Effective Dewatering – Part 1

Editor’s note: This article first appeared in the November/December 2012 edition of MSW Management.

At a staff meeting some decades ago, we began talking about a potential expansion to one of our active landfills. There was a small tract of open ground adjacent to the landfill—actually it was within the buffer zone—that would work very well for a lateral expansion. It was marginal farmland and produced but a few bales of salt grass hay each year. However, as you can imagine, we saw it as being far more valuable as a potential landfill expansion.

There was only one problem: The tract had to be cleaned up a bit.

There were a few seepy areas—we were later to understand that seepy is code for wetland. And that worthless salt grass that didn’t make much hay?…right: also code for wetland indicator.

But the decoding was a lesson for another day. Our first order of business was to dry this ground out.

The task of dewatering fell to me. Could I do it?…Could I do it? No problem.

Step 1: We wanted to minimize surface ponding and get the runoff headed toward a collection point. So we hired a land-leveling contractor—who brought in a laser and several four-wheel Steiger tractors set up to tow drag scrapers. We put an 0.5% slope toward the west edge of the field, and in a week it was as smooth as a parking lot.

Step 2: Then we brought in a large trenching machine that could trench and lay perforated pipe and gravel in a single pass. Along that west edge, we put in an 8-foot-deep French drain that terminated at a precast concrete sump. We also installed a discharge pipe that passed under an access road and into a drainage canal.

Step 3: Finally, we hired an electrician to bring down power and install a submersible pump.

It was quite an impressive bit of work—done quickly and without a hitch. We were dewatering faster than you could say “US Army Corps of Engineers.”

OK, this is about the time we got our first lessons on wetlands and wetland indicators. Through this process we learned several important things:

• Seepy areas may be more accurately classified as wetlands.
• Certain species of salt grass may in fact be wetland indicators.
• Dewatering your own land—even within the boundary of a permitted landfill—may be problematic.
• The US Army Corps of Engineers does not find humor in such projects.
• Well, the upshot was: Nobody got into too much trouble. But I can say that the French drain came out…even faster than it went in. And, as far as I know, that area is today a flourishing salt grass wetland.

Dewatering Is a Good Thing
Now, if we can please move beyond this effective, but ill-advised example of dewatering, you’ll see that dewatering can, in fact, be a good thing.

Dewatering may be used to temporarily lower groundwater to allow for excavation. I was once involved in the design of a landfill that was located near a tidal flat. The groundwater was just a few feet below the surface. It also was at the interface where fresh groundwater met the salty groundwater being recharged from the adjacent ocean bay. In this case, we had to dewater the area so that we could bring in scrapers and begin excavating for a landfill expansion.

Don’t worry. We had properly dealt with any potential wetland issues. As you recall, we’d had some impressive training along those lines and had become quite expert in all things seepy and wet. But even with the appropriate approvals, dewatering and excavating this area posed a unique problem.

We were concerned that, in the process of lowering the groundwater, we could potentially draw saltwater further inland. Saltwater intrusion is a big problem where inland (freshwater) wells draw down the groundwater level, allowing the area to recharge with salt water. This increased salinity will eventually make freshwater supply wells salty—and unusable. Our dewatering wells had the potential to do the same thing.

In order to learn as much as possible about the process, we installed several shallow wells. Actually, they were short backhoe trenches, backfilled with gravel and a vertical perforated pipe. These were installed at various depths and monitored during high and low tides. We didn’t pump from these wells, we just measured the static groundwater level. To our surprise, we were actually able to track tidal variation in some of the wells.

This information helped the geologists and engineers determine how best to dewater…while minimizing the risk of saltwater
intrusion.

Based on our monitoring data, and lots of other hydrogeologic information, we ended up installing a slurry cutoff trench around the project…so that we could dewater inside the trench—minimizing any impact on the groundwater level on either the fresh or saltwater side of the project.

There are three common methods of dewatering, all varying in application and cost. These are dewatering trenches/sumps, French drains, and well points. In order to select the appropriate method, you’ll need to know the answers to several questions:

• How far is it to groundwater?
• How deep do you want to dewater?
• What is the groundwater flow and direction?
• What type of soil(s) will you be dewatering?
• How much time do you have to dewater?
• Finally, do you need any specific approvals or permits to dewater?