Maximum dry density test to quantify pumice content in natural soils

Crushable volcanic soils are well-known for their distinctive texture, vesicular nature and grain fragility. These features of volcanic soils lead to difficulty in interpreting the results of laboratory and field testing because of the occurrence of particle crushing. Sands containing pumice particles are commonly found in the Hamilton Basin in the North Island of New Zealand. The pumice particles originated from a series of volcanic eruptions centered in the Taupo and Rotorua regions. As a result of flooding and erosion along the Waikato River, the pumice particles have become mixed with other materials and have been distributed over the Hamilton Basin; these mixtures are referred to herein as natural pumiceous (NP) sands. This paper initially investigates an appropriate technique for measuring the maximum dry density (MDD) of NP sands; then a modified MDD test is proposed for estimating the pumice contents of these sands. In order to examine the applicability of different standard methods for determining MDD, New Zealand and Japanese standards are employed. The results using the Japanese standard show consistent MDD values when repeating the tests due to negligible particle crushing. On the other hand, the results of MDD tests according to the New Zealand standard indicate that a significant amount of particle crushing occurs after each repeated test and, consequently, it is not possible to get the same result when the test is repeated. NP sands reach their ultimate potential breakage during the modified MDD tests (at least, for the level of loading applied) and they experience different levels of particle crushing which may be a function of their pumice content. As a way forward, the relative breakages of the materials tested are used to estimate the pumice contents of the NP sands.     

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.     

Feasibility of using electrokinetics and nanomaterials to stabilize and improve collapsible soils

Loess as a subcategory of collapsible soils is a well-known aeolian deposit generally characterized as a highly-porous medium with relatively low natural density and water content and a high percentage of fine-grained particles.Such collapsible soil sustains large stresses under a dry condition with natural water content.However,it can experience high and relatively sudden decreases in its volume once it reaches a certain water content under a certain load and therefore,the natural condition of the soil might not be suitable for construction if the possibility of the exposure of the soil to excessive water exists during the lifetime of the project.This research presents the utilization of an innovative method for stabilization and improvement of Gorgan loessial soil.This method uses electrokinetics and nanomaterials to instigate additives to move through soil pores,as an in situ remedial measure.To assess the acceptability of this measure,the deformability and strength characteristics of the improved collapsible soil are measured and compared with those of the unimproved soil,implementing several unsaturated oedometer tests under constant vertical stress and varying matric suction.The result emphasizes the importance of the matric suction on the behavior of both improved and unimproved soils.The test results indicate that the resistance of the soil was highly dependent on the water content and matric suction of the soil.The oedometer tests on samples improved by 3% lime and 5% nanomaterials show considerable improvement of the collapse potential.Results also reveal that stabilized samples experience notably lower volume decrease under the same applied stresses.     

Minimum void ratio characteristic of soils containing non-plastic fines

The minimum void ratio is an index widely used to indicate the contraction characteristics and the densest state of soils. The minimum void ratio obtained by the traditional test method has been utilized to represent the density of soils irrespective of their fines content despite the restriction (FC ≤ 5%). By considering the effect of the blow count, pore water, and their primary properties, the applicability of the minimum void ratio to soils containing fines was examined with an automatic tapping machine. It was confirmed that the blow count of the traditional method is not sufficient for soils with a high fines content. Furthermore, the presence of pore water had a significant effect on the minimum void ratio of soils containing fines. The characteristics of the cyclic minimum void ratio, which indicates the minimum void ratio obtained throughout the repetition of liquefaction and drainage, were also examined.     

Vertical dynamic impedance of suction caissons

Nowadays, suction caissons are being increasingly deployed as foundations to support offshore wind turbines (OWTs). Due to the overturning moment induced by waves and wind, vertical forces are the dominating ones acting on these foundations. In this study the dynamic stiffness and damping coefficients of suction caissons embedded in a viscoelastic soil layer over bedrock, subjected to vertical dynamic load were investigated. Numerical analyses of representative 3D finite element models were performed, while the numerical modelling was validated against existing analytical solutions for end bearing piles. The vertical dynamic response of suction caissons was evaluated by considering the effects of the foundation’s geometry, i.e. the slenderness ratio, and the stiffness of the soil layer on the vertical dynamic impedance of suction caissons. Results showed that the overall dynamic response is profoundly affected by the skirt length and by the variation of soil stiffness with depth.Mathematical expressions of the dynamic stiffness and damping coefficients were derived pertaining to foundations with various slenderness ratios and embedded in different soil profiles. The proposed expressions can be implemented in structural models used for the dynamic analysis of the support structure of a wind turbine, taking thus into consideration the effects of soil-structure-interaction.     

