Unit 4- Ground water, Building Stones and Stability

of Slopes

Groundwater
Ground or subterranean water is water that is found
below the earth’s surface. Groundwater is simply water
under the ground where the soil is completely filled or
saturated with water. This water is also called an
“aquifer.”

Types of Ground Water

The following are four types of ground water:

Connate water. It may also refer as fossil water. This is

water trapped in the pores of the rock during the
formation of the rocks. The chemical composition of
water changes with the changes that the rock undergoes.
Most connate water is saline.

Meteoric water. This refers to ground water which

originate from rainfall and other forms of precipitation
such as hailstorms and snow fall. It is as the result of
precipitation water seeping into the ground.

Juvenile water. It is also referred to as magmatic

water. This is water that is brought closer to the earth’s
surface due to volcanic activities. It usually has high
mineral content.
Oceanic water. This is groundwater that results from
the seepage of groundwater into the ground. It is
common in the coastal areas where ocean water seeps
horizontally into the ground from the ocean.

Water table and depth zones of saturation

Water Table. The water table is the boundary between

the unsaturated zone and the saturated zone
underground. Below the water table, groundwater fills
any spaces between sediments and within rock.

Zones of Saturation. There are various classifications

of groundwater given by different researchers.
However, as per the most popular classification given
by Meinzer (1923), the groundwater has been divided
mainly in two groups: interstitial water and internal
water. The interstitial water is again subdivided into
two divisions. They are vadose water present in the
zone of aeration and groundwater present in the zone of
saturation. The vadose water is further subdivided into
three zones, i.e., soil water zone, intermediate zone and
capillary zone. Fig. 1.2 shows the classification of
groundwater. The soil water zone is adjacent to the
ground surface. The intermediate zone is between the
lower edge of the soil water zone and the upper edge of
the capillary zone. The capillary zone extends from the
bottom edge of the intermediate zone to the upper edge
of the saturated zone. The thickness of the capillary
zone depends on the properties of the soil and also on
the homogeneity of the soil. The depth of capillary zone
is varying from few centimeters to few meters. In
capillary zone, all the pores are field up with water.
However, we cannot draw water by inserting a well up
to that depth. This is because of the negative pressure
developed at this zone due to surface tension effect.
Groundwater zone starts from the bottom edge of the
capillary zone. In this zone, all the pores of the soil
matrix are filled with water. This zone is also known as
zone of saturation. The top surface of the zone of
saturation or groundwater is known as phreatic surface.
This phreatic surface is also known as water table.

The degree of saturation for the soil below the water

table is equal to 1, i.e., the soil is fully saturated. As a
groundwater hydrologist, we are primarily interested for
the water below the groundwater table, i.e., the water
available in the zone of saturation. For the soil above
the water table, the degree of saturation of the soil is
varying between 0 and 1. However, the degree of
saturation will never be 0 due of the presence of
hygroscopic water. The hygroscopic water is the water
that held tightly on the surface of the soil colloidal
particle. Hygroscopic water can be removed from the
soil by oven drying. Fig. 1.3 shows the moisture
distribution in soil column.
 Influence of textures and Structures of rocks on
groundwater storage and movement

Groundwater is water that exists in the pore spaces and

fractures in rock and sediment beneath the Earth's
surface. It originates as rainfall or snow, and then
moves through the soil into the groundwater system,
where it eventually makes its way back to surface
streams, lakes, or oceans.

• Groundwater makes up about 1% of the water on Earth

(most water is in oceans).
• But groundwater makes up about 35 times the amount
of water in lakes and streams.

• Groundwater occurs everywhere beneath the Earth's

surface, but is usually restricted to depths less that about
750 meters.

• The volume of groundwater is an equivalent to a 55-

meter-thick layer spread out over the entire surface of
the Earth.

• It is an important resource for potable water, irrigation,

and industry.

• Because it is largely hidden from view, it is often

forgotten and subject to contamination by careless
humans.

• Groundwater is a primary agent of chemical weathering

and is responsible for the formation of caves and
sinkholes.

The Groundwater System

Groundwater resides in the void spaces of rock,

sediment, or soil, completely filling the voids. The
total volume of open space in which the groundwater
can reside is porosity. Porosity determines the amount
 of water that a rock or sediment can contain.

Porosity

In sediments or sedimentary rocks, the porosity depends

on grain size, the shapes of the grains, and the degree of
sorting, and the degree of cementation.

• Well-rounded coarse-grained sediments usually have

higher porosity than fine-grained sediments, because
the grains do not fit together well.

• Poorly sorted sediments usually have lower porosity

because the fine-grained fragments tend to fill in the
open space.

• Since cements tend to fill in the pore space, highly

cemented sedimentary rocks have lower porosity.
• In igneous and metamorphic rocks porosity is usually
low because the minerals tend to be intergrown,
leaving little free space. Highly fractured igneous and
metamorphic rocks, however, could have high
porosity.

Secondary porosity is porosity that developed after

rock formation. Processes such as fracturing, faulting,
and dissolution can create secondary porosity.

Permeability is a measure of the degree to which the

pore spaces are interconnected, and the size of the
interconnections. Low porosity usually results in low
permeability, but high porosity does not necessarily
imply high permeability. It is possible to have a highly
porous rock with little or no interconnections between
pores. A good example of a rock with high porosity
and low permeability is a vesicular volcanic rock,
where the bubbles that once contained gas give the
rock a high porosity, but since these holes are not
connected to one another the rock has low
permeability.

A thin layer of water will always be attracted to

mineral grains due to the unsatisfied ionic charge on
the surface. This is called the force of molecular
attraction. If the size of interconnections is not as large
as the zone of molecular attraction, the water can't
move.
Thus, coarse-grained rocks are usually more permeable
than fine-grained rocks, and sands are more permeable
than clays.

