Conventional Direct Shear Tests Research Articles

Experimental investigations of the shear strength and deformation behavior of a frozen unsaturated silt

There are limited studies in literature with respect to the deformation and shear strength behavior of frozen unsaturated soils that consider sensitive changes in temperatures below 0 °C. For this reason, the focus of the present study is directed towards investigating the shear strength and deformation behavior of a frozen unsaturated silt. As a part of the study, one-dimensional freezing tests were performed on compacted silt samples with various initial water contents and dry densities. The pulsed nuclear magnetic resonance (P-NMR) method was used to obtain distribution of unfrozen water content of silt samples; this information was used to explain the deformation behavior during freezing. The volume change behavior in unsaturated soils due to freezing can be associated to three primary factors: expansion caused by the ice-water phase transition and shrinkage induced by ice-cementation and dehydration. A critical water saturation was also observed at which the frost heave is minimum, or no volume change occurs. In addition, conventional direct shear tests were performed on compacted unsaturated silt samples both in frozen and unfrozen conditions to determine and quantify the shear strength contribution arising from the different cohesion components. The investigations in this study provide valuable information that can be used in rational explanation of the strength and deformation behavior of unsaturated frozen soils.

Free Head Shear Test on Decomposed Granite Soil

The study presents the test results of Completely Decomposed Granite (CDG) soil tested under drained triaxial compression, direct shear and simple shear tests. Special attention was focused on the modification of the upper halve of conventional Direct Shear Test (DST) to behave as freehead in movement along with vertical strain control during shear stage by using Geotechnical Digital System (GDS). The results show that Free Direct Shear Test (FDST) has clear effect on the measured shear stress and vertical strain during the test. It has been found that shear strengthparameters measured from FDST were closer to those measured from simple shear and drained triaxial compression test. This study also provides an independent check on the consistency of the data by providing an interpretation for angle of dilation together with shearing resistance by using flow rule analysis

Experimental application and accuracy assessment of 2D-DIC in meso-direct-shear test of sandy soil

The examination of the meso-mechanical properties of soils is fundamental to understand its macro-behaviour. This paper aims to evaluate the potential application of Digital Image Correlation (DIC), an optical image processing approach, in direct shear test (DST) for soils. Hence, a new shear box was designed to investigate the close behaviour of granular materials. The noise-floor and shear displacement accuracy were discussed to assess the reliability of DIC, besides the effectiveness of the new box was also examined. The distribution of the displacements, strains, and shear angle were measured under four different normal stresses. From the results, the immediate settlement, and the dilative behaviour of the sand during shearing were observed. Furthermore, the investigation revealed a strain concentration at the interface between the boxes, where a shear band was formed. The use of DIC opens up new ways in soil mechanics, overcoming some limitations of the conventional DST.

Study on Large-Scale Direct Shear Test on Soil–Rock Mixture in an Immersion State under Water

Soil–rock mixture (S–RM) is a natural geomaterial with multiple grain grades. It has dramatic trans-scale and hierarchical natural characteristics, and its mechanical properties are dramatically influenced by water. Various S–RM samples with multiple grain grades in engineering in situ state were prepared for large-scale and conventional direct shear tests in a native state and an immersion state under water to investigate the shear strength and deformation characteristics of S–RMs. The experimental results show that the S–RM in the natural state has high shear strength; the cohesion c is 44.0 kPa, and the internal friction angle φ is 34.7°. The shear strength of the S–RM after water immersion drops significantly: the cohesion c is 45.7 kPa, and the internal friction angle φ is 7.7°. The research results indicate that the cohesion of S–RM is mainly provided by the shear bond strength of the coarse grains, the development of which is governed by the gradation and surface topography of mineral grain and affected by constraint conditions. The interpretation of the sharp drop in the shear strength of S–RM after water immersion is that the sliding friction resistance between coarse grains and clay membrane and the occlusion friction resistance generated by the rotation transfer of coarse grains through clay membrane decreases sharply as the clay membrane around coarse grains becomes substantially soft. The shear strength of S–RM presents a dramatic scale effect and engineering design may come across major deviation and adventure when it adopts shear strength parameters determined by small-scale testing apparatus.

