Yielding of cement-treated marine clay

This paper presents the findings of an experimental study on the primary yielding and post-yield behavior of cement-treated Singapore marine clay. The study was conducted using unconfined compression tests and triaxial tests. The results show that all the primary yield loci for the cement-treated marine clay have a consistent shape regardless of the mix ratio, curing stress or curing period. Three relationships are proposed for determining the size of the primary yield locus. The first two involve the direct determination of the isotropic primary yield stress, whereas the third makes use of the unconfined compressive strength. The first two relations are valid only for 7-day specimens. The third appears to have slightly larger scatter, but it is also applicable over a wider range of curing period and curing stress. Post-yield, over-consolidated samples were obtained by compressing specimens isotropically under effective stress levels higher than their isotropic primary yield stress and then allowing them to swell back to a lower effective confining stress prior to shearing. The normalized yield loci of these pre-yielded samples show a “collapse” from steep arches to more-rounded ellipses, while the yield loci expand with isotropic pre-compression pressure.     

Unconfined compressive strength of loose sandy soils grouted with zeolite and cement

Cement production requires a lot of energy and is also one of the most important sources of carbon dioxide emissions. Consequently, the replacement of part of the cement with a more environmentally friendly material, such as zeolite, is of great importance. The present research involves the conducting of a series of laboratory tests on loose sand specimens ($D_r\approx 30\%$) grouted with cementitious materials (cement and zeolite) to investigate the effect of different parameters, such as the size of the sand particles, the ratio of water to cementitious materials (W/CM) and the replacement of a certain percentage of the cement in the grout with zeolite (Z), on the unconfined compressive strength (UCS) of the grouted sand specimens. The results indicate that for all the grout W/CM and sand grain sizes, when Z is increased from zero zeolite (Z₀), the UCS initially increases. Then, after reaching an optimal amount (Z₃₀), it decreases. Moreover, increasing both the size of the sand particles and the W/CM of the grout is seen to reduce the UCS of the grouted specimens. The UCS of the grouted sand specimens increases with the equilibrium of SiO₂ and Al₂O₃ with CaO elements in the grouting suspension. Finally, equations with a high performance are proposed to predict the UCS of sands grouted with zeolite-cement using a multiple regression model (MRM) and a group method of data handling (GMDH)-type neural network.     

Evaluation of statistical uncertainty of cement-treated soil strength using Bayesian approach

The statistical parameters for the strength of cement-treated soil are evaluated by the strength of cored samples retrieved from cement-treated columns for a quality assurance procedure in the deep mixing method. The sample parameters include the statistical uncertainty associated with the statistical sample size and other factors. Therefore, a probabilistic characterization of the statistical parameters of strength is required to quantify the statistical uncertainty in the quality assurance process. This paper presents a quantitative analysis of the statistical uncertainty for the estimation of the strength of cement-treated columns. The Bayesian approach is adopted to evaluate the statistical uncertainty occurring in the determination of the statistical parameters of the strength from observed data. The inference is performed via a Markov chain Monte Carlo method, in which samples of the parameters are sequentially drawn from a joint posterior probability distribution. An example analysis is performed to illustrate the statistical uncertainty of the unconfined compressive strength of cored samples retrieved from cement-treated columns. The results show that the statistical parameters, inferred from the data with the sample size of approximately 40, include considerable uncertainty. The variability of the estimated statistical parameters is found to depend on both the sample size and the spatial correlation. The influence of the statistical uncertainty, caused in the estimation of the mean and standard deviations in strength, is examined within the framework of quality assurance in the deep mixing method.     

Experimental study on the effects of multiple corrosive ion coexistence on soil-cement characteristics

In some coastal areas, soft soils have high contents of Mg²⁺, Cl^? and SO₄^2?, which negatively affect the soil-cement strength when they are treated with cement. In this paper, laboratory macro- and micro-tests were carried out to study the effects of these three ions on the properties of soil-cement. First, unconfined compressive strength tests were conducted to study the effects of single ions and multiple ions on the strength of soil-cement under different curing times. Then, the soil-cement composition and the microstructure were observed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The action mechanisms of these three ions were explored through an examination of the chemical reaction process. It was found that the strength of the soil-cement decreased with an increasing ion content and that the coexistence of multiple corrosive ions had a greater effect than any single ion. When Mg²⁺, Cl^? and SO₄^2? coexist in soil-cement, they restrain the formation of cement hydration products, reduce the gelling property of these products and destroy the soil-cement structure, resulting in a reduction in strength. The research results provide a reference for developing a modified technique for the production of salt-rich soft soils reinforced by cement.     

