Shear Test Apparatus Research Articles

Direct Shear Testing Apparatus for Saturated Rock Joints

Abstract A novel shear test apparatus has been designed and built to test saturated jointed rock specimens under normal and shear loading, capable of housing seismic transducers to monitor simultaneously the mechanical and geophysical response of the rock joints during shear. The system comprises a sealed pressure chamber and a biaxial compression frame. The internal dimensions of the chamber are 177.8 mm × 228.6 mm × 381.0 mm to accommodate a rock specimen with dimensions 152.4 mm × 127.0 mm × 50.8 mm. The chamber is made of aluminum to reduce its weight and is designed to sustain a maximum chamber pressure of 10 MPa, which is considered sufficient to be able to saturate a wide number of rocks. Structural calculations of the chamber are performed with the finite element method (FEM) software, ABAQUS, with the criterion of a maximum deflection of 1 mm at maximum chamber pressure, which is small enough to prevent the loss of seal between the loading shafts and the chamber. The rocks used in the study are Indiana limestone and Sierra White granite. B-value tests conducted on cylindrical specimens of the rocks placed inside the chamber show that the back pressures required to achieve saturation are 3.5 MPa for Indiana limestone and 5.0 MPa for Sierra White granite. The chamber performance has been evaluated by comparing the changes of volume of the chamber at different pressures, measured in the laboratory, with those predicted with ABAQUS. The successful completion of a number of repeatable direct shear tests, on tensile-induced rock joints in dry and saturated conditions specimens, has further established the correctness of the chamber design and its operation.

A comparative study on the bonding performance of an admixed repair mortar and high-strength flowable micro-concrete with substrate: Considerations on the physico-mechanical compatibility

The physical and mechanical properties of repair mortar (RM), incorporating a multipurpose hybrid admixture and a high-strength concrete matrix phase known as micro-concrete (MiC), have been experimentally investigated. The flexural and compressive strengths, as well as the modulus of elasticity of each mixture, were tested at 7 and 28 days. The dimensional stability of prismatic bar specimens under dry and water-saturated conditions was monitored over a period of 56 days. The thermal coefficient of expansion for each repair material was determined, and the risk of thermal cracking was evaluated. Additionally, the bonding performance of admixed repair mortar and micro-concrete with substrate mortar (SM) was assessed using a modified slant shear test setup. The influence of surface roughness on slant shear strength was also studied. The compatibility of each mix design with substrate mortar was quantified and compared using a “physico-mechanical compatibility” based proximity comparison criterion. The proposed methodology can be used to monitor the compatibility and bonding potential of a repair material for a given substrate. Test results and analysis indicated that the admixed RM, with lower compressive/tensile strength ratio, performed better than MiC in terms of substrate compatibility.

Development of an initial normal-loading direct shear tester of discontinuities, with subsequent passive changes in local normal load/stiffness allowable, and an investigation of the shear behaviours of discontinuities

A widespread case of discontinuity shearing is found among local shallow rock blocks sliding along non-planer, rough, non-anchored discontinuities, which is characterized by an initial normal load in addition to free subsequent changes in local normal load/stiffness, provoked by shear dilatancy (INL). The existing direct shear apparatuses of rock discontinuities fitted for those cases are rare, and there is a lack of understanding of the shear behaviours of discontinuities under such INL boundary conditions. Here, we develop an INL direct shear tester of discontinuities for shear strength. This tester takes advantage of the direct shear test apparatus modified after the YZ-5 model without a servo-controlled system to permit the accomplishment of those boundary conditions. Shear behaviours of unfilled and infilled discontinuities under INL boundary conditions are investigated. Moreover, the possible influence of clay content on those shear behaviours of infilled discontinuities is clarified. The developed tester made it possible to test the shear performance of the local shallow rock blocks sliding along non-planer, rough, non-anchored discontinuities. This work contributes to a limited body of research on the shear behaviours of discontinuities under those INL boundary conditions. Limitations of this work are also revealed.

