The Design Aspects of Typical Service Reservoir, Requirements to Ensure Watertight - Term Paper Example

The design aspects of typical service reservoir, requirements to ensure watertight Name Institution Date Course The design aspects of typical service reservoir, requirements to ensure watertight Introduction Service reservoirs are used for the storage of treated water that is supplied to the populations within the area. They also become critical sources of water during emergencies and when the distribution system has broken down (Bhargava & Gupta, 2004). Reservoirs are accurately designed to ensure stability and durability of services and the protection of the quality of the stored water. For any given project, reservoirs may take one among several acceptable designs that will help achieve these objectives. It is first important to consider the storage capacity required before making any major decisions about the design. In considering the capacity, designers should take into account the five storage components which include: the operational storage, the standby storage, the dead storage, the equalising storage and the fire suppression storage. It should be understood that the total tank volume may not actually represent the effective volume available to the system. The effective volume is obtained by subtracting the dead storage from the total volume. The designers must therefore accurately consider the various volumes as shown in the figure below: Fig: Components of the total storage volume The design of service reservoirs is influenced by the peak per capita demand, pump types, the population to be supplied, the topography of the area and the availability of the power supply (Bhargava & Gupta, 2004). The reservoir capacity will also depend on the mass curve of the demand as well as the duration of the pumping. If the rate of pumping the water and the pattern adopted closely follows the demand pattern of the population, then the reservoir capacity need not be large. Designers have put in efforts towards the variations of the balancing or equalizing reserve by implementing various pumping patterns and schedules. Designers should, however, understand that the total storage capacity of the service reservoir should be able to accommodate the balancing reserve (this takes care of the hourly fluctuations in the demand and supply of the water), the breakdown reserve and the fire reserve. Ghosh (2013) says that the reservoir may be built above the ground or it may be partly or wholly sunk into the ground. For a fresh water reservoir constructed underground, the roof is usually covered using 30cm to 75cm of surfed filling. The walls are normally constructed of reinforced concrete, which helps designers achieve a stable water temperature and protection from pollution. Reinforced concrete also ensures that the structure is protected from the growth of algae since direct sunlight is excluded. It is advisable to use flat slab designs since they provide clear spaces inside the reservoir. In deciding on the type of wall, it is important to keep excavation to a minimum. This may be achieved in a cost effective way by constructing the wall monolithic with the base slab, with the assumption that a section of the base slab will act as a footing of the wall. For buried reservoirs, it will be critical to consider both the maximum earth pressure from the surround soil when the reservoir is empty, and the pressure exerted by water on the inside when the reservoir is full. It is not advisable to depend on the earth filling part to achieve a balance of the water pressure. This is because the filling may not have been placed during the initial testing of the work for water tightness. Designers should understand that in certain soils, especially clay soils, when the weather gets very hot and dry, the surrounding soil will shrink and pull away from the reservoir resulting in an appreciable gap between the soil surface and the structure (Ghosh, 2013). In the design below, the underground reservoir is positioned 400 mm below the ground level with an overburden of 400mm of soil. The vertical wall is designed to stand vertically monolithic with the base and the roof slab. The inner dimension of the reservoir has been designed to be 20m x 20m x 4m high distance from the base slab to the roof slab; the spacing between the internal columns kept at 5m. In this design, the top water level is expected at a height of 3.7m with the maximum ground water level designed to be 400mm below the finished ground level in the rainy season. To ensure that the concrete is totally impermeable of water, the maximum crack width should not exceed 0.2mm. These factors have been considered in the design below: Fig: Underground water reservoir The underground design is preferred for the supply of fresh water supply to local populations. The reinforced concrete used in the design is a great way to ensure that the water does not get polluted or contaminated through leakage Storage Volume Use Varies Designers must understand that water usage is never constant throughout the day. It is used more during the day – usually reaching the peak in the mid-morning as well as the early hours of the evening. During this time the system experiences peak demand and the storage level will fall significantly. It will be critical for the system to be able to replenish the used water during minimum-demand times in the late-night. Depending on the local climates and economies, the shape of the diurnal curve of the stored water is likely to vary significantly when drawn for