Constant Normal Load Conditions Research Articles

Laboratory investigation into effect of bolt profiles on shear behaviors of bolt-grout interface under constant normal stiffness (CNS) conditions

Abstract Rock bolts have been widely used for stabilizing rock mass in geotechnical engineering. It is acknowledged that the bolt profiles have a sound influence on the support effect of the rock bolting system. Previous studies have proposed some optimal rib parameters (e.g. rib spacing); unfortunately, the interface shear behaviors are generally ignored. Therefore, determination of radial stress and radial displacement on the bolt-grout interface using traditional pull-out tests is not possible. The load-bearing capacity and deformation capacity vary as bolt profiles differ, suggesting that the support effect of the bolting system can be enhanced by optimizing bolt profiles. The aim of this study is to investigate the effects of bolt profiles (with/without ribs, rib spacing, and rib height) on the shear behaviors between the rock bolt and grout material using direct shear tests. Thereby, systematic interfacial shear tests with different bolt profiles were performed under both constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions. The results suggested that rib spacing has a more marked influence on the interface shear behavior than rib height does, in particular at the post-yield stage. The results could facilitate our understanding of bolt-grout interface shear behavior under CNS conditions, and optimize selection of rock bolts under in situ rock conditions.

Corrections applied to direct shear results and development of modified Barton’s shear strength criterion for rock joints

In present study, 144 direct shear tests are performed on mated rock joint replicas under constant normal load condition (CNL). For these tests, three natural roughness of joint surface are transferred to the RTV silicon rubber molds. On these molds, mixture of cement, sand, and water in the ratio of 1:1.5:0.45 by weight is poured and joint replicas are made. In this study, the experimental shear strength is corrected with gross contact area (Ac) and incremental dilation angle (i). Further, the peak dilation angle is determined by Barton’s and incremental dilation (dv/dh) approaches and compared. The results showed that Barton’s approach underestimates the peak dilation angle. The roughness quantification of joint surface is done using 3D noncontact type profiler, and morphological parameters of joint surface are determined in each shearing direction as described by Grasselli and Egger (Int J Rock Mech Min Sci 40:25–40, 2003). A new predictive model for joint roughness coefficient (JRC) is developed and Barton’s model is modified. It is observed that modified Barton’s model provides good approximation of shear strength in desired shear direction. Moreover, modified Barton peak shear strength (τPre) is compared with Barton and Grasselli’s experimental peak shear strength, and it is observed that τPre matches closely with Barton’s peak shear strength.

Numerical simulation of experimentally shear-tested contact specimens from existing dam joints

This paper first proposes an original procedure to implement three-dimensional (3D) surfaces of rough existing unbonded contact specimens, i.e. fully open with no chemical bond, into non-linear finite element models to numerically simulate shear tests conducted under Constant Normal Load (CNL) conditions. The developed procedure is applied to 18 contact specimens drilled from concrete, concrete-rock, and rock joints of existing dams. A total of 83 finite element models of the contact interfaces are generated and the results of the numerical simulations are analyzed and validated against corresponding experimental findings. Sensitivity analyses are conducted and highlight, among other things, that the geometric resolution must be selected with caution and that mismatch due to initially unmated conditions or differences between the geometries of the lower and upper contact faces must be considered. A practical non-linear shear strength criterion is also proposed and compared to numerical results. The results presented confirm the major role of roughness and matching properties in the shear response of dam joints. Finally, values of apparent cohesion and friction angle are suggested and applied to dam stability assessment while accounting for joint roughness and matching properties to emphasize the efficiency and relevance of such practice.

