Factors influencing water flow in unsaturated soils

Ng and Shi (1998) and Ng et al. (2001) performed numerical analyses showing the effects of rain infiltration on pore-water pressures and hence on the stability of a given slope. Ng and Shi (1998) used a 2-D finite element mesh for the calculations, while 3-D analyses were conducted in the investigation made by Ng et al. (2001). They considered the intensity of rainfall, duration of rainfall, return period of major rainfall, antecedent rainfall, the saturated permeability (ks) and its anisotropy and the vegetation cover in their analysis. Their study showed that the suction in the soil and the groundwater table are mainly governed by the ratio of the infiltration flux and the saturated permeability (i.e., qb/ks) as well as the initial and boundary conditions. Moreover, the larger the antecedent infiltration rate, the lower the initial negative pore-water pressure will be. From their results, Ng and Shi (1998) also concluded that for a given slope, there exists a critical saturated permeability that may result in the greatest loss of negative pore-water pressure and thus to a lower safety factor.

Tsaparas et al. (2002) also used numerical analyses to study the effect of rainfall intensity, rainfall antecedent, different type of initial conditions and different permeability coefficient on the pore-water pressures in unsaturated slopes. They came to the conclusion that there exists a close interaction between the mentioned parameters and the pore-pressure distributions when performing transient analysis. Their study showed that highly permeable soil slopes (ks>10^-4 m/s) were unlikely to be influenced by small amounts of antecedent rainfall as opposed to soil slopes composed of moderately permeable material (10^-5 m/s). They therefore highlighted the importance of properly choosing these parameters before making any transient simulation.

Cai and Ugai (2004) with similar types of simulations also showed that the stability of a slope is influenced by its initial volumetric water content and the hydraulic properties of the soil as well as the pattern of the water pressure rise due to infiltration. By comparing numerical results with statistical and observational results they concluded that finite element analysis was a good method to evaluate the stability of slopes under rainfall.

Zhang et al. (2004) performed numerical analyses showing that under steady state conditions, the most important factor affecting the matric suction near the ground surface of a slope is the

surface flux (qb) expressed as a percentage of the saturated coefficient of permeability (ks).

They also found that under transient seepage conditions the pore-water pressure profile was dependent on the magnitude of the rainfall flux, the saturated coefficient of permeability, and the water storage function. In addition, they numerically showed that the long term matric suction does not necessarily disappear under steady state rainfall fluxes that are two or more orders of magnitude less than (ks).

Zhan and Ng (2004), from a 1-D analytical study, came to the conclusion that the effects of the desaturation coefficient (α) and the saturated permeability (ks) on the pore-water pressure response were much more significant than that of the water storage capacity of the soil (θs-θr).

They mentioned that the relative sensitivity of (ks) and (α) depends on the range of negative pore-water pressure within the soil. In fact, they showed that during infiltration in a dry soil (early stage of a rainfall), the pore-water pressures are primarily affected by (α). They are controlled by the saturated permeability when the negative pore-water pressures dissipate with the duration of the rainfall. Their analysis showed that for steady state infiltration, the pore-water pressure response is more governed by (qα/ks). However, for the transient infiltration process, the pore-water pressure response depends not only on (qα/ks), but also on (ks/α). The larger this latter ratio, the faster the wetting front advances downwards. Finally, these authors together with Ng and Shi (1998) showed that antecedent rainfall could be much more important than the subsequent as it controls the initial pore-water pressures and the initial coefficient of permeability (kw).

Rahardjo et al. (2001) and Collins et al. (2004) have shown from field observations that rainfall intensity and antecedent rainfall can very often be directly related to slope failures.

However, as stated by Tsaparas et al. (2002), the relation between climatic conditions and slope failures varies with geographical settings due to differences in porosity and permeability of soils. Also from field observations, Tsaparas et al. (2003) showed that the total rainfall and the initial pore-water pressures within a slope were the controlling parameters for the changes in pore-water pressures during infiltration.

Furthermore, the effects of the hysteresis in the SWCC on unsaturated flows were studied by Tami et al. (2004). They showed, from both experimental and numerical studies, that the hysteretic behaviour significantly affected the steady-state matric suctions within the slope profile. This effect was less important on the volumetric water content profile of the slope.

They however conclude with the fact that the appropriate hydraulic properties of the soils (i.e. drying or wetting) should be used in accordance with the actual process the slope is experiencing (i.e. desorption process or adsorption process) even if the slope is under a given steady-state rainfall condition.

Even though much work has been done in recent years, the effects of hydraulic parameters and rainfall patterns on the stability of unsaturated slopes are still difficult to predict and are not fully understood. The reason for this may be: (1) high non-linearity and variability of hydraulic parameters involved in the transient unsaturated flow system, and spatial and temporal variations in the initial and boundary conditions of the system (Zhan et al. 2004).

A recapitulation of the many factors influencing the flow of water through unsaturated soil is presented in Table 2-2.

Table 2-2: Principal factors influencing seepage in unsaturated soil slope.

Initial conditions of the slope Intensity of the rainfall Amount of water infiltrating the soil Rainfall patterns

Surface evaporation Antecedent of the rainfall

Type of vegetation cover Water retention characteristics Rate at which infiltration occurs Coefficient of permeability