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Types of Concrete Gravity Dams

Introduction

Gravity dams are solid concrete structures that maintain their stability against design loads from the geometric shape and the mass and strength of the concrete. Generally, they are constructed on a straight axis,
but may be slightly curved or angled to accommodate the specific site conditions.

Gravity dams typically consist of a nonoverflow section(s) and an overflow section or spillway. The two general concrete construction methods for concrete gravity dams are conventional placed mass concrete and RCC.

1. Conventional concrete dams

Conventionally placed mass concrete dams are characterized by construction using materials and techniques employed in the proportioning, mixing, placing, curing, and temperature control of mass concrete (American Concrete Institute (ACI) 207.1 R-87). Typical overflow and nonoverflow sections are shown on Figures 1 and 2.

Figure 1 : Typical dam overflow section

Figure 2 : Nonoverflow section

Construction incorporates methods that have been developed and perfected over many years of designing and building mass concrete dams. The cement hydration process of conventional concrete limits the size and rate of concrete placement and necessitates building in monoliths to meet crack control requirements.

Generally using large-size coarse aggregates, mix proportions are selected to produce a low-slump concrete that gives economy, maintains good workability during placement, develops minimum temperature rise during hydration, and produces important properties such as strength, impermeability, and durability. Dam construction with conventional concrete readily facilitates installation of conduits, penstocks, galleries, etc., within the structure.

Construction procedures include batching and mixing, and transportation, placement, vibration, cooling, curing, and preparation of horizontal construction joints between lifts.

The large volume of concrete in a gravity dam normally justifies an onsite batch plant, and requires an aggregate source of adequate quality and quantity, located at or within an economical distance of the project.

Transportation from the batch plant to the dam is generally performed in buckets ranging in size from 4 to 12 cubic yards carried by truck, rail, cranes, cableways, or a combination of these methods. The maximum bucket size is usually restricted by the capability of effectively spreading and vibrating the concrete pile after it is dumped from the bucket. The concrete is placed in lifts of 5- to 10-foot depths. Each lift consists of successive layers not exceeding 18 to 20 inches. Vibration is generally performed by large one-man, air-driven, spud-type vibrators.

Methods of cleaning horizontal construction joints to remove the weak laitance film on the surface during curing include green cutting, wet sand-blasting, and high-pressure air-water jet. Additional details of conventional concrete placements are covered in EM 1110-2-2000.

The heat generated as cement hydrates requires careful temperature control during placement of mass concrete and for several days after placement. Uncontrolled heat generation could result in excessive tensile stresses due to extreme gradients within the mass concrete or due to temperature reductions as the concrete approaches its annual temperature cycle.

Control measures involve precooling and postcooling techniques to limit the peak temperatures and control the temperature drop. Reduction in the cement content and cement replacement with pozzolans have reduced the temperature-rise potential. Crack control is achieved by constructing the conventional concrete gravity dam in a series of individually stable monoliths separated by transverse contraction joints.

2. Roller-compacted concrete (RCC) gravity dams

The design of RCC gravity dams is similar to conventional concrete structures. The differences lie in the construction methods, concrete mix design, and details of the appurtenant structures. Construction of an RCC dam is a relatively new and economical concept.

Economic advantages are achieved with rapid placement using construction techniques that are similar to those employed for embankment dams. RCC is a relatively dry, lean, zero slump concrete material containing coarse and fine aggregate that is consolidated by external vibration using vibratory rollers, dozer, and other heavy equipment.

In the hardened condition, RCC has similar properties to conventional concrete. For effective consolidation, RCC must be dry enough to support the weight of the construction equipment, but have a consistency wet enough to permit
adequate distribution of the past binder throughout the mass during the mixing and vibration process and, thus,
achieve the necessary compaction of the RCC and prevention of undesirable segregation and voids.

Dams and Appurtenant Hydraulic Structures

Water, one of the few natural resources without which there is no life, is distributed throughout the world unevenly in terms of place, season and quality. For this reason it is essential to construct dams on rivers, thus forming reservoirs for the storage and the even use of water.

To date, forty-two thousand large dams have been built worldwide, and hundreds of thousands of smaller ones, which have made possible a rational use of a certain amount of river water – the most important water resource for human life and activity. Dams, together with their appurtenant hydraulic structures, belong among the most complex engineering works, above all because of their interaction with the water, their great influence on the environment and their high cost.

Therefore great significance is given to theoretical research relating to dams, to improving the methods of analysing and constructing them, and to the knowledge gained in the course of their exploitation. In the past forty years great progress has been made in this respect.

