Direct Shear Apparatus Research Articles

Frictional Properties and Seismogenic Potential of Caprock Shales

Fractures and faults are critical elements affecting the geomechanical integrity of CO2 storage sites. In particular, the slip of fractures and faults may affect reservoir integrity and increase potential for breach, may be monitored via the resulting seismicity. This paper presents an experimental study on shale samples from Draupne and Rurikfjellet formations from the North Sea and Svalbard, Norway, using a laboratory test procedure simulating the slip of fractures and faults under realistic stress conditions for North Sea CO2 storage sites. The motivation of the study is to investigate whether the slip along the fractures within these shales may cause detectable seismic events, based on a slip stability criterion. Using a direct shear apparatus, frictional properties of the fractures were measured during shearing, as a function of the shear velocity and applied stress normal to the fracture. We calculated the friction coefficient of the fractures during the different stages of the shear tests and analysed its dependency on shear velocity. Information on velocity-dependent friction coefficient and its evolution with increasing slip were then used to assess whether slip was stable (velocity-strengthening) or unstable (velocity-weakening). Results showed that friction coefficient for both Draupne and Rurikfjellet shales increased when the shear velocity was increased from 10 to 50 µm/s, indicating a velocity-strengthening behaviour. Such a behaviour implies that slip on fractures and faults within these formations may be less prone to producing detectable seismicity during a slip event. These results will have implications for the type of techniques to be used for monitoring reservoir and caprock integrity, for instance, for CO2 storage sites.

Effects of Relative Roughness and Particle Size on the Interface Behavior of Concrete Suction Caisson Foundation for Offshore Wind Turbines

The interface behavior between a caisson and the surrounding soil plays an important role in the installation of suction caissons as foundations for offshore wind turbines. A series of shear tests were carried out using a modified direct shear apparatus to study the interface shear behavior between sand and concrete. Sand samples with three particle size ranges (0.63–1.25 mm, 1.25–2.5 mm, 2.5–5.0 mm) and concrete plates with different relative roughness were used to explore the influence of the relative roughness parameter (Rn) and mean particle size (D50) on shear behavior. The responses from the pure sand shear test are also discussed for comparison. Test results show that the higher the relative roughness (Rn), the greater the maximum shear stress (τmax) appeared. The interface shear stress was weaker than that of the pure sand test. Furthermore, the interface friction angle (φ) of sand–concrete was closely related to the relative roughness of the concrete surface. Under the same conditions, the interface friction angle (φ) increased with relative roughness due to the effect of sand particles breakage and redistribution. By contrast, the effect of the mean particle size (D50) on the interface friction angle (φ) was less significant. However, for the pure sand shear test, the friction angle (φ′) obtained from the traditional shear test apparently increased with D50, indicating that the friction angle was more affected by D50 in the pure sand test than in the interface shear test.

Testing and modeling of frozen clay–concrete interface behavior based on large-scale shear tests

The shear behavior of the frozen soil−structure interface is important for accurately predicting the interface responses of structures adopted in the cold regions. The purpose of this study is to experimentally and theoretically investigate the shear behavior of frozen clay–concrete interface under engineering conditions. A large-scale direct shear apparatus with a temperature-controlled shear box is used to test the interface behavior. Test specimens consisting of a cement concrete block and frozen soil with initial water content ranging between 14.6% and 24.6% were prepared at different conditions of temperatures (15.4 to −9.8 °C), shear rates (0.03–0.9 mm min−1), and normal stresses (50–200 kPa). It is found that the peak shear strength is linear developing with increasing of normal stress, initial water content, and temperature. It increased from 67.7 to 133.3 kPa as the initial water content increased from 14.9% to 24.6% at temperature of −6.8 to −6.6 °C, and it increased from 51.2 to 80.6 kPa with temperature decreasing from 15.4 to −9.8 °C at initial water content of 14.6%–14.9%, furthermore it has a power law relationship with shear rate. The final vertical displacement increases with the decreasing temperature, and increasing initial water content. While, it is slight or could be ignored at lower shear rates (e.g. 0.03 mm min−1 and 0.15 mm min−1) and it is −0.25 mm and −0.28 mm at shear rate of 0.3 mm min−1 and 0.9 mm min−1, respectively. In addition, the evolution of vertical displacement also varies with test condition, the growth rate at beginning increases with increasing initial water content and decreasing temperature or ice content, which is because of the ice film effects the particle size. Moreover, a disturbed state concept model combined with linear and nonlinear characteristics is developed to describe the interface shear behavior. The disturbance D reflects the interface mechanical response and responds differently trend for different test conditions, increasing faster with increasing temperature and decreasing initial water content or shear rate. The testing results, including the test and model results, can be used to simulate the performance of engineered geotechnical assets such as earth dams or irrigation channels with concrete linings in cold regions.

