Design Of Concrete Gravity Dams - Introduction

 INTRODUCTION TO DESIGN OF CONCRETE GRAVITY DAMS

Introduction

A dam is an obstruction or a barrier built across a stream or a river for accumulation of water on its upstream side which is used for different purposes. Dams are constructed for deriving various benefits like irrigation, hydropower generation, flood control, domestic/industrial water supply, recreation etc. 

Dams can be classified based on various criteria. As per water resources planning the dams may be classified as storage dams, diversion dams and detention dams. As per hydraulic flow conditions the dams may be classified as overflow dams (spillways) and non-overflow dams. As per materials used they can be classified as earthfill dams, rockfill dams and concrete/masonry dams. 

The concrete/masonry dams can be classified further as gravity dams, buttress dams & arch dams based on their structural behavior and as conventional concrete dams & roller compacted concrete dams as per method of construction.

Conventional concrete dams are constructed by dividing the dam length into blocks of 20-25m long. Concrete placement is done by cableways, cranes, trestles etc. in lifts of 1.5-2m. The compaction of concrete is done by vibrators. Roller compacted concrete dams are constructed using same machinery/equipments as that used for embankment dams. Construction is done from abutment to abutment in lifts of 300-600 mm. Compaction of concrete is done with the help of vibratory rollers. 

Masonry dams were preferred in our country earlier as they were labour intensive, provided more employment opportunities, consumed less cement and did not involve any temperature control measures. However the quality of workmanship and workers are deteriorating now. There are problems of heavy seepage through many of our existing masonry dams. For seepage control, various remedial measures are being adopted these days, viz. guniting on upstream face, upstream concrete membrane, sandwich concrete membrane, prepacked masonry construction etc. Now-a-days, there is therefore a shift in favour of concrete dams. Further, the construction of concrete dams is faster vis-à-vis masonry dams. 

Gravity Dam

A concrete gravity dam is a solid concrete structure so designed and shaped that its weight is sufficient to ensure stability against the effects of all imposed forces. The complete design of a concrete gravity dam includes the determination of the most efficient and economical proportions for the water impounding structure and the determination of the most suitable appurtenant structures for the control and release of the impounded water consistent with the purpose and function of the project. 

General dimensions and definitions

Gravity dams may be straight or curved in plan depending upon the axis alignment. For uniformity, certain general dimensions and definitions have been established and are defined as below:

The structural height of a concrete gravity dam is defined as the difference in elevation between the top of the dam and the lowest point in the excavated foundation area. 

The hydraulic height is the difference in elevation between the lowest point of the original streambed at the axis of the dam and the maximum controllable water surface.  

The length of the dam is defined as the distance measured along the axis of the dam at the level of the top of the main body of the dam from abutment contact to abutment contact including the length of spillway if it lies wholly within the dam. However, the length of the abutment spillway located in any area especially excavated for the spillway is not included in the length of the dam.

The volume of a concrete dam includes the main body of the dam and all mass concrete appurtenances cot separated from the dam by construction or contraction joints. 

A plan is an orthographic projection on a horizontal plane, showing the main features of the dam and its appurtenant works with respect to the topography. A plan should be oriented so that the direction of stream flow is towards the top or towards the right of the drawing.

A profile is a developed elevation of the intersection of a dam with the original ground surface, rock surface or excavation surface along the axis of the dam, the upstream face, the downstream face or other designated location.

The axis of the dam is a vertical reference plane usually defined by the upstream edge of the top of the dam.

A section is a representation of a dam as it would appear if cut by a vertical plane taken normal to the axis and is usually oriented with the reservoir to the left.

Design Considerations

Local Conditions

Collection of data on local conditions will eventually relate to the design, specifications and construction stages of a dam.  Local conditions are not only needed to estimate construction costs, but may be of benefit when considering alternative designs and methods of construction.  Some of these local conditions will also be used to determine the extent of the project designs, including such items as access roads, bridges and construction camps.

Data required to be collected are:

i) Approximate distance from the nearest rail road shipping terminal to the structure site

ii) Local freight or, trucking facilities and rates

iii) Availability of housing and other facilities in the nearest towns

iv) Availability or, accessibility of public facilities or, utilities such as water supply, sewage disposal, electric power for construction purposes, telephone services etc.

v) Local labour pool and general occupational fields existing in the area

Maps and Photographs

Maps and photographs are of prime importance in the planning and design of a concrete dam and its appurtenant works.  From these data an evaluation of alternative layouts can be made preparatory to determining the final location of the dam, the type and location of its appurtenant works and the need for restoration and/or development of the area.

Data to be collected are:

i) A general map locating the area within the State, together with district and township lines.

ii) Map showing existing towns, highways, roads, railways and shipping points

iii) A vicinity map showing the following details:

a) The structure site and alternate sites

b) Public utilities

c) Stream gauging stations

d) Existing man-made works affected by the proposed development

e) Locations of potential construction access roads, sites for Government camp, permanent housing area and sites for Contractor’s camp and construction facilities

f) Sources of natural construction materials

iv) Site topography covering the area of dam, spillway, outlet works, diversion works, construction access and other facilities

Hydrologic Data 

In order to determine the potential of a site for storing water, generating power or, other beneficial use, a thorough study of hydrologic conditions is required.