Influence of applying overburden stress during curing on the unconfined compressive strength of cement-stabilized clay

In the case of cement-stabilized soils cured under the influence of overburden stress, it is necessary to consider two different timelines of cementation and consolidation, along which the structures of stabilized soils evolve. These two timelines are interrelated and ought not to be considered separately, especially in the early stage of curing when significant structural changes occur. In this study, the strength and deformation characteristics of cement-stabilized clay subjected to overburden stress during curing was investigated using an unconfined compression test apparatus. For this purpose, three types of specimens were prepared by stabilizing very soft clay with different amounts of cement. Overburden stress was applied for three different time durations during curing. In the series of unconfined compression tests conducted on the specimens with no overburden stress applied during curing, the axial stress initially increases rapidly, then was maintained before finally dropping. In the series of tests with overburden stress applied during curing, the axial stress continues to increase gradually before rapidly declining. The values of unconfined compressive strength are larger for the specimens subjected to overburden stress during curing. The values of the unconfined compressive strength also increase linearly as the duration of applied overburden stress increases. The increase in strength apparently occurs due to increasing dry density accompanied by a decrease in the water content due to the drainage of pore water during the application of overburden stress.     

水泥稳定碎石混合料强度发展规律研究

对不同级配、水泥剂量为2.5%~5%的水泥稳定碎石混合料进行室内强度试验,分析水泥剂量、级配及养生时间对强度的影响,得到强度发展规律,对试验数据进行回归分析,建立了抗压强度与劈裂强度两者之间的相关方程。     

Particle-size effect of basic oxygen furnace steel slag in stabilization of dredged marine clay

The aim of this study is to provide a comprehensive analysis of the strength development of dredged soil stabilization with different grain sizes of basic oxygen furnace (BOF) slag. Laboratory vane shear (LVS) tests, unconfined compressive (UC) strength tests, and flow value tests were conducted to understand the time-strength behavior of stabilized dredged clay. The results show that the strength was significantly affected by the different maximum particle sizes, despite the same free lime content. An equation to predict the strength development of a larger maximum grain size from a smaller maximum grain size was proposed using a modified BOF slag rate of addition and its calculated specific surface area. The results indicate that the equation is feasible for predicting the strength development of actual construction from laboratory test results.     

Required strength of geosynthetics for reinforced 3D slopes in cohesive backfills with tensile strength cut-off

In extant studies, most of the stability analyses of geosynthetic-reinforced slopes focused on two-dimensional conditions using the Mohr-Coulomb (M-C) failure criterion to describe the strength of backfills. However, in reality, all failures of slopes indicate a somewhat three-dimensional (3D) feature, and the M-C criterion is observed to overestimate the tensile strength of cohesive soils. To partially remedy this shortcoming, the concept of tensile strength cut-off is adopted to include the actual tensile strength of backfills in the yield envelope, and a kinematic approach is presented to evaluate the required strength of geosynthetics for 3D reinforced slopes in cohesive backfills. A 3D rotational mechanism of collapse that is associated with the strength envelope with tension cut-off is developed. The amount of required reinforcement is evaluated and listed as a dimensionless coefficient. The results indicate that the inclusion of the 3D effect and soil cohesion can lead to substantial savings in terms of the reinforcement to be made. In addition, a higher amount of reinforcement is required when the effect of tension cut-off is considered; this effect is more distinct for backfill with a higher amount of cohesion.     

Optimizing the evolution of strength for lime-stabilized rammed soil

In the present study,unconfined compressive strength(q_u)values of two lime-treated soils(soil 1 and 2)with curing times of 28 d,90 d and 360 d were optimized.The influence of void/lime ratio was represented by the porosity/volumetric lime content ratio(η/L_(iv))as the main parameter.η/L_(iv) represents the volume of void influenced by compaction effort and lime volume.The evolution of qu was analyzed for each soil using the coefficient of determination as the optimization parameter.Aiming at providing adjustments to the mechanical resistance values,the η/L_(iv) parameter was modified to η/L_(iv)~C using the adjustment exponent C(to make q_u-η/L_(iv) variation rates compatible).The results show that with the decrease of η/L_(iv)~C.qu increases potentially and the optimized values of C were 0.14-0.18.The mechanical resistance data show similar trends between q_u and η/L_(iv)~C for the studied silty soil-ground lime mixtures,which were cured at ambient temperature(23±2)℃ with different curing times of 28—360 d.Finally,optimized equations were presented using the normalized strengths and the proposed optimization model,which show 6% error and 95% acceptability on average.     

