Understanding Civil Engineering: Dams, Reservoirs, and Geological Considerations

 
CIVIL ENGINEERING
Civil engineering work includes:
 
 dams
 embankments
 motorways
 bridges
 buildings
 cuttings
 quarries
 tunnels
 mines
 
All these need to take into account:
 
1.
Geological factors
2.
Geological rock properties
 
 rock type
 fracture/joints
 weathering
 dip of rocks
 folding/faulting
 cleavage
 water table depth
 porosity/permeability
 rock strength
 
Geological factors
 
Rock properties
 
Dams
&
Reservoirs
 
Water supply
 
Power
generation
 
Irrigation
 
Flood control
Dams & reservoirs are constructed for a wide
range of uses:
 
 
Gravity Dams
Types of Dams
 
There are many different designs of dam, which include
two principal types:
 
 
Arch Dams
 
Gravity Dam
 
 
Arch Dam
 
When building dams &
reservoirs geologists
must take into account:
 
 geological structures
 
 rock properties
 
 
Gravity Dams
Types of Dams Across Constricted Valleys
 
 
Arch Dams
 
These rely on their weight
to hold them in position and
thereby impound the water.
They are usually made from
reinforced earth, masonry or
concrete.
 
The arch squeezes together as
the water pushes against it. The
stress of the impounded water is
also transmitted horizontally into
the rock of the valley sides. These
are made from thin concrete walls.
 
 
Valley shape and rock structure
 
Foundation strength
 
Porosity and permeability
 
Zones of structural weakness and high
permeability
In building major structures like arch dams
and gravity dams 
geological factors
 and
geological rock properties
 must be taken into
account. These include:
 
 
Narrow
, 
deep
 and 
steep-sided
 valleys are the key
characteristics looked for in valley shape. This is due to:
 
Valley
constriction
minimises
dam length
reducing
costs
 
Narrow width
of dam makes
it structurally
stronger
 
Deep valleys
maximise
water
storage
1. Valley Shape:
 
Ideally, above a
chosen dam
site a valley
should 
widen
and remain as
flat
 as possible
 
 
The 
rock structure
 surrounding the reservoir has to be
looked at carefully:
 
Synclinally folded rocks dip
towards the reservoir, reducing
possible leakage but increasing
their liability to slip into the
reservoir. Anticlines increase
leakage but are less prone to slip.
 
Other zones of weakness include fault zones which must be
avoided. Also fault zones may have associated earthquakes.
2. Rock Structure:
 
The rocks of the valley sides must not be liable to slippage
because as the reservoir fills, the water table rises to
lubricate zones of weakness.
 
 
Gravity Dams
3. Foundation Strength:
 
Obviously 
good foundations
 are highly desirable because
the force of the dam must not exceed the strength of the
ground. Or it will fail!
Site to serve as a foundation for major dams depends on
factors:
a.
Existing rock type
b.
Extent of weathering it has undergone
c.
Occurrence of intrusions
d.
Extent of fracturing
e.
Occurrence of geological structures – Faults, Joints
f.
Mode and number of rock types
 
Suitability of Rocks as Foundation
 
Igneous rocks: 
Massive Plutonic and/or Hypabassal igneous rocks
(Granite, Syenites, gabbro etc.,) are most desirable as they are very
strong and durable (not effected by weathering).
Sedimentary rocks
: Bedding and its orientation, thickness of beds,
nature and extent of compaction and cementation, grain size etc.,
influence the strength and durability of the rock. Sandstone -well
cemented types and limestone – if massive, are considered competent
and suitable for dam foundation. Whereas laterites, shales are most
undesirable.
 
Metamorphic rocks: 
Gneisses are generally competent, unless the
posses very high degree of foliation. Quartzite is considered to
desirable as it is very hard and highly resistant to weathering.Schists,
Marbles, Slates etc., are considered unsuitable for dam sites.
 
Gravity Dams
 
Where foundations are weak (clays and mudstones) gravity
dams made out of earth are used, in order to spread the
weight.
 
On firmer foundations (igneous and
metamorphic rocks) gravity dams
made out of concrete and masonry
are used.
 
The foundations for
gravity dams should show
limited compaction when
loaded with the weight of
the dam. Consequently
deeply weathered sites or
poorly consolidated glacial
deposits are avoided.
 
Existing fractures or
bedding should be
minimal, and should
dip upstream.
 
 
Arch Dams
 
The construction of arch dams
relies on the rock mass strength
of the valley sides.
 
Where there is sufficient
strength, thin arch dams, which
are cheap and economical to
build, may be adequate.
 
However, since the stresses imposed in such situations
are horizontal instead of vertical, an absence of fractures
parallel to the valley sides is essential.
 
