Wellpoint dewatering is the temporary lowering of the groundwater table in order to provide dry and safe working conditions during excavation work. BBA Pumps produces and supplies all the necessary products to create a professional wellpoint dewatering system.

What is groundwater?

When using the term “groundwater” we are referring to water in the soil that is below the groundwater table. Groundwater is created by the seepage of rainwater from the surface into the ground, filling the pores within the soil. Groundwater ensures a healthy ecosystem and provides an important source of our drinking water. Our world is highly dependent on its existing water resources. For this reason, it’s critically important that groundwater extraction is carefully planned and controlled when undertaking underground excavation work.
 
Gravity and pressure conditions move groundwater downwards and horizontally within the soil. The water moves at a certain speed. This speed depends on how quickly the water can penetrate into the soil and the soil structure, this is termed the permeability factor. Some terms:

• Permeability factor k, this determines the flow rate of groundwater in meters per day;
• Aquifers are water-bearing soil types such as sand and gravel, where the water flows vertically;
• Aquitards are poorly drained soil types such as clay and peat, where the water flows horizontally;
• Porosity % is the ratio of small pores in, for example, gravel-sand-silt-clay-peat;
• The phreatic zone, is the part of an aquifer, below the water table, in which most pores and fractures are saturated with water.

What is wellpoint dewatering?   

Before excavation work can start the groundwater must be sufficiently lowered. There are several methods for achieving this, the most popular among contractors is wellpoint dewatering due to its efficiency and cost-effectiveness.

The dewatering of large projects is undertaken by specialized dewatering companies. They are responsible for such things as:

• Surveying the soil to understand the composition of the soil layers;
• Establishing the groundwater level, the flow speed and the required pumping capacity;   
• Designing the dewatering system to guarantee optimal groundwater extraction;  
• Monitoring local regulations for any required permits and any potential costs for the discharge;
• Analyzing any possible risks regarding environmental impact or the risk of buildings subsidence.

BBA Pumps does not install wellpoint dewatering systems itself, but supplies specialized dewatering companies who carry out the work. For short-term excavations, such as sewer renovations or swimming pool installations, contractors often install wellpoint systems themselves.

For those who are not regularly involved in groundwater extraction projects we have produced insightful content about wellpoint dewatering. We focus on filter dewatering, where a series of small diameter pipes, fitted with slots at the end of the pipe are placed vertically in the ground around the excavation. Many people use the name risers, riser pipes or spears instead of wellpoint filters.

Installation of wellpoint filters

With wellpoint dewatering, a hole is first made in the ground using a jetting tube that is connected to a high head pump. However, when confronted by hardened layers of soil, dewatering companies will often use a machine to drill the hole. A wellpoint dewatering filter is then placed in the hole to the desired depth in the aquifer (water-bearing soil).

A suitable wellpoint hose ensures an airtight fit between the dewatering filter and the HDPE header pipe. For effective groundwater extraction, place the filters in U-shape around the construction pit at least every two metres (see Figure 1).


The HDPE header pipes are connected via a flexible pump hose to a PT piston pump. As soon as the pump is started, the vacuum in the wellpoint dewatering system increases and the groundwater is drawn away through the slots at the bottom of the filter (riser). The groundwater table falls and a gradient line is formed in the soil between the wellpoint dewatering filters. That is to say: the phreatic zone will be diverted (see Figure 2).

             Figure 2.
             B = HDPE header pipe
             F = Dewatering filter
             G = Gradient
             R = Reach
             S = Slots in the filter
             V = Phreatic zone
             X = Wellpoint hose

Tip: It is important that the wellpoint dewatering filters are placed at the same depth. Otherwise, there is a good chance that the soil in the construction pit or in the trench will retain water in some places.

Multi-stage wellpoint dewatering

With a single stage wellpoint system it is possible to drain a construction pit or trench dry to a depth of about 7 meters. However, should you need to go deeper, we recommend using a multistage dewatering system. This is a system in which you install drainage around the construction pit in several stages. With multi-stage wellpoint dewatering, the construction pit can be dried up to a depth of 20 to 25 meters. Some tips;

• Ensure that there is enough free work space around the construction site. A lot of material will be needed when installing a multistage dewatering system;
• Due to the depth of the pit it may become impractical to discharge water out of the site in one single process. For this reason, a water tank can be placed at a suitable height in the side of the pit into which the PT piston pump can discharge. An additional pump is then used in a second stage to further pump the water upwards and out of the pit;
• It is important to monitor the system pressure on the pumps and ensure that it does not become too high, this can lead to dangerous situations;
• Install non-return valves in the discharge line so that the PT piston pump can be relieved of system pressure if it fails.

