Net Normal Stress Research Articles

Transient electrohydrodynamics of a liquid drop

The transient behavior of a leaky dielectric liquid drop under a uniform electric field of small strength is investigated. It is shown that for small distortion from a spherical shape, the drop deforms to an ellipsoid, and the deformation time history is represented by D=D(∞)[1-exp(-t/τ)], where D(∞) is the steady-state deformation and τ=(aμ(o)/γ)(19μ+16)(2μ+3)/(40μ+40)is the characteristic time, a, γ, μ(o) and μ being the drop radius, the surface tension, the viscosity of ambient fluid, and ratio of the drop viscosity to that of the ambient fluid, respectively. The contributions of the net normal and tangential electrical stresses in the degree of deformation and fluid flow strength are also determined.

Effects of Testing Procedures on the Laboratory Determination of Swell Pressure of Expansive Soils

Abstract There is lack of agreement on laboratory procedures that should be used to measure swell pressure. In this study, effects of apparatus compressibility and applied initial net normal stress on measured swell pressure were revisited, with emphasis on undisturbed specimens. Samples covering a wide range of properties were collected from Texas and Arizona, and effects were explored by examination for consistency of trends in behavior for several “companion” sets of undisturbed specimens. Failure to account for apparatus compressibility results in unintended swell of the specimen, reducing the constant volume swell pressure by as much as half for specimens of this study. Applied initial net normal stress level also affects measured swell pressure. For specimens of this study, swell pressures measured at low initial net normal stress (token load) were found to be about 40%, on average, of swell pressures measured for specimens loaded initially equivalent to field overburden stress. This difference is believed to be primarily due to initial specimen compression. Commonly adopted sampling disturbance corrections to constant volume tests conducted at initial light confining stress (∼ 1–7 kPa) were investigated, and found to give inconsistent values of swell pressure. Comparisons of constant volume to load-back swell pressure are also presented. Findings suggest that application of an initial confining stress level corresponding to field conditions and application of apparatus compressibility corrections as the swell pressure develops are key to the consistent measurement of constant volume swell pressure, and that measured swell pressures from constant volume tests, when carefully performed, are more consistent than swell pressures estimated using methods to account for sampling disturbance.

A Servo/Suction-Controlled Ring Shear Apparatus for Unsaturated Soils: Development, Performance, and Preliminary Results

Abstract In recent years, the key hypotheses of most constitutive frameworks postulated for unsaturated soils have been validated via suction-controlled oedometer, triaxial, and direct shear testing. These techniques only allow for the application of loads along limited modes and levels of soil deformation. It is well known today that a vast majority of geotechnical infrastructure made of compacted soil, or resting on unsaturated ground, involves a wide range of deformations. Calculation of foundation settlement, for instance, requires a good estimation of soil stiffness at relatively small strains. Analysis of slopes, embankments, and soil bearing capacity, on the other hand, requires good estimations of shear strength, from peak to residual. To date, however, there is limited experimental evidence of unsaturated soil behavior under large deformations while being subjected to controlled-suction states. This paper introduces a fully servo/suction-controlled ring shear apparatus that has been made suitable for testing unsaturated soils at large deformations via axis-translation technique. The paper outlines the full development of the apparatus, including details of its main components, assembling, and performance verification against the original Bromhead device. A preliminary series of suction-controlled ring shear tests was accomplished on several statically compacted samples of silty sand. Residual friction angle with respect to matric suction, i.e., residual beta angle, was found to remain virtually constant, regardless of the applied level of net normal stress. The results suggest that a conceptual residual shear strength framework for unsaturated soils, similar to that postulated for peak shear strength, could eventually be devised as more experimental evidence of this kind is made available.

