To understand the role of water in the stability of slopes, there is one thing we need to know. Slopes collapse when gravitational pull overcomes the soil resistance to shear (deformation by sliding between layers). Soil resistance (shear strength) reduces when water pressure increases inside the soil. Rainfall seeping into the slope and leakage from drains cause water pressure to increase. Consider a slope shown on the right. Let us assume that this slope was constructed properly and it is presently stable.
Let us also say that a failure takes place along the black arc of a circle shown, labeled failure surface. By knowing the property of soil in the slope, it is possible in theory to estimate the size and location of this failure surface. The mass of the soil lying above the failure surface, shown in yellow, will try to slip and friction forces generated by the soil along the failure surface, referred to as slip circle (black arc) will try to resist the soil mass above from slipping.
Let us look at the forces in action. The big red arrow represents the weight of the soil mass which generates forces causing the slope to fail. The small red arrows along the slip surface represent the forces causing the soil mass shown in the yellow to slip along the slip surface. Forces preventing the soil mass from slipping are the resisting forces generated by the soil below, represented by the blue arrows.
The slope is stable so long as the resisting forces below the black arc, represented by blue arrows, are greater than that those represented by the red arrows above this arc. That is, the resisting forces are greater than those trying to cause the slip. Almost all slopes are designed for resisting forces at least one and a half to twice those causing slip to occur. Engineers call this a Factor of Safety of 1.5 to 2.0.
When for any reason, the forces represented by the red arrows become the same as those represented by the blue arrows, the slope is said to be in critical state, and failure can occur at any time. When the forces represented by the red arrows become greater than the forces represented by the blue arrows, the soil mass above will move and slip will occur.
Therefore it is important to understand what causes this situation which leads to the forces represented by the red arrows to increase.
Again, note the mass of soil which will slip, the factors causing slip to occur (the red arrows) and the forces preventing slip from occurring (the blue arrows).
As long as forces represented by the blue arrows are greater than those represented by red arrows, the slip is not likely to occur.
However, when the slope is loaded on the top by placing rubbish such as construction debris, the weight on the yellow soil mass is increased. The combined weight of the rubbish and that of the soil mass are now greater than forces resisting the slip and can generate forces large enough to cause the slip.
In another scenario, cracks such as tension cracks, bad drainage or bad construction of the slope allow water to enter the soil in the slope. This adds weight of water to the weight of soil in the yellow soil mass making it heavier. At the same time, due to too much water pressure in the soil, the resisting forces become smaller. Under these conditions, slip will occur.
The surface where probable slip will occur can be estimated by engineers using computers and information obtained with respect to strength characteristics of soils in the slope. However, these design estimates do not take into account bad construction practices or bad maintenance.
Failure conditions discussed here are just some of a number of different modes by which a slope may fail. Geology and the nature of the ground in and under the slope also play a role in its stability.
SO WHAT ARE THE WORST CONDITIONS FOR A SLOPE TO FAIL...
