Showing papers in "Journal of Geotechnical and Geoenvironmental Engineering in 2012"

Factors Affecting Efficiency of Microbially Induced Calcite Precipitation

Abstract: Microbially induced carbonate precipitation (MICP) using ureolytic bacteria shows promise in the field of geotechnical engineering for several different applications, such as ground improvement and groundwater control. This study examined optimal use and efficient control of Sporosarcina pasteurii to induce the precipitation of CaCO3 in open environments. Laboratory tests were conducted to investigate the effect of changing treatment factors, such as chemical concentrations, retention times, and effective input rates (mol/L/h) on chemical efficiency. Chemical efficiency was measured based on weight measurements of CaCO3 precipitation compared with the amount of chemical reactants injected to samples. Based on the experimental results, the optimal time required for the precipitation process to take place in porous media for a specific range of bacterial optical density was determined. Results show that, below a certain urea and CaCl2 input rate (0.042 mol/L/h) and for a bacterial optical density (...

Observed Behaviors of a Long and Deep Excavation Constructed by Cut-and-Cover Technique in Shanghai Soft Clay

Abstract: A 16–18 m deep excavation for a metro station with in-plane dimensions of approximately 20 m wide by 290 m long was constructed by using the cut-and-cover technique in soft clay in the Shanghai metropolitan area, in which many high-rise buildings and utilities exist. To investigate the performance of this deep excavation and the associated effect on the adjacent buildings and utility pipelines, a long-term comprehensive instrumentation program was conducted during construction. Field observations included deflections of diaphragm walls, vertical movements at wall tops, ground settlements, and settlements of surrounding buildings and utilities. Analyses of field data indicated that overexcavation (i.e., removing more soil than designed) and a long construction duration caused the diaphragm walls to develop substantial deflections. The base and middle floor slabs played dominant roles in suppressing postexcavation wall deflections and ground settlements. This excavation in general exhibited different behavi...

Improved Performance of Railway Ballast under Impact Loads Using Shock Mats

Abstract: Impact loads generated because of wheel and rail defects cause accelerated ballast breakage leading to adverse performance of ballasted rail tracks. In the present study, experimental and numerical investigations have been carried out to develop an understanding of the performance of shock mats in the attenuation of dynamic impact loads and subsequent mitigation of ballast degradation. The investigations include different locations of shock mats considering stiff and weak subgrade conditions. With the provision of shock mats, the magnitude of impact forces decreases, and the time duration of impact ges prolonged. In the case of stiff subgrade, the efficiency of the shock mat in reducing the impact forces is greater when it is located at the bottom of ballast rather than at the top, whereas the reverse is true for weak subgrade. However, the provision of shock mats both at the top and bottom of the ballast bed, irrespective of the subgrade condition, is the best solution for minimising the impact force. The shock mats can bring down the impact-induced strains in the ballast bed by as much as 50%, apart from substantially reducing the ballast breakage.

Performance of pile-supported embankment over soft soil: full-scale experiment.

Abstract: This paper describes a full-scale experiment of pile-supported embankments on soft soil, which is conducted within the French national research project, Amelioration des Sols par Inclusions RIgides (ASIRI), to provide a new design method. This experiment shows the great influence of a granular platform reinforced with geosynthetics in load-transfer mechanisms. The measurements of geosynthetic strains highlight that the maximum strain takes place at the vicinity of pile heads during the construction of an embankment. Measurements also show that the load-transfer behavior of a platform reinforced by a single geotextile is different from that of a platform reinforced by two geogrids, although the final settlement is the same in both sections. From the important measurement data of this full-scale experiment, some considerations on the design and the efficiency evaluation have been proposed.

