There are number of dewatering options available for excavations to facilitate construction activities. The choice of best and suitable dewatering system brings economy in construction activities. For this, it is essential to have knowledge on different dewatering systems and each of their features in detail.
The best system is selected from different systems gathered. The choice of dewatering system for a site condition mainly depends on the:
- Location and site features
- Type, size and the depth of excavation planned
- Thickness and the type of stratification
- Permeability of the foundation soul that is below the water table
- Water table level of the area, where the dewatering to be performed
- The potential damage a dewatering system is prone to
- The cost of installation and the operation of the dewatering system
Factors Influencing the Cost of a Dewatering System
The cost invested for a dewatering system or a dewatering method is dependent on four main factors. They are:
- The type and size of the project. This include the pumping requirements for dewatering
- The power type and availability
- The labor resources
- The time period of pumping
The method of slurry cut off walls have helped in controlling the ground water, that have in turn reduced the amount of pumping. Those projects that require high pumping can make use of barrier walls like slurry cut off walls.
Factors Affecting the Selection of Dewatering System
If a project involves the construction of foundation in the ground level that is below water table of that site, it is necessary to undergo dewatering through methods like well point system or deep well system. This problem cannot be solved by methods like trenching or sump pumping.
The equipment operations, or the sliding or the sloughing of the side slopes can result in damage to the foundation. These effects on foundation can be reduced by dewatering systems.
The deep well and well point system does not require any detailed analysis or design before implementing in a site that does not bring huge pressure fluctuation underground. Here conventional dewatering system can be used.
But wherever there is unusual pressure relief, it is advised to have a detailed design prepared by the engineer to perform the dewatering. These details must be specified in detail by the engineer in the contract documents also. The use of unusual dewatering equipment must be specified by the engineer prior to the construction activities.
Major factors that affect the choice of dewatering system are explained in detail:
- The Type of Excavation
- The Geological and the soil conditions
- Reliability Requirements
- The Depth to which the ground water is lowered
- Rate of Pumping
- The Intermittent Pumping
- Adjacent structures affected by ground water lowering
- Dewatering compared with other procedures
1. Type of Excavation
A conventional well point system is found economical and safe when dewatering has to be conducted in a ground area where the water table level is at a lower depth. A lowering of the water table level to a depth of 20 or 30 feet require deep well system or jet-educators to conduct the dewatering.
For excavations that are surrounded by cofferdams, either well point system or a deep well system or both in combination can be employed for dewatering.
A deep well system or a jet-eductor well point system is a best choice for dewatering that requires penetration into a field pervious soil or rock. These are mostly implemented for the construction of deep shafts or tunnels or caissons. The choice between the two is based on the soil formation in the area and the rate of pumping that is desired.
Other related factors include:
- The Interference of the dewatering system with the ongoing construction operations
- The space available to include the dewatering system
- The duration of dewatering
- Installation and the operation cost
2. Geological Conditions
The type of dewatering system and drainage system for an area is best dictated based on the soil and the geological conditions of the area. A conventional well system or well point system can be used if the soil below the water table of the area is deep, free draining sand and more or less homogeneous in nature.
If the soil is highly stratified and there exist an impervious layer of rock, shale or clay then well point systems that are installed closely in space can be employed. Wherever the soil is in need for the relief of artesian pressure, deep wells or jet educators well points in few numbers can be installed.
3. Depth of drawdown
The magnitude of depth to which the drawdown has to be conducted is one of the primary factor based on which the type of dewatering system is chosen. If the drawdown to be made is very deep, the deep well dewatering system or the jet eductor well points are used. The conventional well points system requires many stages of drawdown for a single depth of excavation.
A wide variety of flows can be met by using the deep well system given that appropriate pumps depending on the flow must be chosen. This type of flexibility is not available for jet eductor well point system. The jet eductor pumps are more employed where the flow is small as in the case of silty to find sand cases.
4. Reliability Requirements
The design of dewatering pumps chosen, the standby power, equipment and the power supply is influenced significantly by the reliability of ground water control for a particular project.
For example, if the problem that is faced during dewatering is the relief of the artesian pressure so that blow up during the bottom excavation is prevented. This situation affects the choice of pressure relief system that is selected and the need for a standby equipment with the provision of automatic power transfer.
5. Required Pumping Rate
The pumping rate that is required to undergo dewatering of an excavation ranges between 5 to 50,000 gallons per minute or higher. The selection of walls, piping system, and the pumps are affected by the flow to the drainage system.
For deep well dewatering system, the pumps are available from sizes of 3 ro14 inches that have capacities ranging from 500 to 5000 gallons per minute. These have head value up to 500 feet.
The pumps used in well point system have sizes ranging 6 to 12 in inches with capacities ranging from 500 to 5000 gallons per minutes. This is dependent on the vacuum and the discharge heads.
The jet eductor pumps have pumping ability from 3 to 20 gallons per minute that lift upto 100 feet.
The rate of pumping is largely affected by the:
- The distance to the source of seepage
- The thickness of the aquifer
- The perviousness of the aquifer
- The amount of draw down or the pressure relief required
6. Intermittent Pumping Procedure
Having single or double shifts for dewatering per day brings the labor cost down. This principle cannot be followed wherever the depth of dewatering is very large, the subsoil is pervious and homogeneous. Under these situations, the pumping system can be operated such that large drawdowns are taken during single or double shifts.
7. Effect of draw down procedures on Adjacent Structures and Wells
The foundation soils that is below the ground water table is loaded additionally when the groundwater table of that area is lowered through dewatering system. Application of additional load results in the consolidation of the soil. This consolidation results in settlement of structures that are already constructed within the radius of influence.
Before the application of any dewatering system on a site, it is recommended to check for before mentioned possibility of settlement of active structures around. The water table level of nearby wells has to observed before and after dewatering. If any claims raised due to the dewatering procedure is raised, these observations are the basis on which the evaluation is made.
8. Dewatering Compared with Other Procedures
The control of water during excavation can be best done economically by the process of dewatering. But for certain applications like for the wet side of a cofferdam or a caisson, a tremie sealing is done on the bottom side of the excavation. Any other method like slurry cut off walls or other dewatering procedures are employed.
Other techniques are chosen based on suitability like the use of freezing techniques and rotary drilling machines that is conducted without the lowering of the water table can be employed. The use of concrete cut off walls in a slurry supported trench are other examples.