When contractors encounter groundwater that interferes with construction work, the best question to ask in most cases is, “Can I use wellpoint dewatering?”
Dewatering is necessary when the excavation is deeper than the level of the static groundwater. Wellpoint dewatering is the most frequently used method of keeping a site in a “workably dry” condition. When used in the right soil conditions and designed and installed correctly, wellpoint dewatering systems draw down water faster than other methods. Because wellpoints do not require large drill rigs, they are a cost-effective solution. In addition, they can be installed relatively quickly. Wellpoint dewatering is suitable for a wide range of applications, including digging excavations for foundations, underground parking structures, sewers, pipelines, tunnels, canals, dams and levees. It also enables pinpoint dewatering for small areas, such as the areas around utilities.
How Wellpoint Dewatering Works
A wellpoint dewatering system consists of a series of shallow, small diameter wellscreens placed at predetermined intervals, typically around the perimeter of an excavation or alongside a trench. A vertical riser pipe connected to the wellscreen functions like a straw to drain water from the soil. The riser pipes are connected to a horizontal pipe, called a suction header pipe, which connects to a pump. To prime the system, the pump is filled with water. As the pump expels the water, a vacuum is created. Water is then removed from the ground through the resulting suction and transported to a discharge point.
Valved swing hoses attached to the riser pipes and suction header pipes are used to “tune” the wellpoints by limiting the amount of air that moves through them.
Matching the Dewatering System to the Application
Wellpoint dewatering systems are designed for excavations with a maximum depth of 18 feet. In optimal soil conditions, they can potentially be used for slightly deeper excavations. If the excavation depth exceeds the suction lift limits of the system, the pump will fail to remove water any deeper than 18 feet.
Choosing an Installation Method
The method of installing wellpoints depends on the soil type and other site specifics, including the presence of obstructions such as underground utilities, overhead power lines and adjacent buildings. The availability of personnel and equipment also influences the choice of installation method.
Wells are often installed manually with a jet rod. If an excavator is available, wells may be installed using a jet pump or an auger attached to the excavator, or with a combination of drilling and jetting. A jet casing pipe may be used in silty soil to keep the hole open long enough to place a clean sand filter around the wellpoint screen.
Common Wellpoint Dewatering Mistakes
By sidestepping the common mistakes below, contractors can avoid fines and can get the maximum benefit from a wellpoint dewatering system.
- Neglecting to obtain discharge permits – Many states and localities require permits to discharge the water being removed from a site. A contractor can lose time and money if a government inspector determines work must cease due to noncompliance and levies a fine.
- Drilling without a license – Some states require that any dewatering activity be performed by a licensed well driller.
- Proceeding without geotechnical data – The success of wellpoint dewatering depends in part on the type of soil it is used in. A geotechnical report spells out the types of soil present in the excavation and the depth of each layer, as well as the depth of static groundwater. That information dictates the depth and spacing of the wellpoints.
- Placing too few wellpoints – The size of the area a single wellpoint can dewater depends on the type of soil. On a site with fine soil, the drawdown effect of a wellpoint system will create a radius of influence of approximately 6 1/2 feet per foot of drawdown. Therefore, a large excavation in fine soil may require additional wellpoints in the middle. In coarse gravel, the drawdown effect of a wellpoint system will create a radius of influence of approximately 30 feet per foot of drawdown.
- Choosing an ineffective pump – Contractors sometimes use whatever pump they have on hand for wellpoint dewatering, but the size and type may not be ideal. The best type of pump depends on the application. Any pump chosen should be a high-efficiency vacuum dewatering pump. Size matters, too. A pump of anywhere between 4 and 12 inches may be needed to remove the water at the required flow rate. Drawdown calculations determine the volume of water to be moved, which determines the number of wellpoints and the size of the pump. A pump that can produce higher volumes and higher cubic feet per minute (CFM) may be needed for larger excavations.
- Placing the pump in a suboptimal location – Pump placement is key to effective dewatering. For example, placing a pump at the top of the bank of an excavation, at a higher elevation than the suction header and wellpoints, causes the pump to work harder to maintain a vacuum. Placing it at the same elevation as the intended suction header pipe location is more efficient.
- Using the wrong wellpoint screens – Some applications require stainless steel wellpoint screens and rubber swing hoses rather than plastic screens and hoses. If solvents are present in the groundwater, they can melt plastic materials.
- Skipping the sand filters – When working in fine, silty soil types, it is advisable to place a clean sand filter around the wellpoint screens. This will reduce the pumping of fines and avoid potential subsidence (caving in or sinking) of nearby structures.
- Failing to fine-tune the system – Among the most common mistakes contractors make when running a wellpoint dewatering system is failing to adjust the swing hose valves as needed for system efficiency. Accurately adjusting the valves regulates the amount of air and water drawn through the individual wellpoints to maximize the vacuum in the system. If a pipe is pulling air, it is not pulling water.
Relying on Experience
A qualified full-service provider will identify permits needed, check local discharge limits, obtain geotechnical information, perform drawdown calculations and determine the ideal number of wellpoints and their optimal placement. They will choose or advise on the best type of pump and pump size. In some cases, using several smaller pumps instead of a large pump may result in more effective dewatering.