Experimental Study of Shear Behavior of Rock Joints under Two Types of Boundary Conditions: Constant Normal Load and Constant Normal Stiffness

Shear destruction of rock mass along weakened planes is one of the main reasons of the failure of career boards, underground workings and rock slopes. The paper deals with laboratory testing of rock joints under direct shear loading. Special test and measurement complex was created to study deformation and strength characteristics of rock joints. Test specimens were made from single slab of sandstone of homogeneous structure. Shear tests have been conducted under constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions at three values of normal compressive stress: 0.5 MPa, 1.2 MPa and 1.8 MPa. The analysis of test results allowed to reveal patterns of changes of deformation-strength properties of rock joints depending on the level of normal load, boundary conditions and roughness values. The ultimate shear stress is always higher under CNS boundary condition than CNL for the same values of normal compressive stress. Shear stress practically retains constant value after reaching the ultimate stress under CNL boundary conditions and it decreases under CNS conditions. The values of ultimate shear stresses, shear modulus and the length of linear section on “shear stress–shear displacement” curve increases with increasing surface roughness at the equal levels of normal compressive stress.

Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface

Abstract Shearing behavior and failure mechanism of bolt-grout interface are of great significance for load transfer capacity and design of rock bolting system. In this paper, direct shear tests on bolt-grout interfaces under constant normal load (CNL) conditions were conducted to investigate the effects of bolt profile (i.e. rib spacing and rib height) and grout mixture on the bolt-grout interface in terms of mechanical behaviors and failure modes. Test results showed that the peak shear strength and the deformation capacity of the bolt-grout interface are highly dependent on the bolt profile and grout mixture, suggesting that bolt performances can be optimized, which were unfortunately ignored in the previous studies. A new interface failure mode, i.e. ‘sheared-crush’ mode, was proposed, which was characterized by progressive crush failure of the grout asperities between steel ribs during shearing. It was shown that the interface failure mode mainly depends on the normal stress level and rib spacing, compared with the rib height and grout mixture for the range of tested parameters in this study.

A numerical study to investigate the influence of surface roughness and boundary condition on the shear behaviour of rock joints

The shear behaviours of rock joints with and without rock bolt support are numerically studied using the discrete element code PFC2D. A cohesive contact model was employed to reproduce the damage response of the synthetic intact rock (i.e. asperity degradation). We used the smooth-joint model to simulate the micro-scale roughness of the joint surface. We calibrated and validated the proposed numerical framework against the experimental results. A series of numerical constant normal stiffness (CNS) direct shear tests were conducted on idealised and natural rock joints to investigate the influence of the boundary condition on the shear behaviour of rock joints. In particular, the importance of CNS condition, surface roughness, asperity angle, and the initial normal stress were studied. Additionally, the shear and damage mechanism of bolted rock joints under constant normal load (CNL) and CNS condition was numerically investigated. The results presented show that the shear resistance of the joint increases under both CNL and CNS conditions, but at a high degree of roughness, no significant enhancement was observed on the value of peak shear stress.

Numerical Shear Tests on the Scale Effect of Rock Joints under CNL and CND Conditions

The scale effect of rock joint shear behavior is an important subject in the field of rock mechanics. There is yet a lack of consensus regarding whether the shear strength of rock joints increases, decreases, or remains unchanged as the joint size increases. To explore this issue, a series of repeated and enlarged numerical joint models were established in this study using the particle flow code (PFC2D). The microparameters were calibrated by uniaxial compression tests and shear tests on the concrete material under the constant normal loading (CNL) condition. Three different normal stresses were adopted in numerical shear tests with joint specimen lengths ranging from 100 mm to 800 mm. In addition to the commonly used CNL, the constant normal displacement (CND) condition was established for the purposes of this study; the CND can be considered an extreme case of the constant normal stiffness (CNS) condition. The shear stress‐shear displacement curves changed from brittle failure to ductile failure alongside a gradual decrease in peak shear strength as joint length increased. That is, an overall negative scale effect was observed. Positive scale effect or no scale effect is also possible within a limited joint length range. A positive correlation was also observed between the peak shear displacement and joint length, and a negative correlation between shear stiffness and joint length. These above statements are applicable to both repeated and enlarged joints under either CNL or CND conditions. When the normal stress is sufficiently high and shear dilatancy displacement is very small, the shear behavior of rock joints under CNL and CND conditions seems to be consistent. However, for shear tests under low initial normal stress, the peak shear strength achieved under the CND condition is much higher than that under the CNL condition, as the normal stresses of enlarged joints increase to greater extent than the repeated ones during shearing.

