Natural Soil Deposits Research Articles

Triaxial Strength Properties of Natural Deposits at K0 Consolidation State Using a Precision Triaxial Apparatus with Small Size Specimens

ABSTRACT The strength properties of undisturbed Japanese, Korean and United Kingdom deposits and their remolded soils are examined through K0 consolidated-undrained triaxial compression and extension tests by a new precision triaxial test apparatus (PTA) using small specimens 15 mm in diameter and 35 mm in height. The effect of sample disturbance on the coefficient of earth pressure at rest (K0) under K0 consolidation was quantitatively examined. The K0 value can be explained by the equation, K0 = 0.003 + 0.302Ip, under the normally consolidated region (NC). The K0 values under the effective overburden pressure (σ'VO) were (6∼15)% smaller than those of the NC. The rate of strength increase (cu/p) under σ'VO were in the range of 0.4 to 0.7 and those under the NC were 0.3 to 0.4. The effective angle of friction (φ') obtained from the strain rate of 1%/min was 10% smaller than that of 0.05%/min. The K0 values for the undisturbed soils were 0.07 greater than those using Jaky's formula and the K0 value can be explained by the equation, K0 = 0.912-0.012φ'. Brooker and Ireland and Yamanouchi and Yasuhara's equations can explain the K0 values for the remolded soils. The PTA effectively measured the engineering strength properties of natural soil deposits and their remolded samples. Therefore, it is a valuable tool in diagnosing soil samples and analyzing triaxial strength properties, thus leading to improvements in the accuracy of engineering design.

Liquefaction Resistance of Sand Deposit Improved with Sand Compaction Piles

This paper reports results of in-situ tests and undrained cyclic shear tests on high-quality undisturbed samples obtained by the in-situ freezing method at three sites where foundation soils had been improved with sand compaction piles. Triaxial specimens obtained from the frozen samples were fully saturated in the triaxial cell and then subjected to cyclic loadings. In the current design practice of ground improvement with the sand compaction piles, the SPT N-value at a mid point of a rectangular area surrounded by four adjacent sand piles, at which the N-value, and thus the liquefaction resistance is considered to be the smallest, is set as a target for the degree of compaction. It was found, however, that distributions of N-values and Nd-values in a horizontal plane at a certain depth appeared to be rather random. This suggests that the N-value at the center of a rectangular area does not always provide a conservative evaluation. The relationship between liquefaction resistances and the N-value, which was developed based mainly on field evidences of earthquake-induced liquefaction of natural soil deposits and reclaimed lands, compared quite well with that obtained from tests on fully saturated high quality specimens and mean values of Nd at several locations in the improved sands. Degree of saturation of the frozen sample was revealed to be considerably low, in the range between about 70 and 90%. This fact indicates that the liquefaction resistances of improved sands are significantly higher than those obtained from the relationship which is available only for fully saturated soils.

Pressure-Level Dependency and Densification Behavior of Sand Through Generalized Plasticity Model

Natural soil deposits and man-made earth structures exhibit complicated engineering behavior that is influenced by factors such as the stress level and drainage conditions. The stress conditions within a soil structure vary greatly, ranging from very low to very high values, due to the dead weight, loading and boundary conditions. Saturated sand deposits that exhibit drained conditions under static loading become undrained when subject to earthquake excitations. The Pastor–Zienkiewicz–Chan model has demonstrated considerable success in describing the inelastic behavior of soils under isotropic monotonic and cyclic loadings, including liquefaction and cyclic mobility. This study proposed modifications to the Pastor–Zienkiewicz–Chan model so that effects of stress level and densification behavior are simulated. The proposed model suggested that the angle of internal friction, elastic and plastic moduli are dependent on the pressure levels. Relevant modifications were made to incorporate a power term of mean effective stress on the loading plastic modulus so that a stress-level dependent volume change is obtained in combination with the stress-dilatancy relationship. To better simulate cyclic loading with reference to densification behavior, an exponential term of plastic volumetric strain is included for the unloading and reloading plastic moduli. A total of 11 parameters are needed for monotonic loading, whereas 15 parameters are needed in describing the cyclic behavior. The model simulations were compared with undrained and drained triaxial test results of several kinds of sand under dense and loose states. The predictive capability for monotonic and cyclic loading conditions was also demonstrated.

