Strength Characteristics Of Cement-Soil Admixture And Reliability Analysis Of Deep Mixing Column Composite Foundation

Strength is a key indicator for design and construction control of cement-soil deep mixing columns. It is meaningful to build a strength characterization parameter and a strength predicition model for homogeneous cement-soil admixture, which can significantly reduce the workload of indoor tests. However the presence of soft inclusions (i.e., unmixed or weaker parts) is inevitable due to the non-uniformity of mixing in practice. The heterogeneous nature of cement-soil admixture precludes the application of classical design rules because they are valid only for homogeneous building materials. Therefore, it is necessary to assess the effect of soft inclusions on the performance of cement-soil admixture. In a highly variable environment due to the non-uniformity of mixing, strength of cement-soil admixture is often highly variable in practice. The high variability of strength results in uncertainty for design strength assessment and performance evaluation of geotechnical structures, so it is necessary to investigate the probabilistic design metohd in order to improve the design theory of composite foundation.This study explores the strength characterization parameter of homogeneous cement-soil admixture, establishs empirical prediction formulas of strength and electrical resistivity, and assesses the influence of soft inclusions on the strength of heterogeneous cement-soil admixture by laboratory simulation experiments. Finally, the reliability anasysis method of deep mixing column composite foundation based on local average random field theory is developed. The main works and conclusions can be given as follows:(1) A comprehensive review is conducted on the available literatures on the strength and electrical resistivity of homogeneous cement-soil admixture, and then a series of unconfined compressive strength and electrical resistivity tests are carried out in order to analyze the variation of strength and electrical resistivity of cement-soil admixture. A strength characterization parameter and a strength predicition model are established, and an electrical resistivity empirical formula is proposed based on the Archie electrical resisitivity model. Finally, the relationship between electrical resisitivity and strength is discussed.(2) A series of unconfined compressive tests are performed on cement-soil samples with diameter of 10cm and height of 20cm, which contain soft clay inclusions without mixed cement. The effects of volume percentage, number, shape, and relative position of soft inclusions on the stress-strain curve, unconfined compressive strength, deformation modulus, failure strain, and failure pattern of heterogeneous cement-soil admixture are investigated.(3) A project of soft ground improvement for City Circle Ring Freeway in Wuhan is introduced, which improved by bidirectional deep mixing method, and statistical analysis is performed to study the strength variability of bidirectional deep mixing column samples from this project, then the design strength is defined as a certain percentage one-side lower confidence bound for the strength distribution. A reliability analysis method of composite foundation bearing capacity and a probabilistic settlement calculation method are proposed based on local average random field theory. The research results can promote the development of reliability-based design method of cement-soil mixing column composite foundation.