Experimental Investigation of Hydro-Mechanical Soil Properties of a Slope Failure

A sudden slope failure occurred in Manisa possibly due to the effect of water infiltration because of a slightly damaged sewage pipe. Considering that there was no other evidence such as rainfall or any loading conditions to trigger the slope failure, a laboratory investigation on the soil’s unsaturated hydro-mechanical properties was initiated. Slopes are naturally unsaturated soils, and they may lose their stability with increased saturation degrees with water infiltration. Thus, for a proper investigation, the unsaturated hydro-mechanical properties of soils should be determined. The results presented in this study are focused on determining the key parameters to evaluate the slope failure for unsaturated soil conditions. In this regard, hydraulic conductivity and suction characteristics and the shear strength parameters were determined as well as the classical geotechnical properties of the soil. Classification of soil was determined as silty sand which is known to have slight to moderate suction stresses and mostly affected suddenly by water infiltration. A flexible-wall permeability test was run with a falling head procedure and the saturated hydraulic conductivity of the soil sample was measured as 1x10-7 m/s. Suction characteristics were detected by filter paper method, besides, the soil water retention curve of the soil was constructed. The maximum matric suction of the soil was measured as 2887 kPa for an air-dried sample. Following, the shear strength parameters were measured by conventional direct shear test for both dry and soaked conditions. Finally, the friction angle due to suction was calculated to be 1.7 degrees from the results of dry and saturated shear strength parameters.

Influence of Swelling on Shear Strength of Expansive Soil and Slope Stability

Rainfall is the main factor leading to the landslide of expansive soil slope. To deeply understand the instability mechanism of expansive soil slope under rainfall conditions, it is necessary to clarify the strength attenuation mechanism of expansive soil caused by water absorption and swelling. Therefore, the improved constant volume direct shear test was used in this study to compare with the conventional test, focusing on the variation of effective shear strength parameters with dry density (volume). The micromechanism of expansive soil strength attenuation is further studied, and the influence of expansive soil on slope stability is analyzed by numerical calculation. The results show that the improved direct shear apparatus and method can keep the volume of expansive soil specimens unchanged before and after shear and obtain the real effective shear strength parameter of soil. Under the same conditions, the shear strength parameter of the improved direct shear test is lower than that of the conventional direct shear test. The rapid thickening of the lamellar structure of montmorillonite in expansive soil will lead to the fragmentation and dispersion of clay particles, which is one of the fundamental reasons for the strength difference between expansive soil and ordinary clay. In slope stability analysis, the effect of swelling on the shear strength of slope cannot be ignored and should be considered in engineering design.

Study on Water Characteristic Curve and Shear Characteristics of Typical Unsaturated Silty Clay in Shaoxing

In order to probe into one simplified method to predict the shear strength of Shaoxing unsaturated silty clay, the test method combining unsaturated soil consolidation instrument and conventional direct shear instrument is used to study the shear strength, and the method is compared and verified with the results of equal suction direct shear test. The research results show that the soil water characteristic curve fitted by the measured data points and VG model has obvious stage characteristics in the range of 0~38 kPa, 38~910 kPa, and 910~10000 kPa. The shear strength of unsaturated soil measured by consolidation meter combined with conventional direct shear test is in good agreement with that measured by equal suction direct shear test in the range of 0~500 kPa. The results show that the shear strength, total cohesion, and effective internal friction angle of soil increase slightly with the increase of matric suction in the range of 0~38 kPa. When the matric suction increases from 38 kPa to 500 kPa, the shear strength and total cohesion force of the soil have similar stage characteristics with the SWCC, which first increases and then tends to be stable, while the effective internal friction angle changes slightly. Finally, taking the air-entry value as the demarcation point, an improved model of unsaturated shear strength is proposed by analyzing the error value. Compared with the measured value, the absolute value of relative error is basically kept in the range of 5%~10%, which is close to the measured value.

SHEAR STRENGTH OF UNSATURATED EXPANSIVE SOILS FROM QUEENSLAND NATURAL SLOPES

Expansive soils exhibit significant changes in their swell and shrink behaviour upon variation of moisture content. The majority of soils in Queensland are expansive; consequently, failures in natural slopes with expansive characteristics are commonly reported under climate-induced soil moisture variations. In order to understand the failure mechanism of such soil slopes, it is vital to analyse slope stability under unsaturated conditions. The current study employed undisturbed soil samples from natural slopes in Queensland and determined their basic soil properties, including the soil-water characteristic curve (SWCC), and unsaturated shear strength to facilitate the analysis of rainfall-induced slope failure. A series of conventional direct shear tests were conducted for different moisture contents under various vertical stresses. The test results were analysed to obtain the variation of the apparent cohesion and the effective friction angle of the soil with moisture content and suction. The study presents an empirical equation to predict the cohesive component in the shear strength of unsaturated expansive soils as an exponential function of suction. The formulation originated from multiple linear regression analyses for data sets obtained from shear tests using undisturbed soils with varying moisture contents. The empirical equations can realistically predict the reduction in soil cohesion due to wetting (R2 = 0.8666). The study provides an alternative to the quantitative estimation of unsaturated shear strength of expansive soils.