Aquifers

An aquifer is a large body of permeable material where

groundwater is present and fills all pore space. Good
aquifers are those with high permeability such as
poorly cemented sands, gravels, or highly fractured
 rock. An aquitard is a body of material with very low
permeability. In general, tightly packed clays, well
cemented sandstones, and igneous and metamorphic
rocks lacking fractures are good aquitards. Large
aquifers can be excellent sources of water for human
usage such as the High Plains Aquifer (in sands and
gravels) or the Floridian Aquifer (in porous
limestones) as outlined in your text.

Aquifers can be of two types:

• Unconfined Aquifers - the most common type of
aquifer, where the water table is exposed to the Earth's
atmosphere through the zone of aeration.

• Confined Aquifers - these are less common, but occur

when an aquifer is confined between layers of
impermeable strata (aquitards).

Geologic Activity of Groundwater

• Dissolution - Recall that water is the main agent of

chemical weathering. Groundwater is an active
weathering agent and can leach ions from rock, and, in
the case of carbonate rocks like limestone, can
completely dissolve the rock.

• Chemical Cementation and Replacement - Water is

also the main agent acting during diagenesis. It carries
in dissolved ions which can precipitate to form
chemical cements that hold sedimentary rocks
 together. Groundwater can also replace other
molecules in matter on a molecule-by-molecule basis,
often preserving the original structure such as in
fossilization or petrified wood.

• Caves and Caverns - If large areas of limestone

underground are dissolved by the action of
groundwater these cavities can become caves or
caverns (caves with many interconnected chambers)
once the water table is lowered. Once a cave forms, it
is open to the atmosphere and water percolating in can
precipitate new material such as the common cave
decorations like stalactites (hang from the ceiling),
stalagmites (grow from the floor upward), and
dripstones, and flowstones.

• Sinkholes - If the roof of a cave or cavern collapses,

this results in a sinkhole. Sinkholes, likes caves, are
common in areas underlain by limestones. For
example, in Florida, which is underlain by limestones,
a new sinkhole forms about once each year, gobbling
up cars and houses in process.

• Karst Landscapes - In an area where the main type of

weathering is dissolution (like in limestone terrains),
the formation of caves and sinkholes, and their
collapse and coalescence may result in a highly
irregular topography called karst landscapes.

Pervious and Impervious Rocks

The rocks which are porous and allowed water to pass
freely is known as pervious rocks and whereas the rock
which are not porous and does not allow the water to
pass through due to cracks or defects is known as
impervious rocks. Impermeable rocks include shales
and unfractured igneous and metamorphic rocks. Solid
granite is one example for pervious rocks.

Geological work of Groundwater

The Water Table

Rain that falls on the surface seeps down through the
soil and into a zone called the zone of aeration or
unsaturated zone (also called the vadose zone), where
most of the pore spaces are filled with air. As it
penetrates deeper it eventually enters a zone where all
pore spaces and fractures are filled with water.
This zone is called the saturated zone or phreatic zone.
The surface below which all openings in the rock are
filled with water (the top of the saturated zone) is
called the water table.
The water table occurs everywhere beneath the Earth's
surface. In desert regions it is always present, but
rarely intersects the surface.

In more humid regions it reaches the surface at streams

and lakes, and generally tends to follow surface
topography. The depth to the water table may change,
however, as the amount of water flowing into and out
of the saturated zone changes.

During dry seasons, the depth to the water table

increases. During wet seasons, the depth to the water
table decreases.
Discontinuous aquitards and aquifers may exist in the
subsurface. These arrest downward infiltration to the
water table and form what are called perched water
tables. They overlie unsaturated material and may be
confused with the main water table. Because they are
smaller, they are more easily dewatered or
contaminated.
Movement of Groundwater

Groundwater is in constant motion, although the rate at

which it moves is generally slower than it would move
in a stream because it must pass through the intricate
passageways between free space in the rock. First the
groundwater moves downward due to the pull of
gravity. But it can also move upward because it will
flow from higher pressure areas to lower pressure
areas, as can be seen by a simple experiment illustrated
below. Imagine that we have a "U"-shaped tube filled
with water. If we put pressure on one side of the tube,
the water level on the other side rises, thus the water
moves from high pressure zones to low pressure zones.
The same thing happens beneath the surface of the
Earth, where pressure is higher beneath the hills and
lower beneath the valleys.

The Earth's surface can be divided into areas where

some of the water falling on the surface seeps into the
saturated zone and other areas where water flows out
of the saturated zone onto the surface. Areas where
water enters the saturated zone are called recharge
areas, because the saturated zone is recharged with
groundwater beneath these areas. Areas where
groundwater reaches the surface (lakes, streams,
swamps, & springs) are called discharge areas, because
the water is discharged from the saturated zone.
Generally, recharge areas are greater than discharge
areas.

Groundwater movement is slow relative to that in

surface streams. This is because it must percolate
through pore openings and is further slowed by friction
and electrostatic forces.

For comparison, typical rates of flow are as follows:

Ocean currents - 3 km /hour
Steep river channel - 30 km /hour
Groundwater - 0.00002 km /hour

Groundwater flow occurs on a variety of scales. Local

– Shallow flow occurs over short times and distances,
whereas, deep long-distance flow occurs over time
scales of centuries.

The rate at which groundwater moves through the

saturated zone depends on the permeability of the rock
and the hydraulic head. The hydraulic head is defined
as the difference in elevation between two points on
the water table.

The hydraulic gradient is the hydraulic head divided by

the distance between two points on the water table.

