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This research investigates the influence of matric suction on the variation of shear modulus and damping ratio of a silty soil in very small to medium shear strain levels. A set of laboratory experiments including three bender elements tests in addition to three resonant column-torsional shear tests have been carried out on unsaturated Firuzkuh silt specimens. In this regard, an unsaturated triaxial cell equipped with a set of bender elements and a resonant column-torsional shear device that can apply and control matric suction have been used. All specimens had an initial void ratio of 0.7 and were tested in various matric suctions under a net mean stress of 50 kPa. For this purpose, the axis translation technique has been implemented for applying matric suction, and High Air Entry Value (HAEV) ceramic discs have been used for air-water control of unsaturated silt specimens. According to the results, a significant variation of shear modulus and damping ratio has been observed with changes in matric suction and shear strain level. The output data indicate that shear modulus increases with increasing matric suction while the damping ratio decreases with an increase in matric suction. In addition, shear modulus and damping ratio decrease and increase with increasing shear strain, respectively.

Collapsible soils such as loess are particular types of problematic soils, whose collapse potential have caused severe damages to the buildings and structures. Loess is susceptible to large and sudden volume reduction upon wetting in constant normal stress or upon increasing normal stress in constant water content or a combination of these two stress paths. In this research both of these collapse induced paths: wetting and stress paths are investigated on undisturbed specimens of a Loess to observe the effect of various parameters on the collapse behavior of the tested soil., Seven controlled matric suction unsaturated oedometer tests have been conducted on undisturbed specimens taken from loessial soil of Gorgan, a city in Golestan province in Iran to study the collapse mechanisms of this type of soils. Two identical sets of unsaturated oedometer apparatuses designed and developed at Sharif University of Technology, with the capability of controlling matric suction have been utilized. In these tests, the matric suction, vertical net stress, volumetric water content, and vertical deformation of the samples have been controlled and recorded. Tests have been performed under two separate stress paths: constant matric suction with varying vertical net stress (loading and unloading) and constant vertical net stress with varying matric suction (wetting and drying). Obtained results show the high collapse potential of the tested soil and significant effects of stress path, matric suction, and vertical net stress on the collapse behavior of the tested soil.

The flow dynamics of dry granular flows is significantly affected by a prominent feature of a granular mass known as dilatancy. Although their rheological behaviour has been characterised by way of friction and dynamic dilatancy laws, the role of dilatancy in a granular flow has not been much appreciated. In this study, using a vane rheometer, an experimental investigation was conducted on uniform glass beads of d = 3 mm at different initial relative densities (15 ≤ Dr ≤ 66%) and shear rates that spanned over four orders of magnitude. The flow characteristics in terms of effective friction, volume change and velocity field were obtained and evaluated. The effective friction shows a descending–ascending pattern corresponding to a transitional behaviour from a velocity-weakening solid-like behaviour to a velocity-strengthening liquid-like behaviour. The volume-change measurements show that all specimens dilate and reach almost the same density at each shear rate. The velocity field follows a Gaussian pattern characterised by the slipping velocity at the boundary, the interlayer slippage between particles and the interlayer disorder of the particles. A new non-monotonic friction law and a dynamic dilatancy law are presented as governing rheological laws based on the inertial number and by introducing an effective dilation coefficient. This effective dilation coefficient successfully captures the role of dilation-induced secondary vortex flows in the dry granular flows. [ABSTRACT FROM AUTHOR]

