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