Dynamic Properties of Chemically Stabilized Sulfate Rich Clay

A series of resonant column tests was conducted on chemically stabilized specimens of sulfate-rich expansive clay from southeast Arlington, Tex. Specimens were tested for different stabilizer types, stabilizer dosages, compaction moisture contents, and confining pressures. Three chemical stabilization methods were used: sulfate resistant type V cement, low calcium class F fly ash, and lime mixed with polypropylene fibers. Results in the small-shear strain amplitude range (<0.0001%) were analyzed to assess the influence of compaction moisture content and confining pressure on the linear shear modulus Gmax and material damping Dmin of stabilized soil. Tests were also conducted at small- to mid-shear strain amplitude levels (0.0001–0.01%) to assess the threshold strain limit γth for each treatment method, and to study the effects of torsional shearing on the rate of degradation of normalized modulus G/Gmax of treated soil. A 10%-by-weight dosage of sulfate resistant type V cement was found to give the highest modulus and lowest damping when compacted at 95% of maximum dry unit weight γd-max on the wet side of Proctor optimum.     

Variables Controlling Stiffness and Strength of Lime-Stabilized Soils

Lime treatment is an attractive technique for soil improvement in the construction of rail tracks and pavement layers, in slope protection of earth dams, and as a support layer for shallow foundations. However, there are no dosage methodologies based on rational criteria as in the case of soil-cement technology, where the voids/cement ratio is shown to be a key parameter for the estimation of both strength and stiffness. The present study, therefore, was aimed at quantifying the influence of the amount of lime, porosity, and voids/lime ratio on the initial shear modulus (G0) and unconfined compressive strength (qu) of a lime-treated clayey sandy soil. From the results of unconfined compression tests and bender elements measurements, it was shown, for the soil-lime mixtures investigated, that the voids/lime ratio is an appropriate parameter to assess both initial stiffness and unconfined compressive strength. Also, a unique G0/qu versus voids/lime ratio relationship was established linking the soil-lime mixture initial stiffness and compressive strength.     

Strength Characteristics of Class F Fly Ash Modified with Lime and Gypsum

This paper presents the shear strength characteristics of a low lime class F fly ash modified with lime alone or in combination with gypsum. Unconfined compression tests were conducted for both unsoaked and soaked specimens cured up to 90 days. Addition of a small percentage of gypsum (0.5 and 1.0%) along with lime (4–10%) enhanced the shear strength of modified fly ash within short curing periods (7 and 28 days). The gain in unsoaked unconfined compressive strength (qu) of the fly ash was 2,853 and 3,567% at 28 and 90 days curing, respectively, for addition of 10% lime along with 1% gypsum to the fly ash. The effect of 24?h soaking showed reduction of qu varying from 30 to 2% depending on mix proportions and curing period. Unconsolidated undrained triaxial tests with pore-pressure measurements were conducted for 7 and 28 days cured specimens. The cohesion of the Class F fly ash increased up to 3,150% with addition of 10% lime along with 1% gypsum to the fly ash and cured for 28 days. The modified fly ash shows the values of Skempton’s pore-pressure parameter, Af similar to that of over consolidated soils. The effects of lime content, gypsum content, and curing period on the shear strength parameters of the fly ash are highlighted herein. Empirical relationships are proposed to estimate the design parameters like deviatoric stress at failure, and cohesion of the modified fly ash. Thus, this modified fly ash with considerable shear strength may find potential use in civil engineering construction fields.     

Stress-Strain Responses of Block Samples of Compressible Chicago Glacial Clays

This paper presents the results and analysis of a laboratory investigation of the behavior of lightly overconsolidated compressible Chicago glacial clays over a wide strain range. Each specimen was trimmed from high quality block samples taken from an excavation in Evanston, Illinois. Specimens were instrumented with three sets of bender elements and local LVDTs. After K0 consolidation to the in situ vertical effective stress of the block, drained stress probe tests were conducted. Results of bender elements tests obtained prior to stress probing show that compressible Chicago glacial clay initially is cross anisotropic. Propagation velocities measured by bender elements in axial direction after K0 reconsolidation and drained creep agrees well with the in situ shear wave velocity measured by seismic cone penetration tests. Results of drained stress probe tests are analyzed in terms of shear, volumetric and coupled stiffness, stiffness degradation, and direction of loading. The significant variability of shear, bulk and cross-coupling response depending on stress path direction and strain level provide experimental evidence that the Chicago clays are incrementally nonlinear at the strain levels investigated.     

