BGS238 PROFESSIONAL SKILLS FOR APPLIED GEOCSIENTISTS
SCHOOL OF EARTH & ENVIRONMENTAL SCIENCES Engineering Geology of Dams
Engineering Geology of Dams •
Brain-storm the various geological and engineering aspects of dam construction.
1
Engineering Geology of Dams •
What is the purpose of a dam?
•
Do you need to extend your list above to include aspects to do with the reservoir area?
Engineering Geology of Dams •
What will influence the positioning of the dam?
2
Engineering Geology of Dams •
How can we work out how big (how high) the dam needs to be?
Engineering Geology of Dams Nature of Valley
Chord – Height Ratio
Gorges
Less than 3
Narrow Valleys
Between 3 - 6
Wide Valleys
Greater than 6
Flat Country Plains
Embankments used to create reservoir volume. Eg. Near Heathrow
Modified from Walter, R. C. S (1962) Dam Geology.
3
Engineering Geology of Dams •
Types of Dam Arch or Buttress Dam
Gravity Dam
Embankment Dam
Engineering Geology of Dams Types of dam
Gravity
Principal design factors
Stability derived from self-weight. Ground anchors can be used to increase stability. Masonry or concrete. Used in narrow to wide valleys with Chord / height ratio 5 to 8 eg Vyrnwy (Wales) =7 and the Three Gorges dam in China
Buttress
Reinforced concrete buttresses used to transfer thrust to foundation. Used in very wide valleys eg Errochy =10 and Shira =15 (Scotland)
Arch dam
Stability derived from the curved shape of the dam transferring the forces to the abutments. Reinforced concrete, can be thick (chord/height ratio of 3 – 5) or thin skinned (c/h< 3, radius <150m), single or double curvature. Can withstand large deflections in the foundationsIF the deflections are uniformly distributed. Common in mountainous areas with gorges or narrow valleys eg Vaiont dam, Italy
Earthfill and Rockfill Embankments
Used in wide valleys where foundation materials are deformable and/or variable. Has a very wide footprint with shallow slopes. Constructed from local materials derived from within the reservoir area. The impermeable core can be clay or asphalt concrete. Very common in UK, but also the High Aswan dam (dune sand) in Egypt
4
Engineering Geology of Dams •
Concrete Dams
Typical cross-section for concrete dams: (a) Gravity (b) Buttress (c) Arch Drainage measures for an arch dam. Drainage curtain: downstream (D) of grout (G) cut-off and upstream of stressed zone. The closure of fractures in the stresses zone in-situ reduces permeability and allows development of excessive groundwater pressure.
Engineering Geology of Dams •
Arch Dam
Forces on a Arch Dam
Arch Dam: Forces The arch squeezes together as the water pushes against it. The weight of the dam also pushes the structure down into the ground.
5
Engineering Geology of Dams •
Buttress Dam
Forces on a Buttress Dam
Buttress Dam: Forces Water pushes against the buttress dam, but the buttresses push back and prevent the dam from toppling over. The weight of the buttress dam also pushes down into the ground.
Engineering Geology of Dams •
Embankment Dam
Forces on an Embankment Dam
Embankment Dam: Forces Water pushes against the embankment dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over.
6
Engineering Geology of Dams •
Gravity Dams
Forces on a Gravity Dam
Gravity Dam: Forces Water pushes against the gravity dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over.
Engineering Geology of Dams •
Principal Parts
7
Engineering Geology of Dams •
Dam Location Issues
• • •
Catchment
•
Foundation Rock
•
Silting
Reservoir Slope Stability
Engineering Geology of Dams •
What Geological Input is required for the engineering of a large dam?
•
The stability and safety of the dam on its foundations;
•
The water-tightness of the reservoir basin;
•
The slope stability of the reservoir perimeter
•
The availability of suitable local materials for its construction.
8
Engineering Geology of Dams •
Finally, bearing in mind how the dam might behave, think through appropriate tests and hence decide on what sampling would be required.
Engineering Geology of Dams • •
Additional notes: Stability is obviously the major factor. This involves -
•
The Dam
• •
The dam must function safely
•
The water pressure acting in any discontinuities, cracks or voids in the foundations may cause uplift. Treatment may involve grout and / or drainage curtains
The water pressure acting on the face of the dam may cause sliding and / or trigger earthquakes
9
Engineering Geology of Dams • •
Additional notes: Stability is obviously the major factor. This involves -
•
Upstream : The Reservoir Rim
• • • •
Will flooding impounding water trigger landslides Could a landslide cause a flood wave over the dam Will waves on the water surface cause erosion
Downstream.
• •
Will spillway water cause erosion What would happen downstream in the event of failure or floo d
Engineering Geology of Dams • •
Additional notes: Stability is obviously the major factor. This involves -
•
Other factors are topography and hydrology (which partly determines the capacity of the reservoir and height of the dam), erosion of the catchment and the rate of siltation, measures to allow safe overtopping during flood (spillway design), riparian water and earthquake accelerations.
•
The aim is to build the most cost effective structure, commensurate with safety, which gives, for the particular river discharge, the largest reservoir capacity created by the cheapest dam.
10
Engineering Geology of Dams •
Other Points to note:
•
For undergraduate projects, it is best to focus on either an embankment dam (soil or rock filled) or a concrete gravity dam, as it is possible to do a reasonable analysis in terms of stability.
•
Concrete gravity dam :- sliding failure and toppling failure (see MD lectures at level3)
•
Embankment dams (rock and soil fill):- Sliding failure, failure of upstream and downstream slopes (NK lectures at level 3). Also consider the source of the fill material, its suitability, excavation methods and the temporary stability of the borrow pits, (which should be in the reservoir area so that they are flooded to reduce visual and environment impact).
•
With rock fill, there is also the question of forming an impermeable membrane or core of either clay (if there is some available locally) or asphalt.
Engineering Geology of Dams •
Key references
EM 1110-2-2200
088
CECW-ED
Gravity Dam Design
30 Jun 95
EM 1110-2-2201
238
CECW-ED
Arch Dam Design
31 May 94
EM 1110-1-2908
EM 1110-2-1420
065
148
CECW-EG
CECW-EH-Y
Rock Foundations
30 Nov 94
Hydrologic Engineering Requirements for Reservoirs
31 Oct 97
11
