Important Geotechnical properties related to water Permeability Hydraulic Conductivity Matrix suction Consistency of soil
Permeability Soil permeability is the property of the soil to transmit water and air. The coefficient of permeability (or permeability) in soil mechanics is a measure of how easily water can flow through a porous soil medium Permeability of rock masses is controlled by discontinuity geometry, which includes spacing, direction, discontinuity width and form, as well as the degree of infilling and the roughness of the discontinuity surfaces. The followings are some of the methods used in the laboratory to determine permeability. 1. Constant head permeameter 2. Falling or variable head permeameter 3. Direct or indirect measurement during an Oedometer test 4. Horizontal capillarity test.
Hydraulic Conductivity Hydraulic conductivity of soil is a measure of the its ability to transmit water when submitted to a hydraulic gradient. The coefficient of permeability (k) represents the soil’s ability to transmit and drain water. There are two broad categories of determining hydraulic conductivity: Empirical approach by which hydraulic conductivity is correlated to soil properties like pore size, particle size distributions, and soil texture. Experimental approach in which hydraulic conductivity is determined from hydraulic experiments using Darcy's law The experimental approach is broadly classified into: Laboratory tests using soil samples subjected to hydraulic experiments Field tests that are differentiated into: small scale field tests using observations of water level in soil cavities large scale field tests, like pump tests in wells or by observing the functioning of existing horizontal drainage systems.
Factor affecting soil permeability and hydraulic conductivity 1. Grain-size 2. Void ratio 3. Composition 4. Fabric or structural arrangement of particles 5. Degree of saturation 6. Presence of entrapped air and other foreign matter.
Consistency of soil Atterberg Limits are basic measure of the nature of a fine-grained soil. Depending on the water content in the soil, it may appear in four states: solid, semi-solid, plastic and liquid. Shrinkage Limit: Shrinkage limit (SL) is the water content where further loss of moisture will not result in any more volume reduction. Liquid limit (LL): It is defined as the percentage moisture content at which a soil changes with decreasing wetness from liquid to plastic consistency or with increasing wetness from plastic to liquid consistency. Plastic limit (PL): it is the percentage moisture content at which a soil changes with decreasing wetness from the plastic to the semi- solid consistency or with increasing wetness from the semi-solid to the plastic consistency.
Pore Pressures Subsurface water is divided into zones of positive and negative pore pressures. The dividing line is the groundwater table (also known as phreatic surface) where the pressure is equal to atmospheric pressure. Below the groundwater table, the soil is fully saturated, and the pore pressure is above atmospheric pressure and positive in value.
Simulation of water There are three methods to simulate the ground water: Piezometric lines Water pressure grid Finite element seepage analysis