DAW SWE HNIN MAUNG
LECTURER
•Groundwater is considered a very important natural
resource.
•In arid, semi-arid and dry regions, this may be the only
source of water supply.
• Even in humid areas, groundwater is considered a better
resource for many economic and hygienic reasons.
•Groundwater originates chiefly from the infiltration of
meteoric water, melted snow, and seepage from streams,
lakes ponds and reservoirs.
•Groundwater is not chemically pure H2O but are in solution
of suspension with various.
•Groundwater may contain air and other gases.
•The subsurface water can be distinguished into two main
types: (i) vadose water and (ii) groundwater.
• This type of water occurs from surface downwards
up to a variable depth and is in a state of
downward movement under the influence of
gravity.
• Its movement is commonly described as
INFILTRATION.
• The thickness of soil and rock through which the
vadose water infiltration is called zone of aeration.
• Obviously, in the zone of aeration the soil and rocks
remain unsaturated with water
• This includes all the subsurface water reaching a depth
blow which all the pore spaces, openings and other
cavities of the soil and rock are completely filled with
water.
• The thickness, length and width of the saturated strata,
the aquifer, constitute the groundwater reservoir in a
given area.
• In this zone (of saturation), movement of water is
principally under the influence of a hydrostatic head.
• It is commonly described by the term Percolation, and
is generally lateral in character.
• Subsurface water may be found to exist
in two main zones or environments :
• (1)Vadose water and
• (2) Groundwater.
•This zone can be distinguished into three different types of environments;
soil water, intermediate Vadose water and capillary water.
• The soil water forms a thin zone confined to the near surface depth of the
land.
• It is held up by root zone of vegetable cover and soil chemicals.
• It is lost to the atmosphere by transpiration and evaporation.
•The intermediate Vadose zone occurs immediately below the zone of soil
water.
•It is, in fact, a zone of non-saturation; water in this zone is moving
downwards under the influence of gravity.
•It is generally small thickness and may be absent in many cases.
•The above two zones are sometimes collectively referred as zone of
aeration.
•The zone of capillary water, called capillary fringes is
present only in soils and rocks of fine particle size
underlying the Vadose zone.
• It is absent in the coarse sediments.
• In this zone groundwater is drawn upwards by capillary
action.
• Growth of vegetation observed in some deserts is very
often dependent on the presence of the capillary fringe.
•The cause of rise of water in the capillaries of fine
sediments is the well-known force of surface tension.
•This zone is called the zone of saturation lies below the
capillary fringe, and it is the water held in this zone that is called
groundwater in the real sense.
•The upper surface of water in this zone marks the water table
in the area.
• All the openings are thoroughly interconnected so that a well
dug into this zone receives water from the entire saturated rock
mass, called the aquifer.
•The predominant movement is a type lateral flow controlled by
the head of the water.
• In many cases a significant rise and fall in the level of this zone
is observed as a characteristic feature during the different parts
of the year depending upon rainfall and recharge.
• An aquifer is a rock mass, layer or formation, which
is saturated with groundwater and which by virtue
of its properties, is capable of yielding the stored
water at economical costs when tapped.
• Gravels, limestone, and sandstones generally form
good aquifer when occurring in suitable geological
conditions and geographic situations.
• A rock body of formation, which may be porous
enough to hold sufficient quantity of water but
which by virtue of its other properties does not
allow an easy and quick flow through is, is called an
aquiclude.
• It is to be treated as a practically impermeable rock
mass.
• Compacted clay formations are the best examples
of aquicludes.
• Two basic types of aquifers are distinguished on the
basis of physical conditions under which water can
exist in them:
• (1) the unconfined aquifer and
• (2) the confined aquifer.
• It is also called water table aquifer, is the most common type encountered in the
field.
• In this type, the upper surface of water or the water-table is under atmospheric
pressure which may be acting through the interstices in the overlying rocks .
• Water occurring in this type of aquifer is called Free Groundwater.
• When tapped through a test well, the free water will rise to a level equivalent to the
water table of the area.
• Water table:The depth to upper surface of zone of saturation in free groundwater
is called water table.
• It fluctuates depending on many factors and may be as low as one metre or so or as
deep as 100 metres or more.
• It is the term used for isolated water table in aquifer held
by a small extension of impervious rock within a large
pervious tract.
• In such cases, the main water table is located much
below.
• Supplies from such isolated reservoirs with perched
water table are often unreliable.
• It is a rock formation saturated with water and capable
of yielding water when tapped but unlike unconfined
aquifer, has an overlying confining layer (an
impermeable rock mass) that separates it from the
influence of atmospheric pressure.
• Naturally, water held in this type of aquifer is not under
atmospheric pressure but under great pressure due to
the confining medium.
• The upper surface of water in a confined aquifer is
called piezometric surface.
• For establishing a piezometric surface, level of water in
a number of test wells has to be made.
• It is, in fact, confined aquifer of such a geometry and developed in
suitable geological situations so that the piezometric surface is
above the ground level at many places when projected in elevation.
