Modeling Expansive Soil Movements Beneath Structures

This paper describes the use of an existing numerical model which is capable of generating continuous records over time of 3D soil suction profiles beneath a structure. The model uses recorded climatic data and representative soil properties. The model was used to obtain data required for the simulation of ground movements and the resulting structural response in a separate soil/structure interaction model (the results of the subsequent soil/structure interaction simulations are not reported). The factors influencing the soil moisture distribution beneath a structure were identified and careful consideration was given to quantifying the variability in these factors. The model explicitly captures the long-term moisture redistribution occurring beneath a structure as a result of introducing a ground cover. It also captures the effect of different construction dates on soil moisture conditions. If a range of construction dates are selected at random and 3D simulations over appropriate time periods are conducted, the variability in ground movements due to seasonal and long-term climatic effects and due to the choice of construction date can be quantified.     

Study of Expansive Soils and Residential Foundations on Expansive Soils in Arizona

Construction on expansive soils is challenging and thus prone to some problems and litigation. The engineering community makes extensive use of local experience and empirical procedures to address these problems. Although there has been extensive study of expansive soils and foundations on expansive soils, data related to performance of residential structures are limited in general and limited in the Phoenix area, in particular. In this study, an overview of the Phoenix Valley, Arizona, geotechnical practice and foundation performance related to residential structures on expansive clays, was developed through surveys and interviews with geotechnical engineers, structural engineers, and homebuilders. Using data obtained from files of Phoenix area geotechnical firms and government agencies, the existing Natural Resource Conservation Service map showing expansive soil locations throughout the Phoenix region was updated through the use of correlation developed in this study relating expansion index to common soil index properties such as Atterberg limits and percent passing the No. 200 sieve. Files of forensic investigations linked to expansive soil regions were made available for this study by several geotechnical engineering firms, and Phoenix Valley areas where forensic investigations have been identified, were mapped for comparison to regions identified in the updated map as having expansive soils. Comparison of the forensic investigation map to the updated map of expansive clay locations revealed that most of the forensic investigations were in regions identified with clays labeled as high to moderately high expansion potential, with a few forensic investigations in regions of medium expansion potential. Finally, unsaturated flow analyses were conducted for an Arizona expansive clay profile for two very different landscaped conditions of well-irrigated turf and desert landscape. The results of the numerical analyses were consistent with the reported observations and modes of failure identified through the surveys and interviews conducted with engineering and homebuilder professionals, including the finding that site drainage was found to be extremely important to good foundation performance, regardless of the type of landscape selected.     

Quality Assessment and Quality Control of Deep Soil Mixing Construction for Stabilizing Expansive Subsoils

This paper presents the process and results of a quality management program performed during and immediately after the construction of two deep soil mixing (DSM) test sections. The quality management program consisted of laboratory, in situ, and mineralogical tests to address the effectiveness of the treatment during and after construction. In situ investigations including the down-hole seismic and spectral analysis of surface waves (SASW) test methods were performed to evaluate the degree of improvement achieved through the measurement of compression and shear-wave velocities of the columns and surrounding soils. Scanning electron microscopy and electron dispersive x-ray analysis were performed on raw, laboratory treated and field-treated specimens for qualitative understanding of the degree of mixing achieved in the field and the compounds formed at particle level during stabilization, respectively. Laboratory tests results on field cores indicated that both field stiffness and strength are about 20 to 40% less than the corresponding laboratory prepared soil samples. The down-hole seismic and SASW tests showed considerable improvement in stiffness in and around the DSM columns. Mineralogical studies indicated the formation of silica and alumina hydrates along with interwoven structure of lime-cement treated clay particles in both laboratory and field specimens, suggesting adequate mixing of the soil and binder in both environments.     