The velocity, V, is of groundwater flow is given by:

V = K (h2- h1)/L

where K is the hydraulic conductivity, which is a

measure of the permeability of the material through
which the water is following.

If we multiply this expression by the area, A, through

which the water is moving, then we get the discharge,
Q.

Q = AK (h2 - h1)/L

which is Darcy's Law. It simply states that discharge is

proportional to the hydraulic gradient times the
permeability. Discharge is higher if the hydraulic
gradient is high and/or of the permeability is high.

Note that like stream discharge, Q has units of volume

per time (i.e., cubic meters per second).

Springs

A spring is an area on the surface of the Earth where

the water table intersects the surface and water flows
out of the ground. Some springs occur when an
aquitard intersects an aquifer at the surface of the
Earth. Such juxtaposition between permeable and
impermeable rock can occur along geological contacts
and fault zones.

Hot Spring and Geysers

Hot springs are groundwater discharges of water with

temperatures that range from 30° to 104°C. The waters
are usually rich in dissolved minerals that often
precipitate around the springs. They develop in two
settings: (1) where deep groundwater surfaces along
faults or fractures, and (2) in geothermal regions where
groundwater is heated by shallow magma or hot rock.
Hot springs are distinctive geological features. If the
surface through volcanic ash they become a viscous
slurry called mud pots. If they precipitate dissolved
minerals on cooling, they can form deposits like
travertine made of calcite.

Hot springs can also produce a wide range of colors

due to thermal sensitive bacteria that metabolize sulfur
minerals.

Geysers form when hot water erupts to the surface.

They are caused by boiling of the water at depth which
causes vapor bubbles to rise and reduce the pressure.
This results in rapid boiling which sends the water to
the surface as a geyser. The cycle then repeats after
the empty chamber is refilled with water and is heated
to the boiling temperature.
Hot springs and geysers are common in active volcanic
regions, notably Yellowstone Park in Wyoming.

Wells

A well is human-made hole that is dug or drilled deep

enough to intersect the water table. Wells are usually
used as a source for groundwater. If the well is dug
beneath the water table, water will fill the open space
to the level of the water table, and can be drawn out by
a bucket or by pumping. Fracture systems and perched
water bodies can often make it difficult to locate the
best site for a well.
A special kind of confined aquifer is an artesian
system, shown below. In an artesian system, the
aquifer is confined between aquitards and is included
so that the pressure inside the aquifer can push the
water from a well or spring upward to nearly the same
level as the top of the water table. Artesian systems
are desirable because they result in free-flowing
artesian springs and artesian wells.

Changes in the Groundwater System

When discharge of groundwater exceeds recharge of
the system, several adverse effects can occur. Most
 common is lowering of the water table, resulting in
springs drying up and wells having to be dug to deeper
levels. If water is pumped out of an aquifer, pore
pressure can be reduced in the aquifer that could result
in compaction of the now dry aquifer and result in land
subsidence. In some cases, withdrawal of groundwater
exceeds recharge by natural processes, and thus
groundwater should be considered a non-renewable
natural resource.

Water Quality and Groundwater Contamination

Water quality refers to such things as the temperature
of the water, the number of dissolved solids, and lack
of toxic and biological pollutants. Water that contains a
high amount of dissolved material through the action
of chemical weathering can have a bitter taste, and is
commonly referred to as hard water. Hot water can
occur if water comes from a deep source or encounters
a cooling magma body on its traverse through the
groundwater system. Such hot water may desirable for
bath houses or geothermal energy, but is not usually
desirable for human consumption or agricultural
purposes. Most pollution of groundwater is the result
of biological activity, much of it human. Among the
sources of contamination are:
• Sewers and septic tanks
• Waste dumps (both industrial and residential)
• Gasoline tanks (like occur beneath all service station)
• Biological waste products - Biological contaminants
can be removed from the groundwater by natural
 processes if the aquifer has interconnections between
pores that are smaller than the microbes. For example,
a sandy aquifer may act as a filter for biological
contaminants.
• Agricultural pollutants such as fertilizers and
pesticides.
• Salt water contamination - results from excessive
discharge of fresh groundwater in coastal areas.

Groundwater contamination can result from a point

source where the contaminant plume emanates from 1
spot. Concentrations of the contaminant are highest
near the source and decrease away from the source.
Or, from a widespread source where the pollution is
introduced over a wide area and diffused throughout
the groundwater over a broad region. Nonpoint source
contaminants are difficult to identify and address.
Groundwater contaminant plumes change over time.
They grow in length with groundwater flow. They
grow in width by diffusion and dispersion.
Large plumes pollute large areas and affect many
people.

Remediation of Groundwater Contamination

Problems

In order to begin remediation, contaminant

characterization is first done. Monitoring wells are
installed to assess flow behavior. This allows for
chemical testing to quantify the amount of and
character of the contaminants. Strategies are then
designed to reduce health risks.

Remediation is usually quite expensive. Most

strategies include removing the source of the
contaminant, then pumping the groundwater out and
treating it. Sometimes heat is pumped in to volatilize
the groundwater or steam is pumped in to clean out the
containments. Newly developed techniques use
bacteria to clean the groundwater in a process called
bioremediation.

Prevention of Groundwater Contamination

Contamination is best prevented by managing land

uses. Landfills now require lining the bottom of the
landfill with impermeable clay and plastic liners.
Underground storage tanks require double-lining to
prevent leakage.
Still the best practice is to require that contaminants
not be allowed into the groundwater system.

Deposition

Deposition occurs when the agents (wind or water) of

erosion lay down sediment. Deposition changes the
shape of the land.

