CHAPTER 6: STABILITY ANALYSIS IN PRESENCE OF WATER

Introduction 6.1

Water is the most important factor in most of the slope stability analysis. Pore water in soil can

strongly influence the physical interaction among soil grains. Changes in pore pressures can

directly impact the effective stresses, which in turn, affect both the shear strength and

consolidation behaviour of soil. Therefore, analysis of pore fluid seepage plays an important role

in the solution of many geotechnical problems, especially those concerning the stability analysis

of slopes and retaining structures.

Failure of soil slopes, both natural and man-made, during or shortly after rainfall is a commonly

occuring phenomena. Such rainfall related failures are often associated with tropical areas, where

intense rainfall may occur seasonally, and the soils are residual soils derived from the underlying

rock. Under these conditions, infiltration may result in large volume of water entering into

unsaturated soil slope. Such infiltration may lead to the soil becoming fully saturated, or an

increased degree of saturation, without full saturation being achieved.

Whenever loads are applied to the surface of a soil, they set up stresses within it. As the pore-

fluid has a low compressibility, it will not easily change its volume. As a result, pore-fluid

pressures are set up. In soils of low permeability, these excess pore-fluid pressures cannot escape

except after the passage of much time, and are therefore likely to have a major influence on the

behavior of the soil. Conversely, in soils of high permeability, the excess pore-fluid pressures

escape so readily that to all intents and purposes they may be ignored. The term 'drained' is used

here to denote absence of excess or stress change induced by pore water pressures, whereas, the

term 'undrained' is used to denote their presence. It is also possible to have partly drained

conditions after the escape of some of the excess pore fluid pressures.

There are two types of water flow in surface or subsurface: steady state flow and transient flow.

In the steady state flow, the pore water pressure is constant whereas, it is always chaining in

transient flow. Transient pore pressure is developed in response to short but intense rainfall and

plays an important role in slope failure occurrences.

6.2 Common terms used in simulation of water

Aquifers

Soils and rocks that transmit water with ease through their pores and fractures, respectively, are

call aquifers. Typical aquifers are composed of gravel, sand, sandstone, limestone and fractured

volcanic, igneous, or metamorphic rocks (figure 1 & 2). It is a wet underground layer of water-

bearing permeable rock or unconsolidated materials (gravel, sand, or silt) from which

groundwater can be usefully extracted using water wells.

Aquiclude

Aquicludes consist of strata or discontinuities that are sufficiently less pervious than the

adjoining strata constituting barriers to groundwater. Silt or clay washing into a crack in the

ground can produce an aquiclude that will block the flow in a sandy seam. Typical aquicludes

are clay, shale and unfractured igneous and metamorphic rocks. Aquiclude is a formation which

is although porous and capable of absorbing water, but does not permit its movement at rates

sufficient to furnish an appreciable supply for a well or spring (figure 1 & 2). . Aquiclude is an

impermeable body of rock or stratum of sediment that acts as a barrier to the flow of

groundwater.

Perched Water Table

A perched water table (or perched aquifer) is an aquifer that occurs above the regional water

table. A perched water table is sustained above an aquiclude or an impermeable stratum such as a

clayey layer (figure 1 & 2). It may be transient and rapidly developing in response to heavy

rainfall and dissipating quickly, or permanent in response to seasonal variations in rainfall levels.

Figure 1: Various in situ water conditions

Figure 2: Various in situ water conditions

Artesian aquifer

An artesian aquifer is a confined aquifer containing groundwater under positive pressure. It is

confined between impermeable rocks or clay which causes this positive pressure (figure 3).

Artesian water is derived from an artesian aquifer, in which the piezometric head of the water

pressure is higher than the upper surface of the aquifer, but the water is confined by an overlying

aquiclude. In other words, if piezometers are installed in the artesian aquifer, the water in the

piezometer tubes would rise to greater elevations from the deeper artesian strata than from the

strata nearer to the ground surface. The presence of artesian water should not be overlooked and

must be accounted for any slope stability analysis

Figure 3: Example of an aquifer system with artesian wells

.