Multi-scale analysis of the mechanical improvement induced by lime addition on a pyroclastic soil

An extensive experimental work was developed on a lime treated pyroclastic soil coming from Central Italy, focusing on the link between the mechanical improvement induced by lime addition and the chemo-mineralogical evolution of the system. The shear strength behaviour of lime treated specimens was checked by means of conventional direct shear tests and compared with that of raw samples; lime addition induces an increase of shear strength and soil dilatancy, which is associated to a fragile-type behaviour. The chemo-physical evolution of the soil-lime-water system with time was investigated by XRD, TGA, FTIR, 29Si NMR analyses in order to deepen the knowledge of the modifications responsible for the mechanical improvements. The performed multi-scale analysis of the treated pyroclastic soil revealed the high reactivity of the system at micro level, with the occurrence of pozzolanic reactions since the very short time after the addition of lime. The availability of secondary phases is the main responsible for the observed mechanical improvement, showing the effectiveness of the treatment and the key role of zeolites in the chemo-physical evolution of the system.

직접전단실험시 전단상자의 종류에 따른 모래시료의 전단거동

직접전단시험은 전단상자의 경계조건에 따라 흙의 전단거동이 달라지는 것으로 알려져있다. 본 연구의 목적은 Type-A 형태의 기존 직접전단시험기의 문제점에 대해 분석하고 Type-C 형태의 직접전단시험기의 개발을 통해 신뢰성 높은 직접전단결과를 도출하는데 있다. 기존 직전단시험기와 새로 개발된 직접전단시험기를 사용하여 상대밀도가 60%로 조성된 모래시료에 대해 초기 구속응력 50kPa, 100kPa, 200kPa, 300kPa, 400kPa에서 0.5mm/min의 일정한 전단속도로 직접전단시험을 수행하였다. 일정수직하중 조건에서 수행된 Type-A의 직접전단시험기와 Type-C 시험기의 결과를 비교하여 새로 개발된 시험기의 신뢰성을 평가하였다. 또한 새로 개발된 시험기를 사용하여 일정 수직하중 조건과 정압조건의 두 가지 구속조건에 대한 직접전단시험을 수행하여 그 결과를 비교, 분석하였다. 실험결과, Type-A의 직접전단시험기에서는 하중재하판과 상부 전단상자가 기울거나, 흙의 체적변화로부터 발생되는 시료와 전단상자 내부 벽면간의 마찰이 전단면에 작용하는 응력에 영향을 끼치게 되는 문제가 발생하였다. Type-A의 직접전단시험기에서는 전단상자 형태와 경계조건에 따라 전단강도가 과대 혹은 과소평가 되었다. Type-C의 직접전단시험기에서는 구속조건에 상관없이 일관성있는 실험결과를 나타내었다. 본 연구는 Type-C의 형태로 개발된 직접전단시험기를 통해 Type-A 직접전단시험법의 문제점을 해결하고 신뢰성있는 결과를 도출할 수 있음을 보여준다. Shear behavior obtained by direct shear tests is dependent on shear box and boundary condition. The objective of this study is to analyze problems of conventional direct shear test (type-A) and provide the reliable results by developing type-C direct shear apparatus. Experimental tests are carried out for Ulleung sand by using type-A and -C direct shear devices. The soil specimens, which are prepared at the relative density of 60%, and are applied to vertical confining stresses of 50, 100, 200, 300, and 400 kPa, are sheared at a constant shear strain rate of 0.5 mm/min. By comparing the results obtained by type-A and -C direct shear apparatus under constant normal load (CNL) condition, the performance of new one is verified. In addition, two constrained conditions including constant normal load (CNL) and constant pressure (CP) are applied to type-C one. Experimental results show that type-A direct shear apparatus has some problems such as rotating of loading plate and upper shear box, and the frictional forces between soil and inner wall of upper shear box. Thus, the shear strengths obtained by type-A device are overestimated or underestimated depending on shear box and boundary condition. On the other hand, type-C device produces clear and consistent test results regardless of constrained conditions. This study represents that type-C direct shear apparatus not only can solve the problems of type-A direct shear apparatus but provide the reliable results.

