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Benchmark experiment on shear behavior of ice-filled planar rock joints using a novel direct shear testing apparatus
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The dynamic frictional behaviors of natural discontinuities (joints, fractures, faults) play an important role in geohazards assessment; however, the mechanisms of the dynamic fault weakening/strengthening are still unclear. In this paper, a dynamic shear box was used to perform direct shear tests on saw-cut (planar) and natural (rough) granite fractures, with different normal load oscillation amplitudes. Based on the recorded shear forces and normal displacements, the shear forces, apparent friction coefficients and normal displacements are found to change periodically with oscillated normal loads and are characterized by a series of time shifts. The observed changing patterns are similar for the rough and planar fractures. Compared with the test data under constant normal load (CNL), small/large normal load oscillation amplitude enhances/reduces the peak shear strength, with a critical point. The magnitude of critical normal load oscillation for the rough fractures is smaller than the planer fractures. The results imply that dynamic fault weakening/strengthening can be achieved by both normal load oscillation amplitudes and slip surface topography. The rough fractures with larger normal oscillation amplitude can easily cause frictional weakening under stress disturbance.© 2021 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Samples of rock coupling joints were collected from the Jiangluling Tunnel of the G214 line in Qinghai province. Models with surface topographies similar to these joints were manually created. Freezing shear tests under different normal stress conditions were conducted to study the shear mechanical properties of these models. On this basis, the integral form of the peak shear strength criterion of frozen joints was proposed. Results show that the shear process of the ice layer can be divided into four stages, namely, initial deformation, continuously increasing shear stress, ice shearing, and residual shear. During the continuously increasing shear stress stage, the stress-strain curve is concave, and elastic deformation is not evident. Furthermore, the increase rate of shear stress generally rises as normal stress intensifies. In the ice shearing stage, shear stress does not decrease instantaneously, but plastic deformation is now detectable. When the opening degree is greater than the undulation difference of the joint surface under the action of all levels of normal stress, the shear stress in the ice sharply increases and drops due to local failure and reicing. Then, evident difference between the shear processes under freezing and normal temperature conditions was then obtained. On this basis, the failure forms of joint surfaces, theory of ice adhesion strength under different opening degrees and morphologies, and the shear failure forms of frozen joints under different conditions were considered. The integral form of the peak shear strength criterion of frozen joints was proposed. These results can lay a theoretical foundation for the stability analysis of rock mass engineering in permafrost areas.
A constant-head step injection test using a conventional straddle-packer system was performed for a normal fault in siliceous mudstone. The test applied a new method whereby axial displacements of isolated test sections in a borehole during injection are monitored by measuring the pressures of sliding packers and the pore pressure in the test section. The measured pressures and axial displacement, and the injection flow rate, were used to estimate the hydraulic aperture, shear displacement, normal compliance, normal stress, shear stiffness and hydraulic dilation angle of the fault during the test. The injection successfully yielded a large shear displacement during normal faulting of up to 13.3–49.5 mm (including the estimation error), which left residual shear displacement of 2.8–10.4 mm after a remarkable shear-slip event. The shear stiffness during faulting is estimated to be 2.3 × 10⁷ to 8.4 × 10⁷ Pa m⁻¹ (considering the estimation error), which is consistent with empirically predicted values based on previous studies. The hydraulic dilation angle was inferred to be effectively zero as the residual shear displacement did not leave any increase in hydraulic aperture. The experimental method applied here does not require specialized equipment and could aid in the investigation of the hydromechanical behavior of subsurface fractures or aquifers.
A temperature- and stress-dependent failure criterion for ice-filled rock (limestone) joints was proposed in 2018 as an essential tool to assess and model the stability of degrading permafrost rock slopes. To test the applicability to other rock types, we conducted laboratory tests with mica schist and gneiss, which provide the maximum expected deviation of lithological effects on the shear strength due to strong negative surface charges affecting the rock–ice interface. Retesting 120 samples at temperatures from −10 to −0.5 ∘C and normal stress of 100 to 400 kPa, we show that even for controversial rocks the failure criterion stays unaltered, suggesting that the failure criterion is transferable to mostly all rock types.
For rock fractures, the degradations in the strength of contacting asperities and the surface frictional resistance are responsible for the water-induced weakening in the shear strength. To quantitatively examine their independent roles, direct shear tests on sawtooth fracture samples of granite and sandstone under three moisture conditions: dry, surface wet and saturated, were conducted subject to three levels of normal stresses. The surface wet condition only resulted in the variation in the basic friction angle and the saturated samples underwent the degradation in both unconfined compressive strength (UCS) and basic friction angle, which were obtained via unconfined compression test and direct shear test, respectively. Two weakening coefficients that represent the reductions in UCS and basic friction angle, respectively, were proposed and incorporated into an analytical model, which quantifies the entire shear stress evolutions during shear based on a continuous yielding mechanism. The difference in the shear strength between dry and surface wet conditions originates from the lubricant effect of water represented by the change in basic friction angle with a weakening coefficient less than 10% for both rocks. Under the saturated condition, the weakening coefficient of the UCS varies significantly from 15.17% for granite to 50.39% for sandstone. A series of datasets that characterize the reductions in UCS and basic friction angle induced by water were collected from the literature, which were then incorporated into the analytical model to estimate the general weakening trend in the shear strength of the common rocks in practices. For crystalline rocks, the water-mediated lubrication seems to be the primary mechanism reducing the shear strength, while for sedimentary rocks, the remarkable degradation in UCS may dominate the weakening mechanism. The quantified weakening coefficients and the revealed weakening behavior of various rocks can be directly linked to the fracture shear strength estimation in engineering design.