A local identification technique for geotechnical and geophysical systems

AbstractGeotechnical structures and natural soil deposits are massive multi‐phase particulate systems characterized by the development of localized response mechanisms under extreme loading conditions. Identification and analysis of such systems based on inverse boundary value problem formulations and sparse measurements are generally indeterminate. This paper presents an alternative local inverse problem technique. Point‐wise identification analyses of the constitutive behaviour of water‐saturated geotechnical and geophysical systems are performed using acceleration and pore pressure records provided by a cluster of closely spaced sensors. The developed novel technique does not require the availability of boundary condition measurements, or solution of an associated boundary value problem. The constitutive behaviour at a specific location of a soil‐system is analysed independently of adjacent response mechanisms or material properties. Numerical simulations and centrifuge tests of a soil‐retaining wall system are used to demonstrate the capabilities of the developed technique. Copyright © 2003 John Wiley & Sons, Ltd.

Degree and Extent of Wetting Due to Capillary Rise in Soils

Increased water content of subgrade soils can lead to degradation of their quality and result in pavement distress. Pavement performance depends on the modulus of the compacted unsaturated subgrade soil as well as that of the underlying natural soil deposits. The soil modulus is a strong function of water content; therefore, changes in water content over the life of the pavement must be understood. In this study, it has been found that for a relatively near-surface groundwater table, significant potential exists for capillary rise into subgrade soils. For fine-grained soils in particular, the height of this capillary rise can be quite substantial. Results from capillary rise column experiments have shown that soils wetted above the groundwater table through capillary rise remain at a degree of saturation averaging about 60 percent. It has also been found that soil suction within this capillary zone must be determined through the use of soil water characteristic curves or direct measurement. The conventional assumption that negative pore water pressures can be estimated by backward extrapolation above the groundwater table of a line of slope γ w is only appropriate in a very thin region above the groundwater table, where soils are wetted to a degree of saturation of 85 percent or more.

Assessment of the coefficient of the earth pressure at rest from shear wave velocity measurements

The paper presents an attempt to evaluate the feasibility of predicting the value of the coefficient of the earth pressure at rest K. of natural soil deposits on the basis of the measured velocities of the seismic shear waves. The experimental data presented were obtained from seismic tests performed in a calibration chamber on laboratory prepared specimens of seven granular materials, with grades ranging from fine sand to gravelly sand. The relationship between K. and the shear wave velocity established using the calibration chamber results was used successfully to predict K. at two well-documented sites for which both cross-hole seismic and seismic piezocone penetration test results are available Cet exposé dicrit un essai destiné à évaluer la faisabilité dune pr6vision de la valeur du coefficient de pression au sol au repos (Ko) des dépôts de sol naturels en se basant sur les vélociés mesurées des ondes sismiques transversales. Les donn6es expérimentales qui sont présentées dans cet expos&aute; viennent d'essais sismiques effectués dans une chambre de calibrage sur les spécimens préparés en laboratoire de sept matières en grains allant du sable fin au sable graveleux. La relation entre K. et la vitesse d'onde transversale établie d'après les résultats de la chambre de calibrage a 6t6 utilisée avec succés pour préd1re K. sur deux sites bien documentés oùi des essais de forage transversal sismique et de pénétration piéocone sismique sont disponibles.

A Method of Correcting Yield Stress and Compression Index of Ariake Clays for Sample Disturbance

Ariake clays have been widely deposited around Ariake Bay in Japan. The basic characteristics of Ariake clays are very soft and sensitive. How to assess quantitatively the quality of soil samples and correct the mechanical properties for sample disturbance is a well-known, but a not well-understood issue in geotechnical engineering. In this study, the undisturbed Ariake clays were deliberately disturbed to various levels of disturbance. A series of consolidation tests were then performed in order to investigate the effect of sample disturbance on the mechanical parameters. A simple index for providing a quantitative measure of the degree of sample disturbance is proposed in this paper. In addition, an approach of correcting the consolidation yield stress and the compression index for sample disturbance is also proposed based on the proposed index of sample disturbance. The existing data available are used to verify further the validity of the proposed method. The application of the proposed method is also discussed for a natural soil deposit.

