2.3  Effect of Water

The effect of water on the slope can be considered into two fold. One is ground water or aquifer below the surface that generates porewater pressure and the other is rainwater infiltration that seeps through surface and flows along the slope generating water pressure. It is related to the surrounding precipitation levels, topography, nearby water masses, and the geo-hydrological characteristics of the rock mass (Sj÷berg, 1999).

In medium to hard rock, water occupying the fractures within the rock mass can significantly reduce the stability of a rock slope. Water pressure acting within a discontinuity reduces the effective normal stress acting on the plane, thus reducing the shear strength along that plane. If a load is applied at the top of a slope, the pore pressure increases. Such a load can lead to immediate failure of the slope if it exceeds its shear strength of slope. Water filling in discontinuities can result in lowering of stability condition for natural or artificial slopes. Figure 4 shows a rock blocking resting on an inclined plane and separated from the upper part of the slope by a sub vertical discontinuity plane. The water applies horizontal and vertical pressure along the discontinuities. The uplift force U is also developed due to water at the surface between the block and its base. The water pressure increases linearly with depth down to the intersection of the sub vertical plane with the base and linearly decreases from the intersection point to the lower edge of the block in contact with the surface where the water pressure is zero (Gaine, 1992).  

Addition of water from rainfall and snow melt adds weight to the slope. In addition to it ground water also exists nearly every where beneath the earth surface. Such water fills the pore spaces between the grains or fractures in the rock. Such water can seep into discontinuity present in the rock mass replacing the air in the pore space thus increasing the weight of the soil. It leads to increase in effective stress resulting into failure of the slope. Figure 5 depicts the effect of water content in the rockmass on factor of safety of the slope found on the different slope angles. It depicts for an increase in slope angle from 600 to 800, the factor of safety of the slope under dry rock mass conditions reduces from value of 2 about 1. Whereas, under the saturated rockmass conditions increase in the slope angle makes it unstable when value exceed 700.


 

Figure 4: Diagram of water pressure acting on a block

 

 

Figure 5: Variation in Factor of Safety with slope angle (after Hoek and Bray, 1977)


 

 

In soil and mine waste dump in surface mines, if the unconsolidated material is dry or non-saturated, an increase in load compress the air in the pore spaces thus compacting the mass and bringing grains or rock fragments closer together which increase its shear strength. However, when a rock mass is saturated, an increase in external pressure leads to an increase in the pore pressure, as water is relatively incompressible. This increase in pore pressure has a buoying effect, and can be enough to support the weight of the overlying rock mass, thereby reducing friction and the shear strength.

Unconsolidated sediments behave in different ways depending on whether they are dry or wet (Terzaghi, 1943).  Dry Unconsolidated grain from a pile with a slope angle control by the angle of repose (figure 6a) which generally varies between 30-370. In contrast to this, a slightly wet unconsolidated material exhibits a very high angle of repose because surface tension between water and the grains tends to hold the grains in their places (figure 6b). This is due to capillary attraction resulting into surface tension which holds the wet material together as a cohesive mass. However, when the material is saturated with water the angle of repose reduces substantially (figure 6c). This is because the water gets in between the grains eliminating grain to grain frictional contacts.

 


 

 

Figure 6: Effect of water content in unconsolidated grain of piles.