How to Solve Construction Dewatering in 6 Steps – PART 1

Wellpoint Redundant System

If you are not pumping water out of your construction site fast enough or effectively enough, perhaps you need a detailed dewatering plan from a turnkey solutions and problem-solving provider, according to dewatering pump expert Gary Evans, Sales Manager for MWI Pumps.

“The problem is that you must get the water out of the excavated areas to allow the contractors to go in and do what needs to be done to lay the foundation so that they can go vertical,” Evans says.

Pumping the water out fast and effectively depends on the way the project is engineered. The contractor must know how deep to go in order to build a solid foundation. “A big challenge, especially here in Florida, is that right now we are going through a dry period, so the water table is lower than normal,” Evans explains. “Under normal circumstances, especially during rainy season, the water table is high 90% of the time—especially if it is a residential or commercial build with more than three or four stories.

Here are 6 crucial steps to follow to solve dewatering issues on a construction site:

  1. Identify the method and materials.

The first step is to provide the customer with advice on the best and most cost-effective methods to dewater the project, Evans says. Consider the conditions, the materials in the ground, and the location.

There are three different methods of dewatering:

  • Open-Ditch or Open-Pit Dewatering – The deeper the pit, the more water you will encounter. You must pump it out and keep it out. Generally, submersible hydraulic pumps are effective for this method, which requires removing the water from an open pit while also lowering the groundwater level.
  • Sock Installation – This process uses long runs of actual pipe rather than putting vertical well points into the ground. A specialized trencher digs and installs fabric-wrapped perforated pipe under the construction area. The drain length is determined by the soil conditions and the
  • A Wellpoint Dewatering System around the perimeter of your construction site will take care of lowering the groundwater level and making the ground dry and stable to allow excavation and foundation work to begin. A series of small diameter wellpoint are placed vertically connected via a header pipe, to the suction side of a wellpoint pump which then draws water up and out of the ground. This system is cost effective, and it is optimal for water-bearing soils such as sand and gravel. The method can also keep the ground dry during heavy rain events and flooding conditions.

Generally, the dewatering method can be determined through a geotechnical report, which provides a snapshot to identify for the contractor what they will be facing with regard to materials in the ground.

“This is a crucial step,” Evans says. “For example, if you have hard ground you won’t be able to use wellpoint or sock installation. In that case, you must use the open-ditch method because you won’t be able to punch the materials in the ground in order to move the water.

Sock Dewatering
Foundation Dewatering
  1. Create a Dewatering Plan Guideline (considering already-determined permitting and dewatering requirements).

An equipment and service provider must factor the points of discharge to establish the response rates needed to control the turbidity levels as required by the plan. This action can introduce sediment tanks or sediment bags, ensuring a correct and covered dewatering plan guideline according to the permitting and dewatering plans. This step helps to avoid job shutdowns and potential fines.

The contractor must follow the dewatering plan guidelines and permitting requirements. “Sometimes the permitting and dewatering plan is handled by the contractor or the owner, and MWI works from the information they provide. At times, we are asked to provide that information. In that case, we partner with a consulting company to develop the dewatering plan and permits. We decide what we believe is the best method of dewatering. We build the plan around that and submit it to the city to get approval to move forward.”

To create this, it is important to know the method of dewatering, how much water is being moved, and the point of discharge, which could be a storm drain structure an intercoastal waterway or injection wells in the ground.

“As the dewatering company, we are not responsible for the point of discharge,” Evans says. “The customer is responsible for that, however we take extra precaution to ensure the parameters of the dewatering plan are safe, which usually requires going into a canal, a waterway, or a river in certain areas. Or it may be required to go into deep wet wells somewhere between 80 to 120 feet deep. Then we basically push that water back down into the ground. The dewatering plan gives us permission to discharge the water in the designated place.”

  1. Consider the footprint of the area and the phasing strategy. 

Once the method is established (open-ditch, sock, or wellpoint), the gallons per minute to be pumped must be determined. But first, the footprint of the area to be dewatered must be considered to determine how to best achieve the dewatering goal for that project.

“Usually on the bigger jobs, dewatering is phased,” Evans explains. “We tackle the job in smaller segments rather than trying to deal with too much water at a time by making a larger than necessary hole in the ground. We must create a strategy to phase the work and then determine what size pumps we may need to perform each phase of work. If it is a wellpoint system, for example, we need to know how much we will take on at one time. If it is an open-ditch application, we must determine how many pumps we need. We must determine how many gallons per minute we believe we will be facing and what size pumps we need to be able to move that water via our calculations.”

For more information, please contact MWI Pumps at 954-426-1500 or via the web contact forms.