Evaluation of moisture reduction in aggregate base by wicking geotextile using soil column tests

This study investigated the distance effect on water reduction by the wicking geotextile in a base course experimentally using three sets of soil column tests. In each set of tests, two soil columns were constructed by compacting well-graded aggregate over a non-wicking woven geotextile and a wicking geotextile. A portion of the geotextile specimen was extended outside of the soil column for evaporation. The changes of the water contents in the soil column were monitored by volumetric water content sensors installed at various depths. The experimental results indicate the capillary drainage by the wicking geotextile effectively reduced water content within the soil column up to a distance from the wicking geotextile (i.e., approximately 200 mm for this specific aggregate with 10% fines). The test results also show that the wicking geotextile could reduce more water content of the aggregate below its optimum water content at a faster rate than the non-wicking geotextile.     

Centrifuge and numerical model studies on the behaviour of geogrid reinforced soil walls with marginal backfills with and without geocomposite layers

The aim of this paper is to study the effect of geocomposite layers as internal drainage system on the behaviour of geogrid reinforced soil walls with marginal backfills using centrifuge and numerical modelling. A series of centrifuge model tests were carried out using a 4.5 m radius beam centrifuge facility available at IIT Bombay. A seepage condition was imposed to all models to simulate rising ground water condition. Displacement and pore water pressure transducers were used to monitor the performance of all centrifuge models. A geogrid reinforced soil wall without any geocomposite layer experienced catastrophic failure soon after applying seepage due to the development of excess pore water pressure within the reinforced soil zone of the wall. In comparison, reinforced soil wall with two geocomposite layers at the bottom portion of the wall was found to have a good performance at the onset of seepage and by embedding four geocomposite layers up to the mid-height of the wall from bottom as a result of lowering phreatic surface much more effectively. For analysing further the observed behaviour of centrifuge model tests, stability and seepage analysis were conducted using SLOPE/W and SEEP/W software packages. A good agreement was found between the results of numerical analysis and observation made in centrifuge tests. The effect of number of geocomposite layers as well as its transmissivity was further analysed using parametric study. The results of parametric study revealed that the number of geocomposite layers plays a main role on the good performance of the geogrid reinforced soil walls with marginal backfill.     

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.     

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.     

Strength of sandy and clayey soils cemented with single and double fluid jet grouting

Innovations in jet grouting technology have primarily focused on the cutting efficiency of the jets, with the aim of creating larger columns and increasing the productivity of construction sites. Relatively little attention has been paid to the consequences of the grouting system on the mechanical properties of the formed material. This paper investigates this aspect by analysing the results of two field trials carried out in both sandy and clayey soils, where single and double fluid jet grouting were simultaneously performed, with varied grout composition and injection parameters. Parallel uniaxial compressive tests on samples cored from the columns show that the material formed with the double system is systematically lower in strength than the material formed using the single fluid system. The mineralogical composition of samples cored from the columns was analysed by performing parallel Scanning Electron Microscopy (SEM), X-ray diffraction analysis (XRD), Differential Thermal Analysis (DTA) and Thermo-Gravimetric Analyses (TGA) to determine the reasons for this difference. A lower proportion of cementitious products, an accelerated carbonation of portlandite and a less homogeneous distribution of cement hydration products was found on the surface of the soil particles of the double samples than for the single fluid columns.     

Durability analysis of seashore saline soil bound with a slag compound binder

In this paper, a slag compound binder (hereinafter referred to as the SM binder) was used to bind seashore saline soil. Compressive tests, scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction analytical tests were carried out to measure the unconfined compressive strength, observe the microstructure, analyze the composition of hydration products and evaluate the binding mechanism of the saline soil/SM binder mixture. The results showed that calcium silicate and calcium aluminate hydrates were produced after the hydration of the cinder components in the SM binder. Part of the calcium aluminate hydrate reacted with the gypsum to form ettringite, while the other part reacted with the Cl^? and SO₄^2? in the saline soil to produce Kuzel’s salt. Na⁺ also participated in the hydration reaction and produced zeolite-like substances. These hydration reactions led to the rapid binding of the soil sample. As the surface of the saline soil particles also contained active SiO₂ and A1₂O₃, the Ca(OH)₂ reacted with them to form calcium silicate and calcium aluminate hydrates in a continuously alkaline environment. Such reactions contributed to the third-stage binding of the saline soil, leading to a gradual increase in the strength of the soil samples during the middle and late stages of binding.     

