GEOSKILLS 101: ENGINEERING GEOLOGY :
DAM PROJECT
As an engineering geologist, what are
the pertinent issues or questions that you have to answer in the site
investigation phase for a dam project.
The
engineering geologist must be able to answer the following questions :
1) what is the depth of overburden that
must be removed to reach an acceptable foundation for the dam wall
2) what are the rock types which make up
the foundation and to what extent are they affected by surface weathering
3) what are the engineering properties
of the foundation rock types (important properties are strength, deformability
and durability)
4) what is the geological structure of
the foundation (ie jointing, faulting and folding of the rock strata). A full
description of the defect pattern in the rock mass should include orientation,
spacing, extent or persistence and aperture or openess
5) how permeable is the rock foundation
(ie to what extent are the rock defects such as joints, faults and bedding
open)
6) where can adequate supplies of
construction materials such as clay, sand, gravel and rock fill be obtained,
preferably as close as possible to the dam site
7) will the rock that must be excavated
to provide a spillway for the dam be acceptable for use as rock fill in the
construction of the dam embankment
8) will the spillway require concrete
lining and an energy dissipation structure at its downstream end or is the
spillway rock sufficiently erosion resistant that these can be omitted.
Methods commonly used to explore sites (geological investigation) for dam
construction projects
The primary purpose of
geological/site investigations for a dam project is provide the information
that dam designers require in order to design a safe dam structure and to be
able to estimate with reasonable accuracy how much the dam is going to cost.
The dam site must be explored by an experienced engineering geologist. Methods
commonly used to explore sites for construction projects are:
·
Geological
mapping of surface rock outcrops.
·
Geophysical
surveys. Seismic refraction is often used to determine depth of overburden.
·
Excavation
of trenches and pits using bulldozers, backhoes etc.
·
Diamond
core drilling. As usually carried out this method recovers an undisturbed,
cylindrical sample (a core sample) about 50 mm in diameter from depths of a few
metres to hundreds of metres, if necessary. Other types of drilling which
recover disturbed samples may also be used in some circumstances.
·
concrete
dam and fill (embankment) dam
Dams can be grouped according to the type of material of
which they are constructed as follows; concrete dams are further grouped
according to how they achieve their strength and stability. The difference
between concrete dam and fill (embankment) dam.
Concrete
dam
|
Fill
(embankment) dam
|
Made up
of solid concrete
|
Made up
of soil and gravel
|
Can be
constructed to any height
|
Can be
constructed with moderate height
|
Less
maintenance – low cost to operate
|
Cheaper
to build
|
Example:
Warrangamba Dam in Australia, is a type of concrete gravity dam
|
Example:
Bakun Dam in Malaysia, is a type of concrete faced rock fill dam
|
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