Probabilistic Standard Penetration Test-Based Liquefaction-Triggering Procedure

Abstract: A probabilistic version of the Idriss and Boulanger standard penetration test (SPT)–based liquefaction triggering correlation is derived using a maximum likelihood approach and an updated case history database. Measurement and estimation uncertainties in the cyclic stress ratio (CSR) and SPT (N1)60cs values and the effects of the choice-based sampling bias in the case history database are taken into account. The results of sensitivity analyses show that the position of the most likely triggering curve is well constrained by the case history data and that the magnitude of the total error term is also reasonably constrained. The most likely value for the SD of the error term in the triggering correlation is, however, found to be dependent on the uncertainties assigned to the CSR and (N1)60cs. The results of the sensitivity study appear to provide reasonable bounds on the effects of different interpretations on the positions of the triggering curves for various probabilities of liquefaction. Methods ...

Stratified Response Surfaces for System Probabilistic Evaluation of Slopes

Abstract: The need for probabilistic slope analysis that takes into account the uncertainty of soil properties has been acknowledged by the geotechnical profession. Traditionally, probabilistic slope analysis involves only single-mode failure that is considered based on the critical slip surface. This may result in underestimating the failure probability. In contrast, system reliability analysis for slopes is deemed more rational. This study aims at improving the existing methods of slope reliability analysis by considering system reliability. A stratified response surface method (stratified RSM) is proposed to describe the performance functions of possible failure modes. The proposed method differs from conventional response-surface–based slope reliability analysis (which constructs a single approximate performance function) by generating a group of (stratified) response surfaces. Based on these stratified response surfaces, system reliability analysis can be efficiently carried out by means of either a fi...

Monitoring Twin Tunnel Interaction Using Distributed Optical Fiber Strain Measurements

Abstract: In this field trial, a new monitoring technique using distributed strain sensing known as Brillouin optical time-domain reflectometry (BOTDR) was introduced to monitor the behavior of bolted-concrete linings of a recently completed tunnel when a second bored tunnel was constructed side by side at a distance less than one tunnel diameter apart. This was done by measuring circumferential strains in 12 rings using optical fiber that was installed using the point-fixing method. The strain distributions around the circumference of the rings show a generally similar profile. Maximum compressive strains measured below the tunnel springline nearest to the excavated tunnel were larger than the maximum tensile strains measured at the tunnel crown, distorting the circular tunnel into an oval/ellipsoid that was about symmetrical to the horizontal axis. Several methods were introduced to compare strain measurements made by BOTDR and diameter changes recorded by tape extensometer. This involves the use of a symmetrical tunnel distortion model and the basic differential equation for a circular arch. The calculated results showed some degree of similarity between the two methods. The analysis highlighted the importance of measuring the axial strain in the tunnel ring and subtracting the axial strain component to calculate the actual deflection of the lining as a result of bending.

Field Investigations on Performance of T-Shaped Deep Mixed Soil Cement Column–Supported Embankments over Soft Ground

Abstract: The soil cement deep mixing method has been used to improve soft clayey soils under embankment loading conditions. A compacted granular fill layer or geosynthetic reinforcement layer is placed over the top of soil cement deep mixed (DM) columns to reduce differential settlement between DM soil and the surrounding untreated soil, which, in turn, increases embankment stability. Typically, in conventional deep mixing methodology, the soil cement columns are closely spaced, indicating large area replacement ratios in the con- struction projects. Such practice could increase construction costs substantially. In this research, a new type of DM column, called a T-shaped DM (TDM) column, was designed and used as an alternative to the large-area-replacement-ratio DM columns employed in the field. Unlike in the conventional column, the cross section of the new column varies along the installation depth. Large amounts of cement slurry are injected and thoroughly mixed with the native shallow soil using specially designed mixing blades. At greater depths, deep mixing methodology is applied only to smaller-diameter columns, resulting in large-diameter columns near the surface and smaller-diameter columns deeper. Field trials were conducted to investigate the performance of TDM column-supported soft ground under embankment loading. For comparison, performance of conventional DM column-supported soft ground under similar embankment loading is presented. Differences in quality control studies and in situ plate loading tests on TDM and conventional DM columns are discussed. Under field embankment loading con- ditions, stress concentration ratio, excess pore water pressures generated in the soft clays, total monitored settlement, and lateral soil dis- placement near embankment toes are analyzed and discussed for both treatments. It is concluded that TDM columns have considerable advantages over conventional DM because they both mitigate settlement and enhance the performance of the embankments while reducing construction costs. DOI: 10.1061/(ASCE)GT.1943-5606.0000625. © 2012 American Society of Civil Engineers. CE Database subject headings: Embankments; Cement; Field tests; Pore pressure; Settlement; Soil mixing; Soft soils. Author keywords: Embankment; Cement; Field tests; Pore pressure; Settlement; Soil mixing; Soft soils.