With the multi-stage dewatering system in Figure 3 it can be seen that the top stage will pump less groundwater. This is due to the behavior of the gradient lines of the phreatic zone.

Filter dewatering combined with drainage pipes

With large construction pits, the gradient line may not be sufficiently level to allow dry work. In such cases horizontal drainage pipes can installed in a layer of sand just below the bottom of the construction pit. The drainage pipes are then connected to separate PT piston pumps. BBA Pumps does not supply drainage pipes; however, it does supply drainage connections with quick couplings on the suction side of the pump. The combined drainage is shown schematically in Figure 4.

                   Figure 4.
                   D = The construction site
                   F = Wellpoint dewatering filter
                   G = Gradient line
                   X = Drainage pipe



Tip: It is important that the drainage pipes are properly supplemented with gravel or coarse sand and well-sealed.

Discharging the extracted groundwater

The extraction of groundwater is regulated in some countries. A ditch or sewer system is often enough for use as a discharge point. However, in some territories it is mandatory to return the extracted groundwater to the same soil layer. Specialized dewatering companies are familiar with all of the local regulations and can calculate the potential costs to discharge each m³ of groundwater.

A flexible pump hose is attached to the discharge side of the PT piston pump, which is then connected to HDPE discharge pipes. Before the groundwater reaches the discharge point, it must first flow through a sand collector with a built-in water flow meter.

The sand collector is a settlement tank where the groundwater comes to rest and any sand particles settle. The air bubbles are also removed, so that the water flow meter can very accurately measure how much groundwater is extracted via the wellpoint dewatering system.

Tip: If several PT piston pumps are connected to a collecting discharge pipe, we recommend to place a non-return valve between the pipes after each pump. This prevents unnecessary pump damage in the event of failure of one of the pumps and prevents water from flowing in the wrong direction.

Overview of necessary products for wellpoint dewatering   

All BBA Pumps products for wellpoint systems are of the highest quality and can be reused for many years.

Wellpoint dewatering pump
We recommend the usage of an efficient pump that can run dry and has a high vacuum range. The PT piston pumps have been specially designed for this type of work and can deliver outstanding service for decades with low energy consumption and minimal maintenance requirements.

High head pump
A high head pump is needed to spray a jet of water creating the holes vertically in the ground. For this purpose, dewatering companies use a self-priming high head centrifugal pump. This is often driven by a diesel engine and assembled on a wheelbarrow frame.
 
Jetting tube
A BBA Pumps jetting tube includes a flexible hose and quick coupling to connect to the pressure side of the high head pump.

Wellpoint dewatering filters 
BBA Pumps wellpoint filters (risers) are made from special impact-resistant PVC and available in different lengths. The final meter in length has perforated slots through which the water is drawn inwards. These high quality dewatering filters can be reused for years on a variety of projects.

HDPE header pipes 
BBA Pumps header pipes are made from HDPE, fitted with quick couplings at each end and with multiple branches. The wellpoint dewatering filters are connected to the branches via a wellpoint hose.

Wellpoint hose 
BBA Pumps wellpoint hoses are flexible pieces of hose supplied in one meter lengths, they have the same diameter as the filter. They are available in black rubber or transparent PVC. Wellpoint hoses flexibly bridge the distance between the dewatering filter and the branch on the header piping.

Quick couplings 
BBA Pumps quick couplings come in different forms. We have bends, T-pieces, Y-pieces, reducers, end plugs and much more. Quick couplings are used to connect the pipes of the wellpoint dewatering system together around the construction pit.

Pump hose 
These are flexible, lightweight hoses and have the same size as the header piping. With pump hoses you can easily connect HDPE pipes to the connecting parts of the PT piston pump or to the settlement tank.

Settlement tank
The BBA Pumps sand collector is a settlement tank where fine sand particles settle in the groundwater. This prevents sand from being pumped into a ditch or sewer system. A settlement tank also works to remove air and allows for a Handy flow meter to be fitted.

Water flow meter 
A flow meter allows you to accurately measure the amount of groundwater that is extracted and discharged via the wellpoint dewatering system. This provides additional benefits for the client. Using a water flow meter may in some cases be mandatory for work with governmental bodies or water boards.

Non-return valves
With BBA Pumps non-return valves you professionalize the wellpoint dewatering system and prevent groundwater from flowing back. Installing a non-return valve at the pump prevents the pipe system from emptying its contents over your shoes and work clothes when the quick couplings are loosened.

Pipe and hose racks 
Our pipe and hose racks are useful for the storage and transportation of wellpoint dewatering systems. BBA Pumps produces pipe and hose racks that can be stacked with the mobile wellpoint pumps.