Solution or Pressure Solution

 In structural geology and diagenesis, pressure solution
or pressure dissolution is a deformation mechanism
that involves the dissolution of minerals at grain-to-
grain contacts into an aqueous pore fluid in areas of
relatively high stress and either deposition in regions
of relatively low stress within the same rock or their
complete removal from the rock within the fluid. It is
an example of diffusive mass transfer.

Occurrence. Evidence for pressure solution has been

described from sedimentary rocks that have only been
affected by compaction. The most common example of
this is bedding plane parallel stylolite developed in
carbonates.
In a tectonic manner, deformed rocks also show
evidence of pressure solution including stylolite at high
angle to bedding.[4] The process is also thought to be
an important part of the development of cleavage.

The effects of these processes are as follows:

• Changes in shape, size, and texture of land-forms (i.e.,
mountains, riverbeds, and beaches)
• Landslides
• Buildings, statues, and roads wearing away
• Soil formation
• Washes soil, pollutants, harmful sediments into
waterways
• Causes metals to rust
• Reduces beaches, shorelines
• Delta formation
• Formation of various new landforms

Building Stones and its requirements

Definition. A building stone may be defined as a

sound rock that can be safely used in some situation in
the construction as a massive dressed or undressed
unit. Granites and marbles used in the form of finely
dressed blocks or slabs or columns in monumental and
costly buildings are good building stones.

The following are the quality requirements of good

building stones:

1. Strength: Generally, most of the building stones have

high strength to resist the load coming on it. Therefore,
it is not of prime concern when it comes to check the
quality of stones. But when the stones are to be used in
large structures, it becomes necessary to check the
compressive strength of stones.
Compressive strength of building stones generally falls
within the range of 60 to 200N/mm2.
2. Durability: Building stones should be capable to resist
the adverse effects of natural forces like wind, rain and
heat. It must be durable and should not deteriorate due
to the adverse effects of the above natural forces.
3. Hardness: When stones are used in floors, pavements
or aprons of bridges, they become subjected to wearing
and abrasive forces caused by movement of men or
 machine over them. So, it is required to test hardness
of stone.
Hardness of stone is determined by Mohs scale.
4. Toughness: Toughness of stones means it ability to
resist impact forces. Building stones should be tough
enough to sustain stresses developed due to vibrations.
The vibrations may be due to the machinery mounted
over them or due to the loads moving over them. The
stone aggregates used in the road constructions should
be tough.
5. Specific Gravity: The more the specific gravity of
stone, the heavier and stronger the stone is.
Therefore, stones having higher specific gravity values
should be used for the construction of dams, retaining
walls, docks and harbors. The specific gravity of good
building stone is between 2.4 and 2.8.
6. Porosity and Absorption: Porosity of building stones
depend upon the mineral constituent and structural
formation of the parent rock. If stones used in building
construction are porous then rain water can easily enter
into the pore spaces and cause damage to the stones.
Therefore, building stone should not be porous.
Water absorption of stone is directly proportional to
the porosity of rock. If a stone is more porous then it
will absorb more water and cause more damage to
stone.

In higher altitudes, the freezing of water in pores takes

place and it results into the disintegration of the stone.
 Permissible limits of water absorption for some the
commonly used building stones are as follows:

Type of Stone Maximum limit of

Water Absorption (%)
Sandstone 10
Limestone 10
Granite 1
Shale 10
Slate 1
Quartzite 3

7. Dressing: Giving required shape to the stone is called

dressing. It should be easy to dress so that the cost of
dressing is reduced. However, the care should be taken
so that, this is not be at the cost of the required strength
and the durability.
8. Appearance: In case of the stones to be used for face
works, where appearance is a primary requirement, its
color and ability to receive polish is an important
factor.
Light colored stones are more preferred than dark
colored stones as the color are likely to fade out with
time.
9. Seasoning: Good stones should be free from the
quarry sap. Lateritic stones should not be used for 6 to
12 months after quarrying. They are allowed to get rid
of quarry sap by the action of nature. This process of
removing quarry sap is called seasoning.
10.Workability: Stone should be workable. Stone is said
 to be workable when the work involved in stone working
(such as cutting, dressing & shaping) is economical and
easy to conduct.
11.Cost: Cost is an important consideration in selecting a
building material. Proximity of the quarry to building
site brings down the cost of transportation and hence the
cost of stones comes down.
12.Fire Resistance: Stones should be free from calcium
carbonate, oxides of iron, and minerals having different
coefficients of thermal expansion. Igneous rock show
marked disintegration principally because of quartz
which disintegrates into small particles at a temperature
of about 575°C. Limestone, however, can withstand a
little higher temperature; i.e., up to 800°C after which
they disintegrate.

Building Stones and Their Uses

Based on Geology, stones or rocks are classified into

three types:
• Igneous Rocks. Basalt, Trap, Andesite, Rhyolite,
Diorite, Granite.
• Sedimentary Rocks. Limestones, Dolomite and
Sandstones.
• Metamorphic Rocks. Gneiss, Quartzite, Marble, Slate.

Types of Building Stones

Some of the common building stones which are used for

different purposes in India are as follows:
Granite
It is a deep-seated igneous rock, which is hard, durable
and available in various colors. It has a high value of
crushing strength and is capable of bearing high
weathering.

Granite is used for bridge components, retaining walls,

stone columns, road metal, ballast for railways,
foundation, stonework and for coarse aggregates in
concrete. These stones can also be cut into slabs and
polished to be used as floor slabs and stone facing slabs.

Granite is found in Maharashtra, Rajasthan, Uttar

Pradesh, Madhya Pradesh, Punjab, Assam, Tamil Nadu,
Karnataka and Kerala.