Shaft friction from the DMT and direct shear interface tests

The article presents preliminary attempt to create tri-linear transfer curves for describing pile behaviour under axial loading. Transfer curves would use the parameter measured in dilatometer test, particularly a constrained modulus MDMT. The proposed method is based on concrete rough and smooth interface tests performed in a direct shear apparatus. Based on the obtained mobilization curves, relationships were created between the parameters describing the slope of individual transfer curves and the MDMT modulus. The obtained correlations were used to estimate the capacity of the concrete screw pile subjected to the static load test in tension.

Analysis of Shear Characteristics of Cohesive Soil and Rigid Base: A Case Study of Concrete Base

Taking the concrete as an example, this paper studies the shear characteristics of the cohesive soil and rigid base. A series of shear tests are conducted by the refitted DSJ-2 electric four-joint equal stress direct shear apparatus. The results in the shear tests are contrastively analyzed. The results show that under the condition of simple shear tests, the constitutive relation of structural interface between the clay and concrete is rigid, and the residual strength is obvious. Under the condition of direct shear tests, the shear stress-displacement curves present hyperbolic connection. There is obvious strain hardening phenomenon, no obvious peak strength and residual strength. Under the two experimental conditions, the peak shear strength of the interface increases with an increase in the normal stress. With increasing moisture content, the peak shear strength of the interface decreases. The internal friction angle and cohesion of the structural interface decrease with increasing moisture content, and the relationship between internal friction angle, cohesion of the structural interface and moisture content are nonlinear. The peak shear strength of the direct shear tests is greater than that of the simple shear tests.

Reliability of Shear-Box Tests Upon Soaking Process on the Sand Soil in Al-Najaf City

Accurate prediction of the soil shear strength parameters is essential in the reliable geotechnical design of civil engineering structures. This recent paper investigates the effect of the dry testing condition on the shear strength parameters of the sandy soil using the direct shear apparatus and compared with the saturated condition tests in previous researches on the same soil. The dry soil, usually above the water table, is the principal condition of the Al-Najaf city soil in Iraq. Samples are selected from the site of the University of Kufa, which represents the sandy soil of the city. For reliability purposes, the soil is exposed to different pre-soaking durations (one, two, and four weeks) then air-dried for shear tests. The main results revealed that the angle of internal friction (Φ) tested as a dry sample decreases about -6% up to two-weeks soaking then recovered upon four-week soaking about +6%. Compared to the saturated testing, there are increases in F between 6%-17% from saturated tests. Finally, it is recommended to aware in the selection of testing conditions for calculations of the angle of internal friction.

Effect of Sample Size on TDA Shear Strength Parameters in Direct Shear Tests

Tire-derived aggregate (TDA), a relatively new construction material, has been gaining acceptance as a backfill material for embankments, trenches, and earth-retaining structures because of its lightweight and excellent geotechnical properties. Type A TDA has a basic geometric shape, with particles approximately 12 to 100 mm in size. As a result of the simplicity and accuracy of the direct shear test, most laboratories choose this test in preference to more complex tests. However, TDA requires large-scale direct shear apparatus because of the consistently large size of its particles, and few facilities own this type of apparatus. Depending on the shear box dimensions, the aspect ratio of the particle size to the box dimensions may lead to variations in the shear strength results of the sample being evaluated. This research focuses on studying the effect of TDA sample size on the shear strength results of direct shear tests by using five different shear box sizes. The findings show that the angle of internal friction increases slightly as the dimensions of the shear box decrease. It was found that the maximum variation in the angle of internal friction and the cohesion results of the different shear boxes was only 1.9° and 2.4 kPa, respectively. These differences should be taken into consideration when TDA shear test results are used in the geotechnical design. It is recommended that a shear box with an aspect ratio (W/Dmax) greater than or equal to 4 should be used when evaluating the shear strength parameters of TDA.