Instability and failure of high mountain rock slopes have
significantly increased since the 1990s coincident with climatic warming and
are expected to rise further. Most of the observed failures in
permafrost-affected rock walls are likely triggered by the mechanical
destabilisation of warming bedrock permafrost including ice-filled joints.
The failure of ice-filled rock joints has only been observed in a small
number of experiments, often using concrete as a rock analogue. Here, we
present a systematic study of the brittle shear failure of ice and rock–ice
interfaces, simulating the accelerating phase of rock slope failure. For
this, we performed 141 shearing experiments with rock–ice–rock sandwich'
samples at constant strain rates (10−3 s−1) provoking ice
fracturing, under normal stress conditions ranging from 100 to 800 kPa,
representing 4–30 m of rock overburden, and at temperatures from −10 to
−0.5 °C, typical for recent observed rock slope failures in alpine
permafrost. To create close to natural but reproducible conditions, limestone
sample surfaces were ground to international rock mechanical standard
roughness. Acoustic emission (AE) was successfully applied to describe the
fracturing behaviour, anticipating rock–ice failure as all failures are
predated by an AE hit increase with peaks immediately prior to failure. We
demonstrate that both the warming and unloading (i.e. reduced overburden) of
ice-filled rock joints lead to a significant drop in shear resistance. With a
temperature increase from −10 to −0.5 °C, the shear stress at
failure reduces by 64 %–78 % for normal stresses of 100–400 kPa.
At a given temperature, the shear resistance of rock–ice interfaces decreases
with decreasing normal stress. This can lead to a self-enforced rock slope
failure propagation: as soon as a first slab has detached, further slabs
become unstable through progressive thermal propagation and possibly even
faster by unloading. Here, we introduce a new Mohr–Coulomb failure criterion
for ice-filled rock joints that is valid for joint surfaces, which we assume
similar for all rock types, and which applies to temperatures from −8 to
−0.5 °C and normal stresses from 100 to 400 kPa. It contains
temperature-dependent friction and cohesion, which decrease by
12 % °C−1 and 10 % °C−1 respectively due
to warming and it applies to temperature and stress conditions of more than
90 % of the recently documented accelerating failure phases in permafrost
rock walls.
Rock-ice fracture in cold regions is often an important problem for rock engineering design and construction. In order to assess the safety against failure of ice filled rock joints, a numerical method to study this problem is carried out in this paper. Three models, i.e. ice-rock, rock-rock and rock-ice-rock specimen with known parameters has been used to simulate under static loading, at same time, the damage and fracture processes was modeled and anlysised using RFPA2D (Rock Failure Process Analysis) code. In this study, a newly proposed mechanical model is used to simulate the fracture behavior of ice-rock specimens under the shear loading. In this numerical model, rock and ice is regarded as two kind brittle material (different heterogeneity index) respect, an elastic finite element program is employed as the basic stress analysis tool while the elastic damage mechanics is used to describe the constitutive law of micro-level element. The maximum tensile strain criterion and Mohr–Coulomb criterion are utilized as damage thresholds. The heterogeneous stress field is obtained from numerical simulation, thus it is found that heterogeneity of mechanical properties has significant effect on the stress distribution in rock. The crack propagation processes simulated with this model shows good agreement with those of experimental observations in the pre-exist literature. It has been found that the shear fracture of rock-ice observed at the macroscopic level is predominantly caused by tensile damage at the microscopic level.
The paper presents the results of tilt tests using a large number of three-core samples, aiming to establish a method for determining the basic friction angle of planar rock discontinuities. The three-core tilt test is easy to operate, and the results are comparable with the saw-cut method after modification. Saw-cut and two-core samples were also tested for the purpose of comparison. The test results show that the cylindrical surface of the core is slightly rougher than the surface of saw-cut samples so that the friction angle measured on the cylindrical surface of the core is slightly larger than that measured on saw-cut samples. The basic friction angle of the planar rock discontinuity is obtained by decreasing 2° from the friction angle measured on three-core samples. The appropriate number for test repetitions is 10∼20 in order to obtain a reliable mean friction angle. Grinding or polishing of the cylindrical surface of cores is not recommended, since it will lead to an increase in the friction angle in most of rock types.
Acoustic Emission (AE) is generated in soil and rock materials by rearrangement of particles during displacement or increasing damage in the microstructure preceding a collapse; therefore AE is appropriate for estimation of slope degradation. To overcome the high attenuation that characterise geological materials and thus to be able to monitor AE activity, a system that makes use of a waveguide to transmit AE waves from a deforming zone to a piezoelectric transducer was developed. The system quantifies acoustic activity as Ring Down Count (RDC) rates. In soil applications RDC rates have been correlated with the rate of deformation, whereas the recent application to rock slopes requires new interpretation strategies. In order to develop new strategies the system was installed at two rock slope trial sites in Italy and Austria. RDC rates from these sites, which have been measured over five and 1.5 years respectively, are analysed and clear and recurring trends are identified. The comparison of AE trends with response from a series of traditional instruments available at the sites allows correlation with changes in external slope loading and internal stress changes. AE signatures from the large rock slope in Italy have been identified as generated in response to variations in the groundwater level and snow loading. At the slope in Austria, AE signatures include the detachment of small boulders from the slope surface caused by the succession of freeze-thaw cycles during winter time. The work reported in this paper is contributing to the development of AE monitoring and interpretation strategies for rock slopes. The longer-term aim is to identify approaching failures and derive rules for setting thresholds that can be used to give warning of rock slope failures in time to enable action to be taken.
This paper presents the influence of freeze–thaw
cycles on physical and mechanical properties of Upper Red
Formation sandstones in the southwestern Qom province in
central Iran. For this purpose, five different types of sandstones
were selected. Freeze–thaw test was carried out for 30
cycles and P wave velocity, porosity and uniaxial compressive
strength of specimens were determined after every 5 cycles.