Evaluation of Interface Shear Strength Properties of Geogrid-Reinforced Construction and Demolition Materials Using a Modified Large-Scale Direct Shear Testing Apparatus

AbstractThe interface shear strength properties of geogrid-reinforced construction and demolition (C&D) aggregates were determined using a modified large scale direct shear test (DST) apparatus. Comparisons were made between the results of the various C&D aggregates reinforced with biaxial and triaxial geogrids and with the unreinforced aggregates by means of the modified and conventional DST methods. The modified DST method employed sought to increase interlocking between the C&D aggregates with the geogrids and thus ascertains the true interface shear strength properties of the recycled demolition aggregates. Biaxial and triaxial geogrids were used as the geogrid-reinforcement materials. The C&D aggregates tested with the DST were recycled concrete aggregate (RCA), crushed brick (CB), and reclaimed asphalt pavement (RAP). The modified DST results indicated that the interface shear strength properties of the geogrid-reinforced C&D aggregates were higher than that of the conventional test method and the r...

Measurement of the Shear Strength of an Expansive Soil by Combining a Filter Paper Method and Direct Shear Tests

Abstract The measurement of the shear strength of unsaturated soils in terms of two independent stress state variables is usually difficult, expensive, and time-consuming. This paper presents a proposal to combine a filter paper method and a conventional direct shear test to obtain this measurement. The feasibility of this approach is illustrated through tests on an expansive soil. First, the filter paper method is used to establish the soil-water characteristic curve of the soil, and a series of conventional direct shear tests is subsequently conducted to measure the shear strength of the soil. The matric suction of the soil at failure is estimated from the soil-water characteristic curve based on the water content of the soil tested. The test results show that the failure envelopes of the expansive soil are nonlinear on the shear strength versus the matric suction plane for different net normal stresses. The unsaturated shear strength parameter ϕb is equal to the effective friction angle ϕ′ when the soil is close to being saturated, and ϕb decreases as the soil becomes drier. The combined method proposed in this paper may be a practical technique for analyzing the shear strength of unsaturated soils in terms of two independent stress state variables because it can be conducted in most geotechnical laboratories.

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.

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.

Displacement and load transfer mechanisms of geogrids under pullout condition

Reinforcing elements embedded within soil mass improve stabilization through a load transfer mechanism between the soil and the reinforcement. Geogrids are a type of geosynthetic frequently used for soil reinforcement, consisting of equally spaced longitudinal and transverse ribs. Under pullout conditions, the longitudinal ribs are responsible for tensile resistance, while transverse ribs contribute to a passive resistance. This paper describes a new analytical model capable of reproducing both load transfer and displacement mechanisms on the geogrid length, under pullout conditions. The model subdivides the geogrid into rheological units, composed by friction/adhesion and spring elements, mounted in line. Friction/adhesion elements respond to the shear component mobilized at the soil–geogrid interface. Spring elements respond to the geogrid's tensile elongation. Model parameters are obtained through tensile strength tests on geogrids and conventional direct shear tests on soil specimens. The need for instrumented pullout tests becomes therefore eliminated. Results predicted from this new model were compared to instrumented pullout test data from two types of geogrids, under various confining stress levels. The results revealed that the new model is capable of reasonably predicting load and displacement distributions along the geogrid.

On the interface friction in direct shear test

Distinct element simulations for the direct shear box tests on a dense and a loose 2D sample of a mixture of binary diameter cylinders were performed. Special attention is focused on the friction between the internal surface of the shear box and the sample. In the conventional direct shear test, where the up/downward movement of the upper shear box is restrained and the lower shear box moves horizontally, the frictional force that acts on the sample at the internal surface of the upper shear box is downward for the dense sample and upward for the loose sample, causing the real shear strength to be larger for the dense sample and smaller for the loose sample than the one calculated from the externally applied normal force. Two possible improvements for the conventional direct shear test to reduce the frictional force of the shear box are introduced: one is free of the upper shear box vertically by adding the low friction Teflon rods and a platen between the upper shear box and the bearing ring that measures the shear force; the other is to pull the upper shear box with a flexible rope or wire. The distinct element method simulation and the experimental results show that both these two improvements can reduce the influences of the interface frictional force on the shear strengths.

Vane Shear Behavior of Soft Bangkok Clay

The field vane shear test is one of the most common in situ tests to obtain the undrained shear strength of soft clay. Uncoupling of the torque generated by the soil resistance along the vertical and horizontal planes of the vane has been done by conducting conventional direct shear test and newly developed vertical direct shear test on the soil. From the shear stress-displacement relationship of the direct shear tests, simple analysis is performed to simulate the field vane behavior at various depths. The results of the simulation agree well with those obtained from the field vane tests on soft Bangkok clay. The conventional method of computing the undrained shear strength of the field vane shear based on the maximum torque is close to the equivalent average value from the shear box tests. A special laboratory triaxial vane apparatus was also used to study the shearing behavior of the soft clay with the capability of Ko-consolidating the sample before conducting the vane shear test. The results of the triaxial vane tests were also compared with the predictions. The predicted torque values are lower than the experimental data for the same angle of rotation.