Study on the Effects of Grouting and Roughness on the Shear Behavior of Cohesive Soil-Concrete Interfaces.

Grouted soil–concrete interfaces exist in bored piles with post-grouting in pile tip or sides and they have a substantial influence on pile skin friction. To study the effect of grouting volume on the shearing characteristics of the interface between cohesive soil and concrete piles with different roughness, grouting equipment and a direct shear apparatus were combined to carry out a total of 48 groups of direct shear tests on cohesive soil–concrete interfaces incorporating the grouting process. The test results showed that the shear behavior of the grouted cohesive soil–concrete interface was improved mainly because increasing the grouting volume and roughness increased the interfacial apparent cohesion. In contrast, increasing the grouting volume and roughness had no obvious increasing effects on the interfacial friction angle. Interfacial grouting contributed to the transition in the grouted cohesive soil from shrinkage to dilation: as the grouting volume increased, the shrinkage became weaker and the dilation became more obvious. The shear band exhibited a parabolic distribution rather than a uniform distribution along the shearing direction and that the shear band thickness was greater in the shearing direction, and it will become thicker with increasing grouting volume or roughness. The analysis can help to understand the shear characteristics of soil–pile interface in studying the vertical bearing properties of pile with post-grouting in tip or sides.

Role of normal stiffness in 3D cyclic behavior of gravel–steel interface from large-scale direct shear tests

The shaft capacity of piles is essentially the shear strength response of soil–structure interfaces. The design of piles requires sound knowledge of the three-dimensional (3D) interface behavior under constant normal stiffness (CNS) condition, which has seldom been addressed. In this research, a series of 3D tests of a gravel–steel interface were conducted in two-way circular, cross and beeline cyclic shear paths at different normal stiffness using a large-scale direct shear apparatus. The 3D behavior and constitutive rules of the interface under CNS condition and the influence of normal stiffness were explored in detail. Test results indicate that the normal stiffness remarkably affects the magnitude of normal displacement, normal stress, shear stress, initial shear modulus and degradation of cyclic shear strength. Increased normal stiffness leads to accelerated reduction in the normal stress and therefore accelerated degradation of cyclic shear strength. The variation of normal stress results in “induced aeolotropy” in the shear strength, while it has no impact on the inherent isotropy of strength coefficient. The shaft capacity of the piles degrades slowly and might be lost incompletely if the surrounding soil is relatively hard and the normal stiffness is sufficiently small. By contrast, the normal stiffness is of poor significance in the relationship pattern between performance parameters, in addition to the friction angle of the interface. The resultant stress ratio versus resultant tangential displacement response presents perfect consistency characteristics and can be described using a hyperbola model, independent of normal stiffness and shear path, thereby considerably simplifying the 3D constitutive description of the interface under CNS condition.

Performance evaluation of polyurethane-stabilized railroad ballast under direct shear conditions

The behavior of Elastan-stabilized and unstabilized ballast was explored using the large-scale direct shear apparatus under different normal stresses (σn) and shearing rates (Sr). Fresh granite ballast and Elastan polyurethane having a density of 1100 kg/m3 was used for the study. Further, a triangular aperture geogrid was also used to compare its benefits with that of Elastan treatment of ballast. Elastan-stabilized ballast samples were tested after a curing period of 3 days. Test results established that the shear strength of ballast was profoundly affected by both the applied normal stress and shearing rate. The friction (φ) and dilation (ψ) angles of unstabilized ballast were found to decrease from 65° to 59° and from 21° to 10°, respectively with an increase of σn and Sr. The results further confirmed that the Elastan treatment significantly enhanced the shear strength of ballast. The friction (φ) and dilation (ψ) angles of Elastan-stabilized ballast increases from 65° to 75° and 21° to 22°, respectively. The stabilization efficiency factor (Sef), defined as the fraction of the shear strength of stabilized ballast to that of unstabilized ballast, varies from 1.6 to 1.75 for Elastan-stabilized ballast. The breakage of ballast (Bg) increased from 7.62 to 11.72% with an increase of σn and Sr. Further, the geogrid reinforcement reduced the Bg from 11.72 to 4.24% when compared to unstabilized ballast. On the other hand, visual inspection of the Elastan-stabilized ballast indicated no degradation of ballast particles.

