Properties of Cement-Treated Soils During Initial Curing Stages

The engineering properties of cement-treated soils manufactured by the so-called “Pipe Mixing Method” and “Super GeoMaterial (SGM) Method” were studied. In these methods, clayey soils with high water contents are mixed with cement and used as fill material. Since the cement-mixed soils are transported through a pipeline, whose length at times exceeds 2 km, the properties of the treated soil during the initial stages of the hardening process are important. Bender element, vane shear and fall cone tests were performed to obtain such engineering properties as the shear modulus and the shear strength. The study revealed the following: 1) The minimum shear wave velocity of treated soils is detectable at around 2.8 m/s, corresponding to a shear modulus of about 12 kPa. 2) A linear correlation between the shear modulus and the shear strength exists even in the very early stages of curing, approximately G=300 s, where G and s are the shear modulus and the shear strength, respectively. This relation is similar to that for natural clays. 3) The “setting time” observed for concrete is also apparent in cement-treated soil materials. 4) Fall cone tests comprise a useful and simple technique for measuring very low levels of shear strength.     

Effect of Curing Stress and Period on the Mechanical Properties of Cement-Mixed Sand

Cement mixing is one of the popular ground improvement technologies in geotechnical engineering practice. In order to effectively and confidently design cement-mixed soil structures for specific purposes, its stress-strain behavior needs to be well understood. Though there have been many studies on cement-mixed soils using different types of soils, their behaviors have not been generalized yet. As is the case with concrete materials, the hydration of cement in cement-mixed soil continues with time, thereby improving the strength and deformation characteristics of cement-mixed soil over time. In the field, the cementation bonds are formed under stress in case of in-situ soil. However, in the usual testing techniques, cementation bonds under stress has not been a point of consideration in most of the previous studies. This has led to an underestimation of the stress-strain behavior of cement-mixed soil. On the other hand, soils are subjected to confining stress during loading which has also some effect on the strength and deformation characteristics of soil which has not been considered yet in the case of cement-mixed sand. This study investigates the effect of curing stress and period on the strength and deformation characteristics of cement-mixed sand. The effect of confining stress in the triaxial test is also investigated in another series of specimens. A series of consolidated drained (CD) triaxial compression (TC) tests were done along with the small strain cyclic loading and bender element tests during monotonic loading to determine the small strain Young's modulus (E_v) and shear modulus (G_vh) respectively. The effect of the curing period is significant in the peak strength, stiffness, E_v, G_vh and also in the post peak regime. The curing stress also has a significant effect on the peak strength, E_v and G_vh. The confining stress has an effect on the peak strength, stiffness and in the post peak regime. However, the effect is small compared to clean sand.     

Small strain stiffness and stiffness degradation curve of Bangkok Clays

The small strain stiffness and the stiffness degradation curve of soils are required in advanced numerical analyses of geotechnical engineering problems. The shear modulus at small strain (G_max) and the reference shear strain parameter (γ_0.7) are, for instance, two of the input parameters in a finite element analysis with the hardening soil model with small strain stiffness. The stiffness and strength parameters for the hardening soil model of soft and stiff Bangkok Clays has recently been published (Surarak et al., 2012). This paper is a continuation on the previous study on the stiffness of Bangkok Clay, and focuses on the small strain characteristics. The data are from the Bangkok MRT Blue line project as well as comprehensive studies at Chulalongkorn University and the Asian Institute of Technology. Based on these laboratory and field testing data, the parameters G_max and γ_0.7 can be determined using well-known empirical correlations and the concept of threshold shear strain. Finally, a comparison between the measured data and predictions is made.     

