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.     

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.     

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.     

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.     

Experimental studies on curling development of artificial soils

Soil curling is an important phenomenon associated with volume changes induced by increasing soil suction upon desiccation. The study of soil behaviors associated with drying in soils (e.g. soil shrinkage, desiccation cracks and curling) has received increasing attention over the last few years, which has been mainly driven by the forecast climate change that will warm up our planet. There are significant gaps in the current knowledge related to the factors that control the development of curling deformations in soils. For this, the curling phenomenon is investigated through laboratory desiccation tests on different mixtures of artificial soils. The effects of soil grain size distribution, mineralogy, soil microstructure, and soil water content on the curling deformation are analyzed. Digital photos were taken at regular time intervals during the tests to understand the volume changes in the soil samples during drying. It is found that soil fabric and soil water content have significant effects on curling scenario. It is observed that the percentage of sand particles and the initial water content play a critical role in the development of soil curling. Samples of pure clayey minerals experienced shrinkage without or with minor curling during drying.     

Modeling the stress versus settlement behavior of shallow foundations in unsaturated cohesive soils extending the modified total stress approach

The mechanical behavior of unsaturated soils can be interpreted using either modified total stress or a modified effective stress approach depending on the type of soils and various scenarios of drainage conditions of pore-water and pore-air. Recent studies suggest that the bearing capacity of unsaturated cohesive soils can be more reliably estimated using the modified total stress approach (MTSA) rather than the modified effective stress approach (MESA). In the present study, a modeling technique (extending Finite Element Analysis, FEA) is proposed to estimate the bearing capacity of shallow foundations in unsaturated cohesive soils by simulating the vertical stress versus surface settlement behaviors of shallow foundations extending the MTSA. The proposed technique is verified with the model footing test results in unsaturated cohesive soils. Commercial finite element software, SIGMA/W (GeoStudio 2012, Geo-Slope Int. Ltd.) is used for this study. Details of estimating the unsaturated soil parameters (i.e. total cohesion, modulus of elasticity and Poisson’s ratio) required for the FEA are also presented taking account of the influence of matric suction. Good agreements were observed between the measured bearing capacity values and those from the FEA extending the MTSA.     

Tests on mechanical behavior of unsaturated decomposed granite and its modelling considering finite deformation

Generally speaking, most of the geomaterials in the surface ground are in an unsaturated state. The mechanical and hydraulic properties of unsaturated soil are much more complicated than those of saturated soil. To rationally describe these properties, it is important to couple the stress-strain relation of the unsaturated soil with its water retention characteristics using rational state variables. In this paper, oedometer and triaxial compression tests on decomposed granite under constant-suction and constant degree of saturation conditions were conducted. Based on the test results, a modified constitutive model was proposed to build an incremental relation between the degree of saturation and suction that considers the influence of finite deformation. The modified model was utilized to simulate the corresponding laboratory tests. It is found that the modified constitutive model has satisfactory accuracy to describe the mechanical and hydraulic properties of unsaturated decomposed granite, which verified the reasonability of the assumption adopted in this paper. The test results are also helpful for the understanding of the moisture characteristics of the decomposed granite under constant degree of saturation condition.     

Behaviour of model footing on bamboo mat-reinforced sand beds

Soil reinforcement is one of the most common and cost-effective methods among ground-improvement techniques. Recently, sustainable and natural materials for soil reinforcement have attracted the attention of soil engineers. Bamboo is considered to be one of the most effective materials for soil reinforcement due to it’s excellent mechanical and engineering properties. This paper discusses the results of laboratory model studies conducted on sand beds reinforced with traditionally woven bamboo mats. An improvement in the bearing capacity of about 2.5 times was seen when the reinforcement was placed at the optimum depth. The effective spacing of the reinforcement between two layers of bamboo mats was found to be equal to the optimum depth value, i.e., 0.3B. An improvement in the bearing capacity of about seven times was observed when four layers of reinforcement were placed at a spacing of 0.3B. The results obtained from the present study reveal the excellent capacity of bamboo mat for soil reinforcement.     

Effects of fines on the cyclic liquefaction behaviour in unsaturated,well-graded materials

The liquefaction of cargoes of metallic ores during maritime transportation is believed to have caused a number of ships to capsize during the past 30 years. To minimise the risk of liquefaction, shipping standards specify a transportable moisture limit (TML), which is the maximum moisture content for ore cargoes to be loaded onto a ship. However, the mechanics leading to the liquefaction of these cargoes is not well understood. This study uses an unsaturated soil mechanics perspective to understand the cyclic liquefaction behaviour of partially saturated materials, similar in grading to iron ore fines, a metallic ore that is known to liquefy during shipping transportation. Iron ore fines are transported at relatively low densities and have variable gradings containing a wide range of particle sizes and fines contents. Therefore, the effects of the degree of saturation and the fines content on the cyclic liquefaction behaviour of well-graded materials have been investigated by performing unsaturated, compression-only cyclic triaxial tests on samples prepared with four different gradings containing particle sizes from 9.5 mm to 2 μm with fines (<75 μm) contents of 18%, 28%, 40% and 60%. The trends in the data are discussed and used to develop a simple method that can conservatively estimate the number of cycles that samples with different degrees of saturation and fines contents are able to resist. The use of this method to describe the liquefaction behaviour of cargoes containing iron ore fines, in practice, is discussed.     