Microbially Induced Cementation to Control Sand Response to Undrained Shear

Current methods to improve the engineering properties of sands are diverse with respect to methodology, treatment uniformity, cost, environmental impact, site accessibility requirements, etc. All of these methods have benefits and drawbacks, and there continues to be a need to explore new possibilities of soil improvement, particularly as suitable land for development becomes more scarce. This paper presents the results of a study in which natural microbial biological processes were used to engineer a cemented soil matrix within initially loose, collapsible sand. Microbially induced calcite precipitation (MICP) was achieved using the microorganism Bacillus pasteurii, an aerobic bacterium pervasive in natural soil deposits. The microbes were introduced to the sand specimens in a liquid growth medium amended with urea and a dissolved calcium source. Subsequent cementation treatments were passed through the specimen to increase the cementation level of the sand particle matrix. The results of both MICP- and gypsum-cemented specimens were assessed nondestructively by measuring the shear wave velocity with bender elements. A series of isotropically consolidated undrained compression (CIUC) triaxial tests indicate that the MICP-treated specimens exhibit a noncollapse strain softening shear behavior, with a higher initial shear stiffness and ultimate shear capacity than untreated loose specimens. This behavior is similar to that of the gypsum-cemented specimens, which represent typical cemented sand behavior. SEM microscopy verified formation of a cemented sand matrix with a concentration of precipitated calcite forming bonds at particle-particle contacts. X-ray compositional mapping confirmed that the observed cement bonds were comprised of calcite.     

Shear Strength and Stiffness of Sands Containing Plastic or Nonplastic Fines

This paper presents the results of a systematic laboratory investigation on the static behavior of silica sand containing various amounts of either plastic or nonplastic fines. Specimens were reconstituted using a new technique suitable for element testing of homogeneous specimens of sands containing fines deposited in water (e.g., alluvial deposits, hydraulic fills, tailings dams, and offshore deposits). The fabric of sands containing fines was examined using the environmental scanning electron microscope (ESEM). Static, monotonic, isotropically consolidated, drained triaxial compression tests were performed to evaluate the stress-strain-volumetric response of these soils. Piezoceramic bender element instrumentation was developed and integrated into a conventional triaxial apparatus; shear-wave velocity measurements were made to evaluate the small-strain stiffness of the sands tested at various states. The intrinsic parameters that characterize critical state, dilatancy, and small-strain stiffness of clean, silty, and clayey sands were determined. All aspects of the mechanical behavior investigated in this study (e.g., stress-strain-volumetric response, shear strength, and small-strain stiffness) are affected by both the amount and plasticity of the fines present in the sand. Microstructural evaluation using the ESEM highlighted the importance of soil fabric on the overall soil response.     

Forensic Investigation of a Sulfate-Heaved Project in Texas

This paper focuses on the cause, possible solutions, and future prevention of pavement heave at a new construction project. We speculated that heaving on the east side of the project was caused by a reaction between the lime stabilizer and minerals in the soil. Because of a difference in soil chemistry, the west side of the project (which was still under construction) did not show evidence of heaving. A forensic investigation was initiated to test our hypothesis. The findings of the investigation concluded that the cause of the heaving on the east side of the road was related to the formation of the expansive mineral, ettringite. Ettringite formed due to the reaction of the lime stabilizer with seams of high sulfate soil on the east side. Laboratory testing did not find any effective stabilizer for the high-sulfate soils on the east side. Therefore, reconstruction would involve removing and replacing the treated layer with a select material that has less than 2,000?ppm sulfates. Test results indicate that there was no threat of sulfate heave on the west side. District personnel had performed the field conductivity tests to evaluate and monitor the concentration of the sulfate content on the remaining project. The treatment of 3%/72-h mellowing period/3% lime treatment was employed on the west side. The whole project has been completed for 1?year and no heave has been observed.     