膨胀土地基建筑物裂缝分析与防治

阐述了膨胀土地基建筑物开裂的特征,分析了裂缝产生的机理,提出了相应的预防对策和治理措施。     

Numerical Modeling of Seismic Response of Rigid Foundation on Soft Soil

A numerical model was developed to simulate the response of two instrumented, centrifuge model tests on soft clay and to investigate the factors that affect the seismic ground response. The centrifuge tests simulated the behavior of a rectangular building on 30?m uniform and layered soft soils. Each test model was subjected to several earthquakelike shaking events at a centrifugal acceleration level of 80g. The applied loading involved scaled versions of an artificial western Canada earthquake and the Port Island ground motion recorded during the 1995 Kobe Earthquake. The centrifuge model was simulated with the three-dimensional finite-difference-based fast Lagrangian analysis of continua program. The results predicted with the use of nonlinear elastic–plastic model for the soil are shown to be in good agreement with measured acceleration, soil response, and structural behavior. The validated model was used to study the effect of soil layering, depth, soil–structure interaction, and embedment effects on foundation motion.     

Structural, Construction, and Procedural Failures Associated with Long-Term Pyritic Soil Expansion at a Private Elementary School in Pennsylvania

This paper examines the background, history, and results of multiple investigations associated with pyrite-based expansive soils spanning almost 40 years in conjunction with a private elementary school located in western Pennsylvania. The school was initially designed in 1960. Original construction was completed in September 1961 and the first signs of distress, which were primarily related to slab heave, were reported in early 1962. One wing of the school, a 1965 classroom addition (1965 addition) with different structural and foundation systems, did not experience any expansive soil-related damages and served as a valuable comparison throughout multiple subsequent investigations. Pyritic soil material in the subgrade in conjunction with oxygen-rich groundwater was determined to be the cause of soil movement and building distress. Expansive soil-related problems at the school continued for decades despite an investigation, civil court action, and judgment in the late 1960s followed by a remediation program in the 1970s and 1980s. Following a second round of investigations and litigation in the late 1990s, all of the original classroom, office, and gymnasium building sections, with the exception of the 1965 addition, were demolished in late 2000 and early 2001 based on safety concerns and economic evaluation. Investigation and monitoring to confirm subgrade conditions continued throughout the demolition process. As a part of this paper, the history of this case dating back to one of the early identifications of pyrite as an expansive element of concern in building construction, including one of the earliest comprehensive identifications of the complete chemical-microbiological oxidation process is presented. The initial 1960s investigation and conclusions are identified as well as the series of engineering, procedural, and construction errors that took place during and after the first remediation process that led to ongoing soil expansion and structural damage, including misguided actions and misunderstandings that complicated and delayed a final resolution in this case. Today, the industry is more familiar with the potential for pyrite-related construction problems, nevertheless, the paper incorporates lessons learned for avoiding problems and in particular, the procedural failures that led to the eventual need to abandon and demolish the school facility.     

Foundation Repairs due to Expansive Soils: Eudora Welty House, Jackson, Mississippi

Eudora Welty house was built in the Belhaven District in Jackson Mississippi in 1925. The foundation rests on a surficial nonexpansive 1.5 m (5 ft) clay subgrade overlying highly expansive clays, locally known as Yazoo clay. The house has undergone extensive foundation movement and foundation repairs were made in 1979 and in 2002. Foundation repairs for a portion of Miss Welty’s house were first commissioned in 1979 directly by the renowned and widely acclaimed 1973 Pulitzer winning author. Later, after her passing in 2001, the State of Mississippi, to whom the house had been transferred, and the Eudora Welty Foundation commissioned the restoration of the house. The restoration included extensive foundation repairs. Robert Ewing had the distinct honor and pleasure of meeting with Miss Welty in 1979, and upon her request, of designing and implementing foundation repairs for the rear of the house. The underpinning work performed by Ewing in 1979 and that designed by him in 2002 are described. Fifty drilled piers and more than 100 jacks were used in the underpinning and final restoration of the house. The Eudora Welty House was first declared a Mississippi Landmark by the State. In 2005 the Eudora Welty house was placed on the National Register of Historic Places. Because of Miss Welty’s influence on our national literary heritage and because she did all the writing from her upstairs bedroom, her house was designated to be a National Historic Landmark.     

