INTRODUCTION • Almost all deep excavations are now associated with the removal of subsoil water for laying foundations of structures or underground construction. Most of the structures have deep foundations involving a substantial dewatering job. The overall land area around the world remaining constant, the development arrow has begun to point vertically upwards. Horizontal expansion has given way to vertical expansion. As a result, the importance of using dewatering equipment has increased manifold. • In many projects, dewatering of foundations plays the key role in deciding the overall completion period. Till the time the dewatering equipment is successfully installed and foundations come up, uncertainty prevails over the commissioning date of a project. completion schedule suffers a setback. Thus, it is very important to assess the quantum of dewatering work involved; choose the right dewatering equipment; and keep standby arrangements in place.
WELL POINT DEWATERING A method for draining permeable deposits around an excavation that requires small cones of depression. Tubes about 100 mm in diameter, with wire mesh screens, are sunk into the ground and connected by a header pipe to a suction pump at the top. Usually a series of well-points are connected to one header pipe. When these are used in a staged excavation a considerable depth can be drained and the drawdown restricted, because each well-point acts as a hydrogeologic boundary.
WELL POINT DEWATERING PICTURES SHOWING WELL POINT
AREA OF APPLICATION 1. Hydro projects 2. Excavation of foundations for buildings 3. Basement construction 4. Laying of deep sewer lines 5. Tunnel work 6. Construction of subways 7. Water supply projects 8. Land reclamation projects 9. Canal construction 10.Thermal power project structures with not too deep foundations Underground tank construction 11. Bridge construction
PRELIMANARY REQUIREMENTS • Dimensional plan of area of excavation should be prepared. Proposed depth of foundation should be known and lowest depth of excavation should be ascertained. • Geo-technical investigation data should be collected and position of subsoil water should be known. • If river or stream is running in the vicinity of the site to be dewatered, its distance, discharge, direction and high flood level (HFL) should be known. • Characteristics and type of soil to be dewatered should be ascertained. Thickness of various strata should be known. • Permeability of porous strata should be determined. Coefficient of permeability may vary from 1 cm per second for very fine sands to even 3,000 cm per second for gravel and coarse sands. • Chemical properties of groundwater may be determined only if dewatering equipment has to remain in position for a considerably long period.
MULTI-STAGE WELL POINT SYSTEM • Well-point system is suitable for depths up to 6 m if the pump is installed at ground level. Soils have to be coarser. If the sand content of soil is more than 20 per cent, the well-point system can work in it. For excavations deeper than 6 m, multi-staged dewatering equipment can be used. In this case, each stage has its own pump. Number of stages can vary but more than three stages of well points dont seem practical. More the number of stages, more is the excavation width required. Sometimes, a single-stage well-point system is attached with jet-eductor pump - this system allows dewatering up to 25 to 30 m depth. This is preferable over a multi-stage well-point system. In case of a multi-stage well-point system, care should be taken that the header of the lowermost stage is not more than 4 m above the excavation bed so that vacuum in the lines is not affected.
EDUCTOR SYSTEM The eductor system is generally used in areas with soils of low permeability and to avoid using a multi-stage well-point system. In comparison to deep wells, it proves cost-effective too. It is well suited for deep excavations with stratified soils. The number of eductors in one system activated by a single pump is around 100. The volume of water pumped out by this system is normally low, in the range of 15 to 20 litre per second. Under the system, a series of eductor wells are installed and connected to two parallel headers. One header is a high pressure water supply line and other is a low pressure return line. Both lines run to a central pumping station that feeds water under pressure to eductors located at the bottom of the wells. The system uses a venturi to draw groundwater into the screen of the well and further to the ground surface. While a large diameter pipe forms the well casing, a smaller diameter inner pipe forms the return line. Water is pumped under high pressure between the two pipes and forced through the venturi. The ground water is now recovered through the well and into the return pipe.
SYSTEM DETAILS • Well points are about 1-m-deep slotted pipes carrying brass mesh screens over them. These act as filters or strainers and thus throw out only clear water. The diameter of well points is only 2 to 3 inch. Each well-point has a self-jetting nozzle at the bottom to help it drive into the ground to the desired depth. Sometimes, it takes minutes to sink the well points to the desired depth. Vertical riser pipes connected to the well points are of 2 to 2 ½ inch diameter. These are connected to the horizontal header with flexible swing joints. The header pipe has plug cocks to receive the flexible connections. These connections are equipped with non-return valves. The horizontal pipe connected to the vertical pipes is of 6 inch to 1 ft diameter. In certain cases, it may be of larger diameter. One well-point system has 50 to 60 well points. All the points and pipe system are connected to the pump. A 6 inch diameter header pipe provides a flow of 30 litre per second; a 8 inch diameter header gives 60 litre per second; a 10 inch diameter header gives 110 litre per second; and a 12 inch diameter header draws up to 190 litre per second.
