Effective Dewatering – Part 2

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.