Mechanical effects of biogenic nitrogen gas bubbles in soils

Abstract: The fluid bulk stiffness of a soil is very sensitive to the presence of gas, and a small volume of bubbles can significantly affect the pore pressure response to loading, including Skempton’s B parameter, P-wave velocity, and liquefaction resistance. Biologically mediated processes can lead to the production of gases in soils; nitrogen is particularly advantageous because it is not a greenhouse gas, it is not combustible, and it has low solubility in water. Sands, silts, and clayey sands inoculated with Paracoccus denitrificans were monitored to assess the effects of nutrient availability, fines content, and pressure-diffusion on the evolution of nitrogen gas generation and bulk stiffness. Results show clear evidence of biogas bubble formation, earlier gas generation and entrapment in specimens with higher fines content, and a strong correlation between biogas volume and P-wave velocity. The volume of gas is correlated with specific surface, suggesting that biogas bubble formation develops as heterogeneou...

Centrifuge Modeling of Bridge Systems Designed for Rocking Foundations

Abstract: In good soil conditions, spread footings for bridges are less expensive than deep foundations. Furthermore, rocking shallow foundations have some performance advantages over conventional fixed-base foundations; they can absorb some of the ductility demand that would typically be absorbed by the columns, and they have better recentering characteristics than conventional reinforced-concrete (RC) columns. Foundations designed for elastic behavior do not have these benefits of nonlinear soil-structure interaction. One potential disad- vantage of rocking systems is that they can produce significant settlement in poor soil conditions. Centrifuge model tests were performed to account for the interaction between soil, footing, column, deck and abutments systems. Bridge systems with rocking foundations on good soil conditions are shown to perform well and settlements are small. An improved method for quantification of settlements is presented. The model tests are described in some detail. One of the important factors limiting the use of rocking foundations is the perception that they might tip over; experiments show that tipping instability is unlikely if the foundations are properly sized. In one experiment, a column for a system with large fixed-base foundation collapsed while the systems with smaller rocking foundations did not collapse. DOI: 10.1061/(ASCE)GT .1943-5606.0000605. © 2012 American Society of Civil Engineers. CE Database subject headings: Earthquakes; Shallow foundations; Bridges; Centrifuge models. Author keywords: Earthquake; Shallow foundation; Rocking; Bridge; Centrifuge modeling.

Hybrid Method for Analysis and Design of Slope Stabilizing Piles

Abstract: Piles are extensively used as a means of slope stabilization. Despite the rapid advances in computing and software power, the design of such piles may still include a high degree of conservatism, stemming from the use of simplified, easy-to-apply methodologies. This paper develops a hybrid method for designing slope-stabilizing piles, combining the accuracy of rigorous three-dimensional (3D) finite- element (FE) simulation with the simplicity of widely accepted analytical techniques. It consists of two steps: (1) evaluation of the lateral resisting force (RF) needed to increase the safety factor of the precarious slope to the desired value, and (2) estimation of the optimum pile configuration that offers the required RF for a prescribed deformation level. The first step utilizes the results of conventional slope-stability analysis. A novel approach is proposed for the second step. This consists of decoupling the slope geometry from the computation of pile lateral capacity, which allows numerical simulation of only a limited region of soil around the piles. A comprehensive validation is presented against published experimental, field, and theoretical results from fully coupled 3D nonlinear FE analyses. The proposed method provides a useful, computationally efficient tool for parametric analyses and design of slope-stabilizing piles. DOI: 10.1061/(ASCE)GT.1943-5606 .0000546. © 2012 American Society of Civil Engineers. CE Database subject headings: Slope stability; Soil-structure interactions; Pile groups; Validation; Experimentation; Field tests; Hybrid methods.