Basalt and Trap

They are originated from igneous rocks in the absence of
pressure by the rapid cooling of the magma.
They have the same uses as granite. Deccan trap is a
popular stone of this group in South India.

Limestone
It is a sedimentary rock formed by remnants of seaweeds
and living organisms consolidated and cemented
together. It contains a high percentage of calcium
carbonate.

Limestone is used for flooring, roofing, pavements and

as a base material for cement. It is found in Maharashtra,
Andhra Pradesh, Punjab, Himachal Pradesh and Tamil
Nadu.

Sandstone
This stone is another form of sedimentary rock formed
by the action of mechanical sediments. It has a sandy
structure which is low in strength and easy to dress.

They are used for ornamental works, paving and as road

metal. It is available in Madhya Pradesh, Rajasthan,
Uttar Pradesh, Himachal Pradesh and Tamil Nadu.

Gneiss
It can be recognized by its elongated platy minerals
usually mixed with mica and used in the same way as
granite.

They can be used for flooring, pavement and not for

major purposes because of its weakness. It is found in
Karnataka, Andhra Pradesh, Tamil Nadu and Gujarat.

Marble
It is a metamorphic rock which can be easily cut and
carved into different shapes. It is used for ornamental
purposes, stone facing slabs, flooring, facing works etc.

It is found in Rajasthan, Gujarat and Andhra Pradesh.

Slate
It is a metamorphic rock which can be split easily and
available in black color. It is used for damp-proofing
flooring and roofing.

Quartzite
It is a metamorphic rock which is hard, brittle,
crystalline and durable. It is difficult to work with and
used in the same way as granite but not recommended
for ornamental works as it is brittle.

Laterite
It is decomposed from igneous rocks; occur in soft and
hard varieties. It contains a high percentage of iron
oxide and can be easily cut into blocks.
 The soft variety is used for walls after curing while the
hard blocks are used for paving the pathways.

Use of Building Stones

The stones used for various types of works are as

follows:

• Fine-grained granite and gneiss stones are used for

Heavy engineering works such as building bridge piers,
breakwaters, monuments, etc.
• Granite, quartzite and compact sandstones are used for
masonry works in industrial areas exposed to smoke
and fumes.
• Marble, granite and sandstone are used for facing work
of buildings.
• Limestone and sandstone are used for general building
works.
• Fine-grained granite, marble, and soft sandstone are
used for Carvings and ornamental works.
• Compact limestone and sandstone are used for Fire-
resistant masonry.
• Granite, quartzite stones are used in foundations of
building in places with the high groundwater level.
• Marble, slate, sandstone and granite stones are used for
floor paving.

Slope Stability
Slopes are typically categorized in two types: natural
and artificially-made slopes. Natural slopes are formed
due to physical processes that include plate tectonics
and weathering/erosion of rock masses that result in
material deposition. Artificially-made slopes are
established to facilitate infrastructure projects, ex.,
embankments, earth dams, road cuttings etc.

The stability of a slope is of critical importance in

Geotechnical Engineering applications. A slope
movement (also referred as a landslide) can lead to
severe issues including infrastructure damage or/and
casualties. Slope stability depends on the capability of
the soil mass to withstand its gravitational forces, the
additional loads acting on the slope, as well as potential
dynamic loads (such as that of an earthquake).

A common misconception is that landslides occur in

steep and remote slopes and do not actually impact
human infrastructure. However, statistics show that
most of world’s regions are impacted by (at least) some
types of landslide phenomena that can be triggered by
several factors including erosion, precipitation,
earthquakes, human activity etc.

Landslides may occur rapidly or progress steadily at a

fixed rate. They are common in soils and rock masses
with poor mechanical properties (highly fractured or
weathered). However, a landslide can be triggered also
by deformations along discontinuity layers of strong
rocks. The nature and type of landslide phenomena are
 complex and are further analyzed below.

Types and Examples of Slope Failure

The most common and complete classification system

of landslides is that provided by Varnes (1978), who
introduces a system that requires the definition of the
landslide material and the type of the movement
induced. The ground materials are distinguished in 5
categories:

• Rock: An intact rock mass which used to be located at

its initial position (i.e., it has not been eroded) before
the movement occurred.
• Soil: Soil mass formed or transferred due to weathering
and erosion of rocks. Soils consists of solid particles
and voids filled with liquid and/or air, representing a
three-phase system.
• Earth: The soil material in which more than 80%
consist of particles smaller than 2 millimeters (upper
limit of sand particles).
• Mud: The soil in which more than 80% consists of
particles smaller than 0.06 millimeters (upper limit of
silt particles).
• Debris: The soil material that has 20%-80% of particles
that are bigger than 2 millimeters.

The 5 types of landslide movements that can be

observed are categorized as following:
Falls
Falls are downward movements that progress rapidly
and may not be preceded by initial movements or
warnings. They occur when a rocky mass is detached
from a slope along a discontinuity plane that can be
associated with fractures, joints or bedding (Figure 1).
Falls are controlled by the shear strength of the
discontinuity plane which is reduced with mechanical
weathering propagation and the presence of water. Once
detached, a rock boulder will follow a certain trajectory
which depends on its size and shape and the topography
of the region. The movement type can include free-fall,
bouncing, rolling or a combination of those
components.

Topples
Topples are also failures that occur in rocky materials
and resemble falls. However, this type of failure is
associated with a rotational movement that occurs
around a certain point located in a relatively low
position (Figure 2). Topples are controlled by a
combined action of gravitational forces that induce a
bending moment and external forces (e.g., weathering,
water pressure, freeze-thaw cycles).

