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.

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.

A new theory about single-walled carbon nanotubes

2014 ◽  
Vol 28 (26) ◽  
pp. 1450186
Author(s):  
Yao Yang Tsai ◽  
Shih Chung Fang
Keyword(s):  
Carbon Nanotubes ◽  
Shear Modulus ◽  
Small Diameter ◽  
Small Strain ◽  
Closed Form Expression ◽  
Angle Variation ◽  
Shear Moduli ◽  
Total Potential ◽  
Definition Of ◽  
Walled Carbon Nanotubes

Since carbon nanotubes were discovered, till now no definitive formulation for computing the shear modulus of them was presented. To develop a theoretically rigorous and mathematically elegant expression for the shear modulus, thus, we initially propound a new small-strain theory in which merely small strain will arise when small-diameter carbon nanotubes are formed and thereby conclude the total potential energy including bond elongation and bond angle variation will suffice and the utilization of Quantum Mechanics and certain far complicated potential functions is unnecessary. Then based on it, a closed-form expression derived entirely from the "definition" of shear modulus, which was never published in all other literature, will be evolved. It should be noted that previously there was only one formula by which the shear moduli for all carbon nanotubes with diverse diameters and configurations could be predicted. By comparing the values calculated by the expression in this paper with those reckoned from the article mentioned above, it is obvious that both classes of quantities are similar to each other. It should also be noted that because the expression in this paper is the first (really having no precedent in related study fields) to be derived entirely according to the definition of shear modulus, perhaps this paper can be used as a useful theoretical tool for further study.

scholarly journals Maximum shear modulus of calcareous sand in Dejebel Dahar, Tunisia and its dependency on applied stress

2019 ◽  
Vol 92 ◽  
pp. 04007 ◽  
Author(s):  
Hyunwook Choo ◽  
Minhyuk Kwon ◽  
Lamia Touiti ◽  
Young-Hoon Jung
Keyword(s):  
Experimental Investigation ◽  
Shear Modulus ◽  
Applied Stress ◽  
Shear Wave Velocity ◽  
Small Strain ◽  
Silica Sand ◽  
Particle Crushing ◽  
Bender Elements ◽  
Calcareous Sand ◽  
Stress Dependency

The present experimental investigation aims at investigating the small strain stiffness of calcareous sand as a function of applied stress. The calcareous sand was sampled at Tunisia's Dejebel Dahar region, and the shear wave velocity (Vs) of calcareous sand was measured using modified oedometer cell equipped with bender elements. The results of this study demonstrate that the Vs of the tested calcareous sand is smaller than that of silica sand with minimal crushable particles at relatively low applied stress (σ); however, Vs of calcareous sand is greater than that of silica sand at high σ, reflecting strong dependency of calcareous sand on σ. The applied stress dependency of soils can be expressed as a power function of applied stress (Vs = α (σ / 1 kPa)β, where α = Vs at σ = 1 kPa and β = stress exponent). Generally, the single α-β can capture the dependency of Vs on σ, and the typical β value for sand is around 0.25. The measured β of tested silica sand was around 0.20; while, Tunisia calcareous sand shows β of greater than 0.32, and the dependency of Vs on σ cannot be captured by single α-β. This can be attributed to the fact that the variation of Vs of tested calcareous sand with increasing σ reflects not only fabric change but also particle crushing.

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.

scholarly journals A novel method for determining the small-strain shear modulus of soil using the bender elements technique

2017 ◽  
Vol 54 (2) ◽  
pp. 280-289 ◽  
Author(s):  
Yejiao Wang ◽  
Nadia Benahmed ◽  
Yu-Jun Cui ◽  
Anh Minh Tang
Keyword(s):  
Shear Modulus ◽  
Shear Wave ◽  
Shear Wave Velocity ◽  
High Frequency ◽  
Small Strain ◽  
P Wave ◽  
S Wave ◽  
Bender Elements ◽  
Small Strain Shear Modulus ◽  
First Arrival