Also, long-term durability of sandstones against freeze–thaw
cycles using a decay function model was evaluated. The
results of this study show that an increase in number of
freeze–thaw cycles decreases uniaxial compressive strength
and P wave velocity, whereas the effective porosity increases.
The results obviously indicate that rock strength and petrographic
properties such as grain size and contacts between
grains alone does not provide enough information regarding
sample durability against freeze–thaw cycles. Finally, it was
found that pore size distribution plays the main role on the
resistance of sandstones in freeze–thaw cycles.
Rockfalls pose a significant threat to life and property, thus rockfall protection is a major issue
in areas exposed to severe rockfall hazard. Rockfall protection approaches include quantitative
rockfall hazard and risk assessment, as well as the design of structural countermeasures. These
require a sound and robust quantification of 3D rockfall trajectories, distribution and intensity
of impacts, and magnitude and variability of involved dynamic quantities. Providing highly
reliable modeling inputs to rockfall protection remains a difficult task, because of the
complexity and intrinsic stochastic nature of rockfall physics and the uncertainty of all the
relevant parameters. However, significant advances in rockfall analysis have been made in
the last decade, and modern 3D rockfall modeling techniques now provide effective tools to
support rockfall protection activities, even in difficult design conditions. Nevertheless, their
use can still be complex and raises a series of difficult steps both in the modeling and the
analysis of the results. In this paper, we provide an overview of rockfall analysis aspects
relevant to rockfall protection, as well as examples of applications of modern 3D rockfall
runout modeling techniques in rockfall hazard and risk assessment, and countermeasure
selection, design and optimization.
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, , which represents the friction between the minerals making up the rock surfaces. The basic friction angle, , is the sum of 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.
To assess the safety against failure of rock slopes in cold regions, such as high mountain areas, where stability is potentially maintained by ice in rock discontinuities, the shear strength of ice-filled rock joints was investigated in a series of direct shear-box tests. To permit control and repeatability, the experiments were conducted using simulated rock specimens. These were cast in the laboratory using high-strength concrete. Laboratory measurements showed that at a constant rate of shearing, the interface shear strength between ice and a joint surface of repeatable roughness is a function of both temperature and normal stress.
Direct shear box tests have revealed that the stiffness and strength of an ice‐filled joint are a function of both normal stress and temperature. Comparison of these data with the results of similar experiments conducted on unfrozen joints indicates that at low temperatures and normal stresses the strength of an ice‐filled joint can be significantly higher than that of an unfrozen joint. However, in the absence of sufficient closure pressure, the strength of an ice‐filled joint can be significantly less than that of an unfrozen joint. This implies that if the stability of a slope is maintained by ice‐filled joints, its factor of safety will reduce with temperature rise. This hypothesis suggests that a jointed rock slope that is stable when there is no ice in the joints and is also stable when ice in the joints is at low temperatures will become unstable as the ice warms. Results from the model tests have confirmed this hypothesis. Copyright © 2001 John Wiley & Sons, Ltd.
RÉSUMÉ
Des tests de cisaillement directs ont révélés que la rigidité et la résistance d'un joint rempli de glace est fonction à la fois de la contrainte normale et de la température (Davies et al. , 2000). La comparaison de ces données avec les résultats d'expériences semblables conduites sur des joints non gelés indique qu'à basse température et pour des contraintes normales identiques, la résistance d'un joint rempli de glace peut être plus élevée d'une manière significative que celle d'un joint non gelé. Toutefois, en l'absence d'une pression de fermeture suffisante, la résistance d'un tel joint rempli de glace peut être significativement moindre que celle d'une fissure non gelée. Ceci implique que si la stabilité de la pente est maintenue par des joints remplis de glace, son facteur de sécurité sera réduit avec l'augmentation de la température. Cette hypothèse suggère qu'une pente de roches fissurées qui est stable quand il n'y a pas de glace dans les joints et est aussi stable quand la glace dans les joints est à basse température, deviendra instable quand la glace s'échauffe. Des résultats obtenus par des tests ont confirmé ce résultat. Copyright © 2001 John Wiley & Sons, Ltd.
Eight parameters that have used used to characterize numerically the roughness of surfaces have been measured on the ten profiles presented by Barton and Choubey [1] as typifying different joint roughness coefficients. Values of two of these parameters, Z2 and SF, respectively, the root mean square and the mean square of the first derivative of the profile, are strongly correlated with values of the joint roughness coefficient. The equations avoid the subjectivity of estimates of JRC by comparison with typical profiles. They will be useful checks on values of JRC estimated from profiles of rough surfaces.
The active layer in permafrost region is inevitably subjected to seasonal freezing-thawing actions. The soil freezing and thawing can change boundary conditions of the bridge pile foundation in permafrost region, which will influence the seismic responses of the bridge. In this study, quasi-static tests were carried out by using two 1/8 scaled models to investigate the influence of seasonal freezing-thawing soils on seismic performance of high-rise cap pile foundation in permafrost region. The hysteretic behaviors and damage characteristics of the pier-pile-soil system in permafrost region with thawed and frozen active layer were analyzed. It is found that the freezing-thawing status of the active layer can relocate the vulnerable position of the pile foundation in permafrost region. Furthermore, the frozen active layer limited the deformation capacity of the pier-pile-soil system, then resulted in the redistribution of the pile stress. Although the frozen active layer in winter season enhanced the bearing capacity of the pier-pile-soil system, it weakened the ductility of the pier-pile-soil system. Additionally, the frozen soil temperature and the shear strength of the active layers will affect the adhesive force of the permafrost layer. These results can provide a scientific basis for the seismic design of bridges with pile foundation in permafrost regions.
