NAME : Srithar A/L Krishnan

CLASS : 5 PKA 2

NRIC : 930128-10-6009

SCHOOL : SMT Klang

TEACHER’S NAME : Pn.Saliza Bt. Sahud

Factors Affecting Slope Stability

Factors That Influence The Stability Of Slope

Load Soil cohesion

Drainage Pore- water pressure

Gradient Geology

1. LOAD

Forces acting on the failure plane are the slope load (G) and the applied load (W) (if there is any). W is in a stable condition when resisted by shear stresses acting along the failure surface. If the movement of the soil mass occurs due to the acting load or the soil, slope failure may occur. Figure 6.24 shows the forces considered in the slope stability analysis.

FIGURE 6.24

2. SOIL COHESION

Slope from cohesive soil is more stable because the slope has strong bonding forces among the soil particles. These bonding forces form the shear strength as the result of adhesion and friction and friction among the soil particles. Soil particles are bonded by friction and adhesion forces and are capable to resist failure or mass sliding on its shear surface. The shear strength for slope of non-cohesive soil such as sand and gravel will only depend on the friction force among the particles. Friction force without adhesion sometimes is less capable to withstand the weight of the soil and the applied force that causes the slope to slide. Therefore, slope of cohesive soil can be steeper than slope of non-cohesive soil.

3. DrainageMoisture in slope is due to the water content in the soil. The existence of water either static or dynamic can weaken the bond among soil particles. Rain water that flows through the slope surface or seeps through the soil particles increases the moisture content and the ground-water level. Rain water flow and seepage on slope are shown in Figure 6.26. The shear strength of soil is indirectly proportional to the moisture content of soil. The increases in moisture content increases the weight of soil and reduces the shear strength of the soil. To prevent slope failure or sliding of the soil mass, surface runoff and seepage must be drained properly.

Figure 6.26

Drainage

4. PORE-WATER PRESSURESlope in a normal condition has moisture content and pore-water pressure that can

help in strengthening the bond among soil particles as illustrated in Figure 6.27(a). In other words, the inter-granular stress (effective stress) is in normal condition. When it rains, the rain water and surface runoff will seep into the voids and poses of soil particles. On top of increasing the moisture content, rain also collects water, saturates the slope and increases the existing pore-water pressure in the soil. The excess pore-water pressure will cause the inter-granular stress to reduce and the bond among soil particles become loose. This weakens the soil strength of the slope as shown in Figure 6.27(b).

5. GradientThe steepness of a slope is called gradient. The steepness depends on the type and strength of the sloping soil especially its cohesion. For soil with high cohesion, the gradient can be built steeper, where else, less cohesive soil, the gradient built more gentle to balance the action of applied load and weight of soil with the shear force of soil parallel to the slope surface. Slope in a non-cohesive soil which is built very steep may result in a landslide caused by the agents of weathering.

6. Geology

Slope stability is also influenced by geologic factors such as soil stratification, weathering process and soil erosion. Soil stratification as shown in Figure 6.28 causes slope made in hillside A to be more stable compared to slope made in hillside B. This is due to the direction of soil sliding on the stratum is towards a lower area. The potential for slope A to fail is less than at slope B. Weathering process and erosion on the slope surface can change properties and strength of soil through the following processes:a) Soil mass on the surface of the slope will disintegrate through the action of rain

water, surface runoff, blowing wind and changes in temperature.b) Chemical reaction among original soil particles, water, oxygen and organic acid on

the surface of the slope forms new particle property and affects the particle bonding of the original soil.

c) Erosion of slope is due to river of natural agents such as ice erosion and wave action.

Figure 6.28

By : Srithar A/L Krishnan : srithar_28@hotmail.com : srithar _128@yahoo.com.my