Stiffness and strength mobilisation in steel-slag-mixed dredged clays in early curing

The presence of portlandite (Ca(OH)2) in steel slag leads to potential hydration capacity, and this can be exploited to improve the engineering properties of high-water-content dredged clays. The strength and stiffness mobilisation characteristics of four dredged clays from Japan mixed with two steel slags were continuously investigated from early hours to 28 d of curing in this study, by using direct shear apparatus and bender elements, which allowed testing of mixed specimens from the initial, soft, uncemented states. The stiffness and strength increase rates in the second, main stage of evolution in the clay–slag mixtures were found to be loosely correlated to the amount of amorphous silica. The correlation was also influenced by the amount of portlandite in the slags. The results can be used to screen out clays that are hard to solidify in the medium term, based on quick on-site measurement of the amorphous silica content. For quality control in practice, the strength–stiffness relationship was critically examined as a means to assess the strength using non-destructive stiffness probes. Close observation revealed that to represent a wide range of curing time and mixing conditions by a single line, as proposed in existing studies, could be misleading. A new interpretation for the relationship is proposed.

The behavior of polymer-bentonite interface under shear stress

As new building materials, polymers have gradually been used in infrastructure engineering applications such as civil engineering, water conservancy, and highway repair engineering. The shear behavior of the interface between polymer and bentonite plays an important role in the analysis of the interactions at the interface. This paper investigated the direct shear behavior of polymer-bentonite interface under various conditions. The shear stress, horizontal displacement, and failure mode were observed under four initial normal stresses, four values of the moisture content of bentonite, and three polymer densities by utilizing a large-scale multi-functional direct shear apparatus and a digital image correlation (DIC) system. The results show that the shear strength of polymer-bentonite interface is higher than that of the pure bentonite. An increase in either initial normal stress or polymer density improves the shear strength of polymer-bentonite interface, while a higher moisture content of bentonite reduces it. Moreover, the polymer-bentonite composite fails in the bentonite phase or at the interface between polymer and bentonite. Finally, the concepts of the equivalent angle of internal friction and the equivalent cohesion of the polymer-bentonite interface are proposed and applied to the analysis of the interaction between the polymer and bentonite.

Instability of loose sand in constant volume direct simple shear tests in relation to particle shape

Post-peak flow instability of six sands notably different in terms of average particle shape characteristics, gradation, and mineralogy was investigated. Tests were carried out under constant volume condition using a NGI-type direct simple shear apparatus. The testing program covers wide ranges of initial void ratios and vertical effective stresses. Experimental findings confirm that the critical state parameters of the test sands depend strongly on the average particle morphologic characteristic factors. For specimens in loose state, it is shown that stress ratio at the onset of flow instability, normalized constant volume peak shear strength, brittleness index, and the normalized pore water pressure of the tested sands are interrelated with the particles’ morphologic factors.

Experimental Research on Shear Failure Monitoring of Composite Rocks Using Piezoelectric Active Sensing Approach.

Underground space engineering structures are generally subject to extensive damages and significant deformation. Given that composite rocks are prone to shear failure, which cannot be accurately monitored, the piezoelectric active sensing method and wavelet packet analysis method were employed to conduct a shear failure monitoring test on composite rocks in this study. For the experiment, specimens were prepared for the simulation of the composite rocks using cement. Two pairs of piezoelectric smart aggregates (SAs) were embedded in the composite specimens. When the specimens were tested using the direct shear apparatus, an active sensing-based monitoring test was conducted using the embedded SAs. Moreover, a wavelet packet analysis was conducted to compute the energy of the monitoring signal; thus allowing for the determination of the shear damage index of the composite specimens and the quantitative characterization of the shear failure process. The results indicated that upon the shear failure of the composite specimens, the amplitudes and peak values of the monitoring signals decreased significantly, and the shear failure and damage indices of the composite specimens increased abruptly and approached a value of 1. The feasibility and reliability of the piezoelectric active sensing method, with respect to the monitoring of the shear failure of composite rocks, was therefore experimentally demonstrated in this study.