Survey of Residential Foundation Design Practice on Expansive Soils in the San Francisco Bay Area

The San Francisco Bay Area, California has experienced significant population growth over the last 60 years. The design practices for residential foundations have evolved substantially over this period, as a result of improved geologic characterizations, better engineering understandings of foundation performance, building code changes, and project litigation. The majority of residential foundations are constructed on expansive soils and bedrock, with the primary movement as a result of swell due to wetting of materials in an arid environment. A survey of design practice of practicing geotechnical engineers in the bay area was conducted using a written questionnaire and telephone interviews to compile data regarding the most commonly used design procedures, design details, drainage recommendations, and construction monitoring practices. The results of this survey are compiled and presented in this paper. Three primary foundation systems were identified in the survey as being commonly used in the bay area—rigid footing grids, drilled piers, and mats/slabs. To illustrate problems that have occurred with each of these foundation systems, case histories are presented for recent bay area expansive soil projects for each of these three foundation types.     

Analysis of Deep Moisture Barriers in Expansive Soils. I: Constitutive Model Formulation

Expansive soils cause important economical losses in many arid or semiarid countries in the world. Considering the large economic impact, relatively few efforts have been devoted to develop analytical methods that may help practitioner engineers to adequately design civil infrastructure on this type of soil. A rational design method should be able to quantify the heave or subsidence of the soil associated with the suction changes during water diffusion, as well as the contact pressures on soil-structure interfaces. Accordingly, in this and in a companion paper, the problem of volume changes due to nonpermanent water flow in expansive soils is studied and applied to the case of vertical moisture barriers. In this paper, a constitutive model for expansive soils is proposed. This model is an extension of that developed by Alonso et al. in 1990, in the sense that it can take into account the behavior of expansive soils. The advanced model is evaluated by comparing the numerical results with experimental data.     

Not Making the Grade: Expansive Soils and Higher Education in Texas

Expansive soils are a serious geologic hazard in Texas; accordingly, one would expect Texas engineering colleges to offer a strong educational program on expansive soils at both the undergraduate and graduate levels. However, data from the geotechnical programs within the civil engineering departments of Texas engineering colleges suggest that while Texas educational programs cover the topic of expansive soils in varying degrees, the educational effort is highly skewed toward traditional geotechnical and foundation issues. In most cases, treatment of the expansive soil problem is limited and is not adequate when measured against the scope and extent of expansive soil problems in Texas.     

Seismic Behavior of Soil-Pile-Structure Interaction in Liquefiable Soils: Parametric Study

Nonlinearity of the soil medium plays a very important role on the seismic behavior of soil-pile-structure interaction. The problem of soil-pile-structure interaction is further complicated when the piles are embedded in liquefiable soil medium. A finite-element code was developed in MATLAB to model three-dimensional soil-pile-structure systems. Frequency dependent Kelvin elements (spring and dashpots) were used to model the radiation boundary conditions. A work-hardening plastic cap model was used for constitutive modeling of the soil medium. The pore pressure generation for liquefaction was incorporated by a two-parameter volume change model reported in the literature. In this paper, a 2×2 pile group in liquefiable soil is considered and a parametric study is conducted to investigate its seismic behavior. The effects of loading intensity and stiffness of the soil on the seismic behaviour of the soil-pile system are investigated, considering nonlinearity and liquefaction of the soil medium for a wide range of frequencies of harmonic excitations. The inertial interaction attributable to a structure is analyzed for a system consisting of a four-storied portal frame on the pile group-soil subsystem. The responses of the structure are investigated for harmonic excitation and transient excitations. The importance of consideration of nonlinearity and liquefaction of the soil medium for analysis and design of a pile-supported structure is highlighted. Results from an analysis considering a practical soil-pile problem are presented to demonstrate the applicability of the developed algorithm for a practical problem.     