Geotechnical Characterization and Random Field Modeling of Desiccated Clay

Abstract: An extensive set of in situ and laboratory test data is presented for a footing load test site east of Houston, Texas, in desiccated Beaumont clay. The in situ test program included standard and cone penetration tests (CPTs), the latter of which was selected for statistical analysis to produce vertical and horizontal random field model parameters for corrected cone tip resistance. Given the relatively high sampling frequency of the cone tip resistance in the vertical direction, the vertical random field model parameters were determined using the modified Bartlett’s test statistic with fitted autocorrelation models subject to a strict fitting criterion. Horizontal random field model parameters were generated by collapsing the two-dimensional distribution of the CPTs to a one-dimensional representation and by using less stringent evaluation of the autocorrelation. The results of this study indicate that Beaumont clay exhibits greater inherent spatial variability than previously reported at other cla...

Investigation of Geotechnical Parameters Affecting Electrical Resistivity of Compacted Clays

Abstract: The use of resistivity imaging (RI) in subsurface investigation has increased in recent years RI is a non-destructive method and provides a continuous image of the subsurface However, only qualitative evaluation of the subsurface can be obtained from RI The correlations between RI results and geotechnical engineering properties of soils have become important for site investigation using this method The primary objective of the current study was to determine the geotechnical parameters affecting electrical resistivity of compacted clays Understanding the influential factors will be helpful in determining the correlations between RI results and geotechnical properties of soil The effects of moisture content, unit weight, degree of saturation, specific surface area, percentages of pores, and ion composition on soil resistivity were investigated Soil samples used were classified as highly plastic clay (CH) according to the Unified Soil Classification System High-energy X-ray fluorescence tests

Measurements of drying and wetting permeability functions using a new stress-controllable soil column

Abstract: Direct measurement of a permeability function, k(ψ) (where ψ = suction), of an unsaturated soil is often tedious and time-consuming. Thus, researchers have proposed various semiempirical predictive methods to determine k(ψ) indirectly from a soil-water characteristic curve (SWCC) or a particle-size distribution. Also, stress effects on k(ψ) are generally not considered. To explore any stress effect on k(ψ) , a new 1-m-high stress-controllable soil column is developed to measure stress-dependent SWCC (SDSWCC) and k(ψ) of soils directly. The objectives of this paper are to investigate the effects of (1) two stress-state variables (matric suction and net normal stress) and (2) a drying-wetting cycle on both SDSWCC and k(ψ) of a compacted decomposed silty clay. Each compacted soil column is subjected to an evaporation-ponding cycle under various vertical net normal stresses. The variations of pore-water pressure and volumetric water content (VWC, θw ) profiles with time are measured instantaneously. The k(ψ) ...

Sand Deformation around an Uplift Plate Anchor

Abstract: This paper presents an experimental investigation on soil deformation around uplift plate anchors in sand by using digital image correlation (DIC). The experimental setup consists of a camera, loading frame, plexiglass mold, and computer, which is developed to capture soil deformation during anchor uplifting. A series of model tests are performed to investigate the influence of particle size, soil density, and anchor embedment depth on soil deformation. A set of images captured during anchor uplifting are used to calculate soil displacement fields by DIC. The failure surface is studied by tracking the points with maximum shear strain values. On the basis of this study, it is found that soil deformation and the pullout resistance of plate anchors are substantially influenced by soil density and anchor embedment depth, whereas particle size within the studied range has limited influence. In dense sand, the shape of the failure surface changes from a truncated cone above a shallow anchor to a combine...