Fundamental Parameters for the Stiffness and Strength Control of Artificially Cemented Sand

The treatment of soils with cement is an attractive technique when the project requires improvement of the local soil for the construction of subgrades for rail tracks, as a support layer for shallow foundations and to prevent sand liquefaction. As reported by Consoli et al. in 2007, a unique dosage methodology has been established based on rational criteria where the voids/cement ratio plays a fundamental role in the assessment of the target unconfined compressive strength. The present study broadened the research carried out by Consoli et al. in 2007 through quantifying quantifies the influence of voids/cement ratio on the initial shear modulus (G0) and Mohr-Coulomb effective strength parameters (c′,?′) of an artificially cemented sand. A number of unconfined compression and triaxial compression tests with bender elements measurements were carried out. It was shown that the void/cement ratio defined as the ratio between the volume of voids of the compacted mixture and the volume of cement is an appropriate parameter to assess both initial stiffness and effective strength of the sand-cement mixture studied.     

Stress-Induced Anisotropic Gma x of Sands and Its Measurement

Anisotropy in elastic shear modulus Gma x exists in most soils as the result of either anisotropic soil fabric or anisotropic stress conditions. This paper presents a theoretical and experimental study on the anisotropy in a Gma x of sand due to a K0 stress condition. Elastic shear moduli of two types of sand in multiple stress planes under a K0 condition were measured using bender elements. Stress-induced anisotropy in Gma x of the sands during loading and unloading processes and the important influential factors were investigated. An empirical relationship for the estimation of K0 was proposed based on the experimental data. Shear moduli in nonprincipal stress planes were measured and compared with the results from the theory. The influence of stress cycles on Gma x in multiple stress planes was studied.     

Use of Piezocone to Predict Maximum Stiffness of Venetian Soils

A comprehensive geotechnical in situ and laboratory investigation into the soils forming the Upper Quaternary basin of the Venetian Lagoon has been carried out. In addition to standard tests, measurements of shear wave velocity were performed using crosshole, seismic piezocone, bender element system, and resonant column tests. Despite the incompatible strain levels of the seismic and piezocone tests, it is shown that piezocone tip resistance and excess pore pressure can be used for a preliminary estimate of the small-strain shear modulus Gmax for the Venetian soils.     

Defining Yield from Bender Element Measurements in Triaxial Stress Probe Experiments

This paper describes the elastic response of a block sample of compressible Chicago glacial clay under a variety of stresses and its relationship with the deformation characteristics at relatively large strains. The elastic shear stiffness was obtained from bender element tests during consolidation and shearing in drained triaxial stress probe tests. An empirical correlation was established based on the elastic shear stiffness in a preyield condition. By comparing the empirical correlation with the measured elastic shear stiffness in the stress region during probing, the changes of elastic shear stiffness were investigated. The departure of elastic shear stiffness from values computed by the empirical relation based on K0 loading directly relates to the yielding characteristics of the clay. The large-scale change of soil structure at yielding alters the well-established relationship between the elastic shear stiffness and stresses in the preyield condition. The mechanical yielding response of clays can be detected based on the systematic analysis of the elastic shear wave velocities.     

Modulus-Suction-Moisture Relationship for Compacted Soils in Postcompaction State

Despite clear evidence, changes in mechanical properties (i.e., stiffness or modulus) of compacted subgrades in response to subgrade moisture regime changes after construction have rarely been investigated in the geotechnical profession. In particular, when in-service assessment of pavement subgrade is made, the modulus-moisture variation should be addressed on the basis of unsaturated soil mechanics. This study presents the unsaturated small-strain modulus behavior of five predominately fine-grained compacted subgrade soils. The small-strain shear modulus (Go) of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated using bender elements. A test apparatus was designed to apply two stress state variables, the net confining pressure and matric suction, during the Go measurements. The relationship between Go and the SWCC under a constant mean net stress was developed. Additionally, the effect of compaction moisture content, compaction energy, and soil type on the Go-SWCC relationship was investigated. Finally, a relationship describing the small-strain modulus behavior of unsaturated compacted soils is proposed.     