6.3 Important Geotechnical properties related to water

6.3.1 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. The more permeable the soil is, the greater is the seepage of water

through it. Some soils are so permeable and corresponding seepage is so high that it is not

possible to build a pond without special construction techniques. Permeability is closely related

to porosity. It reflects the capacity of soil sediments to transmit water and is controlled by the

size of the pores and the degree to which they are interconnected.

Generally, materials of larger particle size which are consistently sorted will be more permeable.

A soil can have a high porosity, but low permeability if the open spaces are not well connected.

The sandy soils are often quite porous, since there are a relatively high percentage of void spaces

between the sand grains. These soils are also very permeable, because the pore spaces are usually

large and interconnected, allowing water to flow through them more readily. While clay may

have a higher porosity than sand, clay particles are much finer and the spacing between them is

very small. Clays and silts may not pack together well due to irregular grain shapes and the fact

that certain clay minerals have electrostatic charges which repell each other. In addition,

molecular attraction on the water trapped in the tiny pore spaces between clay particles is much

stronger than that found in the larger sand grain openings. In soil, permeability depends on the

average size of the pores and is related to the distribution of particle sizes, its shape, and

structure.

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. These factors are all variable over small distances and make both

observation and analysis difficult.

Permeability of soil can be measured either in the laboratory or the field. 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.

6.3.2 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. This, in turn, indicates the ability of the soil to change matric suction as a result

of environmental changes (Fredlund and Rahardjo, 1993).Water coefficient of permeability of

saturated soil is a function of void ratio (e) only. For unsaturated soil, the water coefficient of

permeability is a function of void ratio (e) and volumetric water content (θ). This relationship is

commonly expressed by a suction-dependent hydraulic conductivity function, illustrated in

figure 4.

Figure 4: Typical suction dependent hydraulic conductivity function

The hydraulic conductivity function of unsaturated soil can be obtained through direct or indirect

measurement. It depends on the intrinsic permeability and the degree of saturation of the

material. Saturated hydraulic conductivity Ksat, describes water movement through saturated

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

dimensional vertical flow:

U = −Kdhdz

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 gradient is measured as the ratio of vertical distance between the point of intake to the

point of discharge (a distance called head) to the length of flow from the two points. Hydraulic

conductivity depends on grain size, structure of the soil matrix, type of soil fluid, and saturation

of the soil matrix. Important properties relevant to the solid matrix of the soil include pore size

distribution, pore shape, tortuosity, specific surface, and porosity.

Flow through an unsaturated soil is more complicated than flow through continuously saturated

pore spaces. in this case, macropores are filled with air, leaving only finer pores to accommodate

water movement. Movement of water in unsaturated soil is dictated by differences in matric

potential but not the gravity. The matric potential gradient is the difference in the matric potential

of the moist soil areas (high matric potential) and nearby drier areas (low matric potential) into

which the water is moving (Brady and Weil, 1999).

The value of hydraulic conductivity in saturated soils varies within a wide range of several

orders of magnitude depending on the soil material. Table lists the range of expected values of K

for various unconsolidated and consolidated soil materials.

6.3.3 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.

Permeability varies with the square of particle diameter. It is logical that the smaller the grain-

size, the smaller the voids and the lower the permeability. A relationship between permeability

and grain-size is more appropriate in case of sands and silts than that in case of other soils since

the grains are more equidimensional and its fabric changes are not significant.

This suggests a simple method for assessment of permeability of a soil at any void ratio when

values of permeability are known at two or more void ratios. Once the line is drawn,

permeability at any void ratio may be read directly.

Influence of soil composition on permeability is generally of little significance in case of gravels,

sands, and silts, unless mica and organic matters are present. However, this is of major

importance in the case of clays. Montmorillonite has the least permeability with sodium as the

exchangeable ion (less than 10–7 cm/s, even at a very high void ratio of 15). Therefore, sodium

montmorillonite is used by the engineer as an additive to other soils to make them impermeable.

Kaolinite is hundred times more permeable than montmorillonite.