Impact of hydraulic hysteresis on the small strain shear modulus of unsaturated sand

The results of previous studies on silt and clay indicated that variations in the small strain shear modulus, G_max, during hydraulic hysteresis had a non-linear increasing trend with matric suction, with greater values upon wetting. However, due to differences in material properties and inter-particle forces, a different behavior is expected for the G_max of unsaturated sand. Although considerable research has been devoted in recent years to characterizing the behavior of the G_max of sand during drying, less attention has been paid to the effect of hydraulic hysteresis on G_max and its variations during wetting. In the study presented herein, an effective stress-based semi-empirical model was developed to predict the variations in the G_max of unsaturated sand during hydraulic hysteresis. The proposed model incorporated the impact of the possible changes in volume through an empirical void ratio function as well as the effect of the degree of saturation through the use of suction stress. The effective stress was also defined using the concept of suction stress. The efficiency of the proposed model was evaluated by comparing the model predictions with the results of an experimental testing program involving the measurement of the G_max of sand with different grain size distributions during hydraulic hysteresis. Specifically, a suction-controlled triaxial testing device, equipped with a pair of bender elements, was used to define the hysteretic trends in G_max for different values of mean net stress. The model was found to provide satisfactory predictions of the trends in G_max with matric suction, as well as its peak value and the suction corresponding to the occurrence of the peak G_max. It also provided satisfactory predictions of the variations in G_max upon subsequent wetting.     

Behavior of zeolite-cement grouted sand under triaxial compression test

Permeation grouting with cement agent is one of the most widely used methods in various geotechnical projects,such as increasing bearing capacity,controlling deformation,and reducing permeability of soils.Due to air pollution induced during cement production as well as its high energy consumption,the use of supplementary materials to replace in part cement can be attractive.Natural zeolite(NZ),as an environmentally friendly material,is an alternative to reduce cement consumption.In the present study,a series of consolidated undrained(CU) triaxial tests on loose sandy soil(with relative density D_r=30%)grouted with cementitious materials(zeolite and cement) having cement replacement with zeolite content(Z) of 0%,10%,30%,50%,70% and 90%,and water to cementitious material ratios(W/CM) of 3,5 and 7 has been conducted.The results indicated that the peak deviatoric stress(q_(max)) of the grouted specimens increased with Z up to 50%(Z_(50)) and then decreased.The strength of the grouted specimens reduced with increasing W/CM of the grouts from 3 to 7.In addition,by increasing the stress applied on the grouted specimens from yield stress(q_y) to the maximum stress(q_(max)),due to the bond breakage,the effect of cohesion(c') on the shear strength reduced gradually,while the effect of friction angle(φ')increased.Furthermore,in some grouted specimens,high confining pressure caused breakage of the cemented bonds and reduced their expected strength.     

Dynamic properties and liquefaction behaviour of cohesive soil in northeast India under staged cyclic loading

Estimation of strain-dependent dynamic soil properties, e.g. the shear modulus and damping ratio, along with the liquefaction potential parameters, is extremely important for the assessment and analysis of almost all geotechnical problems involving dynamic loading. This paper presents the dynamic properties and liquefaction behaviour of cohesive soil subjected to staged cyclic loading, which may be caused by main shocks of earthquakes preceded or followed by minor foreshocks or aftershocks, respectively. Cyclic triaxial tests were conducted on the specimens prepared at different dry densities (1.5 g/cm³ and 1.75 g/cm³) and different water contents ranging from 8% to 25%. The results indicated that the shear modulus reduction (G/G_max) and damping ratio of the specimen remain unaffected due to the changes in the initial dry density and water content. Damping ratio is significantly affected by confining pressure, whereas G/G_max is affected marginally. It was seen that the liquefaction criterion of cohesive soils based on single-amplitude shear strain (3.75% or the strain at which excess pore water pressure ratio becomes equal to 1, whichever is lower) depends on the initial state of soils and applied stresses. The dynamic model of the regional soil, obtained as an outcome of the cyclic triaxial tests, can be successfully used for ground response analysis of the region.     

Dynamic characterization of EPS geofoam

This paper attempts to describe the dynamic behavior of expanded polystyrene EPS geofoam, and shows the dependence of shear modulus, G, and damping ratio, λ, on shear strain, γ, density, ρ, and confining stress, σ₃, through the results of a series of resonant column and strain- and stress-controlled cyclic compression tests. Shear modulus and damping ratio versus shear strain curves were obtained and a series of equations were developed to model the dynamic behavior of EPS. From stress-controlled cyclic compression tests the effect of the number of cyclic load applications, N, on the maximum axial strain ?_max (for a specific static deviator stress, σ_e, plus the amplitude of the loading cyclic stress, σ_c) and on the dynamic modulus of elasticity E_dyn was evaluated as a function of the EPS density, confining stress, and the applied cyclic stress amplitude σ_c.     