Influence of In Situ Factors on Dynamic Response of Piedmont Residual Soils

The in situ chemical and physical weathering of igneous and metamorphic rocks, indentified as the process of formation of Piedmont residual soils, is a fairly well understood phenomenon. However, the effect this weathering has on the physical, mechanical, and dynamic properties of the rock∕soil is not understood fully. This study focuses on the dynamic shear modulus, G, and material damping ratio, D, of this soil formation for low- to mid-level amplitudes of vibration. The paper presents laboratory test results and correlations that demonstrate the effects that the degree of weathering has on these properties for various levels of confining pressure and shear strain amplitude. A total of 12 specimens of Piedmont residual soils from different depths were tested in a Resonant Column (RC) device. The specimens tested were SM and ML soils according to the USCS classification. The low-amplitude shear modulus and damping values were found to be similar to those reported in the literature from laboratory and in situ tests on the same type of soils. It was found that weathering, void ratio, and apparent overconsolidation ratio exert a noticeable influence on the dynamic response as a result of variations in confining pressure. The understanding of these effects will allow for a better prediction of phenomena such as soil amplification, which may result in damage to existing civil infrastructure founded on these soil deposits. The response in free field soil deposits compared with that of soils experiencing added confining stresses due to foundation loading appears to vary significantly in these geologic formations. Threshold strain and the variation of damping, D, with the normalized shear moduli, G∕Gmax, fall within the same range as those recently reported by other authors in similar soils.     

Treatment of Arsenic-Contaminated Soils. II: Treatability Study and Remediation

Treatment of sandy soils contaminated with arsenic was investigated at a bench scale and carried through to remediation in the field. The initial treatability study looked at many combinations of cement binders and reagents. Salts of iron, barium, manganese, and magnesium were generally effective at reducing arsenic leachability. The most consistently low potential for leaching [toxicity characteristic leaching procedure (TCLP) and a modified version of the American Nuclear Society's ANS16.1] was observed when the soils were treated with a mixture of Type I portland cement and ferrous sulfate. For instance, the average of arsenic concentrations in TCLP leachates in many treated soil samples from four sites was 0.26 mg∕L. Better protection against leaching was observed when the soil was pretreated with FeSO4?7H2O, then with portland cement. In addition to chemical containment, the mixture should prevent ground-water leaching by physical entrapment, because the permeabilities of the treated soils were in the range of 10?9–10?10 m∕s. Scanning electron microscope micrographs showed a dense mass with minimal void space, and a combination of X-ray diffraction, thermal analysis, and solid-state nuclear magnetic resonance (NMR) spectroscopy indicated the formation of a normal hydrated cement matrix, with some excess ettringite being present due to the extra sulfate being added to the formulation. Results from the bench-scale treatability study were reproduced very faithfully in the field, with permeabilities and compressive strengths being similar to those observed in the laboratory and TCLP leachability being even lower than predicted by the laboratory study.     

Characterization of Cemented Sand in Triaxial Compression

This work aims at studying the stress-strain-strength behavior of an artificially cemented sandy soil produced through the addition of portland cement. An analysis of the mechanical behavior of the soil is performed from the interpretation of results from unconfined compression tests, drained triaxial compression tests with local strain measurements, and scanning electron microscopy, in which the influence of both the degree of cementation and the initial mean effective stress was investigated. For cemented sandy soils, it was concluded that the unconfined compression resistance is a direct measurement of the degree of cementation. Consequently, the triaxial shear strength can be expressed as a function of only two variables: (1) the internal shear angle of the nonstructured material; and (2) the unconfined compression resistance. In addition, a logarithmic formulation is adopted to express the relationship between static deformation moduli and axial strain amplitude in axisymmetric conditions. Data from other reported investigation programs give to the proposed correlations a broader acceptance to general geotechnical applications.     

Mechanisms of Small-Strain Shear-Modulus Anisotropy in Soils

In this paper, experimental studies using a true triaxial apparatus and a bender element system, and numerical simulations based on the discrete element method (DEM) were used to investigate the stress- and fabric-induced shear-stiffness anisotropy in soils at small strains. Verified by experiments and DEM simulations, the shear modulus was found to be relatively independent of the out-of-plane stress component, which can be revealed by the indistinctive change in the contact normal distribution and the normal contact forces on that plane in the DEM simulations. Simulation and experimental results also demonstrated that the shear modulus is equally contributed by the two principal stress components on the associated shearing planes. Fabric-induced stiffness anisotropy, i.e., the highest Gxy or Ghh, can be explained by simulation findings in which more contact normals prefer to distribute along the horizontal direction. The experiments and simulations also reveal that the fabric-induced stiffness anisotropy increases with an increasing aspect ratio of the particles. The assumption of transversely isotropic fabric in soils is valid based on the DEM simulation results; however, this assumption is not completely supported by the experimental results.     

