The project design tells you the septic tanks need to be placed in a wet site. The excavation for the tank is almost finished when water starts to enter the hole through the side face.

This means it will be difficult to set the tank level and keep it stable without some additional work. You will have to bring in a solid coarse base to set the tank on and dewater the site. But what does it mean to dewater the excavation?

Dewatering is the removal of ground or surface water from a construction site to allow work to be done “in the dry,” as opposed to under wet conditions. Water is usually removed using well points and power-driven pumps.

The way to do this depends on the site-specific conditions. The key to selecting the right method is to understand the different applications and to know where the water is coming from that is entering the excavation.

Wait and see

If the water is due to recent heavy rainfalls and is flowing in from surface soil layers, the first choice is to wait. It may be possible to just wait a day or two until the flow stops and water drains lower than the bottom of the excavation.

The design should have identified the elevation of seasonal or periodic saturated conditions in the soil, and if so, that will help you decide if waiting is realistic. If you have taken a soils course or know the relationship between soil color and saturated soil conditions, you can make the decision at the site.

By carefully evaluating the soil colors, you can estimate how long the saturated conditions will last and how long it will take for the site to dry out. Of course, this means you need to have flexibility in scheduling, and you need to watch the weather. A good, soaking rain can move this plan from a good idea to a bad one. The good news is that you can move to one of the other choices.

Pump or well points

In general, the two other choices are to pump the excavation as water enters the hole, or to install well points off to the side of the excavation to physically lower the water table. The key to either of these choices is the soil texture and structure in the area. These characteristics will determine which method you should use to keep the water out of the excavation and allow proper tank installation.

When working in an area of sandy soils with high water tables, water movement through the soil can be quite rapid, and trying to keep ahead of the flow with a pump set next to the excavation can be difficult. In this case, well points can be more effective.

The design of the dewatering system will be based on the specifics of the site. Dewatering contractors are experts at this, and it is essential to use a contractor to design and conduct the dewatering effort.

Typically, the contractor will come to the site and evaluate the conditions, then design the dewatering system. The key is that this takes time. Proper planning and scheduling is critical to making it work.

If the soils have a finer texture, the groundwater flow will tend to be slower. By creating a drainage path for the water entering the excavation and placing a pump, you may be able to stay ahead of the flow and keep the working surface dry. In many cases, it is desirable to provide the ability to dewater in the area of the tank for the long-term.

By placing pipe with rock surrounding, and connecting to the pump at the surface, you can allow for backfilling with the dewatering system still operating, and you can enable connection in the future if necessary. When using a drainage material as the base material, this long-term sump design can help protect the integrity of the tank.

Watch the flow

You must watch the water you remove from the excavation to make sure it does not become a problem. Dumping the water on the neighbor’s yard is not an option, and neither is dumping it into the ditch and creating erosion. Be very aware of where the excess water is going and what the potential impacts are, or