Fabric or structural arrangement of particles is an important soil characteristic influencing

permeability, especially of fine-grained soils. At the same void ratio, it is logical to expect that a

soil in the most flocculated state will have the highest permeability and the one in the most

dispersed state will have the lowest permeability. Remoulding of a natural soil invariably reduces

its permeability. Stratification or macrostructure also has great influence on the permeability. it is

more parallel to stratification than that perpendicular to stratification.

Higher the degree of saturation, higher the permeability. In case of certain sands permeability

may increase three-fold when degree of saturation increases from 80% to 100%.

Entrapped air has pronounced effect on permeability. It reduces the permeability of soil. Organic

foreign matter also has the tendency to move towards flow channels and choke them, thus

decreasing the permeability. Natural soil deposits in the field may have some entrapped air or gas

for several reasons. In the laboratory, air-free distilled water may be used as vacuum applied to

achieve a high degree of saturation. However, this may not lead to a realistic estimate of the

permeability of a natural soil deposit.

6.3.4 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.

The capillary rise that will occur within a soil is affected by its particle size and grading, since

they affect the size of the pores within the soil mass. Fine grained soil with corresponding fine

grained pore spaces are able to sustain large pressure differenc between pore water and air,

allowing large capillary rises. in the other hand, coarse grained soil with larger voids tend to

maintain lower pressure differences between air and water, resulting in a lower capillary rise.

The relationship between matric suction and degree of saturation may be presented graphically in

the form of a soil water retention curve (also known as a soil moisture retention curve or Soil

Water Characteristic Curve, SWCC). The form of these curves tends to be similar, regardless of

soil type and are generally ‘S’ shaped, although this shape is poorly defined in clayey soill.

Figure 5 shows typical form of the SWCC with principal characteristics, while figure 6 shows

some soil specific data showing effects of soil grading. This curve also depicts the relationship

between soil water content and soil water pressure potential.

Figure 5: Typical absorption and desorption SWCC (Zhan and Ng. 2004)

Figure 6: SWCC’s for some Dutch soils (after Koorevaar et. al.1983)

6.3.5 Consistency of soil

Soil consistency is the resistance of a soil at various moisture contents to mechanical stresses or

manipulations. It is commonly measured by feeling and manipulating the soil by hand or by

pulling a tillage instrument through it. The consistency of soils is generally described at three soil

moisture levels: wet, moist and dry. Consistency of a soil sample changes with the amount of

water present. Such change in soil consistency may be accurately measured in the laboratory

following standard procedures which determine the Atterberg Limits. This limit corresponds to

the moisture content at which a soil sample changes from one consistency to another. Two of the

Atterberg Limits are important: liquid limit and plastic limit.

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. In each state,

consistency and behavior of the soil is different and so are its engineering properties. Thus, the

boundary between each state can be defined based on a change in the soil's behavior.

Shrinkage Limit: Shrinkage limit (SL) is the water content where further loss of moisture will

not result in any more volume reduction. The testing standard to determine shrinkage limit is

explained by ASTM International D4943. Shrinkage limit is much less commonly used than the

liquid and plastic limits.

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. It is the water content at which a soil changes from plastic to liquid behavior.

The importance of this liquid limit test is to classify soils.

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. Plastic limit is the lower limit of the plastic state. A small

increase in moisture above the plastic limit destroys cohesion of the soil. Plastic Limit test is

defined by ASTM standard test method D 431.

6.4 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. Above the groundwater table where

the soil is unsaturated, the pore pressure is below atmospheric pressure and hence is negative in

value. In this zone, the pore water is continuous or semi continuous and the pore water pressure

is below atmospheric pressure. The magnitude of the negative pore pressure (sometimes called

soil suction) is controlled by surface tension at the air-water boundaries within the pores and is

governed by grain size. In general, the finer the soil particles, the larger the saturation capillary

head, and hence the higher the negative pore pressure. Rainfall infiltration from the ground

surface may rapidly reduce the magnitude of negative pore pressure. Any change in these pore

pressures alters alter the shear strength of soil and therefore has a tremendous effect on the slope

stability.