Slides
Slides refer to ground movements along a specified
surface or zone of weakness. A slide occurs when the
shear stress applied along a surface overcomes its shear
strength. The failure may propagate progressively
initiating from a local failure zone. The main body of
the slide will move downwards separating the stable
from the unstable ground.

There are two main types of slides: rotational and

translational slides. In rotational slides, the failure
surface is curved inwards and points upwards and the
landslide mass is approximately rotating around an axis
transverse to the slide movement and parallel to the
surface of the ground. The movement is usually
associated with shear failure of the ground with its 3-
dimensional geometry being “spoon-shaped”. Certain
features are defined to characterize a rotational slide as
shown in Figure. The surface of rupture is the zone in
which the ground material slides. The main scarp refers
to a relatively steep edge at the head of the landslide
that reveals the undisturbed ground and the visible
component of the rupture surface. The crown is the area
above the main scarp that has not moved downwards.
The main body of the slide is the entire soil mass that
has slipped along the failure surface. The head is the
upper section of the slide between the main scarp and
the displaced ground material. The toe is the most
distanced part from the main scarp where landslide
material has accumulated and the foot refers to the part
of the failed material that has been deposited over the
initial ground surface.

Nevertheless, sometimes the failure surface is

controlled by pre-existing weakness planes (e.g., faults,
fissures, cracks or joints). In this case, an engineering
assessment must recognize those features since the
failure would not be entirely controlled by the
material’s shearing component. A rotational slide will
eventually stop propagating as a stress equilibrium is
restored with the mass movement. An example of a
rotational slide is given in Figure.

Translational slides occur along a pre-defined planar

surface and the ground is subjected to no or little
rotation. Translational slides are mainly controlled by
weakness surfaces (joints, bedding planes etc.) or by the
contact of materials with different shear strength.
Theoretically, a translational slide may propagate
indefinitely given that the rupture surface maintains its
inclination and that its shear strength resistance remains
lower that the driving force.
Lateral Spreads
Lateral spreads are deformational phenomena caused by
liquefaction, the process during which a saturated soil
(usually sands) experiences loss of strength after a
sudden change in its initial stress conditions. Therefore,
the soil tends to behave more like a liquid than a solid.
Such deformations occur on less steep slopes and are
usually triggered by dynamic loads such as that of an
earthquake. Lateral spreading is usually a progressive
process that occurs mainly near shores, riverbanks, and
ports where loose and saturated sandy soils exist.
Infrastructure founded on those type of soils is prone to
extensive damage.
 Flows
According to Varnes (1978), not all types of slope
movements can be categorized in the aforementioned
categories. There are certain landslide phenomena that
take the form of slow or fast-moving flows. In rocks,
there are types of slow movements that result in folding
or bending. Due to the fact that these displacements
resemble viscous fluids, they can be characterized as
rock flows.
Regarding flows in soil materials, Varnes (1978)
distinguished 5 main categories:
1. Debris Flow: Quick soil mass movements that include
less than 50% of fine material. Debris flows are usually
triggered after heavy precipitation saturates and
mobilizes the soil or they can be caused by another type
of landslide upwards (Figure 6a). They spatially occur
near steep gullies where the conditions for such flows
are favorable. An example of accumulated material
after a debris flow is shown in Figure 7a.
2. Debris Avalanche: A rapid debris flow frequently
triggered in steep slopes when the material’s cohesion is
relatively low or/and when the water content is high
(Figure 6b).
3. Earthflow: Flow in fine-grained soils or clayey rocks
under saturated conditions. Liquefaction of the material
leads to the formation of a bowl at the head of the slope
and creates a unique “hourglass” effect (Figure 6c). An
example of an earthflow is illustrated in Figure 7c.
4. Mudflow: A rapid flow consisting of at least 50% clay,
silt and sand particles. They are characterized by a high-
 water content and sometimes they are referred to as
mudslides.
5. Creep: A subtle and steady type of downward flow
caused by shear deformation. Shear stresses are high
enough to cause displacements but not adequate to
result in shear failure. A common feature that reveals
creeping flow is the presence of titled tree trunks
(Figure 7b).

Figure 6

Figure 7
Complex Landslides
A complex landslide comprised of a combination of the
deformational components discussed above. Common
complex landslides include an initial rotational or
translational component followed by certain type of
flow. By combining the type of movement and the type
of the material, Varnes (1978) developed a
classification of landslide phenomena, as presented in
Table.

Causes of Slope Failures

Slope failures can be triggered by natural of human-
induced causes, or a combination of the two. The
natural causes of landslides include: gravitational forces
that tend to destabilize the ground, water saturation,
erosion, dynamic loads (e.g., earthquakes), the sudden
uplift of the aquifer level, volcanic eruptions and
freeze-thaw weathering cycles.

The presence of water is one of the most common

factors that triggers landslides. Water saturation can be
caused as a result of heavy precipitation, snow melt or
changes in the ground water level. Water saturation
reduces the shear strength of soils. In particular, it
decreases the normal effective stress that acts between
the grains and hence, the frictional resistance is
reduced. The Mohr-Coulomb failure criterion suggests
that the shear strength of the ground is proportional to
the normal effective stress as:

Where t is the shear strength, σn is the effective stress,

σt is the total stress, u is the pore-water pressure, c is the
cohesion, and φ the friction angle.

Earthquakes are also triggering factors of landslides.

Ground shaking imposes destabilizing horizontal and
vertical loads (with the first being the most important)
and can also result in liquefaction. Seismic shaking can
also act as a contributing rather than a triggering factor
 as it may cause deterioration of the ground’s shear
strength and destabilize the slope. The slope may
subsequently be prone to land sliding in static
conditions, for example after heavy precipitation, or
when another earthquake occurs.

