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Deep Well Systems For Dewatering of Excavations

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Deep well systems are dewatering methods used to remove the water from pervious sand or rocks formations beneath the excavations. This method can also be employed to remove the artesian pressure of the ground area under consideration. The method of deep well dewatering systems is more suitable in areas where deep excavations are performed. This hence demands higher rate of pumping for dewatering. This is mainly employed for the ground preparation for the construction of tunnels, dams, powerhouses, shafts, and locks. The excavations and the shaft made can be 300 feet in depth. These can be dewatered by pumping from deep wells by using a turbine or a submersible pump. The main significance of deep wells as a means of dewatering is that they can be installed all around the periphery of the excavation. This will let the construction area to free from burden by the equipment that is used for dewatering. This is shown in figure-1. Here the excavation can be pre*drained for the complete depth.

Fig.1: Deep Well Dewatering System

Working and Arrangement of Deep Well System

The deep wells arrangement for the purpose of dewatering is similar to that for commercial water wells. These systems will make use of a screen that have a diameter of 6 to 4 inches with lengths ranging up to 300 feet. When such a system is installed, a filter is placed around the screen. This arrangement helps to prevent the infiltration of the foundation materials into the well. The installation of filter also helps to improve the yield. In order to dewater small deep excavations, the deep well systems can be used in conjunction with the deep wells. This is applied for related works of tunnels, caissons sunk, shafts and the areas with fine grained sand or stratified soils that are pervious. In areas, there are rock layer below the ground table this method work best. An increase in hydraulic gradient to the well because of the use of vacuum creates a vacuum within the surrounding. This phenomenon avoids seepage from the perched water into the excavation. The installation of deep well system incorporating vacuum is shown in figure-2. This type requires adequate vacuum capacity to undergo the dewatering operation efficiently.

Fig.2. The use of deep well with vacuum systems to dewater a shaft over a stratified ground material.

To have sufficient wetted area of intake in the aquifer, adequate well depth have to be provided. This helps to produce yield and interactive drawdown. In most of the civil engineering applications, a depth of 60m with a typical depth value of 20m is used. For a limited distance say 1 to 2m, the well might penetrate an impermeable layer lying below the pumped aquifer. This is to behave as sump for the fines. The pump must be placed such a level in the well so that the water circulation helps it to remain cool. The site layout decides the spacing of the wells. But most commonly, the spacing used is 10 to 30m. The deepening of the well creates drawdown in areas. Sometimes these might be the areas where the wells cannot be sited. Special care and precaution must be taken so that with increase in drawdown no kind of settlement is happening to the adjacent buildings. The gravity flow of water into the well without affecting the unconsolidated strata that is being pumped can be facilitated by the use of well screens. Based on the type of duty performed, the material used can be steel or plastic. Use of different metal will result in corrosion cells.

Operation of Deep Well System for Dewatering of Excavations

The operation of deep well dewatering requires electric supply from three phase generators. For the use in long - term operations, it is recommended to have an automatic cut in generator in case of a power failure. This equipment is considered as an insurance requirement. Where electric power is not available, the diesel operated shaft drive pumps is used. It is advised to have standby pumps in case of any failure. We must calculate the time from the well failure to the ingress of the water. For a short period of time, the adjacent well must have the capability to take over the duty of the failed well. This short period is necessary for remedial action. In order to maintain the drawdown and to stop the pump from drying, level-monitoring electrodes are placed at maximum and the minimum levels inside the borehole. A proper and careful manual tuning of the surface recharge valve will help to maintain pumping in a steady rate. The pumps used for this purpose ranges from simple air lift pumps mainly used for short - term dewatering activities. For large diameter wells, top drive vertical shaft pumps and submersible electric pumps are good choice. Do not use pumps to obtain higher output than that is essential. The use of large shaft driven pipe must have vertical placement accurately with proper vertical liner and screen to have no kind of vibration. An electric submersible bore hole pump is the most versatile pump for deep well pumping. Regular cleaning of pump is necessary. The maintenance will affect the output.

Suitability of Deep Well Dewatering System

The method is more suitable where long term dewatering has to be carried out. This will thus work for large excavations for a variety of ground conditions. These systems can help provide under-drainage of the overlying soil that is less permeable into the permeable stratum that is pumped. This also helps to relieve the pressure below the confining layer of clay.

Design of Deep Well Dewatering System

A detailed conceptual model has to be developed based on the ground investigation carried out. It is very important to determine early the distance, the drawdown and the time results based on the pumping tests carried out. Some of the basic steps that are involved in relation with the design of deep well dewatering systems are:
  1. The drawdown that is required for the well is initially determined. This will help to suit the geometry of excavation.
  2. The inflow into the excavation is calculated. The inflow in large areas can be assessed using the flow nets. This can also check the potential during high inflow velocity that might cause piping or boiling.
  3. The yield and the wetted length of the trial well is calculated.
  4. The number of wells and spacing is calculated. The zone interaction if any is found for those wells at less than 20m centers.
  5. The filter pack and the screen slots are designed to make sure that the aquifer is not drawn into the well.
  6. The entry velocity to the well is checked
  7. Pump size is checked so that it suits the screen or the liner.
  8. Sufficient working space for side slopes and excavation have to be provided.
  9. The calculations are repeated so that an optimum design is obtained.
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