Small strain dynamic behavior of two types of carbonate sands

The study reports on the small-strain dynamic behavior of two types of carbonate sands from Western Australia and the Philippines. Basic characterization of the soils was performed in terms of specific gravity, grading information, angle of shear strength at critical state, particle shape characterization and composition analysis. Piezo-element inserts were utilized to carry out the dynamic tests. For the Western Australia (WA) carbonate sand, both bender and extender element tests were performed, thus the shear modulus, the Young’s modulus and the Poisson’s ratio were examined. Both vertical and lateral bender element tests were performed on a set of specimens from the Philippines (PH) carbonate sand to study the shear modulus and, from which, no fabric anisotropy of the reconstituted specimens was found. It was observed that the overconsolidated specimens had higher stiffness than those during the first loading stages for both carbonate sands. In the pressure range of the study, grain breakage was small and its effect on the behavior of the samples was almost negligible. Empirical equations in the literature proposed from quartz sands could not predict the stiffness of the carbonate soils satisfactorily. In this regard, a preliminary study was carried out adopting the assumed void ratio that only considers the inter-particle voids instead of the summation of inter- and intra-particle voids; based on this concept, the predicted and measured stiffness (including both small-strain shear modulus, G_max and small-strain Young’s modulus, E_max) were found to be satisfactorily close.     

Über die Korrelation der ödometrischen und der “dynamischen” Steifigkeit nichtbindiger Böden

On the correlation of the oedometric and the “dynamic” stiffness of non‐cohesive soils. The “dynamic” shear modulus G_dyn of the soil, i.e. the secant stiffness of the shear stress – shear strain – hysteresis at very small strain amplitudes is often estimated my means of a diagram correlating the dynamic constrained elastic modulus E_s,dyn with the oedometric stiffness E_s for first loading. However, the assumptions and limits of this correlation are not clear. In the context of this paper the correlation E_s ↔ E_s,dyn is checked for four sands with different grain size distribution curves. For this purpose tests with oedometric compression and measurements of the compression wave velocity in a triaxial cell were performed. Partially, significant deviations of the measured data from the correlation approach actually used were obtained. On the basis of the wave velocity measurements and supplementing resonant column tests this paper also discusses the “dynamic” Poisson's ratio ν and presents a modified correlation E_s ↔ G_dyn.     

Modelling tensile/compressive strength ratio of fibre reinforced cemented soils

The present work proposes a theoretical model for predicting the splitting tensile strength (q_t) - unconfined compressive strength (q_u) ratio of artificially cemented fibre reinforced soils. The proposed developments are based on the concept of superposition of failure strength contributions of the soil, cement and fibres phases. The soil matrix obeys the critical state soil mechanics concept, while the strength of the cemented phase can be described using the Drucker-Prager failure criterion and fibres contribution to strength is related to the composite deformation. The proposed developments are challenged to simulate the experimental results for fibre reinforced cemented Botucatu residual soil, for 7 days of cure. While the proposed analytical relation fits well the experimental data for this material, it also provides a theoretical explanation for some features of the experimentally derived strength relationships for artificially fibre reinforced cemented clean sands. A parametric study to analyse the effect of adding different fibre contents and fibre properties is provided. The proposed modelling developments also confirm the existence of a rather constant q_t/q_u ratio with moulding density, cement and fibre contents .     

Gmax of Fine-Grained Soils at Wide Void Ratio Range,Focusing on Time-Dependent Behavior

