The movement behaviour of the Pos Selim landslide

Abstract

This dissertation is concerned with the movement behaviour of the landslide at Ch 23+800 Simpang Pulai-Lojing Highway, Pos Selim, Malaysia (AML, 2007) and makes use of the results of displacement monitoring within the period October 2003 to December 2006. The displacement monitoring involved total station surveys at about weekly intervals of the precise positions of 11 markers on the landslide and one on the crown of the landslide. The apparent movement of the latter, notionally stationary, point at the crown allowed assessment of uncertainties in the coordinate data. Two aspects were studied: marker displacements viewed in plan and section and marker velocity variations as a function of time. The spatial displacement patterns were interpreted by reference to the current understanding of the landslide mechanism (Malone, 2014). Temporal velocity patterns were compared with the temporal rainfall patterns, as measured at the Cameron Highlands meteorological station, for want of a reliable raingauge at site.

The coordinate plan and section plots reveal that motion generally follows an overall constant bearing and dip but significant deviations from straight line motion began at the top of the main body of the landslide in 2005 and at the south toe zone in 2006. Close inspection reveals that in plan, motion throughout is not precisely in a straight line, but shows small undulations, which may relate to survey error. All markers show sudden deviations in the period July to October 2006 which may also relate to survey error.

The velocity plots show post-failure deceleration until about February 2004. Subsequent motion occurred largely in a sequence of long seasonal surges, associated with the temporal rainfall pattern. No summer surge took place in 2006 though the usual summer rainfall pulse did occur. It is conjectured that progressively higher rainfall is needed to trigger a surge as time goes by and the landslide slows down. There was a short surge in late October and early November 2004 which appears to be unrelated to rainfall and is attributed to the simultaneous removal of ground at the Northern Spur with a volume of about 100,000 m^3.