Elastic shear modulus of soft clays from shear wave velocity measurement

Case records comprising the results of down-hole seismic surveys collected at nine sites worldwide, together with comparative results of laboratory bender element tests on reconstituted clay samples, were examined in an attempt to quantify the shear modulus of normally consolidated clays at very small strains of the order of 0·0001%. The shear modulus Gmax under the current state of stresses is given in a formula which includes a newly proposed void ratio function. An empirical expression incorporating the new void ratio function is also proposed for practical use in estimating Gmax profiles with depth in natural soil deposits from routinely available borehole data. In addition, effects of soil structure and ageing on Gmax are discussed on the basis of a case record. On a examiné les dossiers de cas comprenant les resultats de relevés sismiques à foration descendante, obtenus dans dix points du globe, ainsi que les résultats comparatifs d'essais d'éléments de flexion en laboratoire sur des échantillons d'argile reconstitutés, en vue d'essayer de quantifier le module de cisaillement d'argiles normalement consolidées à de très petites contraintes de l'ordre de 0,0001 p. 100. Le module de cisaillement Gmax, à l'état de contrainte existant est donné dans une formule qui comprend une nouvelle fonction proposée d'indice des vides. Une expression empirique comprenant la nouvelle fonction d'indice des vides est également proposée pour l'application pratique a l'estimation des proflls Gmax en fonction de la profondeur dans le dépôts de sol naturel a partir de données de foration descendante couramment disponibles. De plus, les effets de la structure du sol et du vieillissement sur Gmax sont examinés dans le contexte d'un cas d'espèce.

Rotational shear effects on ground earthquake response

Rotational shear has been defined in theory as a class of loading condition under which the second invariant, J 2D, of the deviatoric stress tensor is kept constant during shearing. While rotational shear as strictly defined in theory is seldom seen in real life, its characteristics do exist in many practical loading conditions. Seismic loading is one example. A limited number of undrained tests on saturated sand have shown that rotational shearing yields more pore pressure than other shear paths of the same J 2D magnitude. By intuition, this experimental finding seems to suggest that sand subjected to loading bearing the characteristics of rotational shear may have lower resistance to liquefaction than that under conventional unidirectional shear. However, in contrast to the intuition, a careful examination of the field stress and boundary conditions indicates that rotational shearing may have very little impact on the liquefaction resistance of natural soil deposits. This paper presents the theoretical evidence and numerical results that support the conclusion.

A Pore Pressure Model for Cyclic Straining of Clay

The paper focuses on the development and modelling of residual cyclic pore water pressure, u, in clay during undrained cyclic strain-controlled loading. Data obtained from a series of NGI type cyclic simple shear tests were employed. In addition to the amplitude of cyclic shear strain, γ c , the number of cycles, N, and the overconsolidation ratio, OCR, a new component is incorporated in the conventional characterization of u. This new component is the volumetric threshold shear strain, γ tv , of clayey soils, below which for all practical purposes u does not develop. The lack of understanding of complex clay microstructure and the associated interaction between clay particles during cyclic shearing makes the modelling of u in a manner other than curve fitting rather difficult. This is especially true in regard to the development of negative u in overconsolidated clays. A model based on the systematic curve fitting of the pore pressure data expressed in terms of γ c , γ tv , N and OCR is presented. The possibilities of the incorporation of such a model into existing numerical tools for simulation of the seismic and ocean wave loading response of natural soil deposits are briefly discussed as well.

Archaeological Testing of Site 41BX518 and other High-Potential Areas Along FM 2790 in Bexar County, Texas

The State Department of Highways and Public Transportation (SDHPT) conducted archaeological testing along portions of FM 2790 near Elm Creek in southern Bexar County, Texas, in May 1988. Testing was performed to determine whether portions of the previously recorded prehistoric archaeological site of 41BX518 existed within the right-of-way near the drainage. Results of testing determined that 41BX518 existed beyond the right-of-way limits of the roadway and no subsurface deposits were identified. No further work is recommended within this portion of FM 2790 and a report of testing follows, including comments on natural soil deposits along this portion of the former terraces adjacent to Elm Creek.

Shear Strength of Prestressed Cohesive Soils

The results of an experimental investigation to determine the effects of preloading on the Mohr-Coulomb failure envelopes of cohesive soils are described. In situ shear tests using the Borehole Shear Test (BST) were performed on a number of natural soil deposits. Initially, conventional stage tests were conducted to obtain the failure envelope. Subsequent tests were conducted by consolidating to a higher normal stress than was used in the shear phase of the test. Results are evaluated by examining the relative increase in shearing resistance between the original failure envelope and the prestressed failure envelope. By defining the Preloading Stress Ratio (PSR) as the ratio of the normal stress where shear strength comparisons are being made to the preloading stress, the relationship between relative strength increase and PSR could be determined. The data indicate that this increase may be described as a power function of the form y = axb, where a and b are regression coefficients which indicate the rate ...

