Tame the access challenges that a tunnel project creates.
Access. It’s always the top consideration when you look at options for groundwater control in tunnels and shafts. And those options are determined by how and where you can install dewatering equipment.
What’s more, conventional dewatering systems don’t always work with a combination of horizontal tunnels and vertical shafts. Here are 5 methods that do work, along with detail on what makes each one optimal.
1. Deep wells
Electric submersible pumps lower the groundwater level below your tunnel. The pumps are installed in wells drilled outside the perimeter of the excavation area. This mode of dewatering is a good choice when you need to lower groundwater because of deep excavation.
Deep-well dewatering is less effective in soils with low permeability, which restricts water flow. Many submersible pumps aren’t capable of handling the low rate of flow. In that scenario, dewatering with eductor wells is a better choice.
2. Eductor wells
Tunnels and shafts excavated in sandy or silted areas pose a challenge to dewatering projects. The most efficient way to lower the groundwater level in these conditions is by using an eductor well system, also known as an ejector system.
Water is circulated at high pressure using eductors at the base of each well. This creates a vacuum that promotes drainage. The system works on the venture principle. The wells are drilled outside the excavation area and high-pressure supply pumps at ground level maintain the vacuum. This type of dewatering solution can also help to stabilize the soil in the excavation area.
3. Wellpoints
A wellpoint tunnel and shaft dewatering option only works for excavations up to a depth of 20 feet. Lines of closely-spaced, shallow wells are drilled along the perimeter of the excavation and each wellpoint is connected to a header pipe, to which a wellpoint pump is then attached. These positive displacement pumps can handle both air and water.
The wellpoint dewatering system works in stages. It can control the groundwater level only about 15 to 18 feet below the level of the wellpoint pump heads. If your tunnel excavation is deeper than that, you’ll have to install another ring of wellpoints at a depth equal to that of your first set.
4. Relief wells
Tunnel projects often operate with limited access and space. For example, dewatering techniques may have to be placed in the tunnel itself, while relief wells can be drilled to create an upward pathway for groundwater. As it rises into the wells, it’s removed with sump pumps.
Sump pumps have the ability to handle some solids while they move water. This is helpful for relief well dewatering because the wells are usually filled with sand or gravel to create a permeable path to the area where the sump pump will have access.
5. Tunnel drains
Sump pumps are also used with this system. First, small wells are drilled outward from the tunnel. Groundwater in these areas then flows into the tunnel, where it’s captured by the sump pumps and removed.
Problem solved?
Not quite yet. You’ve selected the right dewatering process for your tunnel, and the groundwater is out of your way. But now you’ve got to dispose of it. And if you’ve used sump pumps, this water will need to be treated to remove the solids.
Know the local environmental rules and regulations about discharging water before you attempt disposal. If you have any doubts, share them with the people who are renting you your pumps. At MWI, we’re just as knowledgeable about local regulations as we are about making sure you get the right pumps for your job. Contact us today with any questions about dewatering, or to learn more about our rental options.
