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. 東京大学 April 3-8, 2011 Vienna, Austria 1. Introduction 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 °C) with dry and rainy seasons. The cyclic effects of the environmental agents physically deteriorate the rock to make the slopes potentially unstable. 2. Objectives 3. Reproduced laboratory weathering tests 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)). 4. In-situ tests for mechanical properties 6. Concluding remarks 1. 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. 2. A reasonable relationship between S-wave velocity and penetration Nd value was established with is a useful tool 3. As an end result, safety can be assessed by knowing the slope angle and S-wave velocity of surface weathered layer. Reference: Geophysical Research Abstracts, EGU CONTACT: Mohsin Usman Qureshi (Ph. D. Student) Geotechnical Engineering Laboratory, Department of Civil Engineering, Tokyo University 7-3-1, Hongo, Bunkyo-ku, , Tokyo, JAPAN, 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. Muzaffarabad, Pakistan Weathered sandstone 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. 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. 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/m 3 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 PKMZDS3 Typical test results from field direct shear tests Silty clay Limestone scree Weathered mudstone Dolomite Scree Limestone scree 5. Factor of safety and shear wave velocity Negative ageing By using the available data from field investigations, infinite stability analysis is performed for weathered slope by assuming slope angle of 15 o, 30 o, 45 o and 60 o, 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.. 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 Weathered surface nn  H θ  C=0 VsVs Laser scanning of surface deterioration Fresh surfaceAfter a freeze- thaw cycle Typical freeze-thaw test results Seismic refractionField direct shear 32cm 8cm No. of drops for 10 cm of penetration is recorded as N d value. V s and H  and H  Cone penetration Seismic refraction analysis was done by using Intercept time method Typical test results from seismic refraction and dynamic cone penetration tests JPNGO JPKOBE PKMZD 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.