6.6 Simulation of water
Water may be simulated for rock slope and soil slope differently. It affects these slopes in different ways. In soil formations, water decreases the frictional shear strength of soil due to the buoyant effect. Modeling rainwater infiltration in slopes is vital to the analysis of slope failure induced by heavy rainfall. To analyze rainwater infiltration into soil, it is important to have an understanding of the hydraulic properties of the soil, in particular the relationship between volumetric water content θ and soil capillary pressure ψ, and the relationship between unsaturated hydraulic conductivity K and ψ.
The primary effect of groundwater pressure in reducing the stability of rock slopes is the resulting decrease in effective shear strength of discontinuities. This phenomenon is described by the effective stress principle, which is fundamental to understanding the influence of groundwater on rock slope stability. Discontinuities and water play important role in rock slope failure. Water filled in discontinuities creates water column which generates water pressure equal to the length of discontinuities. In low rainy season these cracks get filled with rain water which subsequently gets drained out if drainage is allowed. However, during heavy rainy season, the rate of input of water to these cracks is higher than the rate of discharge of water through cracks. It results in filling up of crack with water. Water filled up in the crack creates a water column of variable length, depending on amount of water flowing at the surface. The water column exerts hydraulic pressure depending upon the length of the water column. Tension cracks on top of the rock slope could trap water, which eventually develops hydraulic pressure in the tension cracks along rock discontinuities. Water pressure reduces the normal pressure on the discontinuity and therefore reduces the shear strength. The presence of water may also lower the shear strength of the infill material of the discontinuity.
Ground water is that water which fills up the voids in the soil up to the ground water table and translocates through them. It fills coherently and completely all the voids. In such a case, the soil is said to be saturated. The water zone in the soil mass may be divided into two components: saturated (below water table) or unsaturated zone (above water table). Above the water table, air voids increase as the distance from the water table increases. The water in these zones is held in some place by capillary attraction and exerts relatively large stabilizing forces on the structure of soil creating negative pore water pressure or soil suction. The flow of groundwater is usually very slow and is generally laminar.
Soil water may be in the forms of ‘free' or ‘gravitational water’ and ‘held water’. Free or gravitational water is free to move through the pore space of the soil mass under the influence of gravity. however, the held waeter is locked in the proximity of the surface of the soil grains by certain forces of attraction. Steady-state seepage analysis can be carried out to compute pore pressures as well as flow rates and other hydraulic quantities. There are three methods to simulate the ground water:
• Piezometric lines
• Water pressure grid
• Finite element seepage analysis
Water affects the stability of soil slope by generating pore pressure, modifying density of material and changing its mineral constituents. Groundwater is derived from many sources but primarily originates from rainfall. Some water infiltrates into the ground and percolates downwards to the saturated zone at depth, while some water moves over the surface as surface runoff. Groundwater in the saturated zone moves towards rivers, lakes and seas, where it evaporates and returns to the land as clouds of water vapour, which precipitates as rain and snow. This circulation of water is often known as the hydrological cycle, which also includes precipitation, evaporation, runoff, transpiration, and channel flow.
Groundwater levels are rarely static and vary with the rate of recharge or discharge of groundwater. Fluctuations of ground water may also vary with geology, topography, and proximity to local centers of discharge, such as springs, rivers, and dams that store water or pump water from the ground. Such fluctuations should be studied with observation wells or piezometers and the data presented as maps of groundwater level change.
The unsaturated zone is often located above the main groundwater table (phreatic surface) with voids partially filled with water. This zone is sometimes called the zone of aeration, and extends from the ground surface down through the major root zone. Its thickness varies with the soil type and vegetation. The spaces between particles within this zone, are filled partly with water and partly with air. Molecular attraction is exerted on the water by the soil or rock, and the attraction is also exerted by the water particles on one another.The saturated zone is within the main groundwater regime with voids completely filled with water. Perched groundwater can create saturated zones within unsaturated zones. Different modes of groundwater flow develop in the unsaturated and the saturated zones, and affect the stability of slopes.
Figure 10: Feature of the hydrological cycle
The excavation of an open pit causes groundwater to flow into the pit, setting up hydraulic gradients. The pattern of vertical and lateral variations in hydraulic head is called the groundwater pressure distribution. The flow patterns and the groundwater pressure distribution that are generated within a slope, depend on the following factors;
• Geometry of the slope
• Permeability of the slope material
• Recharge from the surrounding rock mass
• Water storage within the slope
• Local precipitation, runoff, and infiltration characteristics
The most important groundwater parameter for stability purposes is the groundwater pressure distribution within slopes. This distribution can be obtained in two ways: direct measurement of pressure via piezometers or determining pressures from an analysis of the hydraulic properties of the rock mass, e.g., geology and permeability characteristics.