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 a 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?
- And, finally-do you need any specific approvals or permits to dewater?
Trench/Sump
The most economical way to dewater an area is to dig a trench or sump that is deeper than the depth you want to bring the groundwater to…and simply install a pump. This method is limited to fairly shallow depths. If the dewatering process is only to be for a short period of time (i.e., so you can install a culvert or underground tank), it is best not to begin pumping until the last minute. Continual pumping from an open trench or sump will often cause the trench to collapse. This is a result of the groundwater flowing laterally and upward into the trench-thus reducing soil strength.
If you must stop work for a period of time, it is best to stop pumping and allow the trench (or sump) to fill with water. This will minimize the risk (and amount) of collapse until such time as you can return to work.
French Drain
A French drain is essentially a trench or sump that is backfilled with gravel or other drainage media (we’ve actually used crushed glass). A perforated pipe is placed near the bottom of the French drain and groundwater is pumped out.
A French drain is a more permanent means of dewatering than an open trench or sump. The initial installation cost is higher, but when properly installed it is virtually maintenance free.
French drains can typically be much deeper than an open trench.
Well Points
Well points are perforated pipes that are installed vertically in the ground. They are essentially a groundwater extraction well. When installed in a grid or linear pattern, well points can effectively draw down the groundwater level across a large area. Well points can be expensive to install but offer the benefits of less impact than installing a deep French drain. And of course, you have the option to pump from any combination of the wells as needed. With a French drain, it’s usually all or nothing.
Thoughts on Dewatering
Not only is it important to have the appropriate approvals in place before you begin a dewatering project, but you should also consider what happens during and after the dewatering process.
Many landfills have unintentionally created wetlands by discharging water into a temporary pond or other area thus encouraging the growth of some type of wetland indicator species (plant, insect, crustacean, or other animal). Be careful not to create a problem where none existed.
On most dewatering projects, you’ll want to get help from an engineer or geologist to determine things like flow gradient, soil permeability, drainage media, and/or filter design.
Flow gradient refers to how steeply and over what distance the groundwater will slope toward your dewatering point(s). This essentially helps you determine the range of influence of your dewatering system. The flow gradient will vary based on the depth of your dewatering system and the permeability of the surrounding soil. Generally, coarse soils such as sands and gravels will have relatively flat flow gradients. This means your dewatering system will have an increased range of influence. It also means you’ll have to pump a lot more water than if the soils were fine-grained silts or clay.
When installing a French drain, you’ll have to design the backfill material and may also have to install a filter (i.e., geosynthetic) between the soil and the backfill material.
The backfill material must be permeable enough that groundwater would flow from the surrounding soil to the French drain. But that flowing water may also carry small soil particles that could eventually clog the French drain. By installing a filter (usually a geosynthetic fabric) between the surrounding soil and the backfill material, you can allow water to enter the French drain while effectively filtering out the soil particles.
Dewatering projects are common. So too are problems associated with dewatering. Before starting this type of project, consult with an experienced engineer or geologist. Done right, dewatering is a very effective tool for landfill managers.