The Influence of Compaction and Water Conditions on Shear Strength and Friction Resistance between Geotextiles and Ash-Slag Mixture

The paper presents the results of tests of the shear strength of the ash–slag mixture taken from the landfill located in Kraków (Poland) and the interfacial friction resistance at the contact between the ash–slag mixture and woven or nonwoven geotextiles. The tests were carried out in a direct shear apparatus on samples with and without water saturation. The samples for testing were formed in the apparatus box at the optimum moisture by compacting them to IS = 0.90 and 1.00. The test results reveal that the shear strength parameters of the ash-slag mixture were large. It was stated the significant influence of the compaction, the growth of which has resulted in an increase in the angle of internal friction (from 7% to 9%) and cohesion (from 60% to 97%). Whereas the saturation of the samples reduced the shear strength parameters (from 4% to 6%, of the internal friction angle and 30% to 43% of cohesion). The values of the interfacial friction resistance at the contact between the ash–slag mixture and the geotextiles were large as well, but slightly smaller than the values of the shear strength parameters of the mixture itself. The compaction caused an increase in the angle of interfacial friction (from 1% to 5%) and adhesion (from 31% to 127%). The water-saturation of the samples caused a change in the angle of interfacial friction (from −6% to 3%) and decline in the adhesion (from 22% to 69%). Values of the interaction coefficient were about 0.8–1.0 and they tended to rise with increasing the normal stress. Higher values of this parameter were obtained in tests with water saturation and for non-woven geotextiles.

Shear strength and permeability of the cement-casing interface

The shear strength and hydraulic permeability of the interface between well cement and casing was investigated using a triaxial direct shear apparatus. For the first time, these experiments provide measurements under controlled stress conditions with fluid flow measurements along the interface. The low cohesion (1.1 ± 1.1 MPa) and the high friction angle (43.4 ± 2.0°) indicates that the shear strength of the interface is provided by friction. This implies that the state of stress of the cement is critical to well integrity. The hydraulic aperture of the undamaged cement-steel samples was 6.8 ± 1.0 microns. Shear damage to the interface caused a decrease (-20 %) in hydraulic aperture for samples aged up to 1 month, and an increase (+300 %) for samples cured for two years. We performed numerical simulations to estimate the leakage potential from a carbon storage operation. This model predicts negligible leakage amounts (47 tonnes) in a shear-damaged well for the modeled injection of ∼1.26 million tonnes of CO2. Thus, our measurements indicate that the cement-casing interface is not a significant leakage pathway in its intact or damaged state, and that shear-driven failure scenarios for this interface are not a significant risk to CO2 storage security.

Experimental study on the monotonic shear strength of GM/CCL composite liner interface

The interface shear strength between a geomembrane (GM) and a compacted clay liner (CCL) or a geosynthetic clay liner (GCL) is of great importance for landfill stability analysis. An experimental study of the interface shear behaviour between a smooth GM (GMS) or a textured GM (GMX) and a CCL is presented in this paper. A series of monotonic shear tests was conducted using a large direct-shear apparatus. Peak and residual shear strength parameters were obtained by linear fitting of the experimental data. The shear strength of both GMS/CCL and GMX/CCL interfaces was mainly driven by the normal stress level, while the displacement rate had very little influence on the interface shear strength. The residual shear strength of the GMX/CCL interface was even higher than the peak shear strength of the GMS/CCL interface at all normal stress levels. The GM/CCL interface showed considerably higher shear strength over the GM/GCL interface; the residual strength of the GM/CCL interface was even higher than the peak strength of the GM/GCL interface at all normal stress levels. The results presented in this paper are useful for the preliminary design of landfill composite liner systems.