Geometric and mechanical properties of a shear-formed fracture occurring in a rock bridge between discontinuous joints

Rock bridges are commonly encountered in rock engineering practices and play a significant role in the stabilization of rock masses. Under shear loading, they are supposed to provide resistance not only at before-failure stages, but also at after-failure stages, and this is not fully understood yet by now. To promote the solving of this problem, the present study tried to discover the geometric and mechanical properties of a shear-formed fracture occurring in a rock bridge between discontinuous open joints through laboratory experiments. After careful preparation of sandstone specimens containing discontinuous open joints, a series of direct shear tests were carried out under constant normal load (CNL) conditions. Once the rock bridge failed, another two sets of cyclic loading were subsequently applied to the specimen to examine the after-failure properties. The geometric properties of the newly formed fracture in rock bridge area, including undulating angle, fracture roughness, and fracture aperture, were systematically examined and analyzed with respect to the applied normal stress and joint persistence. Furthermore, the failure mode was dependent on the normal stress and joint persistence and also influenced the properties of the shear-formed fracture. Accordingly, different fracture types occurring to the rock bridges were summarized and described in terms of geometric morphology. The findings in present study would enhance our understanding of the influence of rock bridges, especially at after-failure stages, on engineering practices such as rock slopes.

Effects of mica content on rock foliation strength

Foliation is a planar geological structure characterized by the alignment of minerals, including mica grains. Foliated metamorphic rocks exhibit anisotropic mechanical behaviors, wherein the weakest properties are usually oriented parallel to the foliation planes. Rock mechanics research on this subject has been focused on characterizing the full anisotropic behaviors of rocks with stresses applied parallel, oblique and perpendicular to the anisotropy direction (foliation planes). However, less effort has been given to characterizing the mechanical behaviors of isolated foliation planes and describing the influences of mineralogical properties on the weaknesses associated with such geological structures. This paper presents a laboratory testing program, including direct shear tests and pull-off tests, to characterize the influences of mica content on the shear and tensile strengths of foliation planes of the biotite gneiss (which is basically composed of biotite, quartz and feldspar). Direct shear tests were performed under constant normal load and constant normal stiffness boundary conditions, wherein the foliation planes were isolated in 1 cm-thick shear zones. The mica content was estimated by a practical digital image analysis approach that was capable of quantifying the percentage of biotite area on the foliation surfaces that failed in shear and tension. The results showed that the mica (e.g., biotite) content had substantial effects on the mechanical behaviors of isolated foliation planes, especially on the peak shear and tensile strengths. A practical 3D strength envelope was proposed that included the mica content as an independent variable, which can be quantified from foliation surfaces on rock blocks and outcrops. The results provided new insights into the influences of mica-rich foliation planes on the mechanical behaviors of metamorphic rocks, which is directly applicable to rock engineering problems.

Anisotropic surface roughness and shear behaviors of rough-walled plaster joints under constant normal load and constant normal stiffness conditions

Abstract In this context, we experimentally studied the anisotropic mechanical behaviors of rough-walled plaster joints using a servo-controlled direct shear apparatus under both constant normal load (CNL) and constant normal stiffness (CNS) conditions. The shear-induced variations in the normal displacement, shear stress, normal stress and sheared-off asperity mass are analyzed and correlated with the inclination angle of the critical waviness of joint surfaces. The results show that CNS condition gives rise to a smaller normal displacement due to the larger normal stress during shearing, compared with CNL condition. Under CNL conditions, there is one peak shear stress during shearing, whereas there are no peak shear stress for some cases and two peaks for other cases under CNS conditions depending on the geometry of joint surfaces. The inclination angle of the critical waviness has been verified to be capable of describing the joint surface roughness and anisotropy. The joint surface is more significantly damaged under CNS conditions than that under CNL conditions. With increment of the inclination angle of the critical waviness, both the normal displacement and sheared-off asperity mass increase, following power law functions; yet the coefficient of determination under CNL conditions is larger than that under CNS conditions. This is because the CNS condition significantly decreases the inclination angle of the critical waviness during shearing due to the larger degree of asperity degradation.