Impact of Hydraulic Hysteresis on the Small-Strain Shear Modulus of Low Plasticity Soils

Abstract: Experimental studies have observed that the small-strain shear modulus (Gmax) of unsaturated soils measured during hydraulic hysteresis has a greater magnitude during imbibition than during drainage when plotted as a function of matric suction. To capture this behavior, a semiempirical model was developed to interpret the impacts of the stress state and hydraulic hysteresis on Gmax of low plasticity soils. Different from previous empirical relationships for Gmax, this model incorporates elastoplastic constitutive relationships, which integrate the effects of mean effective stress and hardening because of either plastic changes in volume or changes in the degree of saturation. The effective stress is defined as the sum of the net normal stress and the product of the effective saturation and matric suction, facilitating integration of the soil-water retention curve parameters into the model. An experimental testing program involving measurement of Gmax of compacted silt during hydraulic hysteresis w...

Influence of Cement-Voids Ratio on Stress-Dilatancy Behavior of Artificially Cemented Sand

Abstract: The addition of cement is an interesting remediation technique when the project requires improvement of the local soil for the construction of pavement base layers, in slope protection of earth dams and canal linings, as a support layer for shallow foundations and to prevent sand liquefaction. The present study was carried out to quantify the influence of the amount of cement and the porosity in a cement- voids ratio, defined as the ratio between the volume of cement and the volume of voids of a mixture, on the stress-dilatancy behavior of an artificially cemented sand. A program of triaxial compression tests considering three distinct cement-voids ratios was carried out with two combinations of volumes of voids and volumes of cement at each cement-voids ratio. Results showed that the stress-dilatancy relationship is alike for a given cement-voids ratio and that the stress-strain behavior is also similar. The cement-voids ratio is therefore an appropriate parameter to assess stress-dilatancy of the sand-cement mixture studied. DOI: 10.1061/(ASCE)GT.1943-5606.0000565. © 2012 American Society of Civil Engineers. CE Database subject headings: Stress; Dilatancy; Cement; Triaxial tests; Voids; Sand (soil type).

Base Capacity of Open-Ended Steel Pipe Piles in Sand

Abstract: This paper presents a new method for estimating the base capacity of open-ended steel pipe piles in sand, a difficult problem involving great uncertainty in pile foundation design. The method, referred to as the Hong Kong University (HKU) method, is based on the cone penetration test (CPT), and takes into consideration the mechanisms of annulus and plug resistance mobilization. In this method the annulus resistance is properly linked to the ratio of the pile length to the diameter—a key factor reflecting the influence of pile embedment—whereas the plug resistance is related to the plug length ratio, which reflects the degree of soil plugging in a practical yet rational way. The cone tip resistance is averaged over a zone in the vicinity of the pile base by taking into account the failure mechanism of the piles in sand, the condition of pile embedment (i.e., full or partial embedment), and the effect of soil compressibility. The predictive performance of the new method is assessed against a number ...

Influence of Partial Consolidation during Cone Penetration on Estimated Soil Behavior Type and Pore Pressure Dissipation Measurements

Abstract: Estimation of soil behavior type from cone penetration testing, and the interpretation of dissipation tests, is complicated in in- termediate soil types, such as silty sands, sandy silts, etc., where partial consolidation occurs during penetration. This issue is investigated in this paper using results from cavity expansion and finite element analyses as well as field and centrifuge piezocone data. The implications for soil classification are examined using analytical expressions to explore the effect of normalized shear strength, rigidity index, and over- consolidation ratio relative to the influence of partial consolidation and viscous effects under fully undrained conditions. It is shown that partial drainage conditions can affect where data plots on soil behavior charts, thus complicating soil classification. The effect on dissipation tests following partial consolidation during cone penetration is shown to create errors in interpretation using experimental and numerical data. A new approach is developed based in part on manipulation of solutions for pore pressure dissipation (following undrained penetration) to account for these errors when interpreting dissipation tests. Errors can become significant during standard cone penetration testing when the t50 dissipation time is less than about 50 s. Guidelines, including equations and a chart, are presented for practical use. Finally, imple- mentation of this approach is demonstrated in a brief case study. DOI: 10.1061/(ASCE)GT.1943-5606.0000646. © 2012 American Society of Civil Engineers. CE Database subject headings: Soil consolidation; Cone penetration tests; Soil properties; Pore pressure; Sand (soil type); Silts. Author keywords: Cone penetration test; CPT; Cone penetrometer; Penetration rate; Consolidation; Dissipation test; Soil behavior type; Coefficient of consolidation; Intermediate soils.

