Effects of wetting–drying and stress ratio on anisotropic stiffness of an unsaturated soil at very small strains

2009 ◽  
Vol 46 (9) ◽  
pp. 1062-1076 ◽  
Author(s):  
C. W.W. Ng ◽  
J. Xu ◽  
S. Y. Yung
Keyword(s):  
Unsaturated Soils ◽  
Stress Ratio ◽  
Shear Stiffness ◽  
Small Strain ◽  
Matric Suction ◽  
Bender Elements ◽  
Shear Moduli ◽  
Small Strains ◽  
Stiffness Anisotropy ◽  
Stress Dependent

The very small strain shear modulus of soil, G0, is affected by many factors including soil properties, current stress state, stress history, and matric suction. Very little research has been conducted on anisotropic shear moduli of unsaturated soils. In this study, the effects of wetting–drying and stress ratio on anisotropic shear stiffness of an unsaturated completely decomposed tuff (CDT) at very small strains have been investigated using a modified triaxial testing system equipped with three pairs of bender elements. During drying and wetting tests, the measured very small strain shear moduli increased in a nonlinear fashion, but at a reduced rate as the matric suction increased. Similar to the stress-dependent soil-water characteristic curves (SDSWCCs), there was hysteresis between the drying and wetting curves showing the variations in shear moduli with matric suction. Variation in suction on the specimens under isotropic conditions produced changes in stiffness anisotropy (expressed as G0(hh)/G0(hv)) together with anisotropic strains. In shearing tests at constant suctions, significant stress-induced stiffness anisotropy was observed due to a change in the stress ratio. While shearing at a constant stress ratio, G0(hh)/G0(hv) appeared to be constant.

Effects of stress paths on the small-strain stiffness of completely decomposed granite

2005 ◽  
Vol 42 (4) ◽  
pp. 1200-1211 ◽  
Author(s):  
Y. Wang ◽  
C WW Ng
Keyword(s):  
Shear Strain ◽  
Shear Stiffness ◽  
Stress Path ◽  
Small Strain ◽  
Bender Elements ◽  
Small Strain Stiffness ◽  
Small Strains ◽  
Average Shear ◽  
Decomposed Granite ◽  
Completely Decomposed Granite

Research on the small-strain (0.001%–1%) characteristics of sedimentary soils and sands has advanced to the stage where it has been utilized in engineering analysis and design for some time. Despite the progress, the stiffness characteristics of weathered materials such as completely decomposed granite (CDG) at small strains have still attracted relatively little research attention. This paper describes a systematic laboratory investigation of the small-strain characteristics of intact CDG subjected to various triaxial stress paths, including drained compression and extension tests. The small-strain stiffness was measured using bender elements and internal local transducers. Measurements from bender elements illustrate that the elastic shear modulus of CDG increases as the mean effective stress increases and the void ratio decreases. Significant nonlinear shear stiffness – shear strain and bulk modulus – volumetric strain relationships were observed. At 0.01% shear strain, the measured average shear stiffness obtained from the extension tests was about 60% higher than that from the compression tests. The average shear stiffness for the tests with a 90° rotation of the stress path was about 50%–70% higher than that of tests without a change in the direction of the stress path after saturation.Key words: completely decomposed granite, nonlinearity, small strains, extension, compression, recent stress history.

Inherent anisotropic stiffness of weathered geomaterial and its influence on ground deformations around deep excavations

2004 ◽  
Vol 41 (1) ◽  
pp. 12-24 ◽  
Author(s):  
Charles WW Ng ◽  
Erin HY Leung ◽  
C K Lau
Keyword(s):  
Shear Modulus ◽  
Small Strain ◽  
Ground Movement ◽  
Ground Settlement ◽  
Bender Elements ◽  
Shear Moduli ◽  
Wall Deflection ◽  
Ground Deformations ◽  
Stiffness Anisotropy ◽  
Anisotropic Stiffness

The shear modulus G0 at very small strain (0.001% or less) is an important parameter for predicting ground movements of many geotechnical structures. Recent advances in laboratory testing enable the measurement of shear moduli in different planes of a soil specimen for the evaluation of stiffness anisotropy. Most studies of stiffness anisotropy have been conducted on sedimentary soils and clean sands, and the anisotropic stiffness of weathered material has not yet been fully investigated. In this study, the degree of inherent stiffness anisotropy of completely decomposed tuff (CDT) was evaluated through multidirectional shear wave velocity measurements using bender elements. Tests were performed on both natural (undisturbed) Mazier and block samples and the results were compared. CDT clearly exhibits inherent stiffness anisotropy, with a stiffness ratio between the shear modulus in the horizontal and vertical planes (Ghh/Ghv) ranging from 1.26 to 1.36. The stiffness parameters derived from the laboratory tests were utilized in numerical analysis of the influence of the inherent stiffness anisotropy on ground deformations around a hypothetical but typical multipropped deep excavation. For the given soil models and parameters used, the maximum computed wall deflection and ground settlement due to the pumping of groundwater prior to any excavation were 8% and 19% greater, respectively, than those of an isotropic analysis. The maximum wall deflection and ground settlement because of the combined effects of the pumping and recharging of groundwater inside the site and the subsequent multistage excavations were 15% and 10%, respectively, less in the anisotropic analysis.Key words: inherent anisotropic stiffness, shear modulus, excavation, ground movement, volcanic soil, weathered material.