A cohesive grain based model to simulate shear behaviour of rock joints with asperity damage in polycrystalline rock

A cohesive grain based model is proposed to simulate the fracture behaviour of polycrystalline rocks. The model was employed to characterize the cracking response of both inter- and intra-grain contacts in the grain based model (GBM). The model was implemented in distinct element codes and its ability to mimic the mechanical and failure behaviour of polycrystalline rocks was demonstrated through calibration of several granitic rocks including Adelaide black granite, Eibenstock II granite, and Aue granite. The calibrated model of Aue granite was used to assess the effect of joint roughness coefficient (JRC) on asperity damage and shear mechanism of rock joints (i.e. grain crushing) under constant normal load (CNL) and constant normal stiffness (CNS) conditions. The bond-break in intra-grain contacts contributed to asperity damage in form of grain crushing. The numerical observations showed that under CNS condition asperity damage increased with increasing initial normal stress and JRC, and rough rock joints exhibited more dilative response.

Thermal effects on mechanical behaviour of soil–structure interface

Mechanical behaviour of the soil–structure interface plays a major role in the shear characteristics and bearing capacity of foundations. In thermoactive structures, due to nonisothermal conditions, the interface behaviour becomes more complex. The objective of this study is to investigate the effects of temperature variations on the mechanical behaviour of soils and the soil–structure interface. Constant normal load (CNL) and constant normal stiffness (CNS) tests were performed on the soil and soil–structure interface in a direct shear device at temperatures of 5, 22, and 60 °C. Fontainebleau sand and kaolin clay were used as proxies for sandy and clayey soils. The sandy soil was prepared in a dense state and the clayey soil was prepared in a normally consolidated state. Results show that the applied thermal variations have a negligible effect on the shear strength of the sand and sand–structure interface under CNL and CNS conditions, and the soil and soil–structure interface behaviour could be considered thermally independent. In clay samples, an increase in the temperature increased the cohesion and consequently the shear strength, due to thermal contraction during heating. The temperature rise had less impact on the shear strength in the case of the clay–structure interface than in the clay samples. The adhesion of the clay–structure interface is less than the cohesion of the clay samples.

Frictional responses of concrete-to-concrete bedding planes under complex loading conditions

Concrete-to-concrete bedding planes (CCBP) are observed from time to time due to the multistep hardening process of the concrete materials. In this paper, a series of direct/cyclic shear tests are performed on CCBP under static and dynamic normal load conditions to study the frictional behavior effect by the shear velocities, normal impact frequencies, horizontal shear frequencies, normal impact force amplitudes, horizontal shear displacement amplitudes and normal load levels. According to the experimental results, apparent friction coefficient k (k = FShear/FNormal) shows different patterns under static and dynamic load conditions at the stable shear stage. k is nearly constant in direct shear tests under constant normal load conditions (DCNL), while it is cyclically changing with nearly constant peak value and valley value for the direct shear tests under dynamic normal load conditions (DDNL), where k increases with decreasing normal force and decreases with increasing normal force. Shear velocity has little influence on peak values of k for the DCNL tests, but increasing shear velocity leads to increasing valley values of k for DDNL tests. It is also found that, the valley values of k ascend with decreasing impact normal force amplitude in DDNL tests. The changing pattern of k for the cyclic shear tests under constant and dynamic normal load conditions (CCNL and CDNL tests) are similar, but the peak value of k is smaller in CDNL tests than that in CCNL tests. Normal load levels, shear displacement amplitudes, vertical impact frequencies, horizontal shear frequencies and normal impact force amplitudes have little influence on the changing pattern of k for the cyclic shear tests. The tests of this study provide useful data in understanding the frictional behavior of the CCBP under distinct loadings, and these findings are very important for analyzing the stability of the jointed geotechnical structures under complicated in situ stress conditions.

