STABILITY ANALYSIS IN PRESENCE OF WATER
• Pore pressures
• Rainfall
• Steady state flow and transient flow
Common terms used in simulation of water
Aquifers
Aquiclude
Perched Water Table
Various in situ water conditions
Aquifers
Aquiclude
Perched Water Table
Various in situ water conditions
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 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.
Hydraulic Conductivity
Hydraulic conductivity is defined by Darcy's law, which can be written as follows for onedimensional 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-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.
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