Determination of the shear strength of unsaturated soils using the multistage direct shear test

The determination of the shear strength of unsaturated soils is generally more complicated, time consuming and expensive compared to the determination of the shear strength of saturated soils. Although much research has been done on unsaturated soil testing methods, there is still a strong need to translate these studies into practice. Further studies are needed on practical testing methods that can reduce both the cost of and time associated with shear strength testing of unsaturated soils. This paper presents a comprehensive evaluation of the validity of using the multistage direct shear test as a rapid and practical method to determine the shear strength of unsaturated soils. The laboratory tests were performed using a newly-constructed modified direct shear test apparatus that allows independent control of matric suction, referred to as a suction-controlled multistage direct shear test. Unsaturated shear strength was established using multistage loading over a range of net normal stresses and matric suction values. Shear strength parameters obtained from the multistage tests are compared with those from conventional direct shear tests using multiple soil specimens. Recommendations are given on how to carry out multistage direct shear tests to ensure reliable unsaturated shear strength measurements. The tests were performed on undisturbed soil samples obtained from the riverbank of the lower Roanoke River in eastern North Carolina, USA.

Testing Study on the Permeability Characteristics of Unsaturated Soils

The coefficient of permeability of unsaturated soil is an important parameter which is used to analyze moisture migration. For complexity and diversity of unsaturated soil behaviors, it is difficult to perform test of unsaturated soil because of many factors. With GDS system, the permeability characteristics of unsaturated soil are tested and studied in this paper which makes it possible to directly measure the coefficient of permeability at various combinations of net normal stresses and matric suction values. The research shows the coefficient of permeability of unsaturated soil is the function of suction and cell pressure.

Measurements of Drying and Wetting Permeability Functions Using a New Stress-Controllable Soil Column

Direct measurement of a permeability function, k(ψ) (where ψ = suction), of an unsaturated soil is often tedious and time-consuming. Thus, researchers have proposed various semiempirical predictive methods to determine k(ψ) indirectly from a soil-water characteristic curve (SWCC) or a particle-size distribution. Also, stress effects on k(ψ) are generally not considered. To explore any stress effect on k(ψ) , a new 1-m-high stress-controllable soil column is developed to measure stress-dependent SWCC (SDSWCC) and k(ψ) of soils directly. The objectives of this paper are to investigate the effects of (1) two stress-state variables (matric suction and net normal stress) and (2) a drying-wetting cycle on both SDSWCC and k(ψ) of a compacted decomposed silty clay. Each compacted soil column is subjected to an evaporation-ponding cycle under various vertical net normal stresses. The variations of pore-water pressure and volumetric water content (VWC, θw ) profiles with time are measured instantaneously. The k(ψ) ...

Stiffness of a compacted residual soil

Stiffness or Young's modulus of a compacted residual soil can be obtained from numerical analyses that simulate the shear stress versus horizontal displacement of the soil as obtained from the laboratory direct shear tests. The paper presents the simulated Young's moduli of a compacted residual soil from Bukit Timah Granite as obtained from numerical analyses of direct shear test results on saturated and unsaturated specimens. The Young's modulus of the soil appears to increase with increasing net normal stresses and increasing matric suctions. The effect of increasing matric suction on Young's modulus is found to become less significant at high matric suctions.

Field monitoring of an unsaturated saprolitic hillslope

To advance the understanding of the rainfall-induced landslide triggering mechanism, a comprehensive field monitoring programme was implemented in a saprolitic hillslope in Hong Kong. The instrumentation covered the measurements of the two stress-state variables (i.e., net normal stress and matric suction) and their effects. The monitoring results, including pore-water pressure (PWP), volumetric water content, subsurface total horizontal stress, horizontal displacement, and rainfall intensity, are reported. Most instruments recorded reliable and good quality data, which have strong correlation among each other. The site-specific infiltration and deformation characteristics of the hillslope subjected to heavy rainstorms are investigated. Shallow transient perched groundwater tables were believed to be developed at colluvial deposits on the top 3 m, where PWPs up to 20 kPa were typically measured. The main groundwater table probably rose by 6 m when the hillslope was subjected to rainfall intensity of 133.5 mm/h. It is possible that cross-slope groundwater flowed along a shallow, dipped decomposed rock stratum at the central portion of the landslide body. Besides, two distinct types of slope movements were generally observed, namely the “cantilever” and the “deep-seated” mode. Rupture surfaces have possibly been developed at 5 m below ground or deeper, resulting in a multiple translational- and rotational-slide type of failure.