Mechanism of shear deformation, failure and energy dissipation of artificial rock joint in terms of physical and numerical consideration

The physical and mechanical change processes of rock are closely related to energy transformation, and its deformation and failure is an instability phenomena driven by energy exchange. This study investigated mechanism of shear deformation, failure and energy dissipation of joint using both physical and numerical direct shear tests under constant normal load (CNL) condition. Three kinds of joint surface were artificially prepared. An acoustic emission system was employed to monitor acoustic emission in physical test, and rupture frequency was recorded in numerical test to represent micro-crack development. By research of numerical micro-crack development accompanied with physical acoustic emission results, mechanism of shear deformation and failure of joints were illustrated schematically. By definition of dissipation energy, captured using the particle flow code (PFC2D), energy releasing and dissipation were discussed with microscopic damage evolution of joints. Results showed that joints under shearing present a dissipation trend of four stages including a slow rise stage, a rapid rise stage, a shock rise stage and a rapid decline stage.

Velocity-frequency-amplitude-dependent frictional resistance of planar joints under dynamic normal load (DNL) conditions

The problem of how dynamic friction of faults influences the observed earthquakes remains largely unsolved. Our paper presents results of experiments aimed to understand the complex frictional behavior of faults under dynamic normal load (DNL) conditions under consideration of horizontal slip velocity, dynamic normal force amplitude and normal impact frequency using a dynamic shear box device. Data were obtained by continuous measurements of shear and normal forces as well as corresponding displacements. We identified a phase shift between peak normal force and peak shear force with peak shear force lagging. Consequently, the dynamic friction coefficient shows cyclic behavior and the minimum value of the dynamic friction coefficient increases with increasing of slip velocity which is in conflict to the traditional views under constant normal load (CNL) conditions. We found, that dynamic fault friction coefficient during earthquakes may increase or decrease depending on velocity-frequency (VF), velocity-amplitude (VA) and velocity-frequency-amplitude (VFA) parameter. These findings have significant implications for seismic hazard assessment and reliable forecasting of earthquakes.

Laboratory test on foamed concrete-rock joints in direct shear

Foamed concrete has been widely used in tunnels where foamed concrete-rock joints are usually encountered. However, few studies have been conducted on the mechanical properties of the joint. To investigate the shear behaviors of the joint, compressive properties of foamed concrete were first studied by compression tests, and a series of direct shear tests under constant normal load condition were then conducted. Three failure types were identified during the tests: interface failure, foamed concrete failure and the mixed type. Both the normal stress and dry density of the foamed concrete can significantly influence the shear behaviors of the joint, but have little effect on the friction coefficient of the joint. Based on the results, a model was proposed for the joint, which can successfully describe the shear behaviors of the joint. The model can easily be implemented in numerical codes, and then be used to investigate the mechanical behaviors of tunnels in practical cases.

Distinct element method simulations of rock-concrete interfaces under different boundary conditions

The shear behaviour of concrete-rock interfaces has been the aim of extensive research in geotechnical engineering applications such as rock socketed piles, rock bolts and concrete dam arch bridge foundations. Several experimental studies through direct shear tests have been conducted to evaluate the shear behaviour of rock-concrete interfaces under CNL (Constant Normal Load) and CNS (Constant Normal Stiffness) conditions. In this paper, PFC2D numerical simulations of unbonded rock-concrete planar and saw-tooth triangular joints under CNL and CNS boundary conditions are conducted using the Shear Box Genesis (SBG) approach proposed by Bahaaddini et al. (2013b). The numerical simulation results are compared with experimental data published by Gutiérrez (2013) and Gu et al. (2003). Results indicate that the SBG approach reproduces suitably the shear behaviour, failure mode and asperity damage of unbonded (planar and triangular) rock-concrete interfaces, specially under CNL conditions.