Effects of creep as well as initial water content at deposition on the pseudo-elastic shear modulus, G_max, of fine-grained soil are studied in a consolidometer equipped with bender elements. The state boundary (SB) concept is discussed in void ratio (e)-G_max-Effective Vertical Stress (σ′_v) space and the condition of metastability associated with short-term creep and long-term natural creep are exemplified from laboratory creep tests and in-situ soil test results, respectively. In addition to the conventional laboratory reconstitution method adopted for almost all clay samples that use an initial water content of twice the liquid limit, the soil samples from Minato-Mirai (MM) site were also prepared by directly consolidating the slurry with a water content ranging from 1.5 to 5.0 times the liquid limit. The larger primary metastable region or higher degree of on-depositional structuration associated with the samples at higher initial void ratio is confirmed. However, no appreciable difference was found in the increase in G_max with time, represented by N_g, between the samples prepared by the two different methods. In addition to the highly plastic MM clay, the study of N_g also covered other clayey soils of different origin and plasticity index range from 29 to 78. Metastability index, MI(G_max)^e that manifested the aspect of structuration and déstructuration reaches a maximum value at the end of sustained loading and vanishes slowly with any increase in stress level. By using liquidity index instead of void ratio in the e-log G_max plot, the metastability index, MI(G_max)^LI, is found to represent a wider variety of soils with minimum scatter. A slight stress level dependency of metastability increase was observed yielding smaller values at higher stresses. For the present test conditions and duration, subsequent stressing of 1.5-2.0 times the creep stress brought about the complete déstructuration of the creep-added soil-structure formed in the previous creep step.     

Stiffness decay characteristics and disturbance effect evaluation of structured clay based on in-situ tests

The structure of the sedimentary clay influences its mechanical behavior in non-negligible ways. This paper proposes an effective approach for investigating the in-situ stiffness characteristics from shear modulus and strain decay curves (G–γ curves) based on self-boring pressuremeter and seismic dilatometer tests. To evaluate the excavation disturbance effects on the structured clay, the stiffness parameters from pre-bored pressuremeter tests are compared with the results of self-boring pressuremeter tests. The result indicates that the complete in-situ G–γ curves can be acquired by integrating the strain-dependent tangent shear modulus G_t from self-boring pressuremeter tests and the small-strain modulus G₀ from seismic dilatometer tests. Simultaneous observations of the G–γ curves with hyperbolic shapes in semi-logarithmic coordinates at the same strain scale show the similarity of the stiffness decay mode of the soil at different depths. The increase in the measured values of G_t and G₀ with depth can be attributed to the improved consolidation pressures and cemented strength in the structured clay. Additionally, the G/G₀–γ curves measured by the in-situ tests generally agree well with the results predicted by the Stokoe model. The excavation disturbance weakens the stiffness of the structured clay, as evidenced by G_t from the pre-bored pressuremeter data being significantly smaller than that from self-boring pressuremeter tests at the same depth. Based on quantitative analysis, the disturbance degree computed from the measured results has a low sensitivity to the soil depth and a strong negative correlation with the strain level of the soil. This study provides an effective method for predicting the stiffness parameters of structured soil based on in-situ tests.     

Experimental and DEM assessment of the stress-dependency of surface roughness effects on shear modulus

This contribution assesses the effect of particle surface roughness on the shear wave velocity (V_S) and the small-strain stiffness (G₀) of soils using both laboratory shear plate dynamic tests and discrete element method (DEM) analyses. Roughness is both controlled and quantified to develop a more comprehensive understanding than was achieved in prior contributions that involved binary comparisons of rough and smooth particles. Glass beads were tested to isolate surface roughness effects from other shape effects. V_S and G₀ were accurately determined using a new design configuration of piezo-ceramic shear plates. Both the experimental and the DEM results show that increasing surface roughness reduces G₀ particularly at low stress levels; however, the effect is less marked at high pressures. For the roughest particles, the Hertzian theory does not describe the contact behaviour even at high pressures; this contributes to the fact that the exponent in the G₀ – mean effective stress relationship exceeds 0.33 for sand particles. Particle-scale analyses show that the pressure-dependency of the surface roughness effects on G₀ can be interpreted using roughness index α which enables the extent of the reduction in G₀ due to surface roughness to be estimated.     

