Experimental Study on Shear Mechanical Properties of Heterogeneous Concrete Surfaces Under Freeze–Thaw Cycling

Most of the natural and compacted fine-grained soil slopes that are in saturated or unsaturated condition undergo a large deformation prior to reaching failure conditions. Such slopes should be designed taking account of their strain-softening behavior using the residual shear strength (RSS) parameters. In this paper, the slope stability of a recently reactivated Outang landslide near the Three Gorges Dam in China is analyzed based on the RSS parameters of unsaturated soils. In addition, comparisons are provided in the FOS values of slope using both the peak shear strength (PSS) and RSS parameters. Firstly, a series of site investigations of the hydrologic and geologic conditions, ground surface displacements and cracks were described. The PSS and RSS behaviors of the sliding soils derived from a series of direct shear test results performed on saturated and unsaturated soil specimens are summarized. Secondly, a series of slope stability analysis were conducted considering the precipitation and Yangtze River water level variation within a representative period of 7 months, based on thePSS and the RSS properties. In this study, three different scenarios were considered, which include: i) considering only the precipitation with a constant water level; ii) considering only the decrease in water level without rainfall; iii) considering the combination of precipitation and decrease in water level. In each scenario, four steps were included to calculate the values of factor of safety (FOS) at different times. 1) A steady-state seepage analysis was conducted with a constant total head at 525 m on the left boundary and 175 m on the slope surface below the Yangtze River water level. The initial pore water pressures were simulated in the slope under no precipitation and variation of water level. 2) A specific boundary condition was applied on the slope surface to model the precipitation and Yangtze River water level variation. A transient seepage analysis was conducted to calculate pore water pressures at different times based on the initial pore water pressures. 3) The FOS values at different times were calculated by the Morgenstern-Price method taking account of the variation of pore water pressures at different times, using the peak shear strength (PSS) parameters. 4) The last step was repeated replacing PSS parameters with RSS parameters. The RSS parameters were lower than the peak values from laboratory’s direct shear test results for the soils in the sliding zones. The reduction in shear strength from peak to residual state under unsaturated soil condition was greater than that for a saturated soil. The FOS decreased almost linearly with time for the scenario in which only the influence of rainfall infiltration was considered. However, the total reduction in the FOS was relatively small. The FOS decreased rapidly at a linear rate with respect to time with a decrease in water level for the scenario in which Yangtze River water level decrease was considered. The FOS reached to a relatively constant value after Yangtze River water level reached the lowest value. The decrease in Yangtze River water level was the dominant factor that contributed to a reduction in the FOS. The FOS was strongly dependent on the development of the phreatic line after the Yangtze River water level reached the lowest value. The FOS calculated by RSS (i.e. FOSR) is less than unity; they were approximately 16% lower in comparison to that calculated by PSS (FOSP). If PSS parameters were used, the slope would still be stable even under the combined influence of precipitation and Yangtze River water level decrease. These results are inconsistent with the field observations. For this reason, the RSS parameters should be taken into account to evaluate reliably the slope stability of the Outang landslide.

An experimental program has been undertaken to assess both peak and residual shear strength parameters of statically compacted, moderate plasticity clayey soil under suction-controlled conditions, resulting in a defined set of suction-dependent peak and residual failure envelopes over a relatively wide range of suction states, from 0 to 300 kPa. The experimental program was accomplished in a servo/suction-controlled ring shear apparatus, which is suitable for testing unsaturated soils under large deformations via the axis-translation technique. Test results substantiate the crucial role that has been observed to be played by the imposed matric suction on the residual shear strength of compacted clayey soils. For the range of net normal stress (0-200 kPa) and matric suction (0-300 kPa) states investigated, the increase in either peak or residual shear strength, with increasing matric suction, was found to be manifestly nonlinear. Furthermore, a distinct correspondence was observed between the nonlinearity of the peak shear strength envelope, with respect to increasing matric suction, and the soil-water retention properties of the clayey soil. Results, in general, suggest that a conceptual residual shear strength framework for unsaturated soils, similar to that postulated for peak shear strength, can eventually be formulated as more experimental evidence of this kind is made available.

A study of the internal shear strength of adhesive-bonded, stitch-bonded, and needle-punched geosynthetic clay liners (GCLs) is presented. Tests were performed using a large direct shear machine capable of measuring peak and residual (or near residual) shear strengths. For each product, failure occurred at the woven geotextile/bentonite interface and excess pore pressures remained zero on the failure plane during shear. The peak shear strength of the needle-punched GCL increased significantly with increasing normal stress because of the frictional connection of the reinforcing fibers. The peak shear strengths of the adhesive-bonded and stitch-bonded GCLs showed smaller corresponding increases. The residual shear-strength failure envelope was essentially independent of product type. A two-stage procedure for specimen hydration is described, which reduced the required in-machine hydration time to reach equilibrium conditions. For the reinforced products, small decreases in peak and residual shear strengths ...