Soil-Foundation-Structure Interaction with Mobilization of Bearing Capacity: Experimental Study on Sand

Abstract: Recent studies have highlighted the beneficial role of foundation uplifting and the potential effectiveness of guiding the plastic hinge into the foundation soil by allowing full mobilization of bearing capacity during strong seismic shaking. With the inertia loading transmitted onto the superstructure being limited by the capacity of the foundation, this concept may provide an alternative method of in-ground seismic isolation: the so-called rocking isolation. Attempting to unravel the effectiveness of this alternative design method, this paper experimentally investigates the nonlinear response of a surface foundation on sand and its effect on the seismic performance of an idealized slender single-degree-of-freedom structure. Using a bridge pier as an illustrative prototype, three foundation design alternatives are considered, representing three levels of design conservatism. Their performance is investigated through static (monotonic and slow-cyclic pushover) loading, and reduced-scale shaking ta...

Response of Piles with Wings to Monotonic and Cyclic Lateral Loading in Sand

Abstract: Piles are required to withstand large lateral loads compared with the imposed vertical loads in certain applications in the offshore environment, such as for foundations for offshore wind turbines or as anchors for floating facilities. Although typically the soil strength increases with depth, close to the sea bed, the lateral capacity is often low. The requirement to limit pile head deflections necessitates the design of relatively long piles. Increasing the effective pile cross-section through "wings" close to the pile head is shown here with centrifuge model tests to reduce pile head deflections by approximately 50% compared with regular monopiles without "wings" for the same load level. The stiffer initial response of the winged pile also leads to smaller pile head deflections under cyclic loading, although the relative rate of accumulation is similar to that of a monopile. Simple methods for extrapolating from the monotonic pile head deflection to the deflection after thousands of cycles are compared with the results, and are shown to work equally well for piles with and without "wings".

Consolidation and Hydraulic Conductivity of Zeolite-Amended Soil-Bentonite Backfills

Abstract: The effect of zeolite amendment for enhanced sorption capacity on the consolidation behavior and hydraulic conductivity, k ,o f a representative soil-bentonite (SB) backfill for vertical cutoff walls was evaluated via laboratory testing. The consolidation behavior and k of test specimens containing fine sand, 5.8% (dry weight) sodium bentonite, and 0, 2, 5, or 10% (dry weight) of one of three types of zeolite (clinoptilolite, chabazite-lower bed, or chabazite-upper bed) were measured using fixed-ring oedometers, and k also was measured on sep- arate specimens using a flexible-wall permeameter. The results indicated that addition of a zeolite had little impact on either the consolidation behavior or the k of the backfill, regardless of the amount or type of zeolite. For example, the compression index, Cc, for the unamended backfill specimen was 0.24, whereas values of Cc for the zeolite-amended specimens were in the range 0:19 ≤ Cc ≤ 0:23. Similarly, the k for the unamended specimen based on flexible-wall tests was 2: 4×1 0 � 10 m=s, whereas values of k for zeolite-amended specimens were in the range 1: 2×1 0 � 10 ≤ k ≤ 3: 9×1 0 � 10 m=s. The results of the study suggest that enhancing the sorption capacity of typical SB backfills via zeolite amendment is not likely to have a significant effect on the consolidation behavior or k of the backfill, provided that the amount of zeolite added is small (≤ 10%). DOI: 10.1061/(ASCE)GT.1943-5606.0000566. © 2012 American Society of Civil Engineers.

