Construction Dewatering Q&A with the Experts

In a recent webinar, United Rentals Fluid Solutions experts drew on their combined 120 years of experience in pumps, filtration and fluid solutions to answer participants’ questions about construction and industrial site dewatering. Here’s a look at the insights they offered.

Choosing the right dewatering system

• How much does soil type impact which dewatering method is used?
  
  
  
  Soil is the major consideration when looking at system design. You have to know the geography, what’s in the ground, where the permeable layers are and their depth and formations. Since you can’t pull water through non-permeable material such as clay, when you put any system in the ground (wellpoint, deep wells or sock) you must put it in strata that are going to produce water.
  
  
  
  For example, a 15-foot cut in Florida has sand all the way down. When the wellpoint system goes in, there are rarely any issues. In Mississippi, you might have a 25-foot cut where the first 8 to 10 feet are sand, followed by a 5-foot clay layer, another 8 to 10 feet of sand, another 5-foot clay layer and then a couple of confined aquifers. So you have to stage your wellpoints in the sand above and below the clay layers.
  
  
  
  Geography reports and soil formations are crucial in determining the right dewatering solution.

• When would you use a deep well system versus a wellpoint system?
  
  
  
  Wellpoint dewatering systems are best suited for dewatering in shallow foundations, utility work and trench work. They are made up of a series of small-diameter shallow wells placed close together and connected to a common header pipe and pumped with a high-efficiency vacuum dewatering pump.
  
  
  
  In a trench or structure where the final digging depth approaches 20 to 23 feet, we would look at going from a wellpoint to a deep well dewatering system. In a larger structure, at that 20- to 23-foot depth a drawdown to the center of the excavation may also dictate a bench system or well system.

• Would a sock system work for a 12-foot-deep trench, 150 feet long and within 20 feet of a lake?
  
  
  
  It depends on soil formation, especially after 5 feet below ground level. If you have a sand layer the first 12 feet down and a clay layer 15 feet below that, a sock system would not be a good fit.

• What dewatering solutions have you recommended for waterfront projects?
  
  
  
  Waterfront doesn’t dictate the system. It depends on the soil type and geo. Treat it just like anything else. Analyze the soil system, the tidal system and water fluctuation and the effects they could have on the dewatering system.

• What happens to the system after dewatering is complete?
  
  
  
  Once the concrete is poured and cured, the customer or the dewatering company will remove the wellpoints or casings along the deep well, depending on the application. The only instance in which the equipment or materials would be left in the ground is sock dewatering.

• How long do you let a dewatering system run before excavation begins?
  
  
  
  The run time depends on the soil materials. It can take anywhere from overnight to one week. Typically, if the soil is coarse the system can run quickly. If it is fine it will take longer to dewater.

Pumps

• Can you use a regular pump on a wellpoint system?
  
  
  
  Yes, but the pump has to be vacuum assisted. To handle a large system or deeper cuts, we recommend a pump that can provide a high cubic feet per minute (CFM).

• Are there different types of pumps used for wellpoint systems?
  
  
  
  Yes. There are high CFM pumps that can pull and evacuate air quickly within the system. Those can be set up on a centrifugal type pump. There are also rotary-load-style pumps that use positive displacement to pull a vacuum instead of using a secondary apparatus.

Discharge filtration and monitoring

• Can United Rentals take a water sample and provide a filtration solution?
  
  
  
  United Rentals has filtration engineers on staff that can analyze and design a solution to ensure customers are within discharge specifications.
  
   
• Can your team obtain water samples for hydrocarbons and other contaminants
  
  
  
  Yes. We receive the results and identify the best solution within one to two weeks.

• Does United Rentals offer equipment to monitor discharge water?
  
  
  
  Yes. There are many ways to monitor discharge water. United Rentals can monitor pH, turbidity and even solids. When monitoring solids, we look at the differential pressure of water going in and coming out of the vessel so we know when those bags get full and can change them to avoid pass-through and remain within specifications.
  
  
  
  United Rentals can also monitor flow and temperature. For example, for paper mills or any facility where the discharge is elevated in temperature and may be discharged into a creek, we monitor to ensure compliance with the temperature delta specification.

Turnkey solutions

• Can you provide engineer stamp submittals?
  
  
  
  Yes, United Rentals can stamp engineering submittals in every state in the U.S.

• When you provide turnkey services, do you also handle pump fueling?
  
  
  
  Yes. After installation, United Rentals can do pump watch, refuel, monitor levels in auxiliary tanks and internal fuel cell, order and deliver, wet hose or put into your auxiliary tank.

Have questions about dewatering your jobsite? Contact United Rentals Fluid Solutions for the answers you need.