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.     

Predicting settlement along railway due to excavation using empirical method and neural networks

More and more excavation projects are being performed near existing buildings and structures due to large-scale urban construction, in which the excavation unavoidably causes settlement and potential danger to the surrounding construction and buildings. For linear traffic facilities parallel to the excavation, the settlement profile parallel to the excavation, namely, the settlement along the traffic line, should also be considered. Moreover, the precise control of the differential settlement along the traffic lines also plays a very important role. Thus, it is necessary to establish a quick prediction model, which is able to consider both vertical and parallel settlement profiles, using the basic information on the excavation. Based on the large amount of field data, the characteristics of the settlement profiles are analyzed. A simplified empirical method is proposed; it is established based on the Rayleigh and Gauss distribution functions for predicting the ground settlement along railways induced by an excavation. Meanwhile, back-propagation neural networks are also used to predict the settlement behavior. A comparison between the predicted results and the monitoring data is given to verify the feasibility of the proposed method. A good agreement indicates that the proposed method can be employed to predict the settlement along railways due to an adjacent excavation.     

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.     

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.     

The ultimate bearing capacity of shallow strip footings using slip-line method

Based on the limit equilibrium theory, an accurate approach is proposed to solve the ultimate bearing capacity of shallow strip footings under general conditions. The foundation soil is considered to be an ideal elastic-plastic material, which obeys the Mohr-Coulomb yield criterion, and is assumed to be an ideal continuous medium which is isotropic, homogeneous and incompressible or non-expansive. Through analyzing the relative motion and interaction between the footing and soil, the problem of the ultimate bearing capacity of shallow strip footings is divided into two categories. A minimum model with the total vertical ultimate bearing capacity as its objective function is established to solve the ultimate bearing capacity using the slip-line method with no need to make any assumptions on the plastic zone and non-plastic wedge in advance. A convenient and practical simplified method is also proposed for practical engineering purposes. Furthermore, the first category of the problem in the case of the same uniform surcharges on both sides of footings is the focus of the study: the applicable conditions of Terzaghi’s ultimate bearing capacity equation as well as the theoretical exact solutions to its three bearing capacity factors are derived, and a new bearing capacity equation is put forward as a replacement for Terzaghi’s equation. The geometric and mechanical similarity principle is proposed by a dimensionless analysis. The results show that for perfectly smooth footings, the total vertical ultimate bearing capacity obtained by the present method is in good agreement with those by existing methods, whereas the existing methods underestimate the ultimate bearing capacity in the case of perfectly rough footings. The classic Prandtl mechanism is not the plastic failure mechanism of the ultimate bearing capacity problem of perfectly smooth footings on weightless soil.     

Slope reliability and back analysis of failure with geotechnical parameters estimated using Bayesian inference

A Bayesian approach is proposed for the inference of the geotechnical parameters used in slope design. The methodology involves the construction of posterior probability distributions that combine prior information on the parameter values with typical data from laboratory tests and site investigations used in design. The posterior distributions are often complex, multidimensional functions whose analysis requires the use of Markov chain Monte Carlo (MCMC) methods. These procedures are used to draw representative samples of the parameters investigated, providing information on their best estimate values, variability and correlations. The paper describes the methodology to define the posterior distributions of the input parameters for slope design and the use of these results for evaluation of the reliability of a slope with the first order reliability method (FORM). The reliability analysis corresponds to a forward stability analysis of the slope where the factor of safety (FS) is calculated with a surrogate model from the more likely values of the input parameters. The Bayesian model is also used to update the estimation of the input parameters based on the back analysis of slope failure. In this case, the condition FS = 1 is treated as a data point that is compared with the model prediction of FS. The analysis requires a sufficient number of observations of failure to outbalance the effect of the initial input parameters. The parameters are updated according to their uncertainty, which is determined by the amount of data supporting them. The methodology is illustrated with an example of a rock slope characterised with a Hoek-Brown rock mass strength. The example is used to highlight the advantages of using Bayesian methods for the slope reliability analysis and to show the effects of data support on the results of the updating process from back analysis of failure.     