Among other reasons, studies of the liquefaction potential of gravelly soils are limited because of the difficulties involved in preparing uniform specimens, without particle segregation, especially for a well-graded gravelly soil in a dry state for simple shear testing. Errors and difficulties are also involved in compensating for membrane penetration to a gravelly soil specimen in triaxial testing to get reliable data. Thus, innovative approaches for preparing triaxial and simple shear specimens for gravelly soils are introduced and implemented in this study to overcome experimental problems in acquiring accurate test results. In addition to the aim of obtaining reliable testing data on the liquefaction of gravelly soils under initial static shear stress for simulating sloping ground conditions, a study of the effect of various stress paths on the liquefaction resistance of gravelly soils was another goal of this research. In this regard, two sets of cyclic tests using medium-size triaxial and simple shear devices are conducted on a unique soil to compare the liquefaction potential of the tested gravelly soil using these two devices. Results of simple shear tests indicate that as the value of initial static shear stress increases, the cyclic resistance of the tested gravelly soil decreases. However, the results of the triaxial tests show that the variation of cyclic resistance for the tested soil depends on the initial static shear stress level and associated stress reversal conditions. Furthermore, the observed value of the pore water pressure ratio at failure using the strain-based liquefaction criteria for the tested gravelly soil was about 0.85, regardless of the type of testing. In addition, the relationship between the liquefaction resistance of gravelly soils using cyclic triaxial and simple shear devices was obtained. There is a common belief among practitioners that gravels have a far lower potential for liquefaction than sands. Results of experimental tests in this study reveal that gravels containing sands, known as gravelly soils, may have a high potential for liquefaction as well. However, the condition of zero effective stress, typically linked to liquefaction triggering in sandy soils, might not occur in gravelly soils. Rather, it is observed that the condition of excessive soil deformation leads to liquefaction triggering in gravelly soils. In addition, there are several observations on liquefaction triggering in sloping ground conditions. In this condition, initial static shear stress is present in the liquefiable soil; investigating the effects of such initial static shear stress on the liquefaction triggering of gravelly soils is strongly recommended, to ensure careful attention in design processes. [ABSTRACT FROM AUTHOR]

A new analytical method for investigating the stability of unsaturated soil slopes was developed by considering a 3D failure wedge under different steady-state infiltration and evaporation. In this light, the general limit equilibrium approach and concept of effective stress in unsaturated soils were implemented. An extensive parametric study was performed as well, to study the effect of slope geometry, unsaturated soil properties and/or state/material variables, position of groundwater level (GWL), and infiltration and evaporation flow rates on the stability of soil slopes against failure and landslide. In addition, a quantitative comparison was made between the safety factors obtained from the proposed unsaturated 3D analysis and those obtained from two-dimensional (2D) and three-dimensional (3D) classical slope stability analysis. It was revealed that the steady-state infiltration/evaporation has a significant influence on the stability of the silty and clayey soil slopes. In addition, the geometrical aspects of the slope and the hydromechanical parameters of the soil largely affect the safety factor of soil slopes against failure. Results show that the safety factors obtained from unsaturated 3D analyses for the silty and clayey slopes are remarkably different from those obtained from 2D and 3D classical calculations; however, negligible differences were found for the sandy slopes in that respect. In addition, a comparative study was performed on the impact of governing parameters on the safety factors of unsaturated slopes under infiltration and evaporation using an unsaturated 3D column method. It was revealed that infiltration has a pronounced effect on the safety factor compared with that for evaporation and that silty and clayey slopes are more affected by evaporation and infiltration than sandy slopes. In general, this study showed that applying unsaturated soil mechanics concepts to 3D stability analysis not only leads to more precise safety factors against the slope failure and landslide initiation but may also reduce the uncertain conservatism considerations in some practical problems. [ABSTRACT FROM AUTHOR]