Effect of Particle‐Size Gradation on Coarse Sand‐Geotextile Interface Response in Cyclic and Postcyclic Direct Shear Test

In order to investigate the influence of sand particle‐size gradation on cyclic and postcyclic shear strength behaviour on sand‐geotextile interfaces, a series of monotonic direct shear (MDS), cyclic direct shear (CDS), and postcyclic direct shear (PCDS) tests were performed using a large‐scale direct shear apparatus. The influence of cyclic shear history on the direct shear behaviour of the interface was studied. The results indicated that cyclic shear stress degradation occurred at the sand‐geotextile interface. Shear volumetric contraction induced by the cyclic direct shear increased with the increase in cycle number. The lowest final contraction value was observed in discontinuously graded sand. In the MDS tests, there were great differences in interface shear strength due to the different particle‐size gradations, whereas the differences between shear volumes were negligible. In the PCDS tests, the shear stress‐displacement curves exhibited postpeak stress hardening behaviour for different particle‐size gradations, and differences in shear volumes were detected. The well‐graded sand‐geotextile interface had a higher value of shear stiffness and a higher damping ratio relative to the other interfaces. Postcyclic shear stress degradation was observed for the discontinuously graded sand‐geotextile interface.

An Experimental Study on the Basic Mechanical Properties and Compression Size Effect of Rubber Concrete with Different Substitution Rates

An experimental study was carried out on the uniaxial compression, uniaxial splitting, pure shear, and compression size effect of rubber concrete with 5 different substitution rates by applying hydraulic servo and direct shear apparatus. Then, by comparing the failure modes and ultimate strength eigenvalues of rubber concrete under different loading conditions, the following conclusions were drawn: with the increase of rubber substitution rate, the concrete specimens maintain a relatively good integrity under uniaxial compressive failure; on the contrary, the failure sections under uniaxial slitting and pure shear gradually become uneven with an increasing amount of fallen rubber particles. With the increase of specimen size, the integrity of rubber concrete after failure is gradually improved. Affected by an increased rubber substitution rate, the uniaxial compressive strength, splitting tensile strength, and shear strength of the concrete gradually decrease, while the plastic deformation capacity gradually increases. Specifically, the compressive strength is reduced by a maximum of 60.67%; the shear strength is reduced by a maximum of 49.85%; and the uniaxial splitting strength is reduced by a maximum of 58.38%. Then, we analyzed the strength relationship and the underlying mechanism among the three types of loading modes. It is found that, at the same rubber substitution rate, the compressive strength of rubber concrete gradually increases as the specimen size decreases, and the size effect on the compressive strength gradually decreases as the rubber substitution rate increases. Meanwhile, we performed qualitative and quantitative analysis on the equation describing the coupling effect of specimen size and rubber substitution rate on the compressive strength; the results suggest that the proposed equation is of a high level of applicability. Our research has a reference value for the application and promotion of rubber concrete in actual engineering projects.

Volume change behavior of root-permeated soils under partially saturated conditions

Vegetation, particularly roots, serves different functions in relation to increased shear strength under saturated and partially saturated conditions. Quantification of mechanical contribution of roots due to their tensile strength, and relationships of various vegetation parameters and plant-induced suction, as well as shear strength, have been widely studied. Although shear strength is directly related to the volume change characteristics of soil, dilative or contractive behaviour of root-permeated soils has not been of significant interest so far. This study investigates how volume change during shearing is related to the hydrological and mechanical characteristics of vegetated soils relevant to slope stability and shear strength of root-permeated soils under partially saturated conditions. Direct shear tests, on specimens planted with a mixture of species from different plant functional groups, were performed with an Inclinable Large-scale Direct Shear Apparatus (ILDSA). Matric suctions were monitored throughout the test with tensiometers. Vertical and horizontal displacement graphs were plotted to investigate the volume change behaviour. Maximum dilatancy angle was found to be positively correlated with plant-induced suction and net normalised stress, both of which were linked to root biomass and the root:shoot ratio. It was found that maximum dilatancy is controlled by matric suction and net normal stress.