How is the construction of dams

Aggregate Production - The acceptability of natural aggregates is judged upon the physical and chemical properties of the material and the accessibility, proximity to the site and economic workability of the deposit.

Concrete Handling, Placing and Consolidation - The procedure to be adopted for moving concrete from the mixers on to the dam will be governed by site conditions. The problem is to transport it to the dam with the least possible segregation or change in its consistency so it may be compacted uniformly into the dam without unreasonable effort. The cableway is probably the simplest arrangement. The tilting mixers will feed the buckets; these are then moved to a pick up point under the cableway, transported smoothly to the block and emptied quickly through an air operated gate.

Three Tower Cableway

The use of a belt conveyor has also been considered, but problems occur in keeping the belt temperature stable in warm weather and also in windy conditions. The conveyors are usually covered and cold air is blown over the concrete to lower its placing temperature.

The placing of a low-slump concrete, four layers in 2.3m lift

Tractor mounted vibrators at Emosson Dam, Switzerland

Proper consolidation of low-slump concrete is laborious and requires continuous supervision. The most efficient compactor is usually the two man hand-held high-speed vibrator.

Formwork - Probably the most widely used lift is 1.5m, however, on large dams a height of 2.3-3.0m is frequently used. With the larger lifts there are fewer movements of forms and fewer horizontal lift surfaces to be cleaned. The high-lift formwork is unique and expensive with less prospect for re-use, heavier equipment is required for lifting the forms and the heat problems and risks of cracking in the concrete are accentuated. Modern steel formwork is of cantilever design, see figure. Where possible the use of slip forms will expedite the work and lower the costs. At some locations it may be expedient to use precast concrete slabs for formwork with set-retarding agent on the inner surface.

Built in items - The installation of built in items is always a major source of delay on construction. Advance planning is required with close attention to detail. The complication of installation of reinforcement, prestressing, gate hinges, drainage wells and gate wells are common on spillways. There has been a tendency to use precast concrete units for galleries to save time, however this prevents the inspection of the concrete in the interior of the dam. The simplest method of forming galleries is vertical formwork extending the full height of a lift. When this is removed, precast concrete beams or slabs can be laid over the opening and concreted into the next lift. Reinforcement is usually required above and below rectangular galleries and this is best installed as prefabricated units.

Cooling of Concrete - The method of cooling concrete during the first few days after placing can be of the utmost importance if cracking is to avoided. It is essential to give attention to both internal and external factors that may induce cracking;

• Temperature rise, which will depend upon the heat of hydration of the cement, the quantity of cement per cubic metre, the concrete placing temperature and the rate of construction;
• Heat dissipation, which will depend upon the conditions of exposure - including the temperature of the underlying concrete and the thermal diffusivity of the concrete. If it is considered necessary to heat the underlying concrete the rate of rise of its temperature should not exceed 2° Celsius per day;
• The effects of restraint from a cold surface, i.e. rock or concrete say 14 days old, it will depend upon the temperature gradient which can be reduced by placing concrete in half lifts for a predetermined height, say 3m above the cold surface;
• The arrangement of cooling pipes - at 0.25 and 0.75 of the height of the lift may be more efficient than on the top of the old lift and at mid-height of the new lift. The horizontal spacing will depend upon the rate of heat removal required and the temperature of the cooling water (i.e. river water of varying temperature or refrigerated water);
• The local weather conditions - humidity, temperature and wind.

Economical Construction - Concrete dams are expensive, however mechanisation over the last 40 years has reduced by a factor of four the number of man hours required to place a cubic metre of concrete in a mass concrete dam. Although every Engineer strives for perfection, consideration must be given to the degree of perfection that is really necessary. Close co-operation between the Owner and Contractor will save time and money. Questions have to be asked at all stages such as;

• Is it permissible to design for tensile stress in the concrete?
• Will arching of the dam result in overall economy?
• Are longitudinal contraction joints necessary in large gravity dams?
• Can the transverse contraction joints be omiited, located at wider spacing etc?
• What clean up is necessary on horizontal construction joints?
• Should the height of lift be specified by the designer?
• Should the cement content of the concrete be specified or only the properties required in the concrete?
• Can the dams of intricate shape be justified?
• Should ancillary works be separated from the dam to minimise interference with a continuous or cyclic process of dam building?
• What is the optimum layout and design for galleries?

Looking at the 'Construction - General Section' to see the proportion of costs in a concrete dam, assumuing that the materials have been predetermined, the Contractor should concentrate on formwork, the purchase of plant and its operation.

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