Predicting the Onset of Static Liquefaction of Loose Sand with Fines

Abstract: Loose sandy soil subject to undrained shearing manifests deviatoric strain softening, and such a behavior has been referred to as collapse, static liquefaction, or instability. This paper uses the term instability and characterizes its triggering by the corresponding effective stress ratio, referred to as the instability ratio, ηIS. To capture the influence of fines on ηIS, the state parameter, ψ, as originally proposed by Been and Jefferies, was generalized to an equivalent granular state parameter, ψ*. This is achieved simply by replacing the void ratio, e, with the equivalent granular void ratio, e*. The conversion from e to e* was achieved by a predictive approach, and backanalysis is not required. It was hypothesized that, provided the fines content is less than the threshold value, ηIS and ψ*, at the start of undrained shearing, can be described by a single relationship irrespective of the fines content. Two published databases and a series of undrained triaxial tests results are used to eva...

Moisture Retention Properties of Municipal Solid Waste in Relation to Compression

Abstract: Original laboratory setups are used to study the moisture retention properties of municipal solid waste taking into account the porous medium’s structural evolution from compression. A controlled suction oedometer allowed the moisture retention curves (MRCs) of compacted samples to be determined for both wetting and drainage with a matric suction range of 0 to 10 kPa. Another setup utilizing an extraction plate was used to determine a drainage MRC for a noncompacted sample with matric suction varying from 0 to 450 kPa. The experimental results demonstrated the complexity of municipal solid waste (MSW) porous medium compared to soil. The MRC of lightly and uncompacted samples did not exhibit a measurable air-entry suction. Moreover, significant hysteresis between the wetting and drainage MRCs was observed. The experimental MRCs were interpreted with two different models, and a pore size distribution evolution with compression was proposed. Finally, the concept of field capacity in relation to the m...

Instability of Loose Sand under Drained Conditions

Abstract: A type of prefailure instability which occurs under fully drained conditions is studied in this paper. It is observed experimentally that when a specimen is sheared along a drained stress path involving a decrease in effective mean stress, it becomes unstable after the stress path crosses an instability line. The instability occurring under drained conditions is different from the type observed under undrained conditions. The differences and similarities between the two types of instability are elaborated. The practical implication of the study in analyzing the failure mechanisms of granular soil slope is also discussed.

Ground Improvement Using Vacuum Loading Together with Vertical Drains

Abstract: The use of vacuum as a preload together with vertical drains to reduce postconstruction settlement and increase the shear strength of soft ground is increasingly popular. However, conflicting views concerning preloading by vacuum consolidation continue to be disseminated. A review and interpretation of case histories of vacuum loading together with vertical drains indicates that (1) with a novel definition of excess pore-water pressure, existing theories of consolidation, solutions, and associated computer programs that have been developed for fill loading can be applied without any modification to vacuum loading; (2) vacuum that is available in the drainage blanket remains constant with depth within the vertical drains; (3) for vacuum loading as for fill loading, vertical drains may display well resistance; (4) there is no difference in magnitude and rate of settlement for a vacuum load and an equivalent fill load; (5) all empirical concepts of undrained shear strength that have originated from f...

Analysis of Foundations Reinforced with Jet Grouting

Abstract: The use of jet grouting columns to increase the bearing capacity and reduce the settlements of foundations is dealt with, the aim being to formulate a complete design procedure where theoretical analyses and site observations are combined. The study is based on experimental results reported in the literature, which show that jet grouting columns are able to transfer high vertical loads to the surrounding soil but also that structural collapse may locally occur because of a sudden narrowing of their cross sections and/or poor soil cementation. Noticeable attention has been then given to these aspects by interpreting the results of full-scale tests specifically performed to investigate the characteristics of jet grouting columns and their mechanical interaction with the surrounding soil. The results of this analysis form the basis of a numerical method developed to simulate the response of vertically loaded rafts supported by arrays of columns. The nonlinear load-settlement response of axially loade...