Dynamic shear behaviours of granite under coupled static and high-rate loadings

Understanding dynamic shear behaviours of rock under quasi-static confinement and dynamic loads is not only of great importance in underground engineering structures, but also beneficial to reveal earthquake mechanics regarding pre-stressed rocks fracturing from initial intact to rapid shear sliding. Here we propose a dynamic shear test setup and present full-field observations of stressed rock failure at high shear rate (up to γ˙ = 220 s−1) and sliding at high slip velocity over 10 m/s in the order of coseismic slip, with the integration of a novel triaxial Hopkinson bar (Tri-HB) and high-speed three-dimensional Digital Image Correlation (3D-DIC) techniques. Dynamic shear strength and friction coefficient are determined during the dynamic shear test under confinement, with different shear strain rates and pre-stresses. The dynamic shearing process can be divided into three stages, i.e. shear deformation, fracture degradation and dynamic frictional sliding. The high energy density of shear fracture band is observed, the rate and stress state dependences of rock shear strength and dynamic frictional coefficient are revealed. The dynamic shear strength of granite follows the Mohr-Coulomb criteria under varying pre-stresses and strain rates. The gouge materials with size of 1∼10 μm are generated within the high-rate shearing zone, and this appears to control the frictional resistance. The results could help determination of rock shear strength and post-shear failure, as well as explain the shear rupture and the resistance to slip mechanism of rock faults and strong near-surface earthquake events.

Experimental Study of Cationic-Modified Biopolymer for Increasing the Shear Strength of Sand

The application of biopolymers as a more environmentally friendly alternative to cement has emerged as an interesting research subject.The purpose is to enhance the shear strength of sandy soils. In this article, the selected biopolymer is cationic-modified starch. It is expected that cationic starch will have less water absorption properties since modified starch has cationic groups in place of the OH- groups found in the normal starch. This cationic-modified starch namely Amylofax. Five types of samples are created for this testing, including Sample A is prepared with the composition of (silica sand + 2% Amylofax T1100 (w/w) + 20% water (w/w))., Sample B consists of (silica sand + 2% Amylofax T2200 (w/w) + 20% water (w/w))., Sample C is comprised of (silica sand + 2% Amylofax T1100 (w/w) + 2% Glucomannan (w/w) + 20% water (w/w)), Sample D consists of (silica sand + 2% Amylofax T2200 (w/w) + 2% Glucomannan (w/w) + 20% water (w/w)), and Sample E includes (Ottawa sand + 2% cement (w/w) + 20% water (w/w)). The samples were tested using a direct shear test apparatus to determine the soil shear strength parameters (c) cohesion and (Ø) internal friction angle. After conducting the tests on the sand samples with the addition of modified starch biopolymer (cationic starch), it was found that the cohesion value was 961kPa, and the internal friction angle was 63°. These results indicate higher shear strength values compared to sand mixed with natural starch.

Large-scale field test on the shear behavior of concrete pipe-silty soil interfaces in pipe jacking: A case study

With the rapid advancement of urbanization, research on the shear behavior of concrete pipe-silty soil interfaces in pipe jacking has a high engineering application value. In this study, a large-scale field direct shear test apparatus and the corresponding test method were designed and proposed to investigate the shear properties of the concrete-silty soil interfaces in pipe jacking. A series of in-situ direct shear tests were conducted at five normal stress levels under four interface contact conditions. The results show that the shear stress-displacement relationships of the interfaces conform to the typical elastic-plastic constitutive relations, the shear strength-normal stress relationships of the interfaces can be well fitted by the Mohr-Coulomb failure criterion, and the interface shear properties are influenced by soil properties, normal stress, lubrication conditions and stagnation time. Without lubrication, the concrete-silty soil interfaces have a shear stress-displacement curve shape similar to that of the test results obtained from indoor direct shear tests, but their ductility and shear strengths are lower. Under lubrication conditions, the paraffin wax and thixotropic slurry can both effectively diminish the interface friction angle, thereby reducing the concrete-silty clay interface friction coefficient by 62.8% and 72.2% and reducing the concrete-silty sand interface friction coefficient by 60.8% and 59.2%, respectively. Based on the macroscopic and microscopic images observed in the tests, the lubrication mechanisms of the lubricants were analysed. The research results can provide a reference for the prediction of jacking force and construction disturbance in adjacent pipe jacking projects.