PH—Postharvest Technology: Friction of Wheat on Corrugated and Smooth Galvanized Steel Surfaces

A new modified direct shear device was constructed and experiments performed to estimate the coefficient of friction of soft red winter wheat on corrugated and smooth galvanized steel surfaces. The modified direct shear test device is slightly different from most conventional direct shear testers in that the metal test specimen is pulled to create a shear plane as it is moved within stagnant granular material. In most conventional direct shear test devices, the granular material is sheared across the metal test specimen by a force applied laterally to the grain compartment. The effects of steel sample size, sliding speed and normal pressure on the coefficient of friction were studied for the corrugated sample using the modified direct shear device. Four speeds of 0·05, 0·5, 5, and 50 mm/min were tested. Tests were conducted at three different normal pressures of 6·9, 27·6, and 48 kPa. Using the modified shear tester the coefficient of friction of wheat on the corrugated surface increased with an increase in speed and decreased with an increase in normal pressure. ‘Slip-stick’ behaviour was observed for tests with sliding speeds lower than 2 mm/min. Coefficients of friction determined with the shortest and/or the narrowest steel sample were significantly higher (α=0·05) than the values obtained with larger steel samples.The test device was also adapted to enable measurements of the friction force of wheat on smooth galvanised steel. Friction was measured for two sliding speeds (0·5 and 5 mm/min) and three normal pressures (6·9, 27·6 and 48 kPa) At low speeds, slip-stick was observed that ceased after the speed increased to a critical value of 0·2 mm/min. Coefficients of friction were found to decrease with an increase in vertical pressure.Friction was measured using a tilting table test method on the same steel samples which had been used for the tests with the modified shearing device. The coefficient of friction had a tendency to decrease with an increase in normal pressure. Values of the coefficient of friction determined using the tilting table test method were in the same range as those determined for test conditions using the modified shearing device at the lowest normal pressures and sliding speeds.Results from either the tilting table method or the modified shearing method can be applied to design conditions at low pressures and low sliding speeds. For higher normal pressures and greater sliding speeds, the modified shearing method should be applied. For practical applications the modified shearing method should be used because it most closely smulates the actual conditions at the frictional interface in a grain bin when grain slides down the wall.

Direct Shear Interface Test for Shaft Capacity of Piles in Sand

Abstract For precise estimation of shaft capacity of a pile, it is essential to determine accurately the soil-pile interface friction angle (δ). The apparatus available to measure the value of δ, the miniature pile test apparatus and the soil-pile-slip test apparatus are only available for research purposes. This paper presents the details of an investigation carried out using the conventional direct shear test apparatus to measure the value of δ for soil-pile interface. The direct shear interface tests were conducted using four types of surfaces and two types of sands. The values of δ obtained from these tests are compared with the internal friction angle (φ) of the sand and with the results obtained from soil-pile-slip tests. The interface test results are also used to estimate the shaft capacity of a few model piles embedded in sand.

Use of a tilting table to determine the basic friction angle of hard rock samples

A tilting table can be used to determine the basic friction angle of a rock. Dolostone and quartzite sliders with natural and sanded surfaces move at similar angles on sanded plates of dolostone and quartzite. Friction angles measured from conventional direct shear tests on the same specimen are similar.When the natural or sanded sliders are tilted on polished plates, lower friction angles result: about 12° for quartzite and 16° for dolostone. The dolostone sliders leave a loose powder on the polished plates. If the powder is accumulated over successive slides, tilt angles for sliding increase to 30°. Friction angles on polished plates approach a minimum, the mineral friction angle, [Formula: see text], which represents the friction between the minerals making up the rock surfaces. The basic friction angle, [Formula: see text], is the sum of [Formula: see text] and the surface roughness produced by sanding.Tests of a new tilting table design suggest that basic friction angles should be determined with naturally surfaced sliders on plates wet lapped with No. 80 grit. Vibrations from the electric motor driving the table reduce measured friction angles by about 1°. Sliding angles increase up to 2° in very humid conditions. Key words: friction, rocks, tilting table, roughness, humidity, dolostone, quartzite, limestone.