Project

The effects of weathering on the mechanical behavior of soft rocks

Goal: Reproduce the physical weathering process in lab
Apply weathering to the geometrial
elucidate the effects of weathering on strength of geomaterial

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Mohsin Usman Qureshi
added a research item
The reduction in the shear strength of rock exposed on slope surfaces due to mechanical weathering is a ubiquitous phenomenon in regions where extreme environmental conditions prevail, i.e., repeated changes in temperature and moisture. In dealing with the slope instability problems in such regions, the long-term effects of weathering on the strength, deformation and durability characteristics of exposed rock are envisaged in this study. Therefore, in addition to conducting multiple-cycle standard slake durability tests on rock samples taken from the lithologies of Pakistan and Japan, and on artificial soft rock, the decrease in strength and stiffness is also studied by reproducing mechanical weathering in the laboratory. The reproduced laboratory weathering (RLW) is conducted with a new device that enables vacuumed saturation, freezing, thawing, drying and cooling under a maintained level of confining pressure. The decrease in strength, stiffness and durability is elucidated from the test results, which indicate that rock having a very low level of reference strain (shear strength/modulus at small strain) is resistant to RLW and slaking. Intact rock exhibits very low reference strain and this reference strain increases with an increase in the degree of weathering, which is the case of weathered rock. The decrease in the strength of rock is an important property for judging the safety of rock slopes undergoing weathering. Thus, the relation between the strength and the S-wave velocity of rock undergoing weathering is established. The authors recommend the use of this relation for a quick assessment of the strength of rock by briefly measuring the S-wave velocity of the weathered surface layer. The relationship will assist practitioners in quickly screening potentially unstable slopes.
Mohsin Usman Qureshi
added 3 research items
Negative ageing or decay of grains with time is often ignored in conventional geotechnical investigations. Geology is always vital in such a scenario but the micro-scale geotechnical point of concern is the time-dependent loss of strength and deformation characteristics. This paper presents unique data from torsional shear tests on crushed soft rocks from Yokosuka, Japan and 2005-Kashmir earthquake hit areas of Pakistan. Material being sensitive to disintegrate by water-action allowed to simulate the long-term stress-strain and volume change response under saturated conditions whereas dry tests on similar soil represent initial intact response of the material. Negative ageing is manipulated by an enormous decrease in shear strength parameters, changes in grain size curve and increase in volumetric compression. It is concluded that for long-term hazard evaluation of various geotechnical structures, the effects of loss of strength due to decay of grains with time should be incorporated in conventional analysis and design models.
Slope geometry and topographical studies possibly are not the perfect tools for landslide risk assessment in widespread tertiary sedimentary soft rocks in Japan and Pakistan because these rocks are not yet fully lithified and are highly susceptible to weathering and rainfall effects. This paper discusses the strength and deformation response of crushed mudstone to water-submergence because of its extremely high vulnerability to water induced deterioration. Drained torsional shear tests on saturated soils revealed the decay of grains and change in particle shape during consolidation and shearing stages with decrease in particle disintegration at higher confining pressures. Peak shear strengths and friction angles are considerably higher with governing dilatant behaviour for dry specimens as compared to the saturated ones. This study is a caution to conventional soil mechanics in which decay of grains and loss of strength with time are often uncared.
Muzaffarabad is situated close to the epicenter of 2005 Kashmir-Earthquake and many shallow landslides occurred during and after the earthquake. Natural breakdown of seismically-disturbed soft rocks due to weathering is a widespread ongoing phenomenon originating numerous fresh landslides and debris flows. For this reason, seismic refraction surveys, portable dynamic cone penetration tests, Schmidt hammer tests and sampling of residual soils were done in Muzaffarabad outskirts. The analysis of refraction surveys and penetration tests depicted the presence of shallow (0.5-1.5m) and highly weathered material with S-wave velocities in the range of 180 to 300 m/s. Rebound number was as high as 50 for exposed intact rocks and less than 10 at weathered surfaces. The strength and stiffness parameters determined from drained torsional shear tests seem to be largely affected by the sensitivity of soil grains to disintegrate upon saturation. The data presented in this paper can be supportive to practicing engineers for better understanding of the mechanical properties of weathered material for geotechnical analysis and design.
Mohsin Usman Qureshi
added 2 research items