Use of polyethylene terephthalate fibres for mitigating the liquefaction-induced failures

The presence of non-biodegradable plastic waste is a serious concern for the health of endangered species. The present study is based on the sustainable utilisation of polyethylene terephthalate (PET) fibres obtained from waste plastic bottles to enhance the liquefaction resistance of fine sand. After performing a series of stress-controlled cyclic triaxial tests, the cyclic behaviour of PET-fibre reinforced sand has been investigated. The application of PET fibres was found to be more satisfactory in medium dense sand than that in loose sand as observed by residual excess pore water curves. In medium dense sand with 0.6% PET-fibres, the number of cycles to reach liquefaction was about 4 times that of the unreinforced sand. Using the dynamic shear modulus (G), the degradation index was calculated for both reinforced and unreinforced soils to assess stiffness characteristics. After nearly 50 loading cycles, the value of G/G_max increased 2.55 times with the addition of 0.4% PET fibres in unreinforced sand. Based on the results obtained, a regression model has been developed for the calculation of number of liquefaction failure cycles (N_cyc,L) in correlation with several parameters, namely, relative density (D_r), fibre content (FC) and σ_d/σ_c′ (σ_d = deviator stress, σ_c′ = effective confining stress).     

Undrained Shear Strength of Cement-Treated Soils

In order to evaluate the effects of cementation on the mechanical properties of cement-treated soil, a series of isotropic consolidation and undrained triaxial compression shear tests were performed for cement-treated specimens of Ariake clay, Akita sand, Rokko Masado and Toyoura sand. This paper evaluates factors affecting the shear strength of these cement-treated soils. The following conclusions are obtained: 1) Cement-treated soil has a normally consolidated line in e-ln p' space which depends on the mixing cement content. The consolidation yield stress, p'_y, of cement-treated soil increases with increasing cement content and initial specimen density. 2) Changes in cohesive strength due to cement-treatment can be represented by a tensile effective stress, p'_r. Strength properties can then be normalized by the augmented consolidation stress, (p'_c+p'_r). 3) The shear strength properties of quasi-overconsolidated clay can be represented by the yield stress ratio, R=(p'_y+p'_r)/(p'_c+p'_r). 4) The undrained shear strength of cement-treated soils can be represented as a power law relation of the yield stress ratio, R, and the augmented consolidation stress.     

A unified approach to link small-strain shear modulus and liquefaction resistance of pumiceous sand

Natural pumiceous (NP) sands containing pumice particles, a type of volcanic soil, are commonly found in the central part of the North Island in New Zealand. The pumice particles are highly crushable, compressible, lightweight and angular, making engineering assessment of their properties problematic. In this paper, several series of bender element and undrained cyclic triaxial tests were performed on reconstituted and undisturbed NP sands to determine their small-strain shear modulus (G_max) and cyclic resistance ratio (CRR). Furthermore, similar tests were also conducted on normal hard-grained sands (e.g., Toyoura sand) for the purpose of comparison. The results showed that the NP sands have considerably lower G_max compared to normal sands, resulting in their higher deformability during the initial stages of the cyclic loading test. The high angularity of NP sands play an important role toward the end of the cyclic loading and contributed to their higher CRR. Next, the ratio of CRR/G_max for each sample was correlated to a level of strain denoted as cyclic yield strain (ε_ay), which was found to be significantly dependent on the percentages of pumice particles present in the natural soils. On the other hand, the ε_ay was found to be less sensitive to the consolidation stress (σ′_c) and the relative density (Dr) of the materials. For example, over different values of σ′_c and Dr, NP sands have substantially higher values of cyclic yield strain due to their lower G_max and higher CRR when compared with those of ordinary sands.     

Application of Micromechanics Model to Study Anisotropy of Soils at Small Strains

A micromechanics model is used to analyse the stiffness anisotropy of soils at small strains. Five material constants for a cross-anisotropic elastic material are related to micromechanics variables such as fabric anisotropy, contact stiffness, particle radius, and the number of contacts in a given volume of particulate assembly. The analytical results from the model are compared with the published experimental data on small-strain stiffness anisotropy in order to estimate typical soil fabric conditions of sands and clays. The relationship between the small-strain shear modulus obtained from triaxial tests and shear tests is examined using the micromechanics model. The analysis shows that, when a soil is stiffer in the horizontal direction, the shear modulus evaluated from the conventional triaxial drained tests underestimates G_vh and G_hh. The opposite is true when a soil is stiffer in the vertical direction. When a soil is sheared in undrained condition, the measured shear modulus is closer to G_vh than G_hh, especially when the soil is stiffer in the horizontal direction. The effect of soil anisotropy on the stiffness measured from different stress paths in triaxial condition is investigated.     