This work describes field evidence of gravity-induced cracking that has been identified on the failure scar of a small rockslide (volume = 6–7 m³) that occurred in March 2004 in the Rosandra Valley near the town of Trieste (north-eastern Italy). This shallow rock slope failure involved a limestone slope that was re-profiled through blasting about 135 years ago for the construction of an old railway, which has now been reconverted into a cycle path. Field observations ascertained that the slope failure was characterised by a cascading rupture process in which adjacent blocks collapsed in rapid sequence as a result of the progressive breakage of a number of rock bridges (domino-like collapse), thus highlighting a progressive failure mechanism. The rock bridges were localised in eccentric positions on lateral and rear release planes and failed in tension as a result of combined tensile and bending loading conditions (tensile strength of intact rock = 5 ± 2 MPa). The rockslide was triggered by cyclic loading related to freeze-thaw (ice jacking) that occurred over some consecutive days of daily snowmelt and nocturnal frost. The recognition of newly formed fractures also proves the current progressive development of 3D rupture surfaces of some unstable blocks susceptible to failure. This study provides innovative content to detect gravity-induced cracking in the field, which is an important precursory sign relating to the progressive failure of rock slopes. Gravity-induced cracks can be distinguished from pre-existing discontinuities for their more irregular profile (zigzag pattern), rougher surface, lower persistence and potentially different orientation. This study also provides some new insights into the step-path failure process of steep rock slopes. The mechanical process of initiation, growing and coalescence of gravity-induced fractures is strongly time- and stress-dependent. In the absence of external changing factors that profoundly modify the acting stress state (for instance, engineering countermeasures), progressive failure is a dynamic and irreversible process. For excavated slopes, the creation of a new rock face may abruptly provide the kinematic freedom of potentially unstable blocks, thus determining a very quick stress redistribution. If the mechanical system is not able to redistribute the stress on resisting stiff parts without overcoming the intact rock strength, gravity-induced cracking initiates and propagates until reaching slope collapse within times that are much shorter (from hours to 100–200 years) compared to natural rock slopes subjected to long-lasting geological processes (from 1000 to 2000 up to 10,000–20,000 years).
Joint is a crucial factor affecting rock mass stability, but its complex morphology brings challenges to its research. The three-dimensional (3D) scanning technology can obtain the point cloud digital model of the object without contact. The research is to introduce this technology into the analysis of joint characteristics. For this purpose, firstly, cloud plane inclination of joint 3D scanning data is solved by constructing translation matrix M and rotation matrix R. Then, the appropriate sampling interval for 3D scanning technology to analyze the joint morphology is determined by parametric analysis. Next, by integrating 3D scanning, 3D printing, and 3D carving technologies, the two fabrication methods of joint models with the same natural morphology and lithology are proposed, which overcomes the problem of insufficient joint samples in the experiment. Finally, based on the 3D scanning digital model, the recognition method of the potential shear region and the measurement technique of shear failure characteristics are presented. This research provides a new way to analyze the joint morphology and shear mechanism, and is also conducive to the popularization of 3D scanning technology in geotechnical engineering.
An accurate description of the morphology of a joint surface is necessary to interpret its shear behavior. It must be noted that accurately distinguishing the shear behavior differences may be difficult if the defined morphology parameter ignores the micro-slope distribution of the joint surface. The initial climbing angle was therefore defined and used to characterize the micro-slope distribution of the ascent section of joint profile, and a new method was proposed for calculating the morphology parameter, M. The parameter M embodies anisotropy, and the values of M for all profile lines, which are selected on the same joint at equal intervals in one direction, are log-normally distributed. To study the mechanical effects of M, joint replicas with three different strengths were 3D-printed using the scanned point cloud information of the natural joint. An anisotropic direct shear test was conducted to obtain the joint shear parameters in eight directions. The results indicated that the new morphology parameter accurately reflects the anisotropic shear behavior of the joint, and displayed tangential, positive linear, and power relations with shear strength, apparent cohesion, and friction angle, respectively. Furthermore, an index for evaluating the shear failure characteristics, established by using the new morphology parameter, can effectively distinguish the differences between the climbing and shear-off behavior on the joint. The parameter M, which considers the micro-slope distribution, can thus accurately reflect the physical and mechanical properties of a joint, and its application is conducive to the precise quantification of joint shear behavior.
Rock-engineering applications at different depths experience different high-temperature environments, the temperatures of which can reach 400 °C or higher. To investigate the temperature-dependent peak shear strength of rock fractures, a number of direct shear tests were conducted, using granite fractures that were first subjected to thermal treatments at different temperatures (20, 100, 200, 300, 400, 500, 600, and 800 °C). The basic friction angle, joint roughness coefficient (JRC), and uniaxial compressive strength (UCS) demonstrated nonlinear reductions with increasing temperature. However, no simple function could be deduced to capture the evolutions. The peak shear strength exhibited a linear decrease with the increase in temperature under each normal loading condition. However, the thermal effect gradually became less pronounced as the normal stress increased. Using multivariate regression analysis, an empirical equation was developed to describe the ratio between the peak shear strengths of the fractures before and after thermal treatments; this equation was a simple function of the normal stress and treatment temperature. Comparisons illustrated that the new criterion could provide the best evaluation of the peak shear strength of a rock fracture after thermal treatments. Possible reasons for the thermally induced reduction in the fracture peak shear strength were preliminarily analyzed, and the limitations of the developed criterion were also stated. The failure criterion can provide new insights for evaluating the stability of surrounding rock masses for temperature-dependent underground engineering.