Experimental investigation of the relationship between the critical state shear strength of unsaturated soils and the soil-water characteristic curve

This paper presents the comparison of experimental results of the critical state shear strength of an unsaturated glacial till and the predicted shear strength using the Soil-Water Characteristic Curve (SWCC), determined following three different techniques, namely: (i) the conventional SWCC without application of any consolidation stress; (ii) the SWCC measured on a specimen subjected to a net stress equal to the net normal stress of experimental shear tests; (iii) the SWCC derived from the degree of saturation and matric suction of the sheared specimen under critical state conditions. The results of the study suggest that the best predictions of the shear strength are possible using the experimental SWCC information expressed as an apparent SWCC. The study also suggests that the relationship between the apparent degree of saturation and the matric suction of a specimen sheared to critical state is unique, and is not influenced by the shearing mode, or compaction water content (i.e. soil structure).

Shear strength and dilative characteristics of an unsaturated compacted completely decomposed granite soil

Shear strength and dilative characteristics of a re-compacted completely decomposed granite (CDG) soil are studied by performing a series of single-stage consolidated drained direct shear tests under different matric suctions and net normal stresses. The axis-translation technique is applied to control the pore-water and pore-air pressures. A soil-water retention curve (SWRC) is obtained for the CDG soil from the equilibrium water content corresponding to each applied matric suction value for zero net normal stress using a modified direct shear apparatus. Shear strength increases with matric suction and net normal stress, and the failure envelope is observed to be linear. The apparent angle of internal friction and cohesion intercept increase with matric suction. A greater dilation angle is found at higher suctions with lower net normal stresses, while lower or zero dilation angles are observed under higher net normal stresses with lower suctions, also at a saturated condition. Experimental shear strength data are compared with the analytical shear strength results obtained from a previously modified model considering the SWRC, effective shear strength parameters, and analytical dilation angles. The experimental shear strength data are slightly higher than the analytical results under higher net normal stresses in a higher suction range.

Modified Direct Shear Apparatus for Unsaturated Sands at Low Suction and Stress

Abstract Modifications to a conventional laboratory testing system are described for direct shear testing of unsaturated soils at relatively low matric suction and net normal stress. Matric suction ranging from zero (saturated) to about 10 kPa is controlled using a hanging column assembly (ASTM D6836). Net normal stress ranging from about 0.3 to 10 kPa is controlled by directly applying dead loads to the specimen via a series of aluminum top caps machined to varying thicknesses. Precise control of suction and normal stress within these ranges makes the apparatus ideal for examining the shear strength behavior of unsaturated sands, which are characterized by relatively low air-entry pressures and for which the influences of matric suction on mechanical response can be subtle. Soil-water characteristic curves are concurrently obtained during the shear testing program by measuring transient and equilibrium pore water drainage under the imposed suction changes. Testing procedures and recommended protocols are described. Results from a series of tests using saturated and unsaturated specimens of poorly graded fine sand are presented to demonstrate application and performance of the system. Relationships between shear strength and matric suction are non-linear and exhibit peak shear strength at matric suction within the range of the air-entry suction. High friction angles measured for the portions of the failure envelope at low matric suction and normal stress may indicate the effects of dilation on the strength development.

Effects of Dry Density and Grain Size Distribution on Soil-Water Characteristic Curves of Sandy Soils

The unsaturated soil mechanics is receiving increasing attention from researchers and as well as from practicing engineers. However, the requirement of sophisticated devices to measure unsaturated soil properties and time consumption have made the geotechnical engineers keep away from implication of the unsaturated soil mechanics for solving practical geotechnical problems. The application of the conventional laboratory devices with some modifications to measure unsaturated soil properties can promote the application of unsaturated soil mechanics into engineering practice. Therefore, in the present study, a conventional direct shear device was modified to measure unsaturated shear strength parameters at low suction. Specially, for the analysis of rain-induced slope failures, it is important to measure unsaturated shear strength parameters at low suction where slopes become unstable. The modified device was used to measure unsaturated shear strength of two silty soils at low suction values (0 ~ 50 kPa) that were achieved by following drying path and wetting path of soil-water characteristic curves (SWCCs) of soils. The results revealed that the internal friction angle of soil was not significantly affected by the suction and as well as the drying-wetting SWCCs of soils. The apparent cohesion of soil increased with a decreasing rate as the suction increased. Further, the apparent cohesion obtained from soil in wetting was greater than that obtained from soil in drying. Shear stress-shear displacement curves obtained from soil specimens subjected to the same net normal stress and different suction values showed a higher initial stiffness and a greater peak stress as the suction increased. In addition, it was observed that soil became more dilative with the increase of suction. A soil in wetting exhibited slightly higher peak shear stress and more contractive volume change behaviour than that of in drying at the same net normal stress and the suction.

