STABILITY ANALYSIS IN PRESENCE OF WATER

• Pore pressures • Rainfall

• Steady state flow and transient flow

Common terms used in simulation of water

Various in situ water conditions

Aquifers

Aquiclude

Perched Water Table

Various in situ water conditions

Aquifers

Aquiclude

Perched Water Table

Artesian aquifer

Example of an aquifer system with artesian wells

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 permeameter2. Falling or variable head permeameter3. Direct or indirect measurement during an Oedometer test4. 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.

Hydraulic Conductivity

Hydraulic conductivity is defined by Darcy's law, which can be written as follows for one-

dimensional vertical flow:

where U is Darcy's velocity (the average velocity of the fluid through a geometric cross-sectional

area within the soil), h is hydraulic head, and z is vertical distance in the soil. The coefficient of

permeability K, is also sometimes used as a synonym for hydraulic conductivity. Hydraulic

conductivity is defined as ratio of Darcy's velocity to applied hydraulic gradient.

Hydraulic Conductivity

Typical suction dependent hydraulic conductivity function

Hydraulic Conductivity

Factor affecting soil permeability and hydraulic conductivity 1. Grain-size2. Void ratio3. Composition4. Fabric or structural arrangement of particles5. Degree of saturation6. Presence of entrapped air and other foreign matter.

Matrix suction

Matrix suction is defined as the difference between the pore-air and the pore-water pressures

(Fredlund and Rahardjo, 1993b).

It is associated with capillary action, the process where interface tension between water and air

creates a curve interface boundary within a narrow opening, leading to a pressure difference

between the two. The magnitude of the pressure difference between water and air is a function of

the width of the gap between the solid surfaces.

Matrix suction

Typical absorption and desorption SWCC (Zhan and Ng. 2004)

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.

Negative pore pressures increase the effective stresses within a soil mass and improve the

stability of a slope. Ho and Fredlund (1982a) suggested increase in shear strength due to negative

pore pressure as

Where c= total cohesion of the soil

c’=effective cohesion

=matrix suction

= the slope of the plot of matrix suction when - is held constant

Simulation of water

There are three methods to simulate the ground water: • Piezometric lines• Water pressure grid• Finite element seepage analysis

Ground water