Extremely energetic rockfalls (EERs) are defined here as rockfalls for which a combination of both large volume and free fall height of hundreds of meters results in energy larger than about 80 GJ released in a short time. Examples include several events worldwide. In contrast to low energy rockfalls where block disintegration is limited, in EERs the impact after free fall causes immediate release of energy much like an explosion. The resulting air blast can snap trees hundreds of meters ahead of the fall area. Pulverized rock at high speed can abrade vegetation in a process of sandblasting, and particles suspended by the blast and the subsequent debris cloud may travel farther than the impact zone, blanketing vast areas. Using published accounts and new data, we introduce physically based models formulated on analogies with explosions and explosive fragmentation to describe EERs. Results indicate that a portion of the initial potential energy of the block is spent in rock disintegration at impact (typically 0.2%–18%), while other sources of energy loss (air drag, seismic, sound, and ground deformation) are negligible; consequently, more than 80% of the potential energy is converted to kinetic energy of the fragmented block (ballistic projection, shock wave, sand blast, and dust cloud). We also propose simple estimates for the flow of the dust cloud associated with an EER and its long settling time. The areal extent of the affected zone is estimated from the energy balance and an empirical power law relationship.
Recent research has paid little attention to the difference in shear strength between discontinuities with different joint wall material (DDJM) and discontinuities with identical joint wall material (DIJM) and what are the controlling factors of shear properties of DDJM. Bedding planes between argillaceous limestone and mudstone in the Badong formation in the Three Gorges area of China were investigated using physical model tests. The effect of joint wall material combination on the shear properties of DDJM was first revealed by conducting tests on a series of artificial joint specimens that were made of different joint wall materials and with varying joint surface topographies under different normal stresses. The results indicate that: the peak shear strength of DDJM is not equal to the lower peak shear strength of DIJM for two joint wall materials of the DDJM, even so it is closer to the lower peak shear strength than to the higher one; effect of joint roughness and normal stress on peak shear strength of DDJM is similar with DIJM; effect of joint wall material combination on peak shear strength of DDJM is greater with larger roughness and higher normal stresses. Based on the data acquired from the physical model tests, a new empirical equation to estimate the peak shear strength of DDJM was developed. The capability of the equation was validated by comparing estimates with data from direct shear tests on natural DDJM samples gained from the Badong formation. Finally, the proposed empirical equation was applied to the stability analysis of a rock slope in the Badong formation.
To study the mechanical responses of rock joints within a wide range of shear rates, this study develops a horizontal gripping mechanism to improve a double shear test device. Artificial rock joints, including planar and regular asperities with dip angles of 15° and 30°, are produced for conducting double shear tests under constant normal load within the shear rate range of 10⁻²-10¹ mm/s. Experimental results demonstrate that, though normalized by applied normal stress, the shear stiffness of the planar joints has a semi-logarithmic linear relationship with normalized shear rates. For rock joints with regular asperities, the shear rate, asperity dip angle, and normal stress influence the failure modes of asperities. When the shear rate is lower than the threshold shear rate, the asperity dip angles after the shear process and the peak dilation angles of the 15° regular joints increase with increasing the shear rate, whereas these two parameters tend to decrease in the cases of the 30° regular joints. The peak friction angles of both planar and regular rock joints have a semilogarithmic linear relationship with normalized shear rates. The increasing peak friction angles of rock joints at different shear rates after sliding or local cut-off failures are derived mainly from increased basic friction angles. Both the basic friction angles of rock joints and the shear strength of asperities rise as the shear rate increases when asperity cut-off occurs. However, the quantitative effect of these two factors on the increase of peak friction angle requires further study.
The effect of water on the shear behavior of joints in rock is critical for determining the stability of jointed rocks subjected to changes in water levels. In this study, a series of direct shear tests on joints in dry and wet sandstone samples were conducted under constant normal loads to investigate the influence of wetting on joint shear behavior. The results show that the peak shear strength of sandstone joints can be lowered by about 20%–24% owing to wetting, but the saturation-induced reduction of joint shear strength is probably not sensitive to the duration of immersion. The deformation behavior of sandstone joints can also be affected by wetting in that peak shear displacement can increase, joint shear stiffness may be reduced, and shear-induced normal displacement may switch from dilation to contraction. A simplified hypothetical example of a steep rock slope was used to demonstrate the role of wetting in lowering the factor of safety. This example indicated that only considering the purely mechanical effect of pore pressure, but neglecting the wetting-induced weakening of rock joints may overestimate the slope stability and be highly risky for practical rock engineering.
The freeze–thaw action of water inside rocks’ pores, cracks, and joints plays a dominant role in physical rock weathering where the temperature fluctuates across 0 °C. To understand this process in cold regions, artificial weathering was simulated in the laboratory. Diorite, basalt, and tuff were used as specimen, and were maintained in saturated conditions to accelerate the effect of freeze–thaw weathering. X-ray computed tomography (CT) images and scanning electron microscope (SEM) photographs were obtained to investigate changes in the inner microstructure along with repeated freeze–thaw cycles. In addition, physical properties were measured during the cycles. Particle detachment, crack initiation, and propagation and porosity increase due to volumetric expansion of water inside rocks were detected using X-ray CT and SEM. The decrease in P-wave and S-wave velocities, and in weight loss were also observed. Tuff specimens, which have low tensile strength, high porosity, and loose structure, were deteriorated rapidly, while diorite specimens on the other hand, which have high tensile strength, low porosity, and compact structure, showed relatively slight changes. The frost susceptibility of basalt specimens was found to be intermediate between tuff and diorite. The developed process for cycling and freezing set up can be useful to test the various stones in terms of frost susceptibility.
