My Overview on Slope Stability

Slope Stability

Evaluating the stability of slopes in soil is an important, interesting, and challenging aspect of civil engineering. Concerns with slope stability have driven some of most important advances in our understanding of the complex behavior of soils. Extensive engineering and research studies performed over the past 70 years provide a sound set of soil mechanics principles with which to attack practical problems of slope stability (Duncan & Wright, 2005).

Slope stability addresses the tendency of soil masses to attain an equilibrium state between the strength of the soil and the force of gravity.  Department of Irrigation and Drainage Malaysia (DID) mention that their facing of slope stability problems which often occur in the construction of embankment over soft soils, and the instability of waterway slope (e.g. river and pond) due to seepage, drawdown, or erosion by flowing water. Placement of stockpiles, heavy equipment, or other surcharges may also cause instabilities of the slope, particularly during construction stage. In general, altered slope, whether man-made or natural need to be analyzed and checked to ensure that it has adequate factor of safety against slope failure.

Guidelines from Department of Agriculture (DOA) and Malaysia Agricultural Research and Development Institute (MARDI) (1993), and DOA (2000) gives a definition which any area on 300m above sea level, its can consider as ‘highland’, where there have a change in temperature, topography and forest type. Meanwhile, slope more than 25^o where consider as a ‘steepland’.

 Slope Classification

Class	Topography	Degree of Steepness
C1	Flat	0o – 2o
C2	Undulating	2o – 6o
C3	Wavy	6o – 12o
C4	Hilly	12o – 20o
C5	Very Hilly	20o – 25o
C6	Steep	25o – 30o
C7	Very Steep	>30o

Table 1 Slope Classifications

Type of Slope Instabilities

In general, slope stability problems commonly encountered by DID can be categories into three (3) types (DID Manual, 2009), namely:

(a)    Infinite Slope Failure – A slope that extends for a relatively long distance and has a consistent subsurface profile may be analyzed as an infinite slope, the failure plane for this case is parallel to the surface of the slope and the limit equilibrium method can be applied readily;

(b)   Sliding Block Failure – Occurs when the wedge type of sliding mass that cut through the fill and a thin layer of weak soil essentially moves as a block; and

(c)    Circular Arc Failure – occurs when the ground sinks down and adjacent ground rises and the failure surface follows a circular arc.

Slope Stabilization Methods

Slope stabilization methods generally aim to reduce driving forces, increase resisting forces, or both. Driving forces can be reduced by excavation of materials from appropriate part of the unsuitable ground and drainage of water to reduce the hydrostatic pressures acting on the unstable zone. Resisting forces can be increased by introducing soil reinforcements, such as soil nails and geo-synthetic materials, and retaining structures or other supports.