The hydrologic data required include the following:

i) Stream flow records, including daily discharges, monthly volumes and momentary peaks

ii) Stream flow and reservoir yield

iii) Project water requirements, including allowances for irrigation and power, conveyance losses, reuse of return flows, dead storage requirements for power, recreation, fish, wildlife etc.

iv) Flood studies including inflow design floods and construction period floods

v) Sedimentation and water quality studies including sediment measurements, analysis of dissolved solids etc.

vi) Data on ground water tables in the vicinity of the reservoir and dam site.

vii) Water rights, including inter-state and international treaty effects.

Reservoir Capacity and Operation

Dam designs and reservoir operating criteria are related to reservoir capacity and anticipated reservoir operations.  The loads and loading combinations to be applied to the dam are derived from the several standard reservoir water surface elevations.  Reservoir capacity and reservoir operations are used to properly size spillway and outlet works.

Reservoir design data required for the design of dam and its appurtenant works are:

1) Area – Capacity curves and/or tables

2) Topographic map of reservoir area

3) Geological information pertinent to reservoir tightness

4) Reservoir storage allocations and corresponding elevation

5) Required outlet capacities of respective reservoir water surfaces and sill elevations etc.

6) Annual reservoir operation tables or charts

7) Method of reservoir operations for flood control, maximum permissible releases consistent with safe channel capacity

8) Anticipated wave action, wind velocity, fetch etc.

9) Anticipated occurrence and amount of ice, floating debris etc.

10) Physical, economic or, legal limitations to maximum reservoir water surface.

Climate Effects

Climate conditions at a site affect the design and construction of the dam.  Measures to be employed during construction to prevent cracking of concrete are related to ambient temperatures at site.

The data on climate conditions considered as part of design data are :

1) Records of mean monthly maximum, mean monthly minimum and mean monthly air temperatures at site

2) Daily maximum and minimum air temperatures

3) Daily maximum and minimum river water temperatures

4) Amount of annual variance in rainfall and snowfall

5) Wind velocities and prevailing direction

Construction Materials

Construction of a gravity dam requires availability of suitable aggregates in sufficient quantity.  Aggregates are usually processed from natural deposits of sand, gravel and cobbles or, may be crushed from suitable rock.

Data required on construction materials are:

1) Sources of aggregate

2) Water for construction purposes

3) Results of sampling, testing and analysis of construction materials

4) Information on potential sources of soils, sand and gravel to be used for backfill, road surface, protection of slope etc.

Site Selection

The two most important considerations in selecting a dam site are:

1) the site must be adequate to support the dam and appurtenant structures

2) the area upstream of site must be suitable for a reservoir

The following factors should be considered in selecting the best site out of several alternatives:

1) Topography : A narrow site to minimize amount of material in 

the dam, thus reducing its cost

2) Geology : Dam foundation should be relatively free of major 

faults and shears

3) Appurtenant : Selecting a site which will better accommodate the 

Structures appurtenant structures to reduce overall cost

4) Local conditions Sites requiring relocation of existing facilities like 

roads, railway, power lines, canals increase overall cost.

5) Access : Difficult access may require construction of 

expensive roads, thus increasing the cost.

Configuration of Dam

A gravity dam is a concrete structure designed so that its weight and thickness ensure stability against all the imposed forces.

Non-overflow section 

The downstream face is usually a uniform slope, which, if extended, would intersect the vertical upstream face at or near the maximum reservoir level. The upper portion of the dam must be thick enough to resist the shock of floating objects and to provide space for a roadway. The upstream face will normally be vertical. However, the thickness in the lower part may be increased by an upstream batter, if required. The base width (thickness) is an important factor in resisting the sliding and may dictate the d/s slope. 

Overflow section

Spillway may be located either in the abutment or in the dam. Section of spillway is similar to NOF section but modified at top to accommodate the crest and at the toe to accommodate the energy dissipater. The elevation of crest and its shape is determined by hydraulic requirements.

Foundation Investigation

The purpose of a foundation investigation is to provide data necessary to properly evaluate a foundation.  Basic data to be obtained during appraisal investigation, with refinement continuing until construction is complete are:

1) Dip, strike, thickness, composition and extent of faults and shears

2) Depth of overburden

3) Depth of weathering

4) Joint orientation and continuity

5) Tests of foundation rock viz.

Physical Properties Tests

- Compressive Strength

- Elastic modulus

- Poisson’s ratio

- Bulk specific gravity

- Porosity

- Absorption

Shear Tests

- Direct shear

- Triaxial Shear

- Sliding friction

Other Tests

- Solubility

- Petrographic Analysis

Construction Aspects

Construction aspects that should be considered in the design stage are:

- Adequacy of area for construction plant and equipment

- Permanent access roads to facilitate construction activities

- Length of construction seasons