Behavior of a Compacted Completely Decomposed Granite Soil from Suction Controlled Direct Shear Tests

A series of single-staged consolidated drained direct shear tests are carried out on recompacted completely decomposed granite (CDG) soil—a typical residual soil in Hong Kong, under different matric suctions and net normal stresses. Matric suction is controlled by applying air pressure in the pressure chamber and water pressure at the bottom of the high air-entry ceramic disk. The experimental results show that the contribution of suction to shear strength is significant. Shear strength of CDG soil increases with the increase of matric suction. Net normal stress has a remarkable influence on the shear strength of unsaturated CDG soil. The increase in shear strength due to an increase in matric suction (suction envelope) is observed as nonlinear i.e., ϕb value varies with matric suction. No soil dilatancy is observed for zero matric suction (saturated case) but as the suction value is increased, higher soil dilatancy is obvious in lower net normal stresses. The rate of increase of soil dilatancy is greater...

Shear strength of unsaturated soil interfaces

Unsaturated soil interfaces exist where unsaturated soil is in contact with structures such as foundations, retaining walls, and buried pipes. The unsaturated soil interface can be defined as a layer of unsaturated soil through which stresses are transferred from soil to structure and vice versa. In this paper, the shearing behavior of unsaturated soil interfaces is examined using results of interface direct shear tests conducted on a low-plasticity fine-grained soil. A conventional direct shear test device was modified to conduct direct shear interface tests using matric suction control. Further, the results were used to define failure envelopes for unsaturated soil interfaces having smooth and rough counterfaces. Results of this study indicate that matric suction contributes to the peak shear strength of unsaturated interfaces; however, postpeak shear strength did not appear to vary with changes in matric suction. Variations in net normal stress affected both peak and postpeak shear strength. Failure envelopes developed using the soil-water characteristic curve (SWCC) appeared to capture the nonlinear influence of matric suction on shear strength of soil and interfaces.

Is Matric Suction a Stress Variable?

The writer attempts to clarify and address two fundamental questions regarding the appropriate use of matric suction in unsaturated soil mechanics: Is matric suction a stress variable? and Is matric suction a stress state variable? These questions are examined by employing the universally accepted mechanical equilibrium principle, the concept of representative elementary volume (REV) for air-water-solid multiphase porous media, and physical and logical reasoning. It is clarified that matric suction is not a stress variable at a typical air-water-solid REV level, and it can be considered as a stress state variable. However, when it is considered as a stress state variable, there is an interdependency or coupling between matric suction and the net normal stress if both of them are concurrently used to describe the state of stress in unsaturated soils. It is illustrated that the answers to these questions bear important implications for the conceptualization, theorization, and application of unsaturated soil mechanics.

An Oedometer-Type Pressure Plate SWCC Apparatus

Abstract A new oedometer device for SWCC determination has been evaluated by testing a wide range of soil types, and recommendations for best practices for use of this device have been made. The new device allows application and control of the net normal stress, σ−ua, in addition to the matric suction, ua−uw. Thus it can be used to simulate a range of overburden stresses and various other stress states of interest. It also allows the measurement of specimen volume change and the use of a single specimen for determination of the SWCC. The evaluation includes issues of temperature control, air diffusion through high air entry ceramic disks, and study of potential sources of error in water content determination for the SWCC. It is concluded that the new oedometer device can be used to obtain an accurate SWCC for a single specimen up to 1500kPa. General recommendations for appropriate use of the equipment and corrections to SWCC data obtained using axis translation pressure plate-type devices are made.

