Gradation And Water Content Effect On Performance Of Cement-Based Stabilized Soil And Its Control

With the implementation of Chinese coastal development strategy,developing boom initiated in coastal cities,and a large number of infrastructural projects need to be built on soft soil foundations.Chemical reinforcement of cement-based materials(in-situ mixing and fluid solidification)is the most common method of foundation reinforcement.However,the current understandings of cement-based stabilized soil(e.g.,CBS)mainly focus on the external influences on the regular patterns of engineering performance such as cement types,concentrations and curing conditions,but little attentions have been paid on the mechanism of gradations of solid phase and water on the properties of CBS.As such,focusing on the working conditions of in-situ mixing and fluid solidification,a series of lab tests,numerical simulations,in-situ tests,mechanisms investigations,and theoretical models were conducted in this dissertation aiming to solve the following issues:(1)the influence and regulation of particle size distribution on the cohesive behaviors of CBS;(2)the influence and regulation of particle size distribution on the friction and durability of CBS;(3)the micro and meso mechanisms of silt/sand particles as inclusion in CBS;(4)the multiscale characterizing and modeling of the mixture of clay mineral-cement hydration products;(5)the identification of the liquid components in CBS and regulation by vacuum dewatering method;and(6)In situ application of dewatering technique and performance of CBS.The main conclusions of this dissertation can be summarized as following parts:(1)The influence and regulation of particle size distribution of solid phases on the cohesive behaviors of CBS.Silt/sand particle in CBS only acted as inert and hard inclusions.By controlling water content of original clay unchanged,the sands added directly to CBS with different particle sizes,contents,and gradations had no effect on its cohesive strength.While keeping the total soil water content unchanged,the replacement of clay with sand particles will weaken the cohesive strength of CBS.Based on the properties of silt/sand particles and their roles in CBS,a modified cement/water ratio by using the mass ratio of clay fraction to water was proposed to predict the unconfined compressive strength of CBS with different soil gradations.By controlling the flow consistency of CBS,it was found that the addition of silt/sand particles can effectively increase the strength of CBS due to the decrease of clay water content.(2)The influence and regulation of particle size distribution of solid phase on the friction and durability of CBS.When the flow consistency was controlled,it was found that as the increasing concentration of silt/sand particles would result in larger friction coefficient,greater bite force was needed during shearing process,and thus the friction angle of CBS continued increasing.In the meanwhile,the silt/sand particles can also reduce the sensitivity of CBS to the outer environments during drying-wetting cycles,and thus improved the strength.(3)The micro and meso mechanisms of silt/sand particles as inclusion in CBS.The mechanism of silt/sand particles as inclusion in CBS was revealed based on the microstructure observations.Then the numerical simulations of CBS with sands at mesoscale were carried out,finding that sand particles had little influence on the gel strength of CBS,but increased the friction angle,and improved shear strength.At the same time,the addition of sand particles can increase the stiffness of CBS,suppress the development of cracks,and change the distribution of shear bands.When the sand content was over 180 wt.% of the soil,the sand particles in CBS would form a local skeleton and generate skeletal effect,improving the strength of CBS.(4)The multiscale characterization and upscaling models of the mixture of clay mineralcement hydration products.Based on the analysis of microstructure,CBS was deemed as porous and heterogeneous materials.Combining the statistical nanoindentation technique and deconvolution analysis,it was found that CBS mainly contained two phases: clay minerals connected by CSH and cement hydrations aggregates.Through four cross-scale models such as MT,KT,SCA and DEM,the Young’s moduli of individual phases at microscale of CBS can be upscaled to the meso-scale modulus.Based on Young’s modulus at mesoscale,the MT,KT,and DEM models can be further used to establish the relationship of the Young’s modulus from the meso to the macro scale and thus try to establish a multiscale model from micro to macro scales.(5)The identification of the liquid components in CBS and regulation by vacuum drainage method.Research showed that the traditional cement/water ratio was not capable of evaluating the mechanical properties of CBS and thus the concept of free water content was proposed,which equaled to the total water content minus several times of plastic limit.The value increased with curing time as the free water content decreases with the continuing hydration reactions.For this reason,vacuum dewatering method was proposed to reduce the water content of CBS at early stage in order to improve its mechanical strength and reduce the required dosage of cement used in practice.The effective time of vacuum dewatering was the first 12 hours after the mixing of cement and soil,and the mechanical strength of CBS after vacuum dewatering increased by 0.7-1.75 times.(6)The performance of dewatering technique of CBS and in-situ application.According to the performance of the laboratory vacuum dewatering experiments,the in-situ vacuum dewatering technique was proposed and conducted.The results showed that the effective time of in-situ vacuum dewatering was consistent with the laboratory test,and the water content of CBS was reduced by about 6%,which resulted in the in-situ strength increasing by at least 0.6 times.Through XCT observations,it was found that vacuum dewatering can effectively reduce the porosity especially the large pores in CBS.When the same strength of CBS was reached,the vacuum dewatering technique can save about half of the cement consumption,which provided excellent economic,and social benefits.