• When water is tapped from such a confined aquifer, it rushes up to
and even above the surface and may rise to the heights theoretically
equivalent to the projected piezometric surface.
• Such wells are called Artesian Wells, or flowing wells and the type
of groundwater obtained from them, which often needs no pumping,
as Artesian Wells .
• The neighbourhood of ocean or sea, salt water
encroaches on fresh water, and contaminates it and vice
versa.
• Fresh water may over ride and displaces salt water.
• It is defined as volume of voids or openings in a rock
mass expressed in percentage terms of the total volume
of the rock.
• Most rock can hold some water.
• Sedimentary rocks such as sandstones, conglomerate, and
limestone, tend to have a high porosity and therefore can
hold a considerable amount of water.
• A deposit of loose sand may have a porosity of 30% to
50%, but this may be reduced to 10% to 20% by
compaction and cementation as the sand lithifies.
• Sandstone in which pores are nearly filled with cement
and fine-grained matrix may have porosity of 5% or less.
• Crystalline rocks, such as granite, schist, and some
limestones, do not have pores may hold water in joints
and other openings.
• It refers to the capacity of a rock to transmit
fluids (water or petroleum) through it.
• It is often expressed as Intrinsic Permeability,
the unit which is the capacity of a rock of 1 cm
length and 1 cm2 cross sectional area to allow
a flow of 1cm3/ sec, at a difference of 1
atmosphere
• Groundwater is favourable factor if it is used for
human, stock consumption, or irrigation or for
industrial purposes.
• However, contrary to this usefulness of groundwater
it composed problem to stability of engineering
structure, tracks of land, stability of slopes, and
seepage through under the dams.
• Most specificant, such cases involve landslides and
sliding of dams, heaving or flooding from rising water
table that threatens deep excavation , humid
basement of buildings and varieties of other
geological hazards.
• The engineer has first to establish the location of groundwater and
second to estimate the quantity of groundwater survey should be
made.
• Geological aid is of importance at all stages of groundwater
investigation.
• If elevation of groundwater table at sufficient points and at a given
times is known, contour-lines water table maps may be prepared.
• Usually elevations of the water table are determined in wells or
boreholes.
• Both high and low groundwater stages should be recorded in any
groundwater investigation, and because of the water table
fluctuations, the date of each groundwater table determination should
be recorded.
• Geophysical methods also may be used for groundwater investigation.
• If contour map is prepared, using the elevations of the water table, the
direction of groundwater flow would be normal to the contour lines.
• When water percolates into the soil, it enters the voids and starts to fill them up.
• As a result, pore water pressure starts to rise.
• Pore water pressure is the pressure acting on soil grains by water held in the
pores.
• Pore water pressure can be positive or negative.
• It is negative when the voids are only partially filled with water. (This state is
also known as soil suction.)
• Pore water pressure becomes neutral just before the point at which the voids
become completely filled with water.
• Pore water pressure becomes positive at the point when all the air has expelled
from the voids and the water phase in the soil-water mix becomes continuous.
• As that point, the water phase become a column and hydrostatic pressure,
equivalent to the height of the column, is exerted within the pores: The pressure
is transferred to the grains.
•If the hydrostatic pressure is sufficiently high it will force the
grains apart and the mixture will start to behave as a liquid.
•Hydrostatic pressure developed near the soil surface, as when
the upper layer becomes saturated during the heavy rain, caused
the soil to flow.
•When pore water pressure becomes positive along the walls of
a fissure underground, a pipe develops.
• A pipe is an enlarged fissure that forms underground in finegrained, non-cohesive soil, especially silty or fine sandy soil.
•Enlargement of the fissure takes place when water, flowing
along the fissure or into the fissure from the side walls, detaches
particles of soil carries them away in suspension.
• Pipes that have not broken through to the surface can still
sometimes be detected by the presence of an elongated hollow
of subsided ground pointing down the slope.
•The trench may be above the head of a gully and in
the same alignment as the gully, indicating that water
is moving into the gully head as groundwater through
a pipe.
•If water travels downwards to the bottom of the soil
profile, it commonly becomes halted in its path by the
impermeable surface of the rock beneath.
• It then migrates downhill along the interface until it
emerges as a spring at a point where the soil becomes
shallower or the rock outcrops at the surface.
•Power water pressure may become positive at the
base of the soil profile, resulting in a deep transitional
landslide (the commonest deep type) or circular
failure.
•If water goes deeper than the soil profile, it goes into the bedrocks.
• In horizontal rocks, it will move sideways, slowly, along the surface an
impermeable layer.
•In tilted beds it will move more rapidly down the slope. If the rock is
fractured, the water will continue to go deeper.
•Hydrostatic pressure is exerted within the open joint systems of rocks in
exactly the same way as in soils.
•If the water cannot escape as spring water, high pressure can develop and
force the joints apart.
•This is the cause of many rockslides.
•A water movement through the rocks is controlled by: The permeability of
the rock, the angle of bedding, and the number, orientation, openness and
continuity of fractures.
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