Study on strength of artificially frozen soils in deep alluvium

This paper describes a series of stress-controlled uniaxial compressive tests performed on frozen loess and triaxial compressive tests performed on frozen/unfrozen loess, which experienced K₀ consolidated process before freezing, to study the stress–strain–strength behaviour of an artificially frozen soil in deep alluvium. The aim of subjecting the triaxial test samples to K₀ consolidation was to simulate the forming process of deep soils. These tests examined the influence of the initial confining pressure and the temperature of frozen soils on stress–strain–strength behaviour. An analysis of the mechanical behaviour of artificially frozen soil is performed from interpretation of results from the unconfined and triaxial compressive tests of frozen/unfrozen soils, in which the influence of both the degree of cementation arising from the interparticle bonding and the initial confining stress was investigated. For deep artificially frozen soils, it was concluded that the unconfined compressive strength is a direct measurement of the degree of cementation. Consequently, the triaxial compressive strength can be expressed as a function of only two variables: (1) the internal angle of the shearing resistance of the unfrozen soils; and (2) the unconfined compressive strength. Data from additional experiments performed later verified the validity of proposed relationship in evaluating the strength of deep artificially frozen soil.     

Geotechnical behaviour of guar gum-treated soil

Soil stabilization through biological methods is a sustainable, efficient and long-term alternative to conventional techniques. An elaborate study on the engineering behaviour of guar gum-treated soil, a gum-based biopolymer, is carried out through a concise experimental programme. The dry density of the treated soil shows a marginal increase and its optimum moisture content decreases with the increase in guar gum content. The marginal change in dry density over a range of water contents indicates that guar gum-treated soil can be compacted at low energies and that its sensitivity to changes in water content is not appreciable. The stress-strain behaviour of the treated soil indicates that guar gum stiffens the soil matrix, enabling it to resist loads at lower strains. The gain in strength of the soil-guar matrix is seen to depend on both the biopolymer content and the curing time. There is a substantial increase in the unconfined compressive strength of the treated soil – nearly 45% immediately and 131% after 90 days of curing at a 2% guar gum content. The compressibility of the soil is effectively reduced as the addition of guar gum results in the formation of hydrogels that not only clog the pore spaces, but also stiffen the soil matrix. Guar gum is susceptible to degradation; and hence, durability tests are conducted. The results show that there is no appreciable degradation of the treated soil samples within the investigated period of 90 days.     

冻结钙质黏土无侧限抗压强度影响因素研究

对典型的钙质黏土进行人工冻土单轴抗压强度试验,分析了强度曲线的特征,得到了冻结钙质黏土抗压强度随冻结温度、应变速率的变化规律,及在不同养护时间、尺寸效应的影响规律,其结论对冻结法施工、冻结壁设计具有应用价值。     

Influence of sand incorporation on unconfined compression strength of cement-based stabilized soft clay

Cement-based stabilization by deep mixing or grouting is a popular method used to enhance soft clay ground because clay gradation has a significant effect on its unconfined compression strength. However, the functional relationship between the gradation and the strength is currently unclear, which limits the assessment and cognition for the strength formation of cement-based stabilized soft clay. Strength experiments controlling the sand ratio were conducted, in addition to the literature review, to clarify the sand incorporation effect on the strength. An effective cement ratio (the mass ratio of cement to involved clay) was proposed to evaluate the strength of stabilized clay with sand incorporation. In order to combat data dispersion and the labour-intensive nature of these experiments, meso-mechanical numerical simulations were carried out by the finite element method. This served to enrich the data and clarify the mesoscopic mechanism. The numerical result indicates that the strength of stabilized clay at low sand incorporation ratio is mainly controlled by the strength of the cement-clay-water matrix, thus verifying the rationality of studying the effective cement ratio. Subsequently, the threshold sand ratio in the cement-clay-water matrix was discussed to optimize the stabilization of soft clay.     