Aeolian loess deposits are among problematic collapsible soils whose substantial settlements due to the increase in moisture content, stress level, or a combination of both, frequently cause serious structural failures, financial losses, and casualties worldwide. One way to increase the strength and bearing capacity of collapsible soils is to chemically stabilize them with lime. The behavior of lime-stabilized collapsible soils has been investigated mainly by conducting conventional laboratory tests without any measurement or control of unsaturated state variables, such as matric suction. In this research, the hydromechanical behavior of a reconstituted lime-stabilized loessial soil is investigated by conducting unsaturated suction-controlled odometer tests. For this purpose, variable vertical loads have been imposed on the unsaturated lime-stabilized loessial soil specimens (with different amounts of lime content) under constant matric suctions. During all unsaturated odometer tests, unsaturated state variables (e.g., matric suction, degree of saturation, volume change of the tested specimens) have been simultaneously controlled and measured. To capture the hydraulic characteristics of the soil, along with odometer tests, soil-water retention curves (SWRCs) of specimens have been obtained for both wetting and drying paths by conducting filter paper tests. Furthermore, to investigate the effects of adding lime to the evolution of soil pore structures, scanning electron microscope (SEM) micrographs have been captured from the structure of the soil specimens. Results of laboratory tests showed that adding a small amount of lime to collapsible loessial soils not only reduces the collapse-induced volume changes to a large extent, but also can increase the yield strength of the soil. Moreover, it was revealed that further increase of lime content in loessial soil specimens beyond a specific value has no significant influence on the hydromechanical characteristics of the treated soil. In a companion paper, the results from unsaturated odometer tests on treated and untreated loessial soil reported in this paper have been analyzed in an unsaturated effective stress context, and an empirical model for explaining the load-collapse behavior of the soil is presented. Also by implementing the disturbed state concept (DSC), a coupled semiempirical hydromechanical model is developed to predict the disturbance level and calculate the soil strains under applying vertical stress in the Ko condition. [ABSTRACT FROM AUTHOR]

To characterize the role of specimen disturbance and structure in the hydromechanical behavior of collapsible soils, two sets of wetting-induced collapse and suction-controlled triaxial tests were conducted on intact and reconstituted specimens of a loessial soil taken from a loess deposit in Gorgan, a city in the northeastern Golestan province of Iran. The testing approach used an advanced triaxial testing device that was specifically modified to control pressures applied to a soil specimen and to monitor and measure the amount of changes in volume and water content of the soil specimens during testing using highly sensitive digital sensors with an accuracy of 60.01 cm³. Results of the wetting-induced collapse tests show that the collapse phenomenon in intact loessial specimens was mostly a stepwise reduction in volume rather than a continuous decrease with a constant rate or a sudden drop in volume as water entered the voids. This behavior is believed to be attributed to the nonhomogenous distribution of macropores or micropores in intact specimens, which results in the presence of void spaces with different degrees of collapse potential within the soil matrix. A comparison of the experimental measurements during wetting-induced collapse also indicated that soil structure has negligible effects on the behavior of collapsible soils at high values of mean net stress (i.e., 200 kPa or higher) but significantly influences the behavior under low mean net stresses (i.e., 50 kPa or lower). The shear strength values of reconstituted specimens were higher than those of intact specimens when sheared at the same applied mean net stresses and matric suctions. Observations from this study suggest that the use of a critical-state line generated based on reconstituted specimens is not suitable for the characterization of the behavior of natural loessial soils. Finally, the disturbed-state concept (DSC) was used to characterize the stress-strain behavior of the intact and reconstituted specimens of collapsible soils during shear. [ABSTRACT FROM AUTHOR]

Preloading is a popular technique in ground improvement works on soft compressible soils. Observational methods such as the Asaoka or hyperbolic methods are widely used in practical applications to assess final settlement and the degree of consolidation during preloading. In this paper it is argued that the major cause of inaccuracy in the final settlement predicted by the Asaoka method, especially in the early stages of preloading (short assessment periods), is neglect of the effects of creep in Asaoka's solution. This may also be the cause of the lack of agreement between observational methods and the piezometer method at higher degrees of consolidation. In this paper a method is proposed that can improve ultimate settlement predicted by the Asaoka method, by considering the effects of creep. By applying this method to two sets of settlement-time data available in the literature, it was found that the final settlements can be estimated with greater accuracy compared with the Asaoka method, even in the early stages of consolidation. Additionally, the proposed method can predict future settlements accurately. The degrees of consolidation predicted by this method showed good agreement with pore pressures recorded in piezometers. [ABSTRACT FROM AUTHOR]