A relationship describing the shear strength of unsaturated soils

1999 ◽  
Vol 36 (2) ◽  
pp. 363-368 ◽  
Author(s):  
Daud W Rassam ◽  
David J Williams
Keyword(s):  
Shear Strength ◽  
Soil Water ◽  
Nonlinear Regression ◽  
Unsaturated Soils ◽  
Characteristic Curve ◽  
Three Dimensional ◽  
Matric Suction ◽  
Linear Increase ◽  
Soil Water Characteristic Curve ◽  
Stress Dependent

A relationship describing the shear-strength profile of a desiccating soil deposit is essential for the purpose of analysis, especially when a numerical method is adopted where each zone in a discretised grid is assigned an elevation-dependent shear-strength value. The matric-suction profile of a desiccating soil deposit is nonlinear. Up to the air-entry value, an increase in matric suction is associated with a linear increase in shear strength. Beyond air entry, as the soil starts to desaturate, a nonlinear increase in shear strength occurs. The soil-water characteristic curve is stress dependent, as is the shear-strength gain as matric suction increases. In this paper, a three-dimensional, nonlinear regression analysis showed that a power-additive function is suitable to describe the variation of the shear strength of unsaturated soils with matric suction. The proposed function incorporates the effect of normal stress on the contribution of matric suction to the shear strength.Key words: air-entry value, matric suction, nonlinear regression, soil-water characteristic curve, tailings, unsaturated shear strength.

A unified small-strain shear stiffness model for saturated and unsaturated soils

2021 ◽  
Vol 136 ◽  
pp. 104241
Author(s):  
Mahnoosh Biglari ◽  
Iman Ashayeri ◽  
Domenico Gallipoli ◽  
Sara Moradpour
Keyword(s):  
Unsaturated Soils ◽  
Shear Stiffness ◽  
Small Strain ◽  
Stiffness Model ◽  
Small Strain Shear Stiffness

scholarly journals Bender elements in stiff cemented clay: shear wave velocity (Vs) correction by applying wavelength considerations

2019 ◽  
Vol 56 (7) ◽  
pp. 1034-1041 ◽  
Author(s):  
Qasim Khan ◽  
Sathya Subramanian ◽  
Dawn Y.C. Wong ◽  
Taeseo Ku
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Near Field ◽  
Shear Stiffness ◽  
Propagation Distance ◽  
Small Strain ◽  
Bender Element ◽  
Bender Elements ◽  
Sine Signal

For the quality control of cement mixing in clays, small-strain shear stiffness Gmax is now increasingly being used due to enhanced repeatability in shear wave velocity (Vs) measurements. These stiff cemented clays have higher resonant frequencies that require the use of higher input frequencies in bender element testing for reliable Vs measurements. However, the practical requirements for suitable signals (with minimal near-field effects and wave reflections) can often be difficult to implement. To facilitate such Vs measurements, the current study proposes a methodology that can correct Vs values corresponding to lower wave propagation distance to wavelength ratios (Ltt/λ) to more reliable values of Vs at reference Ltt/λ criterion suggested in previous studies (e.g., 2, 3.33, and 4). Two clay types are mixed with ordinary Portland cement and various mix ratios are utilized to cover a wider range of soil stiffnesses. Based on the collected database, it is found that the resulting fitting functions enable the reasonable estimation of the stabilized Vs values corresponding to the suggested Ltt/λ criterion regardless of the nature of the input sine signal.

Small to intermediate strain properties of fly ashes with various carbon and biomass contents

2016 ◽  
Vol 53 (1) ◽  
pp. 35-48 ◽  
Author(s):  
H. Choo ◽  
N.N. Yeboah ◽  
S.E. Burns
Keyword(s):  
Void Ratio ◽  
Small Strain ◽  
Interparticle Contact ◽  
Bender Elements ◽  
Fly Ashes ◽  
Small Strain Stiffness ◽  
Initial Void Ratio ◽  
Constrained Modulus ◽  
Stiffness Anisotropy ◽  
Biomass Particles

High-carbon-content fly ashes with biomass particles are typically landfilled in accordance with the ASTM C618 regulation. To quantify their geotechnical properties relating to storage and disposal, this study evaluates the small to intermediate strain properties of fly ashes with various carbon and biomass contents. Tested fly ashes had carbon contents ranging from 1.1% to 9.6%, resulting from co-combusting coal with biomass (biomass contents ranging from 0% to 8.2% by weight). The small-strain stiffness and intermediate-strain constrained modulus were evaluated using consolidation tests performed in a modified oedometer cell equipped with bender elements. It was found that initial void ratio governed the compressibility (or constrained modulus) of fly ashes, and with an increase in carbon and biomass contents, the small-strain stiffness of fly ashes decreased due to the decrease in number of direct contacts between microspheres. In addition, the interfine void ratio, ef, was employed instead of global void ratio to capture the alteration of interparticle contact or interparticle coordination between microspheres, due to the change in carbon and biomass contents. Finally, the stiffness in an overconsolidated state and the stiffness anisotropy of fly ashes were evaluated.

