Remedy of slope failures along mudstone surfaces requires proper understanding of the shear strength characteristics of that material. Results of studies show that the residual strength of mudstones could be as low as 10 degrees in terms of the effective internal friction angle. Depending on the height and the geometry of the slope, stabilization measures include earth buttress, stabilizing berm, pile wall and anchored wall. This paper presents case histories on the application of the above-mentioned measures.


The Second Northern Freeway is a major transportation development in the northern Taiwan. Extending from Hsihchih to Hsinchu, this 6-lane freeway is 117 km in length. The route comprises 20 interchanges, 40 km of viaduct sections, 8 km of mined tunnels and 70 km of earthwork sections. The construction commenced in 1987 and the freeway became fully operational in 1997. The route runs through the north portion of Western Foothills Area in Taiwan. Major rock formations along the route are composed of the Cholan formation of the late Pliocene and the Toukoshan formation of the early Pleistocene epoch. There are tectonic faults and folds located in close proximity to the route. The Cholan formation is mainly composed of sandstone intercalated with mudstone. The mudstone layer generally has a thickness of less than 1 m. The ground surface is generally covered with a mantle of yellowish brown, highly to completely weathered, sandstone or mudstone which has the thickness of about 10 m. It is commonly recognized that as long as a weak discontinuity of the rock is exposed during excavation, and that the discontinuity is dipping in the same direction as the slope, the overlying block has the tendency of sliding along the weak plane. Provisions of positive preventive measures at the design stage can avoid these unstable conditions.

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