Finite Elements on Generalized Elastic Foundation in Timoshenko Beam Theory

Using the Vlasov foundation model, a modified approach of the continuous beam on elastic supports, leading to both a mechanical model and the proper foundation parameters of the generalized foundation is shown. Two formulations of the beam finite-element with shear deformation effect, resting on a two-parameter elastic foundation, characterized by distinct contributions of normal and rotary reactions are presented. The behavior of the second foundation parameter in the two formulations is governed by the bending cross section rotation of a beam. The first formulation, yielding a free-of-meshing stiffness matrix and equivalent nodal load vector, is based on the transcendental or “exact” solution of the governing differential equation of the beam resting on the elastic layer of constant thickness. Considering a linear variation of the layer thickness along the beam, the second formulation is based on the assumed polynomial displacement field. Numerical comparisons with the exact approach show that the cubic formulation leads to better results when the foundation parameters are variables. The practical utility of the analogy between a tensile axial force and the second foundation parameter is exemplified, too.     

Analysis of Deep Moisture Barriers in Expansive Soils. II: Water Flow Formulation and Implementation

Several alternatives have been proposed to prevent damage to civil infrastructure founded on expansive soils. For example, deep moisture barriers have been used in highways and buildings. However, in some cases, the protected lanes or structures degrade to similar levels as the unprotected ones, although at a smaller rate. In spite of these poor results, relatively few efforts have been devoted to the development of analytical methods for rational designs on expansive soils. This paper couples the constitutive model for expansive soils developed in the companion paper with flow equations in a deforming medium and a finite element code is developed. The resulting numerical tool has the capacity of computing soil suction and volumetric strain changes of expansive soils under a defined wetting–drying regime. To verify the capabilities of this computer code, a laboratory barrier model was built. The model was instrumented to measure soil suction changes and the corresponding surface displacements. The experimental and theoretical results were compared. Finally, the numerical model was applied to a design example of a deep moisture barrier.     

Numerical Solution for Laterally Loaded Piles in a Two-Layer Soil Profile

Piles are often embedded in a layered soil profile, such as sand or clay layer underlain by rock. Several existing solutions are available for laterally loaded piles in a layered soil system. However, these solutions are only applicable to constant soil stiffness for each layer. In this paper, a variational approach is employed to numerically solve the problem of laterally loaded piles in layered soils using beam on an elastic foundation model. The soil stiffness can be either constant with depth or linearly varying with depth. The numerical solution is validated against an existing solution for linearly varying soil stiffness in a single soil layer system and an existing solution for a two-layer soil system with constant soil stiffness. Case studies using the proposed solution for field lateral load tests on full size drilled shafts embedded in weak rock with an overlying sand layer are presented. The simplicity and the relative ease of using the solution make it a good alternative approach for estimating the deflection and moment responses of a laterally loaded pile in a two-layer soil profile.     

深基坑土钉支护数值模拟

基于FLAC3D的基本原理及其求解过程，对深基坑工程土钉墙支护做了数值模拟，对不同土钉长度、倾角的4种支护方案进行了对比选择。对所选支护方案的计算结果进行分析，得出一些有用的结论，对深基坑土钉墙支护设计及合理施工有一定的指导意义。     

Behavior of an Atypical Embankment on Soft Soil: Field Observations and Numerical Simulation

This paper compares the behavior of an embankment with nonsymmetric geometry built on soft soil with that predicted numerically using four elastoplastic soil models. Two of these models are based on isotropic conditions (Modified Cam-Clay on its own or in association with Von Mises) and two other are derived from anisotropic conditions (Melanie on its own or conjugated with Mohr Coulomb). The performance of the models, whose parameters are derived from experimental data, is checked against triaxial tests results. For the embankment, the measured and computed displacements and excess pore pressure are compared, with the isotropic models performing best. The maximum horizontal displacements versus settlements, the change in excess pore pressure versus vertical stress, the extent of the yield domain and the contours of the effective vertical and horizontal stress increments are also examined. The numerical results are explained based on the characteristics of the numerical models, namely the size and shape of the yield surface. The embankment, despite its nonsymmetric geometry, exhibits some similarities with typical behavior.     