Behaviour of soil-infilled rock joints has significant importance with respect to the strength of fractured rock mass. The presence of even a small amount of fine-grained infill material within a joint can reduce its shear strength considerably, depending on the degree of saturation of infill. Therefore, it is crucial to examine how the infill material can adversely affect the joint shear strength. Previous studies of infilled joints have mainly been focused on idealised regular joint patterns owing to the simplicity and reproducibility in laboratory testing. Current literature on infilled rock joints has also neglected the effect of the degree of saturation of infill on the shear behaviour. In most instances, fully saturated infill has been used or assumed, and the contribution of matric suction on the shear strength of joints having unsaturated infill has not been studied. In this study, a series of triaxial tests on natural joint profiles having joint roughness coefficient (JRC) of 10-12 is carried out at constant moisture content. A semi-empirical model is proposed and validated on the basis of laboratory data.
In this paper, we develop a mechanical model that relates the destabilization of thawing permafrost rock slopes to temperature-related effects on both, rock- and ice-mechanics; and laboratory testing of key assumptions is performed. Degrading permafrost is considered to be an important factor for rock–slope failures in alpine and arctic environments, but the mechanics are poorly understood. The destabilization is commonly attributed to changes in ice-mechanical properties while bedrock friction and fracture propagation have not been considered yet. However, fracture toughness, compressive and tensile strength decrease by up to 50% and more when intact water-saturated rock thaws. Based on literature and experiments, we develop a modified Mohr–Coulomb failure criterion for ice-filled rock fractures that incorporates fracturing of rock bridges, friction of rough fracture surfaces, ductile creep of ice and detachment mechanisms along rock–ice interfaces. Novel laboratory setups were developed to assess
the temperature dependency of the friction of ice-free rock–rock interfaces and the shear detachment of rock–ice interfaces. In degrading permafrost, rock-mechanical properties may control early stages of destabilization and become more important for higher
normal stress, i.e. higher magnitudes of rock–slope failure. Ice-mechanical properties outbalance the importance of rock-mechanical components after the deformation accelerates and are more relevant for smaller magnitudes. The model explains why all magnitudes of rock–slope failures can be prepared and triggered by permafrost degradation and is capable of conditioning long para-glacial response times. Here, we present a synoptic rock- and ice-mechanical model that explains the mechanical destabilization processes operating in warming permafrost rocks.
The prime objective of this work is to improve our understanding of the shear behavior of rock joints. Attempts are made to relate the peak shear strength of rock joints with its three-dimensional surface morphology parameters. Three groups of tensile joint replicas with different surface morphology are tested with direct shear tests under constant normal load (CNL) conditions. Firstly, the three-dimensional surface characterization of these joints is evaluated by an improved roughness parameter before being tested. Then, a new empirical criterion is proposed for these joints expressed by three-dimensional quantified surface roughness parameters without any averaging variables in such a way that a rational dilatancy angle function is used instead of
${\text{JRC}} \cdot \log_{10} \left( {{{\text{JCS}} \mathord{\left/ {\vphantom {{\text{JCS}} {\sigma_{\text{n}} }}} \right. \kern-0em} {\sigma_{\text{n}} }}} \right)$
by satisfying the new peak dilatancy angle boundary conditions under zero and critical-state normal stress (not physical infinite normal stress). The proposed criterion has the capability of estimating the peak shear strength at the laboratory scale and the required roughness parameters can be easily measured. Finally, a comparison among the proposed criterion, Grasselli’s criterion, and Barton’s criterion are made from the perspective of both the rationality of the formula and the prediction accuracy for the three groups of joints. The limitations of Grasselli’s criterion are analyzed in detail. Another 37 experimental data points of fresh rock joints by Grasselli are used to further verify the proposed criterion. Although both the proposed criterion and Grasselli’s criterion have almost equal accuracy of predicting the peak shear strength of rock joints, the proposed criterion is easier and more intuitive from an engineering point of view because of its Mohr–Coulomb type of formulation.
A relevant parameter for estimating discontinuity shear strength is the basic friction angle, usually derived from different types of tilt tests. However, the tilt tests described in the literature produce varying basic friction angle values. From a large number of different types of tilt tests on different kinds of rocks, it was possible to conclude that the mechanisms of sliding along cylinder generatrixes and planar surfaces are quite different, and that tests based on sliding along generatrixes are not appropriate for determining reliable basic friction angle values for discontinuity planes. Tests on small specimens are also not recommended, for geometry reasons and because ensuring reliable stress conditions is difficult. To quantify the natural variability in tilt testing, large specimens of the same granite were tested. The results revealed coefficients of variation for the basic friction angle in the range of 5–10 %, a variability which is no greater than that found for other rock mechanics parameters. This observation enables to forward some recommendations concerning the appropriate number of tests needed to obtain reliable results.
The shear strength of rock discontinuities strongly depends on the water content especially when the rocks contain clay materials. To assess the decrease in the mechanical properties of clay-infilled discontinuities due to water saturation, a series of direct shear tests was performed using an advanced shear box that allows the injection of water into the discontinuity. Results show that both the friction coefficient and the cohesion decrease when the discontinuity is saturated. Overall, the shear strength of the discontinuity is considerably reduced to approximately 50% of its original value. This reduction has to be accounted for when conducting stability analyses of rock slopes, dam foundations or underground openings.
Synopsis
The similarities and differences between soil and rock mechanics are discussed with particular reference to the stability of slopes. The effects of constraints and of the stiffness of the system applying stress are of greater importance in rock mechanics. The criteria for failure of rocks are mostly empirical and lead to linear or power laws. Similar laws might be expected to hold for friction. While the Coulomb law is in general adequate for soils, it appears that the frictional behaviour of rocks is described better by a non-linear law and if a Coulomb law is used factors of safety are sensitive to the value adopted for the cohesion. The various methods for measuring friction are described and their limitations discussed. The process of wear and the area of contact between sliding surfaces are considered. It appears that in some cases residual values of friction are attained after small amounts of sliding. Gouge is built up during sliding and its behaviour appears to be time-dependent. At present, numerical values for friction and other parameters of jointed systems are uncertain and so simple formulae are still useful. A number of formulae for factors of safety for sliding on one or two plane surfaces are given. For the case of rock with closely spaced joints the use of soil mechanics theory for circular and other surfaces of sliding is reasonable. In this case, values of friction obtained from single joint surfaces or crushed rock are conservative since the interlocking of rock elements may cause a substantial increase in strength.
