Investigation of Strength and Deformation Characteristics of Cement and Lime Treated Soft Clays

Abstract

This thesis presents experimental and numerical investigations for the effects of cement and lime treatment on compressibility, permeability, stress-strain, strength and stiffness behaviour of three soft clays. Effects of cement treatment on physiochemical and micro-structural properties have also been investigated. X-ray Diffraction, Scanning Electron Microscopy, particle size distribution, pH measurement, organic content, electrical conductivity, cation exchange capacity, exchangeable cation, water content, unit weight, specific gravity, and Atterberg limits tests were conducted to examine physiochemical and micro-structural properties. Compressibility and permeability properties of untreatcd and, cement and lime treated clays were investigated by performing one-dimensional consolidation tests. Stressstrain, strength and stiffness behaviour of untreated and, cement and lime treated clays were evaluated by performing unconfined compression (UC) tests. Consolidated drained direct shear (OS), unconsolidated undrained (UU), isotropically consolidated undrained (CIU) and isotropically consolidated drained (CID) triaxial compression tests were also carried out to assess the stress, deformation and strength properties of cement treated clays. Sets of variables considered in the testing program include a wide range for type of clay (PI = 13% to 47%), type of admixture, clay-water/cement ratio (2 to 30), curing time (1 to 104 weeks), mixing water content (120% to 250%) and effective confining pressure (50 kPa to 400 kPa). Finally, applicability of different constitutive models and Cap models were assessed for the predictions of drained and undrained behaviour.

It has been observed that pH value, electrical conductivity (EC), cation exchange capacity (CEC), increases with decreasing clay-water/cement ratio. pH value, however, decreases while EC and CEC value increases with increasing curing time. Loss on ignition and organic matter decreases due to increasing cement content and with increasing curing time. The relative amount of cementitious product (CSH + CASH) was identified by the XRD analysis and found to increase with the increase of cement content and curing time. Due to the formation of cementitious product, the fabric of the treated clays changed to flocculated type, comprising of clay-cement clusters separated by large inter-cluster voids with smaller intracluster pores as seen from the SEM images of treated clays. These changes were more pronounced with higher cement content and longer curing time.

The significant increase in yield stress and reduction in compression index (Ce) and swell index (Cs) were observed with increasing admixture (cement/lime) content and increasing curing time. Ce and Cs for lime treated clays were found to be greater than those of cement treated clays. Ce and Cs values increase significantly with the increase of mixing clay-water content. The lime-treated clays gained comparatively higher void ratio and volumetric strains and lower yield stress than those of cement-treated clays. The effect of cementation is to increase the values of coefficient of consolidation (cv) and coefficient of volume compressibility (mv). The higher the admixture content and curing time, the lower is the values of cv and mv. cv and mv values for lime treated clay were found to be higher than those of cement treated clay. A substantial increase in the value of cv and mv was found to have occurred within higher mixing water content.

At higher stress level, progressive destructuration of the treated clay particles occurred, which has been verified from the SEM images of cement treated clay compressed at different consolidation pressure. Addition of admixture to the clay increases the permeability and void of the soils, due to flocculation of the soil particles, which has been seen from SEM images. The Intrinsic Compression Line (ICL) and Generalized Compression Line (GCL) have been proposed for the untreated and treated clays. Coefficient of permeability (k) and void ratio relationships have also been proposed for cement and lime treated clays. The values of k of cement and lime treated clays have been found to reduce with increasing cement or lime content and curing period. Values of k for lime treated clays were found to be higher than those of cement treated clays.

Based on the rate of strength development with time, the unconfined compressive strength and cement content relationships have been divided into 3 zones: Inactive Zone, Active Zone and Inert Zone. Since the behaviour of cement treated clays was remarkably governed by we/e, the strength prediction in terms of we/e as well as the interrelationship involving strength, curing time and clay-water content/cement content ratio (we/e) have been proposed.

From stress-strain relationships, the overall behaviour has been categorized into brittle, quasi-brittle and ductile. Comparatively, brittle, quasi-brittle and ductile types for the cement treated clays, while quasi-brittle and ductile types for the lime treated clays have been found. The correlation between yield stress, (σy') and unconfined compressive strength (qu), τf0 and qu axial strain at failure (εf) and qu, and stiffness (Ei and E50) and qu have also been proposed.

In direct shear test, for cement treated clays, vertical expansion (dilation) was observed at low normal stress and experienced vertical contractions or settlement throughout the shearing stage at higher normal stress. The cohesion and friction angle have been increased with increasing cement content and curing time.

The undrained effective stress paths of the cement treated clays obtained from CIU triaxial compression tests indicate that the stress paths belong to different category of states such as normally consolidated, lightly, moderately and heavily over-consolidated state. The degree of alteration have been found different for different samples depending on the amount of cement content, curing time and pre-shear effective consolidation pressure. Upon reaching the peak deviator stress in CIU triaxial test, the progressive destructuration takes place and thus the stress path tends to move either on the I-Ivorslev envelope or envelope of strain softening behaviour.

SEM results have been suggested that complete destructuration takes place only on the shear plane at which the clay-cement cluster crushes. The prevalent role of pre-shear effective consolidation pressure was manifested to annihilate the cementation effect attributing ductility to the treated matrix. The main effect of cement treatment was to modify the behaviour of the soft clay form normally consolidated to over-consolidated state.

Similar natures of deviator stress-axial strain and volumetric strain-axial sirain relationships were observed for all samples having the identical wclc ratio in cm triaxial tests. So, the wclc is a prime parameter governing the engineering behaviour of cement treated samples having different mixing water content. For cement treated clays, peak deviator stress (qmax) criterion of failure envelopes can be used as obtained from CIU and CID triaxial compression tests. The degree of overall curvature of the failure envelope for each type of clay depends on the range of consolidation stresses and hence increases with increasing cement content and curing time.

The values of soil constants λ and κ for the treated clays have been found to be less than those for the untreated clays while the values of the constant N and M are greater for the cement treated clays than those of untreated clays. At a particular curing time, the soil constants λ and κ decreased, while the constant N and M increased with increasing cement content. Correlations between the soil constants λ, κ and N with plasticity index have been proposed. Hvorslev surface was established for the cemented clays. No definite Roscoe surface could be found for cemented clays.

For the untreated clay, it has been observed that the predicted stress paths, deviator stresses, volumetric strains and excess pore pressure responses at small strain levels using the MCC model appear to be significantly close to experimental curves. It appears from the present study that MCC, MMCC, EMMCC and Cap models (Plane Cap and Elliptic Cap) cannot be applied for predictions of drained and undrained behaviour of cemented clays at high water content.