Numerical Validation of an Elastoplastic Formulation of the Conventional Limit Pressure Measured with the Pressuremeter Test in Cohesive Soil

An elastoplastic pressuremeter theory for cohesive soil has been used in the design of construction, such as retaining walls, slope stability, or foundation engineering. This theory takes into account the plasticity along the vertical and horizontal planes and allows for the determination of the conventional limit pressure. We compute here the conventional limit pressure using the Plaxis program to check the validity of the theoretical results. First, we present the theory used for the interpretation of the pressuremeter test in cohesive soil and its extension to the conventional limit pressure, which is defined as the pressure at the borehole wall for a volume increase ΔV equal to the initial volume of the borehole. One of the main results is the theoretical expression of the conventional limit pressure. This volume variation is linked to a radial strain of ?1. This conventional limit pressure can be directly measured with the pressuremeter, whereas the theoretical limit pressure is expressed as an infinite expansion and cannot be directly measured. Then, we validate this theory by using finite elements, and determine the conventional limit pressure with the Tresca standard model of Plaxis, which is compared to the theoretical expression. Conclusions are drawn on the validity of this new theory which allows the measurement and the control of the shearing modulus and shear strength of the natural soil.     

Stability Problems in Soil‐Structure Interfaces: Experimental Observations and Numerical Study

This article presents experimental results of tests on soil‐structure interfaces carried out on a new “ring simple shear” apparatus specially developed at Ecole Nationale des Ponts and Chaussées, Paris, for such studies. In this apparatus strain localization takes place at or near the surface of the rotating steel drum that forms the soil‐structure interface. Depending on the conditions of tests, in terms of surface roughness, special instrumentation is capable of recording local as well as global response. Three tests on Hostun gravel at different confining radial pressures have been conducted and a deviatoric hardening model with nonassociated flow rule has been adopted for their numerical simulations. The point of inception of strain localization based on various theoretical considerations has been discussed and experimentally verified. The post‐peak behavior is simulated by employing a homogenization technique in which the soil sample is treated as a composite material consisting of a shear band surrounded by intact material. A deviatoric strain softening model has been adopted for the shear band. It is shown that the mechanism of failure and the response of the soil sample is reasonably well simulated. Although there are some concerns regarding the homogeneity of the sample, the post‐peak stage and the overall mechanical response of gravel‐steel interface are rather well reproduced.     

Earth Dam on Liquefiable Foundation and Remediation: Numerical Simulation of Centrifuge Experiments

A series of four dynamic centrifuge model tests was performed to investigate the effect of foundation densification on the seismic performance of a zoned earth dam with a saturated sand foundation. In these experiments, thickness of the densified foundation layer was systematically increased, resulting in a comprehensive set of dam-foundation response data. Herein, Class-A and Class-B numerical simulations of these experiments are conducted using a two-phase (solid and fluid) fully coupled finite element code. This code incorporates a plasticity-based soil stress–strain model with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate reduced deformations and increased crest accelerations with the increase in densified layer thickness. Overall, the differences between the computed and recorded dam displacements are under 50%. At most locations, the computed excess pore pressure and acceleration match the recorded counterparts reasonably well. Based on this study, directions for further improvement of the numerical model are suggested.     

微生物在土壤中迁移实验与数值模拟

A thorough understanding of bacteria transport in soil and groundwater is vital to the successful practice of environmental bioremediation. In this work, a dual-process adsorption with growth and decay model of bacterial transport was proposed. The on-site soil and the high efficiency methyl tert-butyl ether (MTBE) degrading bacterium Chryseobacterium sp.A-3, was used in the experiments. The model was validated using one-dimensional soil column experiments. The results show that the dual-process adsorption with growth and decay model proposed well describes the migration mechanism of microorganisms in soil and groundwater environment. According to the model analysis and simulation, the bacterial transport is enhanced as flow velocity and inlet cell concentration increase. Compared with the contaminant MTBE, the bacteria show stronger transport capacity but the irreversible straining in soil prevents the bacteria from transporting longer than MTBE. The results have certain instructive significance to the in-situ contamination remediation operation.