Les similitudes et les différences entre la mécanique des sols et la mécanique des roches sont tuditudiées, avec réference particulière à la stabilité des pentes. Les effets des contraintes et de la rigidité du systéme appliquant les contraintes ont plus d'importance dans la mécanique des roches. Les critères pour la rupture des roches sont pour la plupart empiriques, et suggeèent des lois lintudiaires ou de puissance. On peut supposer des lois similaires pour le frottement. Alors que la loi de Coulomb est gènèralement correcte en ce qui concerne les sols, il semble que le frottement des roches serait plus correctement dtudi;fini par une loi non linéaire, et si l'on applique une loi de Coulomb les facteurs de sécurité sont affectés par la valeur adoptée pour la coh´esion. Les différentes methodes de mesure du frottement sont décrites, et leur champ d'application est étudié. On tient compte également du processus d'usure et de la superficie de contact entre les surfaces en glissement. 11 semble que dans certains cas on obtienne des valeurs résiduelles de frottement aprés des glissements de faible importance. Des cavités se produisent pendant le glissement et leur comportement semble dépendre d'un facteur temps. Actuellement, les valeurs numériques de glissement et les autres paramétres des systémes joints ne sont pas certains, et une formule aussi simple rend encore de grands services. On donne un certain nombre de formules pour les facteurs de sécurité du glissement pour une ou deux surfaces planes. Pour les roches avec des joints rapprochés on peut utiliser la théorie de mécanique des sols pour les surfaces de glissement circulaires et autres. Dans ce cas, les valeurs de frottement obtenues pour les surfaces à joint unique ou les roches écrasées sont plutôt en deçà de la vérité puisque l'enchevêtrement des roches peut résulter en unimportant accroissement de la solidité.
This paper is the second of two papers that present and discusses the results from experiments where artificially created freeze-bonds made from saline ice were tested on direct shear with the freeze-bond oriented horizontally. It discusses the friction forces after freeze-bond failure and the failure energy.The friction force showed increasing linear trends with a non-zero intercept when plotted against the normal force. It shows that for low confinements Amonton's law is insufficient. For larger confinements the values of friction coefficient were in the range of previously reported measurements in ice–ice friction. A slightly decreasing trend of the frictional forces was found when the initial ice temperature increased.A Mohr–Coulomb type of model was proposed to model the ice–ice frictional stresses as function of the normal stresses. An empirical model was obtained to describe freeze-bond failure and subsequent deformation by introducing softening of the cohesion and angle of internal friction.The failure energy had similar trends to those observed for the freeze-bond shear strength when plotted against normal confinement, initial ice temperature and submersion time. Quadratic fitting to the data of failure energy as a function of freeze-bond shear strength allowed the estimation of the elastic shear modulus of the freeze-bond by applying a simple rheological model. The values found were between 2 kPa and 6 kPa which are very low compared with the shear elastic modulus for the ice blocks.
The aim of this study is to investigate the effects of surface roughness and surface energy on ice adhesion strength. Sandblasting technique was used to prepare samples with high roughness. Silicon-doped hydrocarbon and fluorinated-carbon thin films were employed to alter the surface energy of the samples. Silicon-doped hydrocarbon films were deposited by plasma-enhanced chemical vapor deposition, while fluorinated-carbon films were produced using deep reactive ion etching equipment by only activating the passivation step. Surface topographies were characterized using scanning electron microscopy and a stylus profilometer. The surface wetting properties were characterized by a video-based contact angle measurement system. The adhesion strength of ice formed from a water droplet on these surfaces was studied using a custom-built shear force test apparatus. It was found that the ice adhesion strength is correlated to the water contact angles of the samples only for surfaces with similar roughness: the ice adhesion strength decreases with the increase in water contact angle. The study also shows that smoother as-received sample surfaces have lower ice adhesion strength than the much rougher sandblasted surfaces.Research highlights▶ Ice adhesion strength correlates with water contact angle at similar roughness. ▶ Ice adhesion strengths on rougher surface are larger than on smoother surfaces. ▶ Higher water contact angle is not necessarily good for ice adhesion reduction.
The morpho-mechanical behaviour of one artificial granite joint with hammered surfaces, one artificial regularly undulated joint and one natural schist joint was studied. The hammered granite joints underwent 5 cycles of direct shear under 3 normal stress levels ranging between 0.3 and 4 MPa. The regularly undulated joint underwent 10 cycles of shear under 6 normal stress levels ranging between 0.5 and 5 MPa and the natural schist replicas underwent a monotonics shear under 5 normal stress levels ranging between 0.4 and 2.4 MPa. These direct shear tests were performed using a new computer-controlled 3D-shear apparatus. To characterize the morphology evolution of the sheared joints, a laser sensor profilometer was used to perform surface data measurements prior to and after each shear test. Based on a new characterization of joint surface roughness viewed as a combination of primary and secondary roughness and termed by the joint surface roughness, SRs, one parameter termed ‘joint surface degradation’, Dw, has been defined to quantify the degradation of the sheared joints. Examinations of SRs and Dw prior to and after shearing indicate that the hammered surfaces are more damaged than the two other surfaces. The peak strength of hammered joint with zero-dilatancy, therefore, significantly differs from the classical formulation of dilatant joint strength. An attempt has been made to model the peak strength of hammered joint surfaces and dilatant joints with regard to their surface degradation in the course of shearing and two peak strength criteria are proposed. Input parameters are initial morphology and initial surface roughness. For the hammered surfaces, the degradation mechanism is dominant over the phenomenon of dilatancy, whereas for a dilatant joint both mechanisms are present. A comparison between the proposed models and the experimental results indicates a relatively good agreement. In particular, compared to the well-known shear strength criteria of Ladanyi and Archambault or Saeb, these classical criteria significantly underestimate and overestimate the observed peak strength, respectively, under low and high normal stress levels. In addition and based on our experimental investigations, we put forward a model to predict the evolution of joint morphology and the degree of degradation during the course of shearing.