In general, only rocks with very high rock mass strength
are suitable for building arch dams, and in all cases
uniformity of the rock body is desirable.
 
Gravity Dams
4. Porosity & Permeability:
 
The porosity and permeability of the rocks surrounding
the reservoir and the dam indicate sources of 
potential
leakage
. Such as:
 
Permeable soils
 
Aquifers
 
Faults
 
Joints
 
A dam should not be built on a 
permeable base
 because
leakage under a dam generates pressure which lifts the
dam and may cause it to fail.
 
Beds dipping away from the reservoir e.g. anticlines
 
Rocks with calcite cement may become more permeable
as the calcite dissolves away.
 
Parts of a Dam
 
Buttress dams
A buttress dam or hollow dam is a dam with a solid, water-tight
upstream side that is supported at intervals on the downstream side
by a series of buttresses or supports. The dam wall may be straight
or curved. Most buttress dams are made of reinforced concrete and
are heavy, pushing the dam into the ground.
 
 
Earthen dams
Earth fill dam, also called Earth Dam, or
Embankment Dam, dam built up by
compacting successive layers of earth,
using the most impervious materials to
form a core and placing more permeable
substances on the upstream and
downstream sides.
 
These are planned in places
where underlying materials are too weak
to support a masonry dam (where
competent rocks are at deeper depths).
 
 
Geological Factors
 
Rock Properties
 
Hydrogeology
 
Rock type
Weathering
Structural weaknesses
(bedding, joints, faults & cleavage)
Depth to rockhead
 
Rock strength
Porosity
Permeability
 
Depth to water table
 
Geology of Major Dam sites in INDIA
 
Bhakra – Nangal Project – Punjab
It is located on a thick Sandstone beds intercalated with
bands of Silt stones and lies in southern limb of Ramgarh-Dhar
Anticline.
Downstream Dip – 70
Strike – Slightly oblique (NW-SE).
Claystones – 30m (Upstream), 76m (downstream), 9m (band in
middle third section of dam).
 
Nagarjuna sagar Dam
 The rock types exposed in and around the dam sites are the Granite-Gneisses of the
peninsular gneissic complex and the Quartzites and shales belonging to Srisailam stage of
Cudappah systems. Few dolerite dykes intrusive into granite-gneisses.
 Strike – N 60
 W-S 60
 E and N 40
 E-S 40
 W.
 Quartizes and shales are -  3
 to 5
 dip towards downstream.
 Minor Faults rarely exceeding 0.60-0.91m are observed in Quartzites and shales.
Weathering of granite gneisses are limited to 0.6-4.6m.
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Civil engineering work encompasses various structures like dams, embankments, motorways, bridges, buildings, and more. Building dams and reservoirs involves considerations of geological factors, rock properties, and structural design. Different types of dams, such as gravity dams and arch dams, serve different purposes like power generation, water supply, irrigation, and flood control. Geologists play a crucial role in assessing valley shapes, rock structures, and other geological aspects when constructing major structures like dams. Attention to detail in geological features is essential for the successful construction of such projects.


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  1. CIVIL ENGINEERING

  2. Civil engineering work includes: dams embankments motorways bridges buildings cuttings quarries tunnels mines All these need to take into account: 1. Geological factors 2. Geological rock properties rock type fracture/joints weathering dip of rocks folding/faulting cleavage water table depth porosity/permeability rock strength Geological factors Rock properties

  3. Dams & Reservoirs

  4. Dams & reservoirs are constructed for a wide range of uses: Power generation Water supply Irrigation Flood control

  5. Types of Dams There are many different designs of dam, which include two principal types: Gravity Dams Arch Dams

  6. Arch Dam Gravity Dam

  7. When building dams & reservoirs geologists must take into account: geological structures rock properties

  8. Types of Dams Across Constricted Valleys Gravity Dams These rely on their weight to hold them in position and thereby impound the water. They are usually made from reinforced earth, masonry or concrete. Arch Dams The arch squeezes together as the water pushes against it. The stress of the impounded water is also transmitted horizontally into the rock of the valley sides. These are made from thin concrete walls.

  9. In building major structures like arch dams and gravity dams geological factors and geological rock properties must be taken into account. These include: Valley shape and rock structure Foundation strength Porosity and permeability Zones of structural weakness and high permeability

  10. 1. Valley Shape: Narrow, deep and steep-sided valleys are the key characteristics looked for in valley shape. This is due to: Ideally, above a chosen dam site a valley should widen and remain as flat as possible Deep valleys maximise water storage Valley constriction minimises dam length reducing costs Narrow width of dam makes it structurally stronger

  11. 2. Rock Structure: The rock structure surrounding the reservoir has to be looked at carefully: Synclinally folded rocks dip towards the reservoir, reducing possible leakage but increasing their liability to slip into the reservoir. Anticlines increase leakage but are less prone to slip. The rocks of the valley sides must not be liable to slippage because as the reservoir fills, the water table rises to lubricate zones of weakness. Other zones of weakness include fault zones which must be avoided. Also fault zones may have associated earthquakes.