WELL POINT PUPMS-1 • The dewatering pumps are centrifugal pumps driven by electricity or diesel. The pumps are able to produce a high vacuum and have good air handling capacity. For this, self priming centrifugal pumps are attached with vacuum pumps. A diesel set is kept as a standby in case of power failure. A float- actuated air water separator tank is provided with the pump. A vacuum pump throws out the air separated by the separator from the water being removed. The location of the pump in the header line is governed by the fact that the pump is able to develop the required vacuum in the pipeline.
WELL POINT PUPMS-2 • If the header pipeline is too long, i.e. more than 150 m, the pump may have to be located at its centre. If the length is smaller, the pump may be located at its end. In any case, development of maximum vacuum cant be sacrificed. An effort should always be made to set the pump intake at the level of the header line. In any case, it shouldnt be kept more than 5 m above the level of the bottom of the excavation. The pump is equipped with a water discharge pipe that leads to a drain or basin at a sufficient distance from the excavation area to avoid its effect on the dewatering exercise.
WELL POINT PUPMS-3 • The spacing of well points depends upon the type of soil being dewatered. The more the permeability of soil, the less the spacing of well points as more discharge is required to be removed. Thus, spacing is less in sandy soils and more in silt soils. In sandy soils with gravel, spacing may be as less as only 1 ft. Generally, the spacing varies between 1 to 4 ft. The length of the header pipe and rate of discharge also matter in deciding the location and spacing of well points. Well-point systems may be in metal (steel or aluminium) or PVC. Earlier, metallic pipes, brass strainers and MS header pipes were used. Now, more and more manufacturers are switching over to PVC because of its flexibility, corrosion-proof quality and cost- effectiveness. Well-point system in PVC also has the advantage of being lightweight and easier to handle. Another advantage is low friction losses.
INSTALLATION • Well points are equipped with self jetting tips and require water under pressure for self-installation. About 900 litre of water are specified in the IS code for each well-point. Water pressure has to be up to 14.5 kg per sq cm; if the pervious layer is underlain by a clay layer, jetting does not prove effective and augering of clay layer is first done. IS 9759 advises an important safeguard of sanding in of well points to avoid clogging of the system by fine materials from the ground. Under this process, the water pressure for jetting is reduced just to keep the hole around the point open, and coarse sand is filled around the annular space to act as a supplementary filter. Now, the jetting water supply can be safely closed.
MERITS AND DEMERITS • The well-point system has the advantage of getting installed along any flexible line or shape. When a large area is to be dewatered, well points can be installed around the area. For long excavations for pipelines or cables, well points can be installed along a line and prove quite effective. • In such a case, a continuous exercise of installing new well points in the area ahead and taking out well points from the area completed is on. Just one header pipe laid along the length of excavation connects the well points. However, the well-point system is not suitable for very deep areas. Excavation width becomes very large. Sometimes, air leakage into suction pipes is common and this adversely affects dewatering efficiency.
PLAYING SAFE Before beginning the dewatering operation, the well-point system should be checked to ensure it is leak-proof. All leakages, however minor, should be plugged by use of adhesives or paints available for the purpose. Always provide each well-point with a stop cock so that each individual well-point can be connected to or disconnected from the network. See that all vertical risers run full and air leakage is not allowed. Keep all the joints in the system airtight. Try to restrict the number of well points connected to a pumping unit to 50, maximum 60. Provide a solid base to the pump. See that the area of installation doesnt become slushy. Keep an eye on the vacuum gauge fitted to the pump. If any fluctuations are noted, these are an indication of trouble from well points. Discharge from well points may reduce in this case. This problem should be corrected by adjusting the plug cocks and checking the vacuum pump outlet. Always provide each well-point with a stop cock so that it can be connected or disconnected from the network.
QUICK BITES Deep excavations are associated with removal of subsoil water for laying foundations of structures. To complete the projects on time, it is very important to assess the quantum of dewatering work involved. Two types of dewatering equipment are used - well-point equipment and deep bore well equipment. Well-point equipment consists of a number of wells installed around the area of excavation. Well points are equipped with self jetting tips and require water under pressure for self-installation. The well-point system has the advantage of getting installed along any flexible line or shape but is not suitable for very deep areas.