Automatic Static and Cyclic Shear Testing Machine under Constant Normal Stiffness Boundary Conditions

Abstract Dynamic loads, including earthquake, blasting, and vibration loads, induce cyclic shear loads along the joints in rock masses; hence, the risk of failure increases on the joints due to changing shear resistance. On the other hand, joints are under different boundary conditions: constant normal load (CNL) and constant normal stiffness (CNS). Normal stiffness increases on the joints with increasing depth, and it can affect shear resistance. For an accurate assessment of joint shear resistance under varying normal stiffness and number of cycles, advanced laboratory shear apparatus is essential for the shear test. Conventional direct shear apparatuses have limitations such as boundary conditions, working under monotonic (static) shear loads only, or cyclic shear loads with no change of frequency and amplitude of shear loads. Therefore, a new large-scale servo-controlled direct shear testing machine was developed to conduct cyclic shear test (as well as monotonic shear test) under CNL and CNS boundary conditions with varying normal stiffness at different frequencies and amplitudes of shear loads. In the present study, the cyclic shear tests were conducted on nonplanar joints under varying normal stiffness. Moreover, the effects of different frequencies and amplitudes of shear loads were investigated. The test results indicate that peak shear stress increases with increasing normal stiffness at the first cycle, but the influence of normal stiffness decreases with an increase in the number of shear cycles. The frequency of shear load influences peak shear stress, i.e., peak shear stress increases with increasing frequency. The number of cycles does not affect peak shear stress on the joints at low shear amplitude, but peak shear stress decreases with higher amplitude.

Degradation of joint surface morphology, shear behavior and closure characteristics during cyclic loading

In order to investigate the failure mechanism of rock joint, a series of laboratory tests including cyclic direct shear tests under constant normal load(CNL) conditions were conducted. Morphology parameters of the rock joint surface were precisely calculated by means of a three-dimensional laser scanning machine. All test results were analyzed to investigate the shear behavior and normal displacement behavior of rock joints under CNL conditions. Degradation of rock joint surface during cyclic shear tests was also analyzed. The comparison results of the height parameters and the hybrid parameters of the joint surface during cyclic tests show that the degradation of the surface mostly happens in the first shear and the constant normal loads imposed on the joints have significant promotion effects on the morphology degradation. During cyclic shear tests, joints surfaces evolve from rough state to smooth state but keep an overall undulation. Dilatancy of rock joints degrades with the degradation of joint surface and the increase of normal loads. The closure deformation of joint is larger than that of the intact rock, and the normal stiffness increases with the increase of shearing times.

Shear behaviour of a cement grout tested in the direct shear test

Portland cement grouts are widely used in the mining industry to bond cable bolts with the surrounding rockmass. Numerous laboratory and field tests showed that bond failure of the cable/grout interface is the dominant failure mode. Previous research has found that shear behaviour of the grout along a pre-defined plane plays a significant role in determining the nature of the bond failure in a cable bolt reinforcement system. In this study, the shear behaviour of a Portland cement grout was investigated based on a direct shear test. Two different boundary conditions were considered being a constant normal load (CNL) and constant normal stiffness (CNS). Under CNL condition, five different normal pressures between 0.1 MPa and 6.0 MPa were examined. While under CNS condition, the initial normal pressure was set to value within the same range. Also, a CNS of 10kN/m was added. The cohesion, internal friction angle and shear strength of the grout were acquired. The results showed that there is a linear relationship between the shear strength of the grout and resultant normal pressure. However, under the CNS condition, the shear strength of the grout was found to be generally higher comparing to the CNL condition, most likely because sample dilation resulted in an increase in the normal pressure. Consequently, shear strength of the grout also increased.