Bender elements technique has become a popular tool for determining shear wave velocity, Vs, hence the small-strain shear modulus of soils, Gmax, thanks to its simplicity and nondestructive character among other advantages. Several methods were proposed to determine the first arrival of Vs. However, none of them can be widely adopted as a standard and there is still an uncertainty on the detection of the first arrival. In this study, bender elements tests were performed on lime-treated soil and both shear wave and compression wave velocities at various frequencies were measured. In-depth analysis showed that the S-wave received signal presents an identical travel time and opposite polarity compared with that of the S-wave components in P-wave received signal, especially at high frequency. From this observation, a novel interpretation method based on the comparison between the S-wave and P-wave received signals at high frequency is proposed. This method enables the determination of the arrival time of the S-wave objectively, avoiding a less reliable first arrival pick-up point. Furthermore, the “π-point” method and cross-correlation method were also employed and the obtained results agree well with those from the proposed method, indicating the accuracy and reliability of the latter. The effects of frequency on the shear wave velocity are also discussed.

Effects of gravel content on liquefaction resistance and its assessment considering deformation characteristics in gravel – mixed sand

2019 ◽  
Vol 56 (12) ◽  
pp. 1743-1755
Author(s):  
Hirofumi Toyota ◽  
Susumu Takada
Keyword(s):  
Shear Modulus ◽  
Small Strain ◽  
Bender Element ◽  
Liquefaction Resistance ◽  
Liquefaction Potential ◽  
Shear Moduli ◽  
Linear Elastic ◽  
Undisturbed Samples ◽  
Strong Relation ◽  
Secant Shear Modulus

Many reports describe overestimation of liquefaction resistance based on sounding data related to ground materials containing coarse particles such as gravel and cobbles. Better methods of liquefaction potential estimation must be developed using investigation data other than those from sounding. Gathering perfect and undisturbed samples is difficult, but using seismic methods such as PS logging might be effective for assessing liquefaction potential. For this study, bender element (BE) tests and local small strain (LSS) tests were conducted, respectively, to measure the dynamic and static shear moduli of gravel – mixed sand specimens. Subsequently, relations between liquefaction strength and secant shear moduli were examined to provide reliable estimation of liquefaction in gravel – mixed sand. Although the liquefaction resistance increased considerably with overconsolidation, the initial shear modulus exhibited only a slight change with the same overconsolidation. The experimentally obtained results elucidated that the important shear strain level, for which secant shear modulus has a strong relation with liquefaction strength, was not a linear elastic region of 0.001%: it was about 0.01%.

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.

scholarly journals Small-Strain Shear Modulus of Calcareous Sand under Anisotropic Consolidation

2021 ◽  
Author(s):  
Jinquan Shi ◽  
Yang Xiao ◽  
Jian Hu ◽  
Huanran Wu ◽  
Hanlong Liu ◽  
...  
Keyword(s):  
Shear Modulus ◽  
Void Ratio ◽  
Preparation Method ◽  
Stress Path ◽  
Small Strain ◽  
Sample Preparation Method ◽  
Calcareous Sand ◽  
Small Strain Shear Modulus ◽  
Stiffness Anisotropy ◽  
Anisotropic Consolidation

In this study, the small strain shear modulus of a calcareous sand was investigated by conducting bender element tests on both horizontal and vertical planes. The effects of sample preparation method, stress path and stress history on the developing of void ratio, the parameters in the modified Hardin equation and the stiffness anisotropy were examined. The test results show that the moist tamping samples have the least void ratio variation among the five samples. The void ratio recovery in σ'h = 100 kPa tests is higher than that in the σ'v = 100 kPa tests. The samples prepared in dry state have lower stiffness than those prepared in moisture state, which is not influenced by the anisotropic stress state. The stiffness anisotropy induced by the sample preparation method is significant under anisotropic consolidation. In σ'h = 100 kPa tests, the stiffness ratios at the end of the unloading stage are lower than the initial values at the loading stage, which is not found in the σ'v = 100 kPa tests, meaning that the stress history and stress path could affect the stiffness anisotropy and cover the impact of fabric anisotropy.

Sign in / Sign up

Export Citation Format

Share Document