In our previous article (PART 1), MWI Pump’s Steven von Gontard outlined some of the initial tips and precautions when setting up any construction dewatering pumping system, as well as the first two methods. Continued here are an additional three methods, how to determine which one to use and some tips when setting them up:
Wellpoint dewatering consists of installing multiple wellpoints around an excavation to lower the water table. The wellpoints are coupled to a header system through swing joints and the header system is connected to a wellpoint pump. Wellpoint dewatering systems work best in sandy areas. In Central Florida, it is typical to use 1 ½” Sch 40 PVC wellpoints and 6” Sch 40 PVC header pipe. Other areas may require larger wellpoints and header pipe.
The wellpoint itself is a PVC pipe with the bottom 3 feet slotted at .010” (about the size of a business card). Depending on the soil characteristics, a wellpoint can dewater an average of a 15-20-foot radius. The depth of the wellpoint installation is based on the depth of the excavation. The rule of thumb is to install the wellpoints 5-to-6-feet below the desired excavation depth. When installing wellpoints, a filter media of sand, shell, or rock must be installed to keep it from silting or clogging. The type of soil will determine what type of installation will be required.
In good soil with a blow count of N value 10 or less hand installation can be done. Hand installation typically consists of using a jet pump to pump water at high pressure into a jet rod (typically 1 ½” to 2” in diameter) that is operated manually and sunk into the ground at the desired depth. When the desired depth is reached, the jet rod is removed. The wellpoint is stuck into the hole and filter media is applied around it. When soils consist of more clay or rock conditions, a punch or a drill is used. The punched method is similar to the hand installed method except a wellpoint punch is used instead of a jet rod. A wellpoint punch is larger (typically 2” to 4”) and operated with the use of an excavator or another large piece of equipment.
A wet drilled wellpoint system utilizes custom built auger drills and jetting water to reach the desired depth. A dry drilled system utilizes the same auger without water. This method is popular in sites with soil contamination. In extremely poor soil conditions, cased wellpoints are used and require a customized punch (typically 6” in diameter but can go up to 12”). When the punch is at the desired level, the wellpoint and sand are installed inside the punch to create a very large sand column to allow water to permeate through poor soils. Regardless of the wellpoint installation method employed, an MWI Rotoflo™ Rotary Wellpoint pump is used for the most efficient dewatering system.
- Deep Wells
Deep Wells are 6-inch or larger wells (with slotted casings) installed to depths of 25-to-50-feet. These require electric submersible pumps to push the water up the wells and into a manifolded header system. This is a costly system mostly used for deep excavations.
- Sock Dewatering
Sock dewatering is an ideal dewatering method for long pipe runs and ponds in sandy conditions. Sock dewatering utilizes a specialty trencher that installs a 6” sock at a depth of up to 18’. The sock is a flexible corrugated pipe that has small holes in it with a fine mesh cloth around it. This allows ground water to enter the sock and keep other debris out.
When using a sock a 6-inch corrugated sock header pipe (without any perforations) is installed at the beginning of the sock run to the desired depth (about 5 feet below the excavation). The perforated sock is installed for the desired run (a couple hundred feet to couple thousand feet). At the end of the run, another 6-inch corrugated sock header pipe is utilized to bring the run back up to grade.
These sock header pipes are used to allow the sock to dewater the desired depth preventing the pump from catching air and losing vacuum until the ground water is below the desired depth. Depending upon soil conditions, a sock dewatering system can dewater a 20-30-foot radius. One of the benefits of sock dewatering is there is no above-ground pipe. Everything but the start and end of the system is buried below the excavation. For this method to be effective, utility conflicts in the ground must not exist.
Here are a few more tips when choosing the proper dewatering method for a construction project:
- Once a dewatering method is chosen, the key to efficient performance is pump vacuum
- All systems must be airtight to function properly
- As a general rule, you need one inch of vacuum to raise the water by one foot
- If you are using a hydraulic pump, the vacuum is not an issue because the pump is at the bottom of the excavation and pushes the water to the top of the excavation
- Hydraulic pumps are required when your lift is greater than 25 feet
All of these dewatering methods are proven if the correct one is chosen for the specific application.