Stiffness and strength development of the soft clay stabilized by the one-part geopolymer under one-dimensional compressive loading

In this paper, the one-part alkali-activated geopolymer was adopted as the soil binder to stabilize the soft clay under the one-dimensional compressive loading. Factors influencing the stiffness and strength of the one-part geopolymer stabilized soil such as the proportion of silicon-aluminum raw materials, the mass ratio of solid NaOH (NH) to raw materials and the water-binder ratio were taken into account. The stiffness development of the one-part geopolymer stabilized soft clay under one-dimensional compressive loading was investigated by an improved transducer system equipped with a pair of bender extender elements and the tactile pressure sensor, from which the time history of earth pressure coefficient (i.e., K₀) and elastic wave velocity (i.e., the compression wave velocity V_P and the shear wave velocity V_S) of the stabilized soil sample could be measured, respectively. Furthermore, the unconfined compressive strength (UCS) of the stabilized soil sample cured under varied one-dimensional compressive loadings was tested to reveal the strength development of the geopolymer stabilized soil sample. Among different mixing proportions, the alkali-activated binary precursor [90% ground granulated blast furnace slag (GGBFS) and 10% fly ash (FA)] exhibits the highest performance in terms of the V_S, V_P and UCS of the geopolymer stabilized soil sample when the activator/precursor and water/solid ratio are 0.15 and 0.7 (i.e., the 4.67 mol/L of NH solution), respectively. The prediction of UCS for the one-part geopolymer stabilized soft clay was proposed and established based on the elastic wave velocity (i.e., V_S and V_P). The outcome of the current study sheds light on the practical use of the one-part alkali-activated geopolymer as a soil binder in ground improvement.     

Mechanical behavior of lightweight soil reinforced with waste fishing net

Lightweight soil is cement-treated and consists of dredged clayey soil, cement, and air-foam. Reinforced lightweight soil (RLS) contains waste fishing net to increase its shear strength. This paper investigates the strength characteristics and stress–strain behavior of reinforced and unreinforced lightweight soils. Test specimens were prepared with varying admixtures of cement content (8%, 12%, 16%, and 20% by the weight of untreated soil), initial water content (125%, 156%, 187%, 217%, and 250%), air-foam content (1%, 2%, 3%, 4%, and 5%), and waste fishing net (0%, 0.25%, 0.5%, 0.75%, and 1%). Then several series of unconfined compression tests and one-dimensional compression tests were conducted. The experiments with lightweight soil indicated that the unconfined compressive strength increased with an increase in cement content, but decreased with increasing water content and air-foam content. The stress–strain relationship and the unconfined compressive strength were influenced by the percentage of waste fishing net. In addition, the strength of RLS generally increased after adding waste fishing net due to the bond strength and the friction at the interface between waste fishing net and soil mixtures, but the amount of increase in compressive strength was not directly proportional to the percentage of waste fishing net. The results of testing indicated that the maximum increase in compressive strength was obtained for a waste fishing net content of about 0.25%. The bulk unit weight of lightweight soil was strongly dependent on the air-foam content. The compression characteristics of lightweight soil, including the yield stress and compression index, did not depend greatly on whether the samples were cured underwater or in air.     

Rate dependence on mechanical properties of unsaturated cohesive soil with stress-induced anisotropy

The strain rate during shearing has been shown in experimental studies to strongly affect the mechanical behaviour of soil. For saturated soil, sufficient knowledge has been obtained to achieve equilibrium conditions for the pore water pressure. Nevertheless, little is known about unsaturated soil. Therefore, this study used a hollow cylindrical torsional shear apparatus to investigate the rate dependence on the deformation and strength properties of unsaturated soil. First, unsaturated specimens were anisotropically consolidated with different directions of major principal stress to assess the rate dependence of the anisotropic behaviour. Then, the shear stress was removed to produce an isotropic stress state. Shearing was applied using the specimens to evaluate the strain rate effects on the mechanical properties of unsaturated cohesive soil. The results indicate that the secant shear modulus increased with the strain rate in both constant suction (CS) and constant water content (CW) conditions. The shear strength did not change with the strain rate under a CW condition, but it decreased with the strain rate under a CS condition.     

