Infiltration through an unsaturated zone is vertical and causes no positive pore pressures. If the infiltrating rainfall it, during its descent encounters a material of lower permeability, flow will be impeded if the permeability of this lower zone is less than the rate of infiltration. Under this situation, a perched water table is formed on the surface of the impermeable zone, and a lateral flow takes place along the upper surface of the impermeable zone. Below the impermeable zone, the infiltration rate reduces to the value of permeability of the zone.
When the infiltrating rainfall meets the groundwater table (phreatic surface), most of the vertical component of flow is destroyed and the lateral flow in the general direction of groundwater flow takes place. Under these circumstances, the groundwater table rises by an amount equal to the depth of saturation caused by the descending.
Above the water table, the infiltrating rainfall raises the degree of saturation of the soil, which reduces the negative pore pressure and the shear strength of soil. As lateral flow develops, pore pressure increases and as a result, effective stress and shear strength are reduced. Increase of positive pore pressure occurs when the infiltrating rainfall forms a perched water table or has caused a rise in the groundwater table. Deposits of gravel and sand are able to infiltrate water without difficulty, whereas clay-rich mantles retard the ingress of water and characteristically remain wet after periods of rainfall.
6.5.1 Rainfall Infiltration Model.
Rainfall or rainstorm is one of the most significant triggering factors for slope failure. Study of rainfall-induced landslide mechanics is one of the most important and difficult issues for slope stability. In general, the effect of rainfall infiltration on slope could result in changing soil suction and positive pore pressure on main water table as well as raising soil unit weight thus reducing shear strength of rock and soil.
Infiltration is defined as the movement of water from the ground surface into the soil or rock via the pores or interstices of the ground mass. It can be further divided into two parts, first contributeing to the water content of the unsaturated zone, and the second part recharges the saturated groundwater system. in this process, some recharge to the unsaturated zone may be lost by transpiration or evaporation.
Subsurface water may be divided into zones of positive and negative pore pressure. The dividing line is the groundwater table where the pressure is equal to atmospheric pressure. The groundwater table is generally determined from the level of water in an open standpipe.
Rainfall may be separated into four components: runoff, infiltration, interception, and evapotranspiration (ET). Interception and ET are often disregarded when identifying rainfall components because they represent a small portion of the total rainfall (Joel et al. 2002). These simplifications leave the approximation of rainfall as nearly equal to addition of the infiltration and the runoff.
One of the earliest physical infiltration models was developed by Green and Ampt (1911). Based on the model, the time (t) required to saturate the soil to a depth (Lf) is given by:
Where u = difference between the volumetric water content before and after wetting
Kw =hydraulic conductivity of wetter zone
S = Wetting front capillary suction
The infiltration rate (If) is the rate at which water entres the soil
surface. The green-Ampt model predicts:
In infiltration model, water from precipitation is assumed to enter the soil as a sharp wetting front. The soil above the front is assumed to be saturated. The soil below of the front is assumed at some uniform initial moisture. This model gives a very reasonable prediction even when compared with other more rigorous approaches based on unsaturated flow (Bouwer, 1966).
In actual condition, the infiltration-runoff system sustains much more complexity than those expressions in a simple physical or empirical model. The infiltration rate could be affected by the distribution of rainfall, soil initial condition, rearrangement of soil particles due to the impact of raindrops, swelling of clayey soils, activities of worms and other soil fauna etc. (Bouwer 1966). The simulation of infiltration process as result of a rainfall event is still possible. However, the threshold rainfall for a slope failure could be a combination of a number of rainfall events or a prolonged antecedent rainfall. Under such circumstances, simulation of rainfall infiltration could be extremely time consuming if not impossible.