Slope failure and landslide mechanism, Murree area, north Pakistan

Abstract

The area around Murree town is inherently vulnerable to mass movement processes, most commonly due to land sliding. It is characterized by high degree of erosivity and erodibility due to both natural and man induced factors. The area is part of a young orogenic belt that is experiencing one of the world's most rapid uplift rates, and is located in the near vicinity of major thrust faults. Shear zones associated with still active faults, such as Murree Thrust and also the folding generated due to thrusting induce an inherited weakness and natural instability in the rocks of the area. This compounded with ever increasing population pressure, deforestation, large-scale construction work in the form of residential and commercial buildings and road network have further aggravated the problem. Lithologically the area is comprised of shale, siltstone and sandstone belonging to the Oligocene-Miocene Murree and Kuldana formations.

In order to study the landslides and other mass movement problems in Murree, an area around the main town and along the Murree-Kashmir Highway is selected for detailed analysis of the problem. The eastern slopes of the Murree ridge, and the road section between Jhika Gali and Aliot village are worst affected by the land sliding. Around Murree town, major landslides such as Shifang Hotel, Midway and MIT landslides, located on the upslopes of the Jhika Gali-Lawrence College Bypass road are caused by inadequate drainage system from the city centre (Murree ridge). In most part of the area around Murree ridge, creep is a common phenomenon in thick colluvium coverdue to obvious loss of vegetation. The area between Jhika Gali and Aliot village is also affected by land sliding, and Some of the largest landslides such as Aliot, Birgran and Kasseri landslides have inflicted great damage to farm and forestland. This area is located in the footwall of the MBT and is deformed by folding and termination splays associated with it. Most of the landslides initiated at the colluvium-bedrock interface, but once initiated the tension cracks spread into the bedrock and therefore causing the failure in it. Major landslides in the area exhibit a complex array of joint/fracture pattern both parallel and perpendicular to the bedding, and in various ways contribute to the mass movement.