The water level measured in a piezometer within the saturation zone coincides with the water

table. However, the pore pressures are no longer hydrostatic if there is a flow. In this instance,

the pore pressure from any point within the soil mass is computed by means of a flow net, from

the difference in head between the point and the free water surface.

By lowering effective stress, positive pore pressure reduces the available shear strength within

the soil mass thereby decreasing the slope stability. Increase in positive pore pressure can be

rapid after a period of heavy rainfall. That is a major reason why many slope failures occur after

heavy rainfall. The rate of increase, however, depends on many factors such as the rate of

rainfall, the nature of the ground surface, the catchment area, and the soil permeability.

Pore pressure below the groundwater table can be assessed using analytical, numerical, and

graphical methods. Various analytical methods are available for determining flow nets and pore

pressure distributions in slopes. Numerical techniques using finite difference or finite element

method provide powerful tool for obtaining pore water distributions in slopes. They are the only

means by which transient flow situations can be fully modeled.

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

c = cʹ + (ua − uw)tan∅b

Where c= total cohesion of the soil

c’=effective cohesion

(ua − uw)=matrix suction

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

Here, a matrix suction (ua − uw) increases the shear strength by (ua − uw) tan∅b. The increase

in soil strength can be represented by a three-dimensional failure surface using stress variable

( σ − ua )and (ua − uw) , as shown in Figure 7. These negative pore pressures reduce in

magnitude when the degree of saturation increases and become zero when the soils are fully

saturated, The major problem in evaluation of stability in unsaturated soils is associated with the

assessment of reduction in negative pore pressure and possible increase in positive pore pressure

as a function of rainfall history.

The total stress is either due to self-weight of the soil or the externally applied forces or both. the

neutral stress at any point inside a soil mass is defined as the stress carried by the pore water and

is same in all directions when there is static equilibrium, since water cannot take static shear

stress. This is also called ‘pore water pressure’ and is designated as u. This is equal to γw.z at a

depth z below the water table i.e.

u = γw z

Effective stress is defined as the difference between the total stress and the neutral stress. This is

also referred to as the inter granular pressure and is denoted by:

σ' = σ – u

let us consider full saturation of the rock, including the joint, where no drainage of water is

allowed. If we assume that water is incompressible and that no flow of water into or out of the

joint is allowed, the volume of the test specimen including the joint must remain constant. under

this condition, the water must sustain stresses sufficient to prevent volume change of the

specimen. The total applied stress across the joint will be transmitted by the rock asperities and

by the water. If the water carries some of the normal stress, then the rock asperities carry less

normal load and therefore has less shear strength than it would be if drained. The normal stress

transmitted by the water is equal to the joint water pressure. The stress transmitted through the

rock asperities is, therefore, equal to the applied stress minus the joint water pressure. The joint

shear strength will now be reduced proportionally. The reduced normal stress acting through the

rock contacts is termed as the effective normal stress and is given by

σʹn = σn − u

Where

σ′n =effective normal stress

σn =normal stress

u =water pressure

The total stress imposed on such a soil will be sustained by the soil (the effective stress,) and the

pore water (the pore pressure, u). A reservoir can be used to create an upward seepage through

the soil sample. For this purpose, we assume that the valve leading to the upper reservoir is

closed. Thus, there is no water flowing through the soil sample (figure

8). This is the case of no seepage the Effective stress is σ′ = H2(γsat − γw)

Figure 8: Effective stress when there is no water flow

Upward seepage conditions can be induced in the laboratory using constant-head permeability

test apparatus, in figure 9. The upper reservoir causes the water to flow upward through the soil

sample. If the hydraulic gradient is large, the upward-seepage force will cause the effective stress

within the soil to become zero, thus causing a sudden loss of soil strength in accordance with the

effective-stress principle. However, if downward seepage is allowed, effective stress sigma' is

σ′ = H2(γsat − γw).

Figure 9: Effective stress when there is no water flow

In the analysis of stability of slopes in terms of effective stresses, the pore water pressure

distribution is of fundamental importance and its evaluation is one of the prime objectives in the

early stages of any stability study.