different cities. Designers must therefore obtain local design information for any water system to be erected so that the demand needs are properly understood. With a dependable source of supply capacity, however, equalization storage of about 22 % of the peak daily demand would work well for small residential areas (HDR Engineering Inc, 2001). Service Reservoir location A service reservoir serves as the storage for water before the water is distributed to the populations in the furthest points of the water system. Considering the cost of the pipelines as well as the need to distribute pressure uniformly throughout the system, it is advisable to locate the reservoir at or near the centre of the area to be served. If the area is relatively flat, it is easy to have the water tower built at the centre of the population. In areas with sloppy gradient, it may be beneficial to find the highest point within the area and locate an elevated tank; this may be at one end of the area instead of the centre and gravity will help in the distribution of the water to the whole area. Construction materials Steel and concrete are the major materials used for the construction of the service reservoirs. All the piping work, fittings and joints must be done according to standards, with particular attention drawn to the fact that these areas are the points where leakages and breakages will mostly begin to occur. Standard exist for regulation of all constructions, including the use of welded steel water tanks, factory-coated bolted steel tanks as well as the wire and strand wound, circular pre-stressed concrete tanks. Safe and durable paint should also be applied on the tank surface, internally and externally. Paint that might cause toxification to the stored water should not be used. Waterproofing Several factors have been known to contribute to cracks and leakages in structures used to hold water. These include movements resulting from shrinkage and creep, movements from temperature variations and air humidity, damage of the concrete material due to percolation of liquids with aggressive chemicals, damage as a result of uneven settlement of foundations, and cracking of concrete caused by rusted bars. To reduce the effect of temperature variations as far as possible, the tank can be partially buried partly into the ground so that there can be enough soil to cover the top and to create embankments on the external surface so that the soil can cover the tank completely. Particular design considerations must also be directed towards ensuring that the tank is guarded against evaporation and movements that may result from extreme temperatures before the covering is made (Hobbs, 1969). At the design stage, particular interests should also be directed towards observing serviceability, design, joints, loads and durability. For durable construction, appropriate selection of materials, cementitious component, additives, strength and W/C ration and curing of the components needs to be stressed. The joints must be carefully done with careful decisions regarding types of the joints, joint materials, spacing, joint design and its construction. The structure itself must also be made waterproof. This can be achieved by reducing the permeability of concrete by introduction of water-reducing agents, pore blocking additives, organic blinders, high performance PCE and super plasticizers. The concrete itself should also be properly cured. This will enhance the water-proofing capabilities of the concrete. During curing, it will be important to keep the concrete moist especially during the first few days. The construction joints should also be set at right angles relative to the general direction of the member. Vertical joints should however, be avoided as far as possible by completing a layer of concrete not exceeding 600 mm high in a continuous operation that is worked around the circumference in the two directions from the starting point; this is repeated for the day’s operation (see fig below). Before the job is closed for the day, a rebate will have to be formed on the concrete at the top surface to form ‘key’ to achieve construction joint as seen in the figure below for the operations of the following day. All fixed joints e.g. new-to-aged concrete, casting interruption joints and floor to wall joints will have to be preventively sealed using water bars. This will also be done for all the through-wall penetrations. These considerations will greatly contribute to the water-proofing ability of the structure and the designers will achieve a highly effective water system that will reliable and efficient (Joints & Sealants, 2010). Fig: Construction joints for water-proofing Key 1. Plain cement concrete 2. Flexible waterproofing coating/membrane 3. Protective screed 4. RCC raft Protective method on vertical surface C1. First construction joint minimum at 300 mm above the raft top C2. Subsequent construction joint (As minimum as possible) List of References Bhargava DV & Gupta PK, 2004, Variation effects on the economical design of service reservoirs, Indian Journal of Engineering & Material Sciences, 11: 107 – 112 Ghosh KM, 2013, Analysis and Design Practice of Hydraulic Concrete Structures, PHI Learning Pvt. Ltd, Delhi HDR Engineering Inc, 2001, Handbook of Public Water Systems 2nd Edition, John Wiley and Sons, New York Hobbs, AT, 1969, Manual of British Water Engineering Practice, vol. 2. W. Heffer and Sons, Ltd, Cambridge Joints & Sealants, 2010, Healthy Construction Manual-1, Dr. Fixit Institute of Structural Protection & Rehabilitation Read More