Effect of artificial cementation on cone tip resistance and small strain shear modulus of sand

A series of cone penetration and bender element tests were performed on sands artificially cemented with gypsum in a calibration chamber to investigate the effect of cementation on the cone tip resistance (q _c) and small strain shear modulus (G _max) of sand. It was found that both the q _c and G _max of cemented sand are significantly affected by the degree of cementation while the effects of stress and density are reduced due to the cementation bonds. As the degree of cementation increases, the relationship between the q_\textc -D_\textR -s_\textv^￠ q_{{\text{c}}} {-}D_{{\text{R}}} {-}\sigma _{{\text{v}}}^{\prime } of cemented sand is observed to be similar to that of quartz sand with low compressibility. As the density and stress level affect q _c more significantly than G _max, the G _max/q _c of cemented sand decreases with increasing q _c. However, as the cementation causes a larger increase in G _max than q _c, the G _max/q _c ratio of cemented sand increases as the gypsum content increases. It was also observed from the G_max /q_\textc - (q_\textc /p_\texta )(p_\texta /s_\textv^￠ )^0.5 G_{{\max }} /q_{{\text{c}}} - (q_{{\text{c}}} /p_{{\text{a}}} )(p_{{\text{a}}} /\sigma _{{\text{v}}}^{\prime } )^{{0.5}} relation that the G _max/q _c ratio of cemented sand locates above the upper bound suggested by previous studies.     

Modelling tensile/compressive strength ratio of artificially cemented clean sand

The present work proposes a new theoretical model for predicting both the splitting tensile strength (q_t) and the compressive strength (q_u) of artificially cemented sand and assesses their ratio for a given material. The proposed model is based on the concept of the superposition of the failure strength contributions of the sand and cement phases. The sand matrix obeys the concept of critical state soil mechanics, while the strength of the cemented phase can be described using the Drucker-Prager failure criterion. The analytical solutions are compared against the results of tests on three different types of cemented clean sand cured for different time periods. While the analytical relation fits the experimental data well, it also provides a theoretical basis for the explanation of some features related to the experimentally derived strength relationships for cemented clean sand. The value of the power relationship between the strength and the porosity/cement ratio index seems to be governed by the soil matrix properties, while the interdependency of the strength and the curing time can also be captured. For a given cemented sand, the model equally confirms the existence of a unique tensile/compressive strength ratio (q_t/q_u), independent of the curing time and primarily governed by the compressive to tensile strength ratio (or the friction properties) of the cement. It is also confirmed that the q_t/q_u ratio changes within a narrow range for different frictional properties of the cementing phase.     

Loading-unloading shear behavior of rammed earth upon varying clay content and relative humidity conditions

This paper investigates the impact of clay and moisture contents on the shear behavior of compacted earth taking into account loading-unloading cycles. Fine sand was added to a natural soil, thereby obtaining three different soils with clay contents of 35%, 26%, and 17%, respectively. A series of triaxial tests was conducted on samples previously equilibrated at three different values of relative humidity (RH). The evolution of failure strength f_c, Young's modulus E, and residual strain ε_res was investigated according to the clay content and the RH, the last two parameters being measured during the loading-unloading cycles. Firstly, the relative humidity at which the samples were fabricated and conditioned was seen to have a strong impact on the mechanical characteristics of the earthen material. An increase in RH led to a decrease in both failure strength f_c and Young’s modulus E, and an increase in plastic strain. The tendencies were found to depend on the clay content of the samples. Secondly, with an increasing stress level, a progressive decrease in Young’s modulus and an increase in residual strain ε_res (after a loading-unloading cycle) appeared. Thirdly, within the range of the investigated clay contents, both failure strength f_c and residual strain ε_res increased with an increasing clay content at constant values of RH and confining pressure, the rate of this increase being a function of the RH. Young’s modulus E was relatively insensitive to changes in the clay content, its variation being less than 20% for all cases. Finally, based on a particular definition of Bishop's effective stress, involving a specific functional form χ(s), the failure states of all the samples were observed to lie approximately on a unique failure line crossing the origin in the (p′-q) plane regardless of the matric suction and confining pressure.