6.5 Infiltration

Infiltration through an unsaturated zone is vertical and causes no positive pore pressures. If the

infiltrating rainfall it, during its descent encounters a material of lower permeability, flow will be

impeded if the permeability of this lower zone is less than the rate of infiltration. Under this

situation, a perched water table is formed on the surface of the impermeable zone, and a lateral

flow takes place along the upper surface of the impermeable zone. Below the impermeable zone,

the infiltration rate reduces to the value of permeability of the zone.

When the infiltrating rainfall meets the groundwater table (phreatic surface), most of the vertical

component of flow is destroyed and the lateral flow in the general direction of groundwater flow

takes place. Under these circumstances, the groundwater table rises by an amount equal to the

depth of saturation caused by the descending.

Above the water table, the infiltrating rainfall raises the degree of saturation of the soil, which

reduces the negative pore pressure and the shear strength of soil. As lateral flow develops, pore

pressure increases and as a result, effective stress and shear strength are reduced. Increase of

positive pore pressure occurs when the infiltrating rainfall forms a perched water table or has

caused a rise in the groundwater table. Deposits of gravel and sand are able to infiltrate water

without difficulty, whereas clay-rich mantles retard the ingress of water and characteristically

remain wet after periods of rainfall.

6.5.1 Rainfall Infiltration Model.

Rainfall or rainstorm is one of the most significant triggering factors for slope failure. Study of

rainfall-induced landslide mechanics is one of the most important and difficult issues for slope

stability. In general, the effect of rainfall infiltration on slope could result in changing soil

suction and positive pore pressure on main water table as well as raising soil unit weight thus

reducing shear strength of rock and soil.

Infiltration is defined as the movement of water from the ground surface into the soil or

rock via the pores or interstices of the ground mass. It can be further divided into two parts, first

contributeing to the water content of the unsaturated zone, and the second part recharges the

saturated groundwater system. in this process, some recharge to the unsaturated zone may be lost

by transpiration or evaporation.

Subsurface water may be divided into zones of positive and negative pore pressure. The

dividing line is the groundwater table where the pressure is equal to atmospheric pressure. The

groundwater table is generally determined from the level of water in an open standpipe.

Rainfall may be separated into four components: runoff, infiltration, interception, and

evapotranspiration (ET). Interception and ET are often disregarded when identifying rainfall

components because they represent a small portion of the total rainfall (Joel et al. 2002). These

simplifications leave the approximation of rainfall as nearly equal to addition of the infiltration

and the runoff.

One of the earliest physical infiltration models was developed by Green and Ampt (1911).

Based on the model, the time (t) required to saturate the soil to a depth (Lf

) is given by:

t =µ

kw[ Lf − S ln �

S + LfS

�]

Where u = difference between the volumetric water content before and after wetting

Kw

S = Wetting front capillary suction

=hydraulic conductivity of wetter zone

The infiltration rate (If

) is the rate at which water entres the soil surface. The green-Ampt model

predicts:

𝐼𝑓 =𝑆 + 𝐿𝑓𝐿𝑓

In infiltration model, water from precipitation is assumed to enter the soil as a sharp wetting

front. The soil above the front is assumed to be saturated. The soil below of the front is assumed

at some uniform initial moisture. This model gives a very reasonable prediction even when

compared with other more rigorous approaches based on unsaturated flow (Bouwer, 1966).

In actual condition, the infiltration-runoff system sustains much more complexity than those

expressions in a simple physical or empirical model. The infiltration rate could be affected by the

distribution of rainfall, soil initial condition, rearrangement of soil particles due to the impact of

raindrops, swelling of clayey soils, activities of worms and other soil fauna etc. (Bouwer 1966).

The simulation of infiltration process as result of a rainfall event is still possible. However, the

threshold rainfall for a slope failure could be a combination of a number of rainfall events or a

prolonged antecedent rainfall. Under such circumstances, simulation of rainfall infiltration could

be extremely time consuming if not impossible.

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

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.