixing pile is not easy to mix uniformly in the deep and complex soft strata in the coastal area when the foundation is treated by cement-soil mixing pile method,due to the large sedimentary thickness of argillaceous soft soil,large clay content and a large number of biological remains such as shells in the soil,the influence of the existence of non-uniform bodies(clay group and shells)on the bearing characteristics of cement-soil and cement-soil mixing pile composite foundation is systematically studied by means of field investigation,indoor and outdoor tests and numerical simulation.The main work and achievements are as follows :(1)The physical and mechanical properties of muddy soft soil layer in coastal area were studied through laboratory tests and vane shear tests.The results showed that the natural moisture content of argillaceous soft soil was high,the pore water was developed,and it was saturated.The soil moisture content was mainly distributed in the range of 40% ～ 55%,and the average saturation was 95.5%.The argillaceous soft soil was soft plastic-flow plastic in the natural state,which belonged to high compressibility and medium sensitivity soil.The thixotropy was strong,the permeability of the formation was poor,and the water was not easy to discharge.Under the load,the consolidation speed of the soil was slow,and the later settlement was large.It was suggested that the standard value of shear strength of muddy soft soil layer in this area was about 28.7 k Pa.(2)The pile quality of cement-soil mixing pile was evaluated through the unconfined compressive strength test of cement-soil core sample and the load test of composite foundation.The results showed that the average strength of cement-soil core sample was1.575 MPa when the cement-soil mixing pile was constructed in the muddy soft soil layer in the coastal area.The strength obeys the normal distribution,and the overall strength was high.The bearing capacity of composite foundation met the design requirements.However,the dispersion of unconfined compressive strength of core samples was relatively large.According to the observation of core drilling of cement-soil,it was found that some core samples of cement-soil had obvious inhomogeneity,and there were non-homogeneous bodies such as clay group and shell,and they existed in cement-soil in various spatial distribution forms,which was the main reason for the large dispersion of cement-soil strength.(3)Considering that the content and spatial position of clay group will significantly affect the strength and deformation characteristics of cement-soil mixing pile,the test conditions of cement soil containing different clay group content and spatial position were designed,and 12 groups of cement soil samples were made for unconfined compressive strength test to study the influence of clay group on the mechanics and deformation characteristics of cement soil.The results showed that the existence of clay group reduced the volume of cement soil,resulting in obvious structural characteristics of cement soil samples.When the content of clay group reached 6%,the strength and deformation modulus of the sample would be greatly reduced.Under the action of stress,the cement soil area around the clay group would produce stress concentration,and the compressive stress inside the sample will produce transverse tensile stress.The failure mode of cement soil changed from overall shear failure to brittle tensile failure.When the relative position of clay group is1/3 of the sample height,the middle part of the sample was not completely extruded,and the cracks developed completely around the clay group,and the sample was more likely to be destroyed.(4)Considering that the shell content and diameter size will affect the strength and deformation characteristics of cement-soil mixing pile,the test conditions of cement soil containing different shell content and diameter size were designed,and 12 groups of cement soil samples were made for unconfined compressive strength test to study the influence of shell on the mechanics and deformation characteristics of cement soil.The results showed that the shell with flake shape had the effect of dispersing stress.When the shell content increased from 7% to 19%,the strength of cement soil samples continued to increase,and the deformation modulus was also improved to varying degrees.When the shell diameter was below 16 mm,the shell had the effect of strengthening cement soil.When the shell diameter was above 16 mm,the shell had the effect of weakening cement soil.(5)The three-dimensional discrete-continuous medium model of cement-soil mixing pile composite foundation with spatial variability was established to simulate the load test of composite foundation;the results showed that the clay group was a weak structure,which would change the internal stress distribution of the pile.With the increase of the clay group content and the decrease of the spatial location burial depth,the settlement of the pile,the additional stress and the settlement and additional stress of the contact force chain and the soil around the pile were all changing towards the decrease of the bearing characteristics of the composite foundation.Compared with the influence of clay group on the bearing characteristics of composite foundation,the influence of shell was very limited.Combined with the numerical simulation results,when the load requirement was less than or equal to360 k Pa in the complex soft strata in the coastal area,it was suggested that the content of clay group in the cement-soil mixing pile should be controlled below 15% to ensure that the overall bearing capacity of the composite foundation met the load requirement,thereby reducing the overall settlement in the later period.