Table of Contents
What Do You Mean by Term Well Point?
A Well Point is also known by the term Drive point. Well Point is a well of small diameter that is used for dewatering.
Parts consisting in a Well Point are a shallow shaft, a dewatering pump to draw water, a riser pipe that connects the Well Point to the surface, and a small well that fills up.
The diameter of a riser pipe is around 38 to 50 mm. It has slots or openings at the bottom that enables water to be drawn up using the pump.
The size of the slots or openings in the riser pipe is determined by the relative grain size of the material used for the water-bearing formation.
The opening in the riser pipe should be large enough to allow water to enter the tube, but it should be small enough to keep the water-bearing formation in place. The riser pipe of a Well Point is often connected through a flexbow to a common header main pipe.
The flexbow incorporated in the riser pipe acts as an adjustable valve which serves two purposes, firstly it controls the flow of water and air entering the Well Point system, and secondly it allows others to have a clear view of what is being pimped.
The common header main pipe of the riser pipe is then connected to the well point dewatering system. Well Points are very versatile and it can be used for a wide range of applications like shallow foundation and trench works.
Well Point may be installed in parallel for particularly long pipeline trenches with the help of special trenching machines. Well Point can be used for irrigation purpose of small to medium size fields like residential gardens.
If sufficient water is not yielded from one well for running the irrigating system them a second well point can be installed and connected with the first one which will help them function as one system.
As an alternative for boreholes Well Point can also be used around the perimeter of excavations, which will act as a means of lowering and controlling groundwater levels.
Well Point is more cost and time-efficient than a borehole in cases where shallower depth is required. A Well Point can be installed very quickly in an appropriate ground condition.
While the installation process of Well Point traditional drilling techniques such as jetting or auguring is typically used. Large quantities of water cannot be yielded from Well Point, typically we can get around 5 gallons of water per minute.
There is also a general limit of 5 to 6 m in height over which water can be drawn in Well Point as the vacuum is limited in practice to 0 bars. The second set of Well Point can be installed at a lower level for the requirement of greater depts.
Well Points are capable of dealing with water from very low flows in silty sands as well as from large flows in coarse sands and gravel.
What Is Well Point System?
Well Point system can be used to lower the groundwater levels of an excavation site in order to provide stable working conditions. Well Point system is mainly suited for a shallow foundation, trench work and dewatering utility.
A Well Point system consists of several closely spaced shallow wells of small diameter. The components of a Well Point are connected to common header main pipe and water is pumped with a high-efficiency vacuum dewatering pump.
Further stages of Well Point system are required for drawdown when it exceeds 15 feet and installed at successively lower levels as excavation proceeds.
Types of Well Point System
Here. mainly two types of Well Point system are as follows.
- Single-Stage Well Point System.
- Multi-Stage Well Point System.
1. Single-Stage Well Point System
In a single Well Point system the well point consists of a pipe about 1 m long and 50 mm in diameter. In order to prevent clogging of the pipe, the perforations are covered with a screen.
A jetting nozzle is provided at the lower end. The jetting nozzle also acts as a drive point. During the installation process of the well point, a ball valve is provided near the lower end which allows the water to flow only in one direction.
At the bottom end of the well point a riser pipe having the same diameter as the well point is connected. At the ground level, the riser pipes are connected to a horizontal pipe also known as header pipe.
Then the pumping unit is connected to the header pipe. The spaces between well points are 0.6 to 1.5 m. Water is jetted through the riser pipes in the downward direction during the installation of well point at the required depth.
Till the excavation work is complete the installed pump work continuously to regulate the water level.
2. Multi-Stage Well Point System
Multi-stage well point systems are used if excavation exceeds 4.5 m and there is failure due to slope. In Multi-stage well point systems the well points are installed in stages.
At the first installation stage, the well point is installed at a depth of around 4.5 m and put into action till the water table is lowered by about 4.5 m.
Then the surrounding area is excavated up to the depth of 4.5 m. Then in the second installation stage well points are installed in the area which is already excavated.
In this stage the water table is furthered lower by another 4.5 m.