Triaxial extension and tension tests on lime-cement-improved clay

This paper presents the results of a series of undrained and drained isotropic consolidated triaxial extension, tension and compression laboratory tests on lime-cement-improved very soft clay. The main objective of these tests was to investigate the material strength and stiffness properties for stress conditions similar to those expected on the passive side of excavations where a retaining structure is supported by Deep Mixing columns. The different stress paths to failure were obtained by varying the directions of the major and minor principal stresses in a conventional triaxial test cell. The undrained tests conducted at low consolidation stresses, corresponding to depths of approximately 0–10 m below the ground surface, revealed significant differences in undrained strength depending on the directions of the major and minor principal stresses, indicating anisotropic material behavior. Based on the undrained triaxial test results, the relationship among the undrained strength, the effective consolidation stress and the over-consolidation ratio (OCR) is presented for different stress paths to failure.The experimental data from the drained tests show that a failure surface comprised of a shear failure function based on the Mohr-Coulomb failure criterion and a tensile failure function based on the tensile strength and the confining stress can be applied for lime-cement-stabilized clay.     

Estimating the unconfined compressive strength of intact rocks from Equotip hardness

In order to utilize the Equotip hardness tester, originally developed in the field of metallic engineering, as an indirect method to predict the unconfined compressive strength (UCS) of rock, laboratory tests were undertaken to establish the UCS, Equotip hardness (L-value) and porosity of nine rock types. Using existing data from Verwaal and Mulder (Int J Rock Mech Min Sci Geomech Abstr 30:659–662, 1993) and the results of laboratory tests, an equation relating UCS (MPa) and L-value obtained from single impacts (L _s) and porosity (n %) was derived, which provides a close approximation of the UCS value. An equation to relate UCS and Equotip hardness is also presented, although this is less accurate. It is considered Equotip testing has advantages over the commonly used Schmidt hammer test.      

Effect of fiber reinforcement and distribution on unconfined compressive strength of fiber-reinforced cemented sand

A series of unconfined compression tests were carried out to examine the effect of fiber reinforcement and distribution on the strength of fiber-reinforced cemented sand (FRCS). Nakdong River sand, polyvinyl alcohol (PVA) fiber, cement and water were mixed and compacted into a cylindrical sample with five equal layers. PVA fibers were randomly distributed at a predetermined layer among the five compacted layers. The strength of the FRCS increases as the number of fiber inclusion layers increases. A fiber-reinforced specimen, where fibers were evenly distributed throughout the five layers, was twice as strong as a non-fiber-reinforced specimen. Using the same amount of fibers to reinforce two different specimens, a specimen with five fiber inclusion layers was 1.5 times stronger than a specimen with one fiber inclusion layer at the middle of the specimen. The fiber reinforcement and distribution throughout the entire specimen resulted in a significant increase in the strength of the FRCS.     

Evaluation of the mechanical degradation of carbonate aggregate by rock strength tests

Aggregate degradation (AD) is one of the major reasons accounting for failure of aggregate materials, and the mechanical degradation of aggregate materials can be determined by different test methods. This process basically requires many aggregate samples and special instruments, and thus is time-consuming. The main purpose of this research is to evaluate the possibility of estimating the AD characteristics using rock strength tests and to investigate the relationships between AD properties and rock strength tests. For understanding the relationships, two common rock strength tests are employed, i.e. unconfined compressive strength (UCS) and point load index (PLI) tests. In the tests, the AD properties of 40 kinds of carbonate aggregates sampled from Iran were studied. The AD properties were determined by Los Angeles abrasion value (LAAV), aggregate impact value (AIV) and aggregate crushing value (ACV). Also, the samples are classified according to the strength and rock types, and the effect of this classification is investigated based on the relationship between rock strengths and AD properties. The results indicate that the PLI is better than UCS for evaluation of AD properties. Among rock strength tests, PLI has a closer relationship with AIV (R² = 0.832). Also, UCS has relative larger effects on the ACV (R² = 0.812) under the same loading condition. The weakest correlation occurs between LAAV and UCS (R² = 0.679). In view of the rational AD properties in the predictive procedure, it is possible to predict AD properties based on the strength tests and rock types. The results also show that the prediction of AD properties using rock strength test based on rock types yields better correlations than that using unclassified samples. The classification based on rock types can extrapolate the different relationships of AD prediction from rock strength tests. The results in this context could be used for preliminarily selecting proper rock aggregates with a limit of allowable AD tests for practical applications by PLI.