混凝土环收缩开裂试验研究及理论预测

为了揭示受约束混凝土内部的应力发展情况，以便更好地预测混凝土的开裂时间，本文在前人理论分析混凝土环收缩开裂的基础上，综合考虑了自由收缩、徐变、约束度和弹性模量等因素对混凝土环开裂的影响，推导出混凝土环应力公式。将混凝土环应力公式与最大抗拉应力破坏准则相结合，预测混凝土环开裂时间。预测得到的开裂时间与混凝土环约束试验的实测结果吻合良好。     

An artificial neural network approach to inhomogeneous soil slope stability predictions based on limit analysis methods

The assessment of soil slope stability is an important task in geotechnical designs. This study uses finite element upper bound (UB) and lower bound (LB) limit analysis (LA) methods to investigate inhomogeneous soil slope stability on the basis of the conventional Mohr–Coulomb parameters. The obtained stability numbers are presented in inhomogeneous soil slope stability charts. In order to minimize manual reading errors when using the chart solutions, an artificial neural network (ANN) is employed to develop a stability assessment tool for the slopes investigated in this paper. The slope stability analysis using the ANN-based tool is convenient, and the predictions it provides are highly accurate.     

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.     

The role of undrained clay soil subgrade properties in controlling deformations in geomembranes

Strains were evaluated in a 1.5 mm HDPE geomembrane from overlying coarse uniform drainage gravel when placed above six different compacted clayey soils while keeping pressure, protection, loading rate equal. In each case, a protection layer consisting of 400 g/m² nonwoven geotextile was placed over the geomembrane. Vertical load of 300 kPa was applied in a relatively short duration. A photogrammetry procedure was used to develop a digital elevation model for each deformed geomembrane surface and the distribution of resulting strain in the geomembrane was evaluated on a percent area basis. The proportion of the overall geomembrane area in which the localised strain exceeded 3% was related to the compacted water content, index soil properties, and undrained shear strength of the six different clayey soils. It was found that an increase in moulding moisture content resulted in increased geomembrane strain in all cases, but the magnitude of the increase in strain varied considerably, depending on the plasticity and silt content of the soil used.     

Utilization of carbide slag-activated ground granulated blastfurnace slag to treat gypseous soil

Treating gypseous soils with lime or cement may induce remarkable swelling, resulting in the deterioration of pavement subgrade or other foundation layers. To mitigate this swelling, two industry by-products, carbide slag (CS) and ground granulated blastfurnace slag (GGBS), were utilized in this study. The CS was used to activate the GGBS, which was used to treat an artificial gypseous soil with different binder contents and CS:GGBS ratios, compared to ordinary Portland cement. The treated soils were soaked after a 7-day curing period. A series of tests was performed to examine the properties of the treated soils, including swelling, strength, water content, X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). It was found that the CS-GGBS-treated soils experienced slightly higher swelling (0.2%–1.0%) than the cement-treated soils (0.1%–0.3%) during the 7-day curing period. However, the following soaking process significantly increased the swelling of the cement-treated soils (>5.0%), caused cracks on the specimen surface, and reduced the strength, whilst the swelling of the CS-GGBS-treated soils after soaking was much lower (<0.3%), no cracks were observed, and the decrease in the soaking-induced strength was much less. The XRD, SEM, and MIP results indicated that the formation of ettringite was primarily responsible for the swelling. For the CS-GGBS-treated soils, the activation of GGBS and the formation of ettringite at an early age (within 7 days) rapidly consumed the Ca(OH)₂ in the CS; and hence, the further formation of ettringite after soaking was very limited. For the cement-treated soils, the cement hydration continuously supplied Ca(OH)₂ for the ettringite formation until completion, resulting in longer and higher swelling after soaking.     