The human-induced causes of landslides are actions that

can destabilize slopes, including: toe excavations,
infrastructure loads acting on a slope, machine
vibrations that apply dynamic loads, construction of
weak embankments or earth dams, and deforestation
that may exacerbate extensive flooding and debris/earth
flows.

Role of Water
Although water is not always directly involved as the
transporting medium in mass movement processes, it
does play an important role.
Water becomes important for several reasons-
1. Addition of water from rainfall or snow melt adds
weight to the slope. Water can seep into the soil or rock
and replace the air in the pore space or fractures. Since
water is heavier than air, this increases the weight of the
soil. Weight is force, and force is stress divided by
area, so the stress increases and this can lead to slope
instability.
2. Water has the ability to change the angle of repose (the
slope angle which is the stable angle for the slope).
Think about building a sand castle on the beach. If the
sand is totally dry, it is impossible to build a pile of
 sand with a steep face like a castle wall. If the sand is
somewhat wet, however, one can build a vertical wall.
If the sand is too wet, then it flows like a fluid and
cannot remain in position as a wall.
• Dry unconsolidated grains will form a pile with a slope
angle determined by the angle of repose. The angle of
repose is the steepest angle at which a pile of
unconsolidated grains remains stable, and is controlled
by the frictional contact between the grains. In general,
for dry materials the angle of repose increases with
increasing grain size, but usually lies between about 30
and 45°.

• Slightly wet unconsolidated materials exhibit a very

high angle of repose because surface tension between
the water and the solid grains tends to hold the grains in
place.
• When the material becomes saturated with water, the
angle of repose is reduced to very small values and the
material tends to flow like a fluid. This is because the
water gets between the grains and eliminates grain to
grain frictional contact.

3. Water can be adsorbed or absorbed by minerals in the

soil. Adsorption, causes the electronically polar water
molecule to attach itself to the surface of the minerals.
Absorption causes the minerals to take the water
molecules into their structure. By adding water in this
fashion, the weight of the soil or rock is increased.
 Furthermore, if adsorption occurs then the surface
frictional contact between mineral grains could be lost
resulting in a loss of cohesion, thus reducing the
strength of the soil.

In general, wet clays have lower strength than dry clays,

and thus adsorption of water leads to reduced strength
of clay-rich soils.

4. Water can dissolve the mineral cements that hold grains

together. If the cement is made of calcite, gypsum, or
halite, all of which are very soluble in water, water
entering the soil can dissolve this cement and thus
reduce the cohesion between the mineral grains.
5. Liquefaction - As we have already discussed,
liquefaction occurs when loose sediment becomes
oversaturated with water and individual grains loose
grain to grain contact with one another as water gets
between them.
This can occur as a result of ground shaking, as we
 discussed during our exploration of earthquakes, or can
occur as water is added as a result of heavy rainfall or
melting of ice or snow. It can also occur gradually by
slow infiltration of water into loose sediments and soils.

The amount of water necessary to transform the

sediment or soil from a solid mass into a liquid mass
varies with the type of material. Clay bearing sediments
in general require more water because water is first
absorbed onto the clay minerals, making them even
more solid-like, then further water is needed to lift the
individual grains away from each other.
6. Groundwater exists nearly everywhere beneath the
surface of the earth. It is water that fills the pore
spaces between grains in rock or soil or fills fractures
in the rock. The water table is the surface that separates
the saturated zone below, wherein all pore space is
filled with water from the unsaturated zone above.
Changes in the level of the water table occur due
changes in rainfall. The water table tends to rise during
 wet seasons when more water infiltrates into the
system, and falls during dry seasons when less water
infiltrates. Such changes in the level of the water table
can have effects on the factors (1 through 5) discussed
above.

7. Another aspect of water that affects slope stability is

fluid pressure. As soil and rock get buried deeper in the
earth, the grains can rearrange themselves to form a
more compact structure, but the pore water is
constrained to occupy the same space. This can
increase the fluid pressure to a point where the water
ends up supporting the weight of the overlying rock
mass. When this occurs, friction is reduced, and thus
the shear strength holding the material on the slope is
also reduced, resulting in slope failure.

Safe and Unsafe Slopes

Safe. Slope stability refers to the condition of inclined

soil or rock slopes to withstand or undergo movement.
 A slope can be globally stable if the safety factor,
computed along any potential sliding surface running
from the top of the slope to its toe, is always larger
than 1.

Unsafe. Slopes considered unstable due to their incline

(or critical angle of repose), applied to slopes made of
unconsolidated material. Unstable slopes are prone to
failure in the form of rockfalls, rock flows, plane
shears, or rotational shears.

Types of Slope Failures

Soil slope failures are generally of four types:

1. Translational Failure
2. Rotational Failure
3. Wedge Failure
4. Compound Failure

Translation Failure
• Translation failure occurs in the case of infinite slopes
and here the failure surface is parallel to the slope
surface.
• A slope is said to be Infinite, when the slope has no
definite boundaries and soil under the free surface
contains the same properties up to identical depths
along the slope.
• A slope is said to be Infinite, when the slope has no
definite boundaries and soil under the free surface
contains the same properties up to identical depths
 along the slope.
• A slope is said to be Infinite, when the slope has no
definite boundaries and soil under the free surface
contains the same properties up to identical depths
along the slope.