Corrosion of Steel Piles Driven in Soil Deposits

ABSTRACT About 130 steel piles of 15 m length with L-shaped cross-section were driven into natural soil deposits at 10 stations for corrosion tests under a wide variety of soil conditions. Prior to driving, thicknesses of the test piles were measured and physical, chemical and electrochemical characteristics of the soil deposits were investigated. The piles were withdrawn approximately 2, 5, and 10 years after driving and the corrosion rates were evaluated. The test results obtained in this 10 years research indicate that (1) the withdrawn piles were generally in excellent condition, corrosion effects were minimal and independent of soil condition, (2) the average corrosion rate was approximately 0. 01 mm/year per both faces of a pile over the period of 10 years, (3) the corrosion of the inner face of a pipe pile seems to be the same as the outer face, (4) it is difficult to find any particular soil parameter which influenced the corrosion rate most decisively, and (5) any effect on corrosion of steel composition (mild steel, copper-bearing steel or weathering steel), cathodic protection or protection by painting was not evident. Influences of welding, cold-working, and underground electric leakage current were insignificant.

Anisotropic Strength of Cohesionless Sands

Natural soil deposits are generally anisotropic as well as heterogeneous. In order to provide a guideline for estimating a reasonable strength parameter applicable to such complex soils, plane strain tests on uniform-anisotropic, uniform-pseudoisotropic, layered-anisotropic, and layered-pseudoisotropic samples have been performed. Anisotropic strength response, caused by the anisotropic distribution of contact normals, should be expected even in a uniform mass of spheres. It is always necessary to assume that sand is generally anisotropic. Tests on samples prepared by the plunging-tapping method are advisable for obtaining a reasonable estimate of the mean strength for anisotropic sands.

Cam-Clay Predications of Undrained Strength

The validity of the critical-state Cam-Clay theory to predict undrained shear strength is substantiated with data from 96 different clay and silt soils reported in the geotechnical literature. The study investigates normally-consolidated and overconsolidated strengths, which is important in that a range of strengths is more common in natural soil deposits. Attention is given to establishing the critical-state pore pressure parameter from routine consolidated-undrained shear tests. The critical-state theory is shown to be an effective stress method that incorporates total stress approaches similar to SHANSEP.

Uncertainty, Safety, and Decision in Soil Engineering

Probability and decision theory are used to solve the decision problems in subsoil exploration, design of a foundation on clay, and design of a foundation on sand. The uncertainties that arise out of inaccuracies in measurement of soil strength and variability of natural soil deposits are considered and a model of the design and decision process is developed to account for the available information, the experience and judgment of the engineer, and the costs of investigation and failure.

Stresses and Displacements in a Cross-Anisotropic Soil

Synopsis The cross-anisotropic elastic half-space, with isotropy as a special case, is suggested as an improved mathematical model of natural soil deposits. A solution by Michell is modified to produce the important stresses and displacements in terms of the five elastic constants required to define cross-anisotropy. Where the exact expression for stress is strongly dependent on the values of Poisson's ratios, and hence of doubtful practical value in soil mechanics, an approximate expression is suggested. It is shown that expressions for surface settlement can be easily modified to include anisotropy. Consideration of the type of anisotropy existing various undisturbed soils indicates that anisotropy contributes to the over-estimation of settlements overconsolidated clays, and is partly responsible for the observed stress distribution in sand. Le semi espace élastique anisotropique simultané, en traitant l'isotropie comme un cas spécial, est proposé comme modèle mathématique amélioré des dépots de sol naturel. Une solution par Michell est modifiée pour exprimer les tensions et déplacements importants selon les cinq constantes élastiques nécessaires pour déterminer l'anisotropie simultanée. Lorsque l'expression exacte sur la tension dépend largement des valeurs des coefficients de Poisson, et par conséquent d'une valeur pratique douteuse en mécanique des sols, on propose de l'exprimer d'une manière approximative. On montre que pour exprimer les tassements de surface on peut facilement faire les modifications afin de comprendre l'anisotropie. L'étude du type d'anisotropie existant dans divers sols non remaniés indique que l'anisotropie contribue à la survaluation des tassements dans les argiles surconsolidés, et qu'elle est partiellement responsable de la distribution de tension observée dans le sable.