Micromechanical aspects of the shear strength of wet granular soils

AbstractThis paper presents a micromechanical model for the analysis of wet granular soils at low saturation (below 30%). The discrete element method is employed to model the solid particles. The capillary water is assumed to be in a pendular state and thus exists in the form of liquid bridges at the particle‐to‐particle contacts. The resulting inter‐particle adhesion is accounted for using the toroidal approximation of the bridge. Hydraulic hysteresis is accounted for based on the possible mechanism of the formation and breakage of the liquid bridges during wetting and drying phases. Shear test computational simulations were conducted at different water contents under relatively low net normal stresses. The results of these simulations suggest that capillary‐induced attractive forces and hydraulic hysteresis play an important role in affecting the shear strength of the soil. These attractive forces produce a tensile stress that contributes to the apparent cohesion of the soil and increases its stiffness. During a drying phase, capillary‐induced tensile stresses, and hence shear strength, tend to be larger than those during a wetting phase. The proposed model appears to capture the macroscopic response of wet granular materials and revealed a number of salient micromechanical mechanisms and response patterns consistent with theoretical considerations. Copyright © 2008 John Wiley & Sons, Ltd.

A constitutive model for unsaturated soil interfaces

AbstractA constitutive model based on the disturbed state concept is presented to describe the behavior of interfaces in unsaturated soil. The model is an extension of an existing model developed for a sand–steel interface. As opposed to the original model, the modified model incorporates two independent stress variables, which are the net normal stress and matric suction. The saturated and dry state of the interface can be modeled as a special case using the constitutive model presented in this paper. The modified model is capable of capturing the main features of unsaturated interfaces observed during laboratory testing, including increasing shear strength and strain softening with increasing suction and net normal stress and increasing dilatancy with increasing suction. Laboratory tests were carried out on unsaturated interfaces in a modified direct shear test apparatus. The observed behavior of interfaces between unsaturated soil and steel plates (rough and smooth) is presented in comparison with model predictions. Copyright © 2008 John Wiley & Sons, Ltd.

Interface Direct Shear Testing of Unsaturated Soil

Abstract A commercially available direct shear apparatus was modified for testing unsaturated soil and interfaces between unsaturated soil and stainless steel. Major modifications include the addition of apparatus for suction-controlled testing using the axis translation method, and construction of shear boxes for testing unsaturated soil and interfaces. The major features of the device are described and results of performance tests are presented in this paper. In addition, typical results of tests on unsaturated clayey silt and interfaces are presented for drained tests conducted under different matric suction and net normal stress conditions. The Extended Mohr-Coulomb Failure Envelope for the soil is compared to that obtained for the rough interface. The soil/interface test results demonstrate the successful implementation of the device.

Soil-water characteristic curve and consolidation behavior for a compacted silt

Measurement of the soil-water characteristic curve (SWCC) in the laboratory is commonly conducted under zero confining pressure. However, in the field, the soil is under a confining stress. Therefore, it is important to study the effects of the confining stress on SWCC. In addition, the consolidation curve is normally generated under saturated conditions. However, the soil above the water table is usually unsaturated. Hence, it is also necessary to investigate the effects of matric suction on the characteristics of the consolidation curves. This paper presents the SWCCs under different net confining stresses and the isotropic consolidation curves under different matric suctions that describe the volume change characteristics of unsaturated soils with respect to stress state variables, net normal stress, and matric suction. A series of SWCCs was determined for statically compacted silt specimens in a triaxial cell apparatus under different net confining stresses. Isotropic consolidation tests under different matric suctions were also carried out. The results of the SWCC tests show that the air-entry value increased with increasing net confining stress. The yield points (i.e., yield suction, s0) obtained from the SWCC tests also increased with increasing net confining stress. The results of isotropic consolidation tests indicate the strong influence of matric suction on compressibility and stiffness of the compacted silt specimens.Key words: soil-water characteristic curve, isotropic consolidation, pore-water pressure, volume change, NTU mini suction probe, matric suction.