Erosional behavior of gravel-sand mixtures stabilized by microbially induced calcite precipitation (MICP)

The objective of this study is to evaluate the effectiveness of Microbially Induced Calcite Precipitation (MICP) for improving internal erosion resistance of gravel-sand mixtures. Two gravel-sand mixtures with 25% sand/75% gravel and 50% sand/50% gravel were used; the former was susceptible to suffusion whereas the latter was internally stable. The MICP treatment was conducted by either mixing a urea-calcium solution with the tested soils prior to bacteria injection (the pre-mixing method) or injecting the bacteria prior to the urea-calcium solution injection (the injection method). A series of pressure-controlled erosion tests was performed on specimens placed inside a column erosion test apparatus under different levels of axial stress. During the erosion test, the erosion rate, axial deformation, and hydraulic conductivity were measured. Without the MICP treatment, the specimens with 25% sand/75% gravel exhibited much faster backward erosion and suffusion. In contrast, the specimens with 50% sand/50% gravel showed slow backward erosion only. Within the tested conditions, MICP was very effective in mitigating internal erosion for the soil with 25% sand/75% gravel. However, for the soil with 50% sand/50% gravel, the MICP treatment was only successful when the injection method was applied and the erosion test was performed at a low axial stress.     

Geo-mechanical behavior of clay soils stabilized at ambient temperature with fly-ash geopolymer-incorporated granulated slag

Geopolymer is a cementitious material that can replace ordinary Portland cement in several geotechnical engineering applications, such as soil stabilization, with the advantages of much lower harmful emissions and energy consumption. This paper presents a rigorous evaluation of the geo-mechanical behavior of different types of clay soils treated with geopolymer, including the influence of soil characteristics and mineralogy. Two natural clay soils in addition to a commercially available kaolin clay were used for this investigation. Laboratory experiments were performed including unconfined compressive strength (UCS) and consolidated undrained (CU) triaxial compression tests under different confining pressures. The UCS and triaxial tests indicated that the addition of geopolymer considerably increased the yield stress and initial stiffness of all examined clays. With the increase of geopolymer content, the stress–strain behavior of treated clays was found to develop progressively from ductile response into a post-peak brittle fashion. The CU tests also demonstrated that the addition of geopolymer changed the initial characteristics of remolded clays from quasi-over-consolidated to heavily over-consolidated, rendering high yield surface and more effective shear strength parameters (i.e., cohesion and friction angle). Moreover, although the overall qualitative stress–strain and stress path responses of the clays were similar, significant quantitative differences were observed, particularly in terms of the attainable yield strength, stiffness, and shear strength. These differences can be attributed mainly to the heterogeneity associated with the soil mineralogy and the corresponding differences in the interaction between the clay/non-clay minerals and geopolymer.     

活性MgO碳化固化土的干湿循环特性试验研究

碳化固化技术是一种利用二氧化碳对搅拌有活性氧化镁的土体进行碳化,以达到快速提高强度的低碳搅拌处理软土的创新技术。通过室内试验研究干湿循环对碳化固化土物理力学特性的影响,并与相同掺量下水泥固化土进行对比。结果表明:活性Mg O固化粉土碳化3 h试样的无侧限抗压强度可达5 MPa,粉质黏土碳化24 h试样可达2.6 MPa;干湿循环后碳化固化土的干密度降低,而水泥土干密度基本不变;6次干湿循环后粉土碳化试样的无侧限抗压强度仍然能达到4 MPa以上,为水泥固化粉土强度的2倍,具有较好的抗干湿循环性能;经过6次干湿循环后,粉质黏土碳化试样的残余强度仅为35%,而水泥固化粉质黏土降到65%,表明固化粉质黏土的抗干湿循环性能均较差,且粉质黏土碳化试样的抗干湿循环能力不及水泥固化粉质黏土试样。通过X射线衍射(XRD)、电镜扫描(SEM)及压汞试验(MIP)测试表明干湿循环对粉土碳化试样的累计孔隙影响不大,因此粉土试样仍然具有比较大的密实度来保证试样强度;粉质黏土碳化试样因孔隙增加明显而变得疏松,因此强度显著降低。     

软土蠕变特性试验研究

针对广州南沙原状软土进行了一系列室内试验研究,包括三轴压缩试验、三轴蠕变试验和一维固结试验,系统地探讨了软土的蠕变变形特性。结果表明:软土的蠕变特性与多种因素有关,包括土体的初始固结度、土层排水条件、加荷比等;次固结系数与固结压力的关系取决于土体的先期固结压力和试验中的加荷比。