Strain-Dependent Dynamic Properties of Remolded Sand-Clay Mixtures

A series of undrained cyclic torsional simple shear tests using hollow cylindrical torsional shear apparatus was carried out to investigate the dynamic shear moduli and damping properties of clayey specimens with various sand contents and plasticity indices. The clayey soils used were collected from various sites along the coast of west Japan. Among these clayey soils, a clay sample with intermediate plasticity and another with high plasticity were mixed with silica sand at different proportions in order to examine the dynamic properties of sand-clay mixtures. In addition, experiments were carried out on undisturbed and remolded natural clay specimens with various plasticities. The effects of plasticity, loading frequency and confining pressure on the strain dependent normalized shear modulus and damping ratio were examined. Based on the results, empirical correlations for predicting the normalized shear modulus and damping ratio of remolded sand-clay mixtures at various shear strain levels were proposed.     

Rapid Construction and Settlement Behavior of Embankment Systems on Soft Foundation Soils

The I-15 Reconstruction Project in Salt Lake City, Utah required rapid embankment construction in an urban environment atop soft lacustrine soils. These soils are compressible, have low shear strength, and require significant time to complete primary consolidation settlement. Because of this, innovative embankment systems and foundation treatments were employed to complete construction within the approved budget and demanding schedule constraints. This paper evaluates and compares the construction time, cost, and performance of three embankment/foundation systems used on this project: (1) one-stage mechanically stabilized earth (MSE) wall supported by lime cement columns; (2) expanded polystyrene (geofoam) embankment with tilt-up panel fascia walls; and (3) two-stage MSE wall with prefabricated vertical drain installation and surcharging. Of the technologies evaluated, the geofoam embankment had the best performance based on settlement and rapid construction time considerations, but is more costly to construct than a two-stage MSE wall with PV drain foundation treatment. The one-stage MSE wall with lime cement treated soil was the most costly, and did not perform as well as expected; thus, it had only limited use on the project.     

Field and Lab Investigations of Prematurely Cracking Pavements

A forensic study was conducted to identify the cause of the premature cracking on three recently completed projects that were built with the same design. Nondestructive [ground penetration radar, falling weight deflectometer (FWD), GeoGauge, and Portable FWD], nuclear density gauge, dynamic cone penetration, and extensive laboratory tests were performed. It was found that the initial stiffness of the treated base was found to be excessively high by FWD backcalculation. Some sections of the backcalculated base moduli were over 20.7?GPa. This indicates that the layer is excessively brittle for a base material, similar to lean concrete. Six specimens (that were made without a mellowing period) exhibited cracks. There was no cracking for six specimens that had two days of mellowing. It was concluded that the culprit of the transverse cracking in the main lanes was the shrinkage of lime treated base layers. The longitudinal cracks are related to the edge drying and the transverse cracks are related to the insufficient mellowing period. Based on the findings of this study, the District implemented a 2-day mellowing period for Quicklime treated caliche base. Three newly constructed pavements (age 8, 5, and 2?months) were surveyed. No cracking can be observed so far, and the District thinks the cracking problem has been mitigated by the 2-day mellowing period. Without the mellowing period, cracking had normally occurred 1?to?2?months after construction..     

Small Strain Stiffness of Natural Granitic Saprolite in Hong Kong

Nonlinear stress-strain characteristics and shear stiffness-shear strain relationships of sedimentary soils and recompacted sands at small strains have been reported by many researchers. However, research work on the behavior of granitic soils at small strains has not attracted much attention, despite the fact that many countries around the world are underlain by granitic saprolites. In the study reported in this paper, shear stiffness of a natural granitic saprolite from Hong Kong has been investigated in the field, using the self-boring pressuremeter and geophysical techniques such as the suspension P-wave and S-wave logging method, and in the laboratory, using a triaxial apparatus equipped with internal displacement measuring devices. The observed stiffness-strain relationships of the natural granitic saprolite are highly nonlinear at small strains. Shear stiffness decreases significantly as shear strain increases. At very small shear strains (in the order of 0.001%), the elastic shear moduli deduced from the suspension S-wave logging method are generally consistent with the predictions made using an empirical correlation based on standard penetration test N values and also with the results of triaxial tests incorporating local displacement measurements. For shear strains larger than 0.01%, reasonable consistency can be found between the normalized shear stiffness-shear strain relationships obtained using the self-boring pressuremeter and from the triaxial apparatus.