A new bonding agent for the Near-surface mounted Fibre-reinforced polymer strengthening system for concrete structures

Over the past few decades, the implementation of Near Surface Mounted (NSM) Fibre Reinforced Polymers (FRP) has become a promising strengthening method for improving the performance of concrete structures. Despite the common use of epoxy adhesives in NSM FRP systems, concerns have arisen due to their drawbacks, including the emission of toxic fumes and reduced mechanical strength when subjected to elevated temperatures and moisture. To address these issues, cement-based adhesives have been introduced as a substitute bonding agent with the goal of enhancing the performance of concrete structures in high-temperature settings and minimizing environmental and health hazards. In this paper, a new cement-based adhesive was introduced for the NSM FRP system for concrete structures. The efficiency of this adhesive was assessed through an experimental investigation involving pull-out testing using a single-lap shear test setup to study the bond behaviour of the NSM Carbon Fibre-Reinforced Polymer (CFRP) laminate for concrete structures. The results highlighted the potential of the developed cement-based adhesive as a promising alternative for strengthening systems requiring ductile bond performance, achieving approximately 65% of the maximum bond stress compared to epoxy adhesives in the NSM FRP system.

A novel pipe-segment shear test apparatus: polypropylene pipe behaviour over sand beds versus element interface tests

This study describes a novel laboratory test apparatus for investigating the axial interaction between pipeline and soil. Contrary to the majority of existing pipe–soil shear rigs, the proposed apparatus applies a relative pipe–soil shear displacement through driving a soil box below an axially restrained pipe segment, which is instrumented to measure the pipe settlement and the axial resistance at the pipe–soil contact surface. Through axial shear tests of polypropylene pipe segment on sand, this study explores the effect of vertical loads, soil types, and densities on the resulting axial resistance and estimates the interface stress evolution. The coefficients of axial resistance obtained from large-scale pipe–soil shear tests are compared to those obtained via planar element interface shear tests with consideration of the wedging effect. Their consistency suggests that under the low stress levels investigated in this study, the effect of both pipeline curvature and settlement on the resultant data are minor. The agreement provides validation for the novel apparatus to generate high-quality data under controlled conditions for future studies. The findings of this study will also potentially help reduce the uncertainties around subsea pipeline design when linking the interface shear behaviour at element scale to large-scale pipe–soil interaction.

Experimental and Numerical Assessment of Bonding Between Geopolymer Concrete and CFRP Sheet Using NSM Techniques

Abstract Geopolymer concrete is a new concrete technology that emerged as an alternative to conventional concrete by replacing traditional Portland cement with geopolymer binders. Strengthening of concrete structures using CFRP composites has become an essential technique last decades. Most of the studies were conducted on conventional concrete of Portland cement. In this research, an experimental and finite element analysis has been conducted to assess the bonding properties between geopolymer concrete substrate and CFRP composites using near-surface mounted strengthening system. The program consisted of concrete prisms bonded with NSM CFRP laminate and tested using single-lap shear test set-up with different bond lengths. Bond-slip behaviour has also been evaluated. The experimental results exhibited significant bonding properties, achieving an average bonding stress between 8.97 and 15.58 relative to the bonding length. The numerical analysis showed comparable results and a good correlation with that of experimental work.

Bond tests on concrete elements externally bonded with steel plates and assessment of bond strength models

The use of steel plates as externally bonded reinforcement for existing reinforced concrete (RC) elements can be still considered a reliable typology of strengthening intervention thanks to their relatively lower prices in comparison with fiber reinforced plastic materials, FRPs, largely used in the last decades, the ductile stress–strain behavior, and the high stiffness properties. Despite steel plates represented the first attempt of external strengthening for RC elements, design indications and practical execution rules concerning the use of such a technique nowadays are still very few. There is, indeed, a lack of experimental and theoretical studies in literature for predicting accurately the behavior of existing RC members externally strengthened with steel plates. In particular, special attention should be paid to the bond behavior at the steel–concrete interface, since it can strongly influence the performance of the strengthened elements. To this aim, an experimental campaign concerning bond tests on steel plates bonded over concrete elements and realized according to the single shear test set-up was carried out by the Authors. The experimental results are analyzed in the paper in terms of failure loads and modes, load–displacement behaviour, axial strain distributions along the steel plates, and distribution of shear stresses and slips. Then, the experimental results are compared with some well-known literature bond strength models for external FRP reinforcements in terms of failure load, transfer length and bond law. The comparisons with existing models in terms of debonding loads are developed also for an extended database of bond tests on steel plates available in literature and were prodromal to calibrate new coefficients specifically assessed for externally bonded steel plates.

