“If more than 30 minutes are lost each morning due to wet conditions and delays, or there is a need to add additional pumps again and again, professional help may be needed,” says Williamson. “Some contractors or estimators assume their dewatering project will be simple, and only put more thought into system design and dewatering planning after the project is underway. Then they need to catch up. They may not have budgeted for the cost of proper dewatering. Most reputable pump companies will offer design help both before and during a project, oftentimes at no charge.”
A dewatering expert can be helpful in assessing the proper approach to match jobsite soil conditions. For instance, you may encounter perched water that sits on top of clay layers or compacted materials while the soil underneath is dry. Then there is the issue of soil permeability. Some soils, such as sand, allow water to flow through them faster than soils such as clay. This all needs to be taken into account when a dewatering solution is chosen.
According to Nute, the rate the water recharges is the No. 1 factor in pump sizing. This means you need to accurately calculate the recharge rate. “Operate the system for a minimum of 24 hours prior to starting work to be sure that the dewatering system is adequate and safe,” he advises. “Be prepared for changing environmental conditions and build a system with some level of redundancy in both hardware and capacity.”
“Soil has a strong influence on performance as well as installation,” says Aldridge. “Recharge sources can substantially impact a project because they change the flow conditions, as well as can impact the structural stability of the soils. So based on the combination of soil permeability and recharge source, there may be significant changes. It will also impact the overall drawdown curve of the site.”
Some soils will be harder to dewater. And then there is the issue of recharge sources. “Soil consistency is very important in any dewatering system design,” says Williamson. “Rocky or clay-rich soil is challenging. If the jobsite includes a natural spring or underground stream of water, a larger and more complex dewatering solution will be needed.”
There are basically two ways to deal with groundwater — prevent it from reaching the excavation or handle it after it enters the excavation. “Very often, the nature of the work will determine whether you need a system that removes the water prior to entering the work space or whether you can deal with it as it enters the site,” says Nute. “The two types of systems can be completely different in design, equipment used and layout.”
“Preventing groundwater from reaching a site is preferred, but also requires a more elaborate dewatering solution, such as a wellpoint system,” says Williamson.
Aldridge agrees, noting, “It is always more advantageous to minimize flow into a site, but it’s not always that easy or the most cost-effective method. However, a properly designed dewatering system will allow for better site management, more efficient work methods, and provide a safer environment to the employees. In reality, a properly designed dewatering system is a useful tool that can provide a contractor insurance for a successful project.”