Aluminum Waste Reaction Indicators in a Municipal Solid Waste Landfill

Abstract: Subtitle D landfills may contain aluminum from residential and commercial solid waste, industrial waste, and aluminum pro- duction wastes. Some aluminum-bearing waste materials, particularly aluminum production wastes, may react with liquid in a landfill and cause uncontrolled temperature increases, significant changes in gas composition and pressure, nuisance odors, and changes in leachate composition and quantity. Such reactions may also cause degradation of leachate quality (e.g., increased ammonia, sodium, potassium, chloride, and TDS concentrations), combustion of the surrounding waste, damage to engineered components (gas collection systems, leachate collection systems, and liner system materials), and slope instability. Temperatures exceeding 150°C (300°F), generation and accumulation of undesirable explosive and toxic gases (e.g., hydrogen, acetylene, ammonia, carbon monoxide, and benzene), and gas pressures exceeding 210 kPa (30.5 psi) have been observed. Water from leachate recirculation, precipitation, the waste, or groundwater infiltration can initiate the exothermic reaction if aluminum production wastes are present. This paper uses a case history to illustrate some indicators of an aluminum reaction and problems that can develop from such a reaction in a Subtitle D landfill. DOI: 10.1061/(ASCE)GT.1943-5606.0000581. © 2012 American Society of Civil Engineers. CE Database subject headings: Solid wastes; Leaching; Aluminum (chemical); Waste disposal; Landfills. Author keywords: Solid waste; Leachate; Stability; Leachate recirculation; Aluminum; Exothermic chemical reaction; Waste disposal; MSW; Subsurface fire.

Modified UH model: Constitutive Modeling of Overconsolidated Clays based on a Parabolic Hvorslev Envelope

Abstract: Most clays, either naturally deposited or man-made, possess a certain degree of overconsolidation owing to tamping, cyclic loading, erosion, excavation, and/or changes in groundwater tables. An easy-to-use constitutive model for overconsolidated clays is useful for relevant engineering applications. In this paper, a simple model is proposed for overconsolidated clays based on the unified-hardening (UH) model. To evaluate the potential peak stress ratio of overconsolidated clays, a parabolic Hvorslev envelope rather than a straight envelope (used in the original UH model) is adopted. The proposed parabolic Hvorslev envelope passes through the origin of the mean stress-deviatoric stress plane. It has a slope of 3 as the overconsolidation ratio (OCR) approaches infinity and intersects with the critical state line as the OCR reaches unity. This modification leads to more realistic predictions for highly overconsolidated clays than does the original UH model with a straight Hvorslev envelope and is consistent with the critical state soil mechanics in which the higher peak stress ratio in overconsolidated clays is a result of interlocking (or dilatancy) rather than cohesion. The modified UH model retains the same parameters as those in the modified Cam-clay model. Reasonable agreement between the model predictions and experimental data dem- onstrates that the modified model is capable of addressing the fundamental behavior of overconsolidated clays. The present model is de- veloped for reconstituted clays with an isotropic fabric. The potential improvement of the model, taking into account anisotropy and structural effects, is discussed. DOI: 10.1061/(ASCE)GT.1943-5606.0000649. © 2012 American Society of Civil Engineers.

Effects of Reinforcement on Liquefaction Resistance of Solani Sand

Abstract: A study on liquefaction resistance of Solani sand reinforced with geogrid sheet, geosynthetic fiber, and natural coir fiber is reported. Tests were carried out on shake table (vibration table) with sand samples prepared at a relative density of 25%, with and without reinforcements. Synthetic geogrid sheets were used in three different combinations of three, four, and five layers. In case of fibers, the percentage of fibers by weight of dry sand were taken as 0.25, 0.50, and 0.75% and mixed randomly with the sand sample. The liquefaction parameters, such as the maximum pore water pressure (Umax⁡), maximum pore water pressure built-up time, stay time for Umax⁡, and pore water pressure dissipation time were measured corresponding to various levels of accelerations varying from 0.1–0.4 g. The frequency of the dynamic load was kept constant at 5 Hz. The liquefaction resistance of sand was evaluated in terms of maximum pore water pressure ratio (rumax⁡). Test results indicated that on inclusion of fiber...