Development of a Field Torsional Direct Shear Test Apparatus

Development of a Field Torsional Direct Shear Test Apparatus

Soil Structure in Volcanic Pumice Soil of Dozou-sawa River Evaluated from In Situ and Laboratory Tests

Abstract Volcanic pumice soils are widely distributed in many countries and sometimes cause severe geo-disasters including large-scale slope failures and long-distance debris flows triggered by seismic ground motion. However, the understanding of the mechanical properties of volcanic pumice soils is still limited because volcanic pumice soils generally have a sensitive structure and are typically distributed in mountain areas that are sometimes difficult to approach. In this study, a portable in situ direct shear test apparatus was developed to evaluate shear strengths of a volcanic pumice soil without disturbing the natural soil structure. A series of in situ direct shear tests was conducted for the volcanic pumice soil in Dozou-sawa river, where a large-scale slope failure followed by debris flow occurred in the 2008 Iwate-Miyagi Nairiku Earthquake. A series of laboratory direct shear tests and triaxial tests were also carried out with intact and reconstituted specimens. The in situ direct shear tests and laboratory direct shear tests using intact specimens showed consistent shear strength. Intact specimens had an extremely loose and sensitive structure because of highly crushable porous particles and a very loose soil skeleton sustained by weak cementation between particles. Such a fragile soil structure could never be reproduced in reconstituted specimens. It was confirmed that the soil structure significantly affected peak shear strengths. For the evaluation of mechanical behavior of such sensitive soils, it is important to use as few disturbed specimens as possible. The newly developed in situ direct shear test apparatus was confirmed to be an effective tool to simply evaluate the shear strength of sensitive volcanic pumice soils on site under low disturbed conditions.

Shear Behaviour of Fibreglass Rock Bolts for Various Pretension Loads

In this paper, after determining an appropriate experimental design and testing scheme to test the shear performance suitably, the impacts of pretension and shear interfaces on fibreglass rock bolts were investigated. For this purpose, at the first stage, the double shear testing apparatus was modified to address the issues and shortcomings of the established testing methodology. Then, 20-tonne and 30-tonne rock bolts were tested with pretension loads of 0 kN, 10 kN, 15 kN, and 20 kN for each test scheme. For this, a 40 MPa small aggregate concrete was used for all samples to simulate the host rock. The grout used to anchor the rock bolts was mixed to the product’s recommended guidelines to ensure close resemblance to field mixtures. To maintain consistency, all samples were manufactured with the same concrete and grout recipes. The results of double shear tests revealed that increasing the pretension also increased the confining pressures at the shear interfaces for clean joints. This, in turn, reduced the damage propagating from the bolt at the shear interface and decreased the hinge point bending. The results can be applied for further analytical and numerical studies, which could utilise for a better understanding of the shear behaviour of fibreglass rock bolts.

Anchorage performance of large-diameter FRP bolts and their application in large deformation roadway

In underground coal mines, fibre reinforced polymer (FRP) bolt is ideal for mined rib reinforcements as it can prevent gas explosions caused by shearer frictional spark. With increasing mining depth, small diameter FRP bolts used in shallow underground mining cannot fulfil the rib support requirements. Under the engineering background of deep underground shortwall mining in Wudong coal mine, this paper systematically studies Φ27 mm FRP bolt support for large deformation coal rib. Specimens with a fan-shaped cross-section were used to enable the tensile testing of the bolt rod, the measured average tensile strength of the studied FRP bolt was (486.1 ± 9.6) MPa with a maximum elongation of 5.7%±0.6%. The shear strength of the bolt was measured as approximately 258 MPa using a self-made double shear testing apparatus. Based on the equivalent radial stiffness principle, a laboratory short encapsulation pullout test (SEPT) method for rib bolting has been developed undertaken consideration of the mechanical properties of the coal seam. Results showed that the average peak anchorage forces of the Φ27 mm FRP bolt and Φ20 mm steel rebar bolt were 108.4 and 66.4 kN, respectively, which were agreed with the theoretical calculations and field measurements. Based on theoretical analysis of the loading states of the bolt under site conditions, bolting method of full-length resin grouting was adopted to offset the weaknesses of the FRP bolt. Numerical method was employed to compare the bolting effect using Φ27 mm FRP bolts and steel rebar bolts. Large diameter FRP bolting was determined as the optimum rib support scheme to increase the productivity of the coal mine and to enhance the ground control capability for +425 level mining roadways. This study provides the laboratory testing design and theoretical prediction of large diameter FRP bolts used for rib support in large deformation roadways.