Degradations of the artificial undulated joint and the natural schist joint enable us to verify the proposed model with a relatively good agreement. Finally, the model of Ladanyi and Archambault dealing with the proportion of total joint area sheared through asperities, as, once again, tends to underestimate the observed degradation. Copyright
Barton, N., 1973. Review of a new shear-strength criterion for rock joints. Eng. Geol., 7: 287–332.The surface roughness of rock joints depends on their mode of origin, and on the mineralogy of the rock. Amongst the roughest joints will be those that formed in intrusive rocks in a tensile brittle manner, and amongst the smoothest the planar cleavage surface in slates. The range of friction angles exhibited by this spectrum will vary from about 75° or 80° down to 20° or 25°, the maximum values being very dependent on the normal stress, due to the strongly curved nature of the peak strength envelopes for rough unfilled joints.Direct shear tests performed on model tension fractures have provided a very realistic picture of the behaviour of unfilled joints at the roughest end of the joint spectrum. The peak shear strength of rough—undulating joints such as tension surfaces can now be predicted with acceptable accuracy from a knowledge of only one parameter, namely the effective joint wall compressive strength or JCS value. For an unweathered joint this will be simply the unconfined compression strength of the unweathered rock. However in most cases joint walls will be weathered to some degree. Methods of estimating the strength of the weathered rock are discussed. The predicted values of shear strength compare favourably with experimental results reported in the literature, both for weathered and unweathered rough joints.The shear strength of unfilled joints of intermediate roughness presents a problem since at present there is insufficient detailed reporting of test results. In an effort to remedy this situation, a simple roughness classification method has been devised which has a sliding scale of roughness. The curvature of the proposed strength envelopes reduces as the roughness coefficient reduces, and also varies with the strength of the weathered joint wall or unweathered rock, whichever is relevant. Values of the Coulomb parameters c and Φ fitted to the curves between the commonly used normal stress range of 5–20 kg/cm2 appear to agree quite closely with experimental results.The presence of water is found in practice to reduce the shear strength of rough unfilled joints but hardly to affect the strength of planar surfaces. This surprising experimental result is also predicted by the proposed criterion for peak strength. The shear strength depends on the compressive strength which is itself reduced by the presence of water. The sliding scale of roughness incorporates a reduced contribution from the compressive strength as the joint roughness reduces. Based on the same model, it is possible to draw an interesting analogy between the effects of weathering, saturation, time to failure, and scale, on the shear strength of non-planar joints. Increasing these parameters causes a reduction in the compressive strength of the rock, and hence a reduction in the peak shear strength. Rough—undulating joints are most affected and smooth—nearly planar joints least of all.
When Barton's JRC-JCS shear strength criterion was used for the interpretation and prediction of shear strength of natural joints, it was found that the JRC-JCS model tends to overpredict the shear strength for those natural joints with less matched surfaces. To overcome this shortcoming, a new JRC-JMC shear strength criterion is proposed in order to include the effects of both joint surface roughness and joint matching, in the form of τ = σn·tan[JRC·JMC·log10 (JCS/σn) + r]. The JMC should be set at 0.3 for any measured JMC < 0.3. It is a modification of the existing JRC-JCS criterion. The new JRC-JMC model provides appropriate fitting of the shear test results on various joints with different degrees of surface, roughness and matching, and gives a better interpretation and prediction, particularly for natural joints that do not have perfectly matched surfaces.
Ice formation and accretion may hinder the operation of many systems critical to national infrastructure, including airplanes, power lines, windmills, ships, and telecommunications equipment. Yet despite the pervasiveness of the icing problem, the fundamentals of ice adhesion have received relatively little attention in the scientific literature and it is not widely understood which attributes must be tuned to systematically design "icephobic" surfaces that are resistant to icing. Here we probe the relationships between advancing/receding water contact angles and the strength of ice adhesion to bare steel and twenty-one different test coatings (∼200-300 nm thick) applied to the nominally smooth steel discs. Contact angles are measured using a commercially available goniometer, whereas the average strengths of ice adhesion are evaluated with a custom-built laboratory-scale adhesion apparatus. The coatings investigated comprise commercially available polymers and fluorinated polyhedral oligomeric silsesquioxane (fluorodecyl POSS), a low-surface-energy additive known to enhance liquid repellency. Ice adhesion strength correlates strongly with the practical work of adhesion required to remove a liquid water drop from each test surface (i.e., with the quantity [1 + cos θ(rec)]), and the average strength of ice adhesion was reduced by as much as a factor of 4.2 when bare steel discs were coated with fluorodecyl POSS-containing materials. We argue that any further appreciable reduction in ice adhesion strength will require textured surfaces, as no known materials exhibit receding water contact angles on smooth/flat surfaces that are significantly above those reported here (i.e., the values of [1 + cos θ(rec)] reported here have essentially reached a minimum for known materials).
The temperature-dependent shear strength of ice-filled joints in rock mass considering the effect of joint roughness, opening and shear rates
- Huang
Study on criterion of peak shear strength of frozen rock joint surface
- Shen