Rotational Failure
• In the case of rotational failure, the failure occurs by
rotation along a slip surface and the shape thus
obtained in slip surface is curved. Failed surface moves
outwards and downwards.
• In homogeneous soils, the shape is circular while in
case of non-homogeneous soils it is non-circular.
• Rotational failure may occur in three different ways :
1. Face failure or slope failure
2. Toe failure
3. Base failure
• Face failure occurs when soil above the toe contains
weak stratum. In this case the failure plane intersects
the slope above toe.
• Toe failure is the most common failure in which
failure plane passes through toe of slope.
• Base failure occurs when there is a weak soil strata
under the toe and failure plane passes through base of
slope.
• Rotational failure can be seen in finite slopes such as
earthen dams, embankments, man-made slopes etc.

Wedge Failure
• Wedge failure, also known as block failure or plane
failure, generates a failure plane that is inclined.
• This type of failure occurs when there are fissures,
joints, or weak soil layers in slope, or when a slope is
made of two different materials.
• It is more similar to translational failure but the
difference is that translational failure only occurs in
case of infinite slopes but wedge failure can occur in
both infinite and finite slopes.

Compound Failure
• A Compound failure is a combination of translational
slide and rotational slide.
• In this case, the slip surface is curved at two ends like
rotational slip surface and flat at central portion like in
translational failure.
• The slip surface becomes flat whenever there is a hard
soil layer at a considerable depth from toe.

Prevention of Landslides

To reduce the chances of landslides: vegetation

cover protects land from land slides and soil erosion.
Therefore, efforts should be made to maintain
greenery particularly on slopes. Provisions should be
made at community level to prevent people from
excavating, removing materials from the soil or
cutting trees. Trees should be planted on slopes and
slope base to prevent erosion. Records of erosion,
landslide masses and falling rocks should be
maintained. Before building house, information
should be gathered about site and history of landslides
in the area. During constructing a building on a slope,
design that suits the natural slope should be adopted.
Vegetation and large trees should not be removed
while constructing. Natural streams or drainage paths
should not be obstructed during construction. Surface
water should be diverted towards the natural galley
enabling water to quickly drain away from the slope.

In landslide prone areas: listen to weather forecast

on the radio, TV etc. about heavy rains. During nights
residents should remain awake of heavy continuous
rain and be ready to move immediately to a safer
location. Abnormal sounds of soil and rock movement
or breaking of trees may be followed by landslides
hence, these should be listened attentively and
considered seriously. To observe cracks on the slope
one should not move closer to the slope. If residents
have to evacuate place it should be done immediately
without wasting time to collect belongings. While
evacuating, efforts should be made to avoid possible
landslide paths because landslide can occur suddenly.
If rocks are falling one should immediately seek cover
behind trees and other solid objects. Efforts should be
made to stay together and support each other as far as
it is possible and useful. Special attention should be
paid for very small children, very old people and sick
or disabled people.

During Landslide: Preferable one should stay where

people are available around. During disaster children
need special attention and comfort than normal
situation. Attentively all their questions should be
answered up to maximum possible extent even if they
ask the same question again and again. One should
talk children and assure them that they are safe. In
fact, more courage, strength and energy is needed to
face the situation successfully so one should conserve
energy and postpone unnecessary work and divert the
energy to accomplish necessary work only.

After Landslide: After landslide one should not enter

the area without permission from the authorities.
Although field and buildings may seem to be as
before landslide but one should not enter damaged
area or buildings until the authorities declare them as
safe. Persons engaged in the removal of debris or the
digging up of bodies buried under in the mass should
do their work in an organized manner. If water is
available in the debris deposit it should be removed
first and all water paths should be diverted away from
the affected slope area and the debris. Children should
not be allowed to go through the loose and new
 deposits of debris because the surface may appear to
be dry but the wet conditions can prevail within the
mass. Damaged area should be replanted or priority
during next season of planting to avoid erosion.

Retaining walls as a landslide solution

Retaining walls are structures designed to restrain the
soil. They are normally used in areas with steep slopes
or where the landscape needs to be shaped severely
for construction or engineering projects. However,
retaining walls have been found to be a very efficient
solution against landslides. There are various ways of
constructing a retaining wall, the most common types
being:
➢ Gravity walls: they manage to resist pressure from
behind due to their own mass.
➢ Piling walls: made of steel they are usually used in
tight spaces with soft soil having 2/3 of the wall
beneath the ground.
➢ Cantilever walls: they have a large structural footing
and covert horizontal pressure from behind the wall
into vertical pressure on the ground below.
➢ Anchored walls: they use cables or other stays
anchored in the rock or soil behind to increase
resistance.

The type of wall that will be used depends on the

circumstances of every case. Soil type, slope angle,
groundwater characteristics and other specifics will be
considered before deciding on the proper solution.
Geological Consideration in Alignment
Natural slope stability should be preserved while
constructing hill roads. In view of this, geological and
hydrological conditions should be studied and taken
into consideration.

Stability of a slope depends on the nature of the rock,

inclination or dip of the strata, geological defects like
folds and faults, and the ground water conditions.

Hill roads are constructed by cutting along the face of

the hill. The stability of the hill face or slope is vital
for the safety of the road.

The alignment should be such that the bedding planes

in the case of sedimentary rocks dip away from the
cut slopes.

Cuts in granite, limestone and rubble are generally

stable, while those in loam, clay, clay loam and shale
are vulnerable.

Cuts for hill roads should not cause conditions that

trigger landslides; for example, steepening of cuts in
fill or unstable rock, overloading of bedding planes in
stratified soils or relatively weak underlying soils
made of fill material, and restriction of ground water
flow by the fill will cause a wide range of ground
movements and failure of slopes.
The judgment and technical expertise of an
engineering geologist and/or a geotechnical engineer
is considered valuable in the alignment and
construction of hill roads.

Case Histories (can be studied from the Internet)