Experimental investigation of factors affecting the characterisation of soil strength properties using a Bevameter in-situ plate sinkage and shear test apparatus

The Bekker-Wong soil-wheel interaction model has been widely adopted in the terramechanics field. This model requires the soil to be characterised using a Bevameter, which entails performing in situ plate sinkage and shear stress tests. Bevameter soil characterisation is not a standardised test procedure, and the test setup may influence the identified soil model parameters. This study investigates the influence of the following five factors for partially saturated sandy soil: I) soil preparation method on pressure-sinkage, II) soil preparation method on shear stress, III) torsional vs. translational shear mechanism, IV) shear contact area, and V) the influence of shear velocity. The results indicate that the influence of soil preparation on pressure-sinkage response is substantial, exhibiting an order-of-magnitude difference. The influence of soil preparation on shear tests is notable, but less significant. The shear mechanism, shear contact area and shear velocity exhibited a maximum absolute shear stress difference of 18%, 20% and 10%, respectively. Moreover, depending on the test setup configuration and data processing decisions, the estimated internal soil friction angles ranged from 16.5 to 37.5° for the same soil. The findings are expected to have significant implications for the prediction of vehicle drawbar pull using the Bekker-Wong model.

Hydrate dissociation and mechanical properties of hydrate-bearing sediments under local thermal stimulation conditions

Considering the interaction between hydrate dissociation and the mechanical properties of hydrate-bearing sediment (HBS) facilitates a deep understanding of the relationship between hydrate extraction efficiency and reservoir stability during hydrate exploitation. In this study, a series of tests were carried out to study the effects of hydrate dissociation under local thermal stimulation and the mechanical properties of HBS by using the plane strain shear test apparatus. The characteristics of the local maximum shear strain and volumetric strain were also investigated using the particle tracking velocimetry (PTV) method. The results show that the tested water-saturated host sands have the same heat transfer characteristics under local thermal stimulation, despite having different particle size distributions and mineral compositions. The temperature transfer inside the HBS was slower than that of the host sand sample, and the temperature distribution of the HBS after hydrate dissociation was the same as that of the host sand sample. HBS under an axial loading force higher than that of the peak strength of the host sand would fail after hydrate dissociation. Additionally, even partial dissociation of the hydrate can lead to catastrophic failure of HBS under this condition.

A new shear test setup for determining shear strength of normal and high strength concrete

Since the start of the 20th century various investigators proposed different shapes, sizes of specimens to determine the shear strength of concrete and proposed several models. Several disagreements and discussions were surrounding the concept of shear strength, particularly in case of High strength concrete (HSC). In order to compare the shear strengths of Normal Strength Concrete (NSC) with High Strength Concrete, a “New shear test setup” was proposed in this study. From literature and different standard codes, it was found that, the shear strength of plain concrete becomes constant after certain compressive strength. Different authors and codes proposed different strength of concrete beyond which the shear strength becomes constant. The aim of the study was to find the compressive strength beyond which the shear strength becomes constant using the proposed shear test setup. In order to obtain the shear strength, 42 cubes of 150 mm × 150 mm × 150 mm of concrete of varying strength of 20 MPa to 80 MPa were cast and tested. The experimental results were compared with a numerical model based on FEM software ATENA and the existing models proposed in the literature. It was found from the experimental work that the shear strength became constant after a compressive strength of 50 MPa- 60 MPa. According to the shear strength results, there was a significant correlation between experimental results and results predicted using ATENA software. Moreover, the experimental results were compared with the shear strength values predicted by various national building codes and various shear models proposed using the different shear test setups. It was found that the experimental results closely match with the values obtained by AS: 3600–2018 equation and Mattock equation's based on the push-off specimen.