Modulus−suction−moisture relationship for compacted soils

2008 ◽  
Vol 45 (7) ◽  
pp. 973-983 ◽  
Author(s):  
Auckpath Sawangsuriya ◽  
Tuncer B. Edil ◽  
Peter J. Bosscher
Keyword(s):  
Shear Modulus ◽  
Empirical Relation ◽  
Small Strain ◽  
Matric Suction ◽  
Unit Weight ◽  
Bender Elements ◽  
Compacted Soils ◽  
Subgrade Soils ◽  
Dry Unit Weight ◽  
Propagation Technique

The ultimate parameter of interest in engineering design of compacted subgrades and support fills for highways, railroads, airfields, parking lots, and mat foundations is often the soil modulus. Modulus of compacted soils depends not only on dry unit weight and moisture but also on matric suction and soil structure (or fabric) resulting from the compaction process. However, these relationships in the as-compacted state (i.e., immediately after compaction) have not yet been extensively explored. This paper presents an experimental laboratory study of the shear modulus – matric suction – moisture content-dry unit weight relationship using three compacted subgrade soils. Compacted subgrade specimens were prepared over a range of molding water contents from dry to wet of optimum using enhanced, standard, and reduced Proctor efforts. A nondestructive elastic wave propagation technique, known as bender elements, was used to assess the shear wave velocity and corresponding small-strain shear modulus (Go) of the compacted subgrade specimens. The matric suctions were measured with the filter paper method. An empirical relation that takes into account the effect of compaction conditions is proposed for the Go – matric suction – molding water content relationship of compacted subgrade soils.

Determination of small-strain stiffness of Shanghai clay on prismatic soil specimen

2012 ◽  
Vol 49 (8) ◽  
pp. 986-993 ◽  
Author(s):  
Q. Li ◽  
C.W.W. Ng ◽  
G.B. Liu
Keyword(s):  
Stress State ◽  
Shear Wave ◽  
Small Strain ◽  
Triaxial Tests ◽  
Shear Moduli ◽  
Small Strain Stiffness ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Stiffness Anisotropy ◽  
Elastic Shear

Although a large number of tunnels and deep excavations have been constructed in Shanghai, small-strain stiffness properties of natural Shanghai clay have rarely been reported in the literature. In this study, the degree of inherent stiffness anisotropy of natural Shanghai clay was investigated in a triaxial apparatus equipped with local strain transducers and a shear-wave velocity measurement system. Three sets of side-mounted bender elements, consisting of one transmitter and two receivers each, were installed on a prismatic specimen. Two series of triaxial tests on prismatic specimens of intact Shanghai clay were carried out under an isotropic stress state. Shear-wave velocities and hence elastic shear moduli in different planes were determined from bender element measurements. The cross-correlation method using two received signals gives rise to the most objective and repeatable results on shear-wave velocities in comparison with other commonly used methods. Intact Shanghai clay clearly exhibits inherent stiffness anisotropy in terms of its elastic shear modulus ratio (G0(hh)/G0(hv)) of about 1.2 for a mean effective stress varying from 50 to 400 kPa. The measured higher stiffness in the horizontal plane may be attributed to the stronger layering structure in the horizontal bedding plane. A unique relationship is found that relates the normalized shear moduli to the stress state in each plane by incorporating a void ratio function in the form of F(e) = e–2.6.

Small strain stiffness for granite residual soil: effect of stress ratio

2021 ◽  
Author(s):  
Xianwei Zhang ◽  
Xinyu Liu ◽  
Lingwei Kong ◽  
Gang Wang ◽  
Cheng Chen
Keyword(s):  
Stress Ratio ◽  
Small Strain ◽  
Residual Soil ◽  
Resonant Column ◽  
Residual Soils ◽  
Column Tests ◽  
Small Strain Stiffness ◽  
Deviator Stress ◽  
Granite Residual Soil ◽  
Stress Ratios

Most previous studies have focused on the small strain stiffness of sedimentary soil while little attention has been given to residual soils with different properties. Most studies also neglected the effects of the deviator stress, which is extensively involved in civil engineering. This note considers the effects of the deviator stress on the small-strain stiffness of natural granite residual soil (GRS) as established from resonant column tests performed under various stress ratios. Although increasing the stress ratio results in a greater maximum shear modulus for both natural and remolded residual soils, remolded soil is more sensitive to changes in the stress ratio, which highlights the effects of soil cementation. The data herein offers new insights to understand the stiffness of residual soil and other weathered geomaterials.

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