There are two basic types of reservoir:
- Open impounding reservoirs are for the catchment of 'crude' water.
- Service reservoirs are used in water distribution and should be enclosed for protection.
A similar type of structure, in terms of design and construction, liquid-retaining tanks which are generally used for industrial and public utility storage of liquid natural gas (LNG), oil products, and so on.
The principle of all open impounding reservoirs is the same, in that they consist of a dam across a valley to impound a natural stream of water. Typically they include overflow weirs, draw-off points and cut-off walling, or similar constructions, to intercept stray water which may flow beneath the dam. The water quality is improved by a treatment plant.
They tend to be constructed in an elevated position, as this reduces the pumping requirement, and means that the structure can take a very simple form in the ground.
The construction of reservoirs and liquid-retaining tanks is determined by:
- The type (and temperature) of the liquid being stored.
- The size and shape of the tank.
- Whether it will be in or above the ground.
- The type of tank, i.e. open or closed, lined, unlined, and so on.
Open impounding reservoirs can be constructed from a range of materials, such as rock, earth, concrete or composites, depending on the reservoir size and earth type. An earthen embankment should have a watertight concrete face or a core of clay or concrete.
The geological conditions must be assessed, as this will determine the type of material to be used in constructing the tank or floor to prevent degradation, cracking, settlement or movement. Cracking can be controlled by the use of construction joints to accommodate movement. The type of cement used, its temperature during construction, and the wall dimensions will determine the spacing of the joints.
Rectangular tanks are commonly designed as simple slabs cantilevered from the floor. The corners restrain against deflection. Circular tanks can be designed as ring tension structures with ring reinforcement restricting the wall’s outward deflection, or as simple cantilever structures.
Service reservoirs require the construction of a roof structure. Where the roof is to be connected to the top of the walls, they should be able to resist against pressures caused by the expansion of the roof slab. Examples of such measures include:
- A layer of compressible material, such as foamed plastic, placed between the tank walls and the ground.
- A roof slab rested on sliding joints made of bituminous material, stainless steel, multi-layer rubber, and so on.
- Cantilevered from columns, allowing roof and walls to move freely.
The floor of reservoirs and tanks must be constructed so as to control and reduce shrinkage. The techniques used in the control of shrinkage include:
- Laying a floor in alternate bays or ‘chess-board’ squares, with intervals of a few days in between; however, while this can eliminate a lot of the primary cooling shrinkage it will not control the hardening shrinkage.
- Allowing the slab to move freely on a sliding layer of synthetic material with adequate expansion joints.
- Restraining the slab on a rough concrete sub-floor, restricting shrinkage cracks. Continuity is provided at the joints by rebar.
In-ground storage tanks or reservoirs used for storing substances such as LNG may be lined or unlined, depending on the water table height and the nature of the soil.
The most common linings are metal-lined tanks with concrete walls and floors. Excavation is undertaken, a concrete tank cast and a metal lining fixed. The metal lining is normally nickel-steel or aluminium alloy, as these are capable of withstanding temperatures of below -160-degrees (the temperature at which natural gas liquefies).
Other linings are used depending on the type of liquid to be stored. In particular, concrete must be protected from acids and sulphates which are very damaging. The types of lining can be either:
- Protective linings designed to resist corrosion, which can be made of glass, acid-resisting asphalt, wax, lead, and other soft metals.
- ‘Sacrificial’ linings that do not resist corrosive action but can be renewed at certain periods when the tank is emptied. This is usually a cheap form of cover such as cement mortar rendering.
Common types of waterproofing that are used include:
- Plastic sheeting.
- Renderings which contain a waterproofing additive.
- Geomembranes such as high and low-density polyethylene, Butyl, EPDM, PVC, and synthetic rubbers.
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- Sewer construction.
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- Types of water.
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