Conventionally, discussion on seismic slope instability has been concentrated on the possibility of slope failure and its consequence during expected earthquakes. Recently, a concern is arising on what is called the compound effects of earthquakes and other action of nature, which is, for example, rainfall. Field investigations of seismically affected regions during a recent decade demonstrate that strong ground shaking generates cracking and other types of minor damages, even if no failure occurs, and those minor damages lead finally to slope instability. The ongoing failures in Kashmir are noteworthy. After the 2008 Wenchuan earthquake, debris flow increased in numbers in the rainy season. After the 1999 ChiChi earthquake in Taiwan, mountain slopes are significantly less stable and there are many debris deposits in valleys subject to failure. Thus, when a typhoon attacks, failure occurs profoundly. The strategy for damage mitigation should bear this fact in mind.
Mohsin Usman Qureshi
added 2 research items
Shear strength is one of the key parameters in geotechnical engineers for slope stability analysis. A slope is more vulnerable in the state when the material changes from the state of a solid material to that of granular soils as a consequence of physical weathering. However, it is not an easy task to determine shear strength of the thousands of slopes in the field. Therefore, it is desired to correlate shear strength with other parameters that can be easily measured in the field. Shear wave velocity is chosen for this purpose in the present study. For the uniformity of test data, artificial cement-treated sand (CTS) samples were utilized for the study on variation of shear wave velocity and shear strength after Reproduced Laboratory Weathering (RLW) cycles. Unconfined compression tests and triaxial compression tests were conducted at the fresh state as well as after different number of RLW cycles. As a consequence, a logarithmically linear correlation was detected between shear strength (τs) and shear wave velocity (Vs) undergoing weathering cycles. This finding is useful for risk assessment of weathered slopes
The present study is a part of a greater research project for disaster mitigation in unstable mountain slopes subjected to heavy rainfalls. The idea of disaster mitigation is composed of the following three stages. 1) Primary screening of potentially hazardous slopes on the basis of local geology and topography, which is brief. 2) Periodical simple investigation on the extent of slope instability by running S-wave velocity measurement in the surface layer and assessment of shear strength therein that enables a simple evaluation of factor of safety during heavy rains. 3) Monitoring of ground movement during heavy rainfalls in such slopes where the factor of safety suggests insufficient stability; if monitoring suggests abnormal movement, caution or evacuation order is issued to local people. Shear strength is one of the key parameters in for slope stability analysis in Stage 2). Noteworthy is that a slope is more vulnerable in the state when the material changes from the state of a solid to granular as a consequence of physical weathering. In this regard, the process of physical weathering was recreated in the laboratory by considering repeated formation of ice in rock joints, which is a typical cause of weathering and deterioration.
Mohsin Usman Qureshi
added 2 research items
A huge number of slope failures had been reported following the 2005, Kashmir earthquake of magnitude 7.6. Thorough examination of the disturbed slopes indicated that the rock is extensively weathered and lost its shear strength. Mechanism of weathering is repeated temperature and moisture changes. The authors demystify the problem by conducting the laboratory tests to observe the change in mechanical properties and durability of soft rocks due to reproduced laboratory weathering and performed seismic refraction survey and intrusive tests to delineate the extent of weathering. Laboratory tests indicated the reduction in mechanical properties due to weathering. Field studies dictated the loose/weathered layer to be of shallow extent. The authors also suggested the remedial measures for such highly weathered area.
The Slake durability test has been accepted as a standard test for assessing the weathering potential of rocks in the form of a slake durability index. But still its wide range acceptability and reproducibility has inquisitiveness. In this paper, authors addressed the potential mechanisms effecting the authenticity and reproducibility of this test and suggested some alternate interpretation approaches. In this regard, soft sedimentary rocks have been tested up to five slaking cycles using standard slake durability test apparatus. Index obtained by the initial slaking cycles was representative of the state of weathering with some inclusion of shape effects, whereas the continuation of slaking cycles gave an idea about the long-term weathering potential, i. e. rate of weathering. To avoid misleading interpretations due to shape effects, it has been recommended to perform multiple slaking cycles instead of standard 2 slaking cycles.
Mohsin Usman Qureshi
added 19 project references
Mohsin Usman Qureshi
added a project goal
Reproduce the physical weathering process in lab
Apply weathering to the geometrial
elucidate the effects of weathering on strength of geomaterial