  12. 3. Foundation Strength: Obviously good foundations are highly desirable because the force of the dam must not exceed the strength of the ground. Or it will fail! Gravity Dams Site to serve as a foundation for major dams depends on factors: a. Existing rock type b. Extent of weathering it has undergone c. Occurrence of intrusions d. Extent of fracturing e. Occurrence of geological structures Faults, Joints f. Mode and number of rock types

  13. Suitability of Rocks as Foundation Igneous rocks: Massive Plutonic and/or Hypabassal igneous rocks (Granite, Syenites, gabbro etc.,) are most desirable as they are very strong and durable (not effected by weathering). Sedimentary rocks: Bedding and its orientation, thickness of beds, nature and extent of compaction and cementation, grain size etc., influence the strength and durability of the rock. Sandstone -well cemented types and limestone if massive, are considered competent and suitable for dam foundation. Whereas laterites, shales are most undesirable. Metamorphic rocks: Gneisses are generally competent, unless the posses very high degree of foliation. Quartzite is considered to desirable as it is very hard and highly resistant to weathering.Schists, Marbles, Slates etc., are considered unsuitable for dam sites.

  14. Gravity Dams Where foundations are weak (clays and mudstones) gravity dams made out of earth are used, in order to spread the weight. The foundations for gravity dams should show limited compaction when loaded with the weight of the dam. Consequently deeply weathered sites or poorly consolidated glacial deposits are avoided. Existing fractures or bedding should be minimal, and should dip upstream. On firmer foundations (igneous and metamorphic rocks) gravity dams made out of concrete and masonry are used.

  15. The construction of arch dams relies on the rock mass strength of the valley sides. Where there is sufficient strength, thin arch dams, which are cheap and economical to build, may be adequate. Arch Dams However, since the stresses imposed in such situations are horizontal instead of vertical, an absence of fractures parallel to the valley sides is essential. In general, only rocks with very high rock mass strength are suitable for building arch dams, and in all cases uniformity of the rock body is desirable.

  16. 4. Porosity & Permeability: The porosity and permeability of the rocks surrounding the reservoir and the dam indicate sources of potential leakage. Such as: Gravity Dams Permeable soils Aquifers Faults Joints Beds dipping away from the reservoir e.g. anticlines A dam should not be built on a permeable base because leakage under a dam generates pressure which lifts the dam and may cause it to fail. Rocks with calcite cement may become more permeable as the calcite dissolves away.

  17. Parts of a Dam

  18. Buttress dams A buttress dam or hollow dam is a dam with a solid, water-tight upstream side that is supported at intervals on the downstream side by a series of buttresses or supports. The dam wall may be straight or curved. Most buttress dams are made of reinforced concrete and are heavy, pushing the dam into the ground.

  19. Earthen dams Earth fill dam, also called Earth Dam, or Embankment Dam, compacting successive layers of earth, using the most impervious materials to form a core and placing more permeable substances on the downstream sides. These are planned in places where underlying materials are too weak to support a masonry dam (where competent rocks are at deeper depths). dam built up by upstream and

  20. Geological Factors Rock Properties Rock type Weathering Structural weaknesses (bedding, joints, faults & cleavage) Depth to rockhead Rock strength Porosity Permeability Civil Engineering Hydrogeology Depth to water table

  21. Geology of Major Dam sites in INDIA Bhakra Nangal Project Punjab It is located on a thick Sandstone beds intercalated with bands of Silt stones and lies in southern limb of Ramgarh-Dhar Anticline. Downstream Dip 70 Strike Slightly oblique (NW-SE). Claystones 30m (Upstream), 76m (downstream), 9m (band in middle third section of dam).

  22. Nagarjuna sagar Dam The rock types exposed in and around the dam sites are the Granite-Gneisses of the peninsular gneissic complex and the Quartzites and shales belonging to Srisailam stage of Cudappah systems. Few dolerite dykes intrusive into granite-gneisses. Strike N 60 W-S 60 E and N 40 E-S 40 W. Quartizes and shales are - 3 to 5 dip towards downstream. Minor Faults rarely exceeding 0.60-0.91m are observed in Quartzites and shales. Weathering of granite gneisses are limited to 0.6-4.6m.

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