Well Point Dewatering System
In excavation sites a simple yet efficient method of lowering the water table is using well point dewatering. In a well point dewatering system, there are many series of shallow wells also known as well points, which are installed at a predetermined depth and at appropriate spacing all around the excavation site.
All the series of well points are connected to the surface through a riser pipe, and which in turn is connected to the common header main pipe through a flexbow.
The flexbow which is incorporated in the header pipe acts as an adjustable push-fit valve which allows the control of water and air entering the well point system. This is known as trimming and it gives a clear view of what is being pumped.
The header pipe of the well point system is connected to the well point dewatering pump, which pumps the water and discharges it to the designated point.
PVC Well Point System
PVC well points are economical well points which are used when a jet rod or punch casing reused in a well point system. The well point riser can be pulled and reused because an inside slip coupling is used.
In order to create a well point screen which allows maximum water flow with minimum passage of fine material, precision slotting of PVC plastic pipes is used. This PVC plastic screen is self-cleaned by reverse flushing during jetting.
The PVC well point screen totally eliminates clogging due to corrosion or deterioration by electrolytic action. Maximum drawdown can be achieved by using an optional inner tube.
Installation of Well point System
High pressure jetting pumps are used in the installation process of self jetting well points and placing tubes that are used in disposable well points.
The jetting pump most commonly used in well point system supplies water via the jetting hoses at a rate of about 20 L/s and at a pressure of 6 to 8 bar.
A standard 63 mm fire hose is usually connected to the flexible jetting hoses with instantaneous male and female connectors. For heavy-duty uses like hole puncher and pile jetting, other more powerful jetting pumps and hoses are often used.
Well Point for Shallow Well
In good condition water can be drawn from the maximum depth of 25 inches in a shallow well. The well point that is used in a shallow well is generally not used for a good source of potable water.
A shallow well is often exposed to hazards like pollution from fertilizers and animal wastes and sometimes becomes dry wells. If a well point is to be used for a source of potable water, one should refer to their applicable local well and plumbing codes.
To ensure if the water is safe for drinking, regular testing should be necessary.
Application of Well Point System
At the following construction or project sites well point system can be adopted,
- Hydro projects
- Water supply projects
- Land reclamation projects
- Canal construction
Structures for thermal power plant with not too deep foundations
- Underground tank construction
- Bridge construction
- Excavation of foundations for building
- Basement construction
- Laying of deep sewer lines
- Tunnel work
- Construction of subways
Advantages of Well point System
Followings are the unique benefits of well point system due to which this method is commonly more preferable over other dewatering methods;
- Groundwater Management: The main purpose of any dewatering practice is to draw down the water table below the working platform and maintain
- the depressed level for the duration of the dewatering period.
- Area of Application: Well point system is usually most efficient to cover more land area than other as this system cover more area with a constant
- capacity of 100 m3/h of water, air or water, and air mixture is discharged against a head of 20 meters.
- Design Criteria: Well point system gives you a different method to adopt and selection of the most efficient method for your particular job or site.
- Technical Background: Dewatering is commonly required on-site when excavations are to extend beyond the subsoil water table. For shallow
- excavations (up to 4m) wellpoint dewatering is often the most effective method to achieve an adequate drawdown in permeable soils. (i.e. sand, clayey
- sand, etc.)
Quick Deploy: Well-point systems can be placed on the project site very efficiently and effectively to reduce the downtime of the construction project.
- Less Cost: The system of wellpoint method and dewatering equipment can be taken on lease for short and long periods, and individually, to serve the
- builder’s dewatering requirements.
- More Effective: At construction projects where deep well dewatering is not required, a wellpoint dewatering system is an effective and economical way
- to reduce the level of the water table to the required excavation level.
Disadvantages of Well point System
Followings are some cons of well point system
- It is essential to continue pumping once it has started until the excavation is complete. If it is stopped in between, it may prove to be tragic.
- In the case of ground consisting of stiff clay, gravel, or boulders, well points are installed in drilled holes, which increases the installation cost.
- Wellpoint dewatering system is suitable for lowering water table by 5 to 6 meters only. For deeper excavations, where the water table is to be lowered for a greater depth caisson method required for that particular site.