Experimental study on direct shear behavior of frozen soil–concrete interface

Understanding the shear strength and failure mechanism of a rock joint is essential in rock engineering. This study performed a series of direct shear tests and discrete element modelings on artificial joint specimens to investigate the effect of roughness [randomly generated joint profiles with joint roughness coefficient (JRC) = 20, 19.6, and 10] on the joint strength. The results of the numerical simulation were consistent in the peak shear strength with the laboratory tests and Barton’s equation. From a microscopic viewpoint, the rock joint’s peak and residual shear strength were mainly mobilized from the friction property of such a joint profile. The contribution of friction to the shear strength at the residual stage was reduced because of dilation behavior and decreasing contact area along the joint surface. Therefore, the mobilized friction angle decreased from the initial basic friction angle to a certain value depending on the initial JRC value. The mobilized JRC of a rock joint was found to be related to the initial JRC, the unconfined compressive strength (UCS) of joint material, and the applying normal stress. The surface of joint models with high UCS is less damaged than that with low UCS. Finally, a new model for predicting the residual shear strength of a rock joint was also proposed, which can be applied for the joint using both randomly generated profiles and Barton’s standard profiles.

The mudded weak interlayer is a geotechnical sandwich material exhibiting strain softening behavior, which plays an important part in the slope stability. The present work primarily focuses on the shear strength of the mudded weak interlayer in rock slopes. To determine the peak and residual shear strengths of the mudded weak interlayers, the particle flow code (PFC) is used to simulate the failure behavior during the direct shear tests. Laboratory investigations including uniaxial compression test, SEM, and 3D deformation measurement are employed to calibrate the essential micro parameters of the mudded weak interlayer during the simulation process in PFC. The numerical model is built based on these parameters and both the peak and residual shear strengths can be predicted from the model. The prediction results show that the peak and residual internal friction angle are 19.36° and 14.61°, while the peak and residual cohesion are 22.33 kPa and 2.73 kPa, respectively. Moreover, to validate the obtained peak and residual strengths, the results are compared with literature data. The peak and residual shear strengths of the mudded weak interlayer can serve as an important benchmark to evaluate the stability of side slopes and provide guiding suggestions for their reinforcement.

Dredging operations in European harbors for maintenance of navigable water depth produce vast amounts of harbor mud. Between 2005 and 2007, the second largest harbor construction project in Germany was designed as a pilot study to use dredged harbor mud as backfill material to avoid expensive disposal or ex situ treatment. During this project, a partial collapse of the backfill highlighted the need for an improved assessment of undrained shear strength of naturally occurring liquid harbor mud. Using vane shear testing, this study evaluates the effect of shear rate on the undrained shear strength of harbor mud. It is shown that measured values for both peak and residual shear strength are significantly influenced by shear rate effects. Furthermore, the influence of shear rate on the peak shear strength is found to be independent of water content while the influence of the shear rate on the residual shear strength strongly depends on water content. New shear rate dependent correction factors μ are proposed ...

A preliminary experimental program has been undertaken to assess suction-controlled peak and residual shear strength properties of a statically compacted, moderate plasticity clayey soil. The experimental program was accomplished in a fully servo/suction-controlled ring shear apparatus, suitable for testing unsaturated soils under large deformations via the axis-translation technique. The test results highlight the important role played by matric suction on residual shear strength behaviour of compacted clayey soils. For the range of net normal stresses and suction states investigated, the increase in peak shear strength with increasing suction was found to be significantly nonlinear for the clayey soil. A distinct correspondence was also observed between the nonlinear nature of peak shear strength envelope, with respect to increasing matric suction, and the soil-water retention curve. The residual failure envelope, however, remains reasonably linear for the range of suction values induced in the present work via axis-translation.

Effect of freeze-thaw cycles on engineering properties of biocemented sand under different treatment conditions

In designing a foundation of dam, a tunnel or an underground structure, it is necessary to estimate the mechanical properties of rock mass. However, rock mass is not a uniform body, but contains joints and other weak planes, and these weak planes reflect on the mechanical properties of the rock mass. The strength value obtained in laboratory leads to an over-estimation of strength of rock mass, because rock specimens are almost collected from rock mass between weak planes.This study was undertaken for the purpose of estimating shear characteristics of rock mass containing weak planes, and a direct shear testing machine, which is applicable in the study, has been developed. As a preliminary research, the process of forming shear surface and the shear behaviours of rock specimens were investigated on OGINO tuff.The results in this study are summarized as follows;1) A distribution of the principal stresses in rock specimen was analyzed by F.E.M.. In this analysis, the tensile stresses appear near the shear edge of the shear testing machine. From this result, it is thought that a crack initiation in rock specimen is originated by the tensile stress, and the shear stress is then concentrated at the tip of the tension crack. Consequently, the crack may progress along the direction of shear load and the shear surface becomes undulated.Also, as normal stress is inceased in the direct shear test, peak and residual shear strengths take higher values and shear displacement at the point of the peak shear strength is larger.2) The direct shear tests were conducted on the rock specimens containing the artificial discontinuities.From the results of the shear tests which were carried out at a constant normal stress, it is found that peak shear strength takes remarkably low value in the region of θ=45°.On the other hand, it is found from the results of the shear tests done under the confined normal displacement that peak shear strength takes high value in the region of θ=45°.From the above findings, it is clear that shear behaviours of rock containing discontinuities are affected by the angle (θ) between the direction of shear load and that of discontinuities, and the condition of restraining normal displacement.

