Safety assessment of weathered slopes by measuring shear wave velocity

1. Seismic refraction analysis was done by using Intercept time method No. of drops for 10 cm of penetration is recorded as Nd value. 8cm 32cm t and H t Vs and H Seismic refraction Cone penetration Field direct shear Weathered sandstone Muzaffarabad, Pakistan JPNGO PKMZD JPKOBE Silty clay Limestone scree sn t H Weathered mudstone Weathered surface Dolomite Scree Vs Fresh surface After a freeze-thaw cycle Limestone scree C=0 g, f Typical test results from field direct shear tests θ Laser scanning of surface deterioration Negative ageing Safety assessment of weathered slopes by measuring shear wave velocity Mohsin U. Qureshi, Ikuo Towhata and SuguruYamada Department of Civil Engineering, The University of Tokyo, Tokyo, Japan. April 3-8, 2011 Vienna, Austria 東京大学 1. Introduction 3. Reproduced laboratory weathering tests 4. In-situ tests for mechanical properties 5. Factor of safety and shear wave velocity Change in mechanical properties is studied by subjecting soft rocks to freeze-thaw cycles in a triaxial system which is capable to maintain a temperature range of -5 to 45°C under pressure in the confining cell. Field direct shear tests, Dynamic cone penetration tests and seismic refraction tests were performed to evaluate shear strength, shear wave velocity and depth of surface weathered layer. Tests were performed at four localities in Japan (Yokosuka (JPYZ), Nagano (JPNGO), Izu (JPIZU) and Kobe (JPKOBE)) and two sites in Pakistan (Muzaffarabad (PKMZD) and Taxila (PKTXL)). The reduction in shear strength of slope surfaces due to weathering is ubiquitous phenomenon in the regions where extreme environmental conditions prevail i.e. repeated change of temperature and moisture. Natural disasters such as earthquakes or heavy rainfalls thwart the potential stability of weathered slopes. Close examination of rock slope surfaces in Kashmir, Pakistan (hit by 7.6 magnitude earthquake in 2005) indicated that the rock is highly weathered and lost its shear strength. The area is also under the cyclic temporal effects (in winter temperature falls below 0°C and in summer average temperature is 35-40°C) with dry and rainy seasons. The cyclic effects of the environmental agents physically deteriorate the rock to make the slopes potentially unstable. By using the available data from field investigations, infinite stability analysis is performed for weathered slope by assuming slope angle of 15o, 30o, 45o and 60o, for both wet and dry conditions. The calculated factor of safety is shown in a relationship with the measured S-wave velocity in the field which is measured in dry conditions.. A typical freeze-thaw cycle consisted of freezing the partially saturated rock to a maintained temperature of -6°C. Freezing allows the water in matrix porosity and cracks to expand by 9% in volume, creating sufficient pressure which is if greater than the tensile strength of rock, results in expansion of pores and widening of cracks and joints. Freezing is followed by thawing up to a maintained temperature of 45°C. Soft rock specimen having a dry unit weight of 16-17kN/m3 was saturated under vacuum (-100kPa). These tests were performed at confining levels of 30, 60 and 100kPa to study the effects of confinement on mechanical deterioration of soft rock during freeze-thaw process. Plot of normalized stiffness against freeze-thaw cycles indicated that confining pressure resists the deterioration due to freeze-thaw process. 32cm Moreover the relationship between slope and S-wave velocity is developed for the FOS=1 from wet and dry cases and stable, potentially stable and unstable zones are marked. For some recent problems, slope angels and S-wave velocity of some jeopardizing slopes are verified . 6. Concluding remarks In dealing with the slope instability problems for such regions, present as well as future mechanical properties of those slopes have to be elucidated. Therefore, envisagement of negative ageing behavior of geo-material in laboratory, and elucidation of in-situ mechanical properties and depth of weathered surface layer in field takes precedence. • Freeze-thaw weathering in laboratory deteriorates the stiffness of soft rock. However confinement thwarts this deterioration which is rational with natural weathering process taking place at shallow depths. • A reasonable relationship between S-wave velocity and penetration Nd value was established with is a useful tool • As an end result, safety can be assessed by knowing the slope angle and S-wave velocity of surface weathered layer. Typical test results from seismic refraction and dynamic cone penetration tests 2. Objectives • Reproduce the mechanical weathering process in laboratory to evaluate the change in mechanical properties at various confining levels. • Propose a method to evaluate the safety factor of weathered slopes by measuring the shear wave velocity in field. Typical freeze-thaw test results 7. Acknowledgement The University of Tokyo and the Ministry of Education, Culture, Sports, Science and Technology (MEXT: Government of Japan) are gratefully acknowledged for the research facilities and financial support PKMZDS3 CONTACT: Mohsin Usman Qureshi (Ph. D. Student) Geotechnical Engineering Laboratory, Department of Civil Engineering, Tokyo University 7-3-1, Hongo, Bunkyo-ku, 113-8656, Tokyo, JAPAN , Email: qureshi@geot.t.u-tokyo.ac.jp Reference: Geophysical Research Abstracts, EGU2011-1576-4