Water plays an exceptionally significant role in the economy and in the life of all countries. It is of crucial importance to the existence of people, animals, and vegetation. The settling of people in different regions of the Earth has always been closely dependant on the possibilities for water supply, parallel with those for providing food, shelter, and heat.

The increase in population, as well as the development and enrichment of mankind, in a number of places has reached a level at which the water supply, needed for the population, industry, irrigation, and production of electric power, has been brought to a critical point.

On the other hand, reserves of water on Earth are very large. They have been estimated to amount to 1.45 billion km3 (Grishin et al., 1979). If we assume that the above quantity of water is uniformly spread over the Earth’s surface, then the thickness of such a water layer would be almost 3,000 m. As much as 90% of that quantity is attributable to the water of oceans and seas, while the remainder of barely 10% belongs to lakes, rivers, underground waters, and glaciers, as well as moisture from water in the atmosphere. Only 1/5 of the freshwater, which is suitable for man’s life and activities, is available for use.

More than twenty large dams and over a hundred smaller ones have been built in the Republic of Macedonia, which have still only partially exploited the available water, and flood control remains incomplete. The majority of the large dams were built in the period from 1952 to 1982 while, principally because of the lack of investment, the past twenty years have seen the construction mainly of smaller dams with a height of up to twenty metres and a reservoir volume of 300,000 cubic metres.

In the next few years some two or three more large dams will be completed which will still not
satisfy the need for water for the water supply, for irrigation and for the production of electrical energy, which are continually on the increase. The situation in all developing countries is similar, so that dams will continue to be built in the future despite the resistance on the part of devotees of the unobstructed flow of rivers.

An important unfavourable circumstance, which renders difficult a more complete utilization of water, is the fact that it is very not uniformly distributed on the Earth’s surface – considering space, time, and quality. That is to say, particular countries and regions suffer from drought, while others possess too large quantities of water. Also, the very same region could, in the course of a particular period of the year, be exposed to drought, while suffering from floods in another period. In that way, water, that common nationwide wealth without which no life is possible, can be an irreplaceable friend to man, but also his great enemy if he is not able to utilize it in a correct manner and to keep it under control.

Hydraulic land reclamation, i.e. irrigation of land, or else drainage of excess water from a specific territory. At the moment, irrigation systems cover approximately 270 × 106 ha, or 20% of the total cultivated areas. In many countries, especially in developing ones, increased food production is only possible by improving or increasing irrigation. The greatest amount of water is spent on irrigation – 3⁄4 of total consumption in the world. Great efforts are made to develop effective ways of saving water by avoiding losses in distribution networks and by applying more skillful irrigation techniques.

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Dam Failures and Incidents

I. Reasons Dams Fail

Dams can fail for one or a combination of the following reasons:

1. Overtopping caused by water spilling over the top of a dam. Overtopping of a dam is often a precursor of dam failure. National statistics show that overtopping due to inadequate spillway design, debris blockage of spillways, or settlement of the dam crest account for approximately 34% of all U.S. dam failures.

2. Foundation Defects, including settlement and slope instability, cause about 30% of all dam failures.

3. Cracking caused by movements like the natural settling of a dam.

4. Inadequate maintenance and upkeep.

5. Piping is when seepage through a dam is not properly filtered and soil particles continue to progress and form sink holes in the dam. [See an animation of a piping failure.] Another 20% of U.S. dam failures have been caused by piping (internal erosion caused by seepage). Seepage often occurs around hydraulic structures, such as pipes and spillways; through animal burrows; around roots of woody vegetation; and through cracks in dams, dam appurtenances, and dam foundations.

II. History of dam failures around the world

Here are some cases of dam failures around the world

1. Malpasset arch dam failure in France in 1959 (421 deaths)

The causes:

High uplift pressures following heavy rainfall & a weakness in the left abutment rock

Lessons learnt:

Appropriate SI and assessment by experts in all areas of dam design

2. Vaiont dam overtopping incident in Italy in 1963 (2600 deaths)

The causes:

Instability of reservoir slopes causing a landslip & 125m high wave over the dam

Lessons learnt:

Measure pore water pressures & movements at depth as well as at the surface

3. Dale Dyke dam breach in 1864 ( 244 deaths )

The causes:

Internal erosion possibly caused by hydraulic fracture of the core

Lessons learnt :

Designs include wider cores, use of cohesive & compacted fill and placing pipes in tunnels through natural ground

4. Eigiau & Coedty dam failures in 1925 (16 deaths)

The causes :

Foundation failure of Eigiau & overtopping failure of Coedty

Lessons learnt :

Dams need to be designed, supervised and inspected by qualified engineers

Slide Failure at Dam – Association of State Dam Safety Officials (ASDSO)