Chapter 7 - Modeling of unsaturated soils slopes considering the residual shear strength behavior

Shallow slope failure repeatedly occurs in many highway slopes in Mississippi due to the abundance of expansive Yazoo Clay. The highly plastic clay soil undergoes repetitive wet–dry cycles, which reduces the shear strength to fully soften state. The current study focused on the progressive change in shear strength and the safety factor of slopes constructed out of Yazoo Clay. Undisturbed and remolded specimens were used to determine the peak, fully softened and residual shear strength, with effective normal stresses of 25, 50, and 100 kPa. The variations in shear strength were investigated with the 2D slope stability analysis software, which uses the Finite Element Method, Plaxis 2D. A highway slope in Jackson, MS was considered as the reference slope. Different rainfall volumes, 70.8 mm (2.78 in.) to 312.4 mm (12.29 in.), with a rainfall duration (30 min–7 days) based on 100-year return periods of Jackson, MS were utilized. Furthermore, three slope ratios 2H:1V, 3H:1V and, 4H:1V were selected for this study. The safety factor of the slope was determined based on peak shear strength soil test data. Later, the topsoil layer, which gets weathered within the active zone due to the repeated wet–dry cycle, was varied to fully softened and residual shear strengths. The slope stability analysis results showed that the safety factor reduces progressively from peak to residual shear strength. In addition, the factor of safety was critical when the soil reached its fully softened shear strength for 2H:1V and 3H:1V slopes with progressive rainfall. On the other hand, the 4H:1V slope reached failure at the residual phase with the presence of rainfall.

The effect of weathering processes in decreasing the shear strength of clay shale had been done in this study. The drying process of clay shale with sunlight in the laboratory up to 80 days had been conducted to create the conditions of weathered sample. The peak and residual shear strength parameters of unsaturated and saturated clay shale were obtained from triaxial laboratory test, and all samples were tested on each 8 days of weathering process. Decrease of shear strength in peak and residual condition was obtained during 80 days of the drying process. The residual shear strength parameters were distinguished between residual shear strength without stress release and with stress release of confining pressure. The results up to 80 days of unsaturated clay shale showed that the cohesion at peak stress conditions reduced to 30 % based on initial shear strength before the occurrence weathering, while the internal angle friction reduced to 64 %. Residual cohesion without and with stress release reduced to 4 % and 1 %, respectively while residual internal angle friction without and with stress release reduced to 15 % and 5 %. Similar situation also occurs for the saturated clay shale samples.

Current design guidelines for geosynthetic-reinforced soil structures disagree over the shear strength parameters that should be selected to characterize the backfill material. Most geosynthetic reinforcing materials are classified as extensible inclusions for almost all practical applications. The extensible nature of geosynthetic reinforcements has led to the recommendation by several agencies and reinforced soil designers toward the use of the residual shear strength instead of the peak shear strength for design. However, common practice in the US has been the use of the peak shear strength. The main purpose of this paper is to provide experimental evidence regarding selection of either peak or residual shear strength to characterize the backfill material for the design of geosynthetic-reinforced soil structures. Specifically, experimental results from reduced-scale models tested in a geotechnical centrifuge indicate that the stability of geosynthetic-reinforced slopes is governed by the peak soil shear strength.

Soils with Extremely High Plasticity (EHP) are improper for structural applications due to their low shear strength and high compressibility. Therefore, the development of soil stabilizers has become a primary focus in the efforts to enhance the engineering properties of soil, thus making it suitable for construction. Recent advancements in sustainable engineering have drawn attention to using environmentally friendly stabilizers. Bamboo Leaf Ash (BLA), obtained from abundant bamboo resources, offers a great potential for soil stabilization. With silica content up to 70% as one of its main chemical components, BLA has significant potential as a pozzolanic material for soil improvement. In this study, the residual shear strength of cohesive soil under drained conditions is measured and the optimal BLA mixture variation for improving soil stability is determined. Bentonite was used as the soil sample, and the residual shear strength was evaluated utilizing a ring shear test under drained conditions. Torque was applied to the remolded specimen to generate a shear plane until a constant shear strain was achieved, representing the residual shear strength. The results revealed that the optimal variation for enhancing residual shear strength is achieved at a 5% BLA mixture, where a balance is found between plasticity reduction and soil stability improvement. The experiment results also demonstrate that BLAs can significantly enhance the engineering properties of bentonite by reducing the Plasticity Index (PI) and increasing both peak and residual shear strength.