A procedure to predict the precise seismic response of arch dams in time domain using boundary element formulation

In this context,a new boundary element algorithm based on the time-convoluted traction kernels is employed to evaluate the spatially varying earthquake ground motions of the Pacoima dam in the USA subjected to SH,SV and P incident waves.An accurate three-dimensional(3D)model of the dam canyon is implemented into the computer code BEMSA to investigate the seismic response of the dam.The analyses are performed in time domain with a linearly elastic constitutive model for the medium.This modeling procedure has been validated by the results reported in the existing literature.According to the results of this study,the response of the dam to earthquake waves is generally influenced by predominant frequency of the incident motion,surface topography,relative distance of observation points,and type of the incident seismic wave.For the cases considered,the incident SV wave has led to the maximum amplification of incident motions,especially at the left side of the dam.The results indicate that the proposed procedure can be employed for accurate prediction of a dam response during an earthquake.     

Electro-osmosis treatment techniques and their effect on dewatering of soils,sediments, and sludge: A review

Electro-osmosis is an established method of dewatering fine soils, sediments, and sludge (SSS). The efficiency of electro-osmotic treatment is controlled by the electrical resistance of the system. Due to an increase in SSS resistance during treatment, its cost efficiency is reduced, limiting the widespread use of this technique. The aim of this paper is to discuss the main reasons for the increase in SSS resistance during treatment and then to analyze the most recent and widely spread modifications to classical electro-osmotic treatment that attempt to combat these issues and improve the efficiency of the technique. These modifications to electro-osmotic treatment are polarity reversal, an intermittent current, the injection of chemical solutions at the electrodes, and the use of geo-synthetics. The paper discusses the relevant research on the above adaptations, and the advantages and disadvantages of each are evaluated and compared using the available laboratory and field tests in the literature on electro-osmotic dewatering research. All four methods of modification are shown to provide significant improvements and can be successfully translated to the field for greater use. However, the improvements they bring about may not be sufficient to warrant their general use in geotechnical applications.     

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.     

A comparison of seismic response to conventional and face destress blasting during deep tunnel development

A novel design of development face destress blasting was implemented during the construction of an experimental tunnel at great depth.A second tunnel was developed nearby using conventional blasting as a control.The tunnels were developed parallel to one another and perpendicular to a high subhorizontal stress.High resolution seismic monitoring was used to record and compare the seismic response generated by each excavation.Analysis of the seismic data from the conventionally blasted tunnel indicated that the seismogenic zone of stress-driven instability extended up to 3.6 m ahead of the face.Destress blasting within the corresponding zone of the adjacent tunnel had the effect of reducing the rock mass stiffness,primarily due to weakening of the pre-existing natural discontinuities.The reduction in rock mass stiffness was inferred from the spatial broadening of the seismogenic zone and associated reduction in the measured spatial density of events,radiated energy and seismic potency ahead of the face.High strain gradients around the unsupported portion of the conventionally blasted excavation were implied by the rate at which the spatial density of seismicity changed with respect to the tunnel face position.In contrast,the change in the spatial density of seismicity around the destressed development face was much more gradual.This was indicative of lower strain gradients in the rock there.A reduction in rock mass stiffness following destress blasting was also indicated by the much wider variety of seismic source mechanisms recorded adjacent to the destressed tunnel.Seismic source mechanisms associated with destress blasting were also more clearly characteristic of compressive overstressing with fracture closure.The source mechanism data also indicated that destress blasting induced instability on all natural joint sets.When compared to conventional development blasting,destress blasting typically reduced violent strain energy release from the rock mass and the associated seismicity,but not always.     

Monotonic,cyclic and post-cyclic performances of single-helix anchor in residual soil of sandstone

Helical anchors are commonly used in Brazil for guyed transmission towers subjected to static and cyclic wind loads.In most cases,these anchors are installed in tropical residual soil,a micro-structured material in which the shear strength is provided by soil bonding.During installation of a helical anchor,as the helical plate moves downward into the ground,the soil penetrated is sheared and displaced.Consequently,in this type of soil,anchor installation affects the soil shear strength significantly associated with a bonded structure.However,the cyclic responses of helical anchors in this type of structured soils are rarely reported.To address this problem,tests were conducted in a Brazilian residual soil to investigate the monotonic,cyclic and post-cyclic performances of single-helix anchors.Field tests used two instrumented single-helix anchors installed in this typical residual soil of sandstone,which is frequently observed in large areas in the southern Brazil.The testing results indicate that the disturbance caused by the anchor installation affected the monotonic uplift performance markedly.The results of cyclic loading tests also show no significant degradation of helix bearing resistance and reduced displacement accumulation with increasing load cycles.This is perhaps due to the soil improvement caused by previous loading,which then increases the stiffness response of the anchor.