Development of a new cyclic shear test setup for characterizing thin metallic foils

Recent developments aimed at reducing cumulative CO2 emissions in the energy sector and e-mobility are leading to an increase in the production volumes of formed components made from thin metallic foils used for alternative energy supply concepts. Thus, components such as bipolar plates in fuel cells and a variety of parts in e-motors and batteries are being manufactured in ever higher quantities and thinner material thicknesses. As a consequence, this leads to increased challenges regarding the feasibility and robustness of required production processes. Finite Element Analysis in fact do represent a useful tool in this context to predict and to optimize expected outcome of forming processes virtually at an early stage of development. The prediction accuracy of such simulation codes thereby significantly depends on the precision of modelling the sheet metal material behavior, which is derived from special characterization methods. However, classical approaches to mechanical characterization usually tend to fail in case of thin metallic foils, especially when the structural stability of the specimen becomes an important factor for the validity of the characterization test. The cyclic shear test, for example, is used to determine the hardening behavior of the material to be characterized, but appear unsuitable for metallic foil investigations unless a special anti-wrinkling device is used. Current anti-wrinkling devices proposed in literature must be attached directly to the deforming area of the specimen and cause further weaknesses. Attaching such devices to prevent the specimen from wrinkling requires remarkable skills of the operator to avoid deforming of the specimen even before testing. Against this background, this paper presents a novel experimental setup, which increases the structural stability of the cyclic shear specimen and thus prevents wrinkling. Structural stability of specimen is enhanced by curving the gauge area of the specimen to drastically suppress the tendency to wrinkle. As a proof of concept FE-simulations with LS-Dyna were performed in this study to verify this novel idea and to design a suitable experimental setup.

Analysis of the Shear Strength of a Soil-Geosynthetic Interface

Abstract The paper presents the results of the shear strength measurements of a soil-geosynthetic interface. The tests were executed using a large-size direct shear test apparatus. A total of 5 different samples of materials were tested, i.e., ash, sand, well-graded gravel, fine poorly-graded gravel, and medium poorly-graded gravel. These materials were reinforced using different types of geosynthetics, i.e., Thrace WG80 black woven geotextile, Tencate Miragrid GX55/30 woven geogrid, and Thrace TG3030S rigid polypropylene geogrid. An interface coefficient α, which represents the ratio of the soil-geosynthetic interface shear strength to the shear strength of unreinforced material sample, was determined for given combination of the material and geosynthetics. The coefficient α reached a greater value in the critical stress state than in the peak stress state for sands and gravels reinforced using GX55/30 and TG3030S geogrids. The value of the coefficient α was in a range of 0.87 - 1.04 for gravels and 1.03 - 1.19 for sand. The black woven geotextile was used as the reinforcement only in samples of sand and ash. The results pointed to the different behaviour of these materials in the testing of the interface shear strength.

Effects of moisture content and landfill age on the shear strength properties of municipal solid waste in Xi'an, China.

With the continued expansion of waste landfills, accidents may occur if the landfills are not properly stabilized. In this study, samples of municipal solid waste (MSW) from a waste landfill in Xi'an, China were collected through on-site drilling. Considering the effects of nine landfill ages (1, 2, 3, 11, 12, 13, 21, 22, and 23 y) and six moisture contents (natural, 20, 40, 60, 80, and 100%), 324 groups of MSW were tested in the laboratory using a direct shear test apparatus. The results indicate the following: (1) with an increase in horizontal shear displacement, the shear stress of MSW gradually increases without a peak stress phenomenon, which is a displacement hardening curve; (2) with an increase in landfill age, the shear strength of MSW increases; (3) with an increase in moisture content, the shear strength of MSW increases; (4) with an increase in landfill age, the cohesion (c) decreases and the internal friction angle (φ) increases; and (5) with an increase in moisture content, the c and φ of MSW increases. The c range found in this study was 6.04-18.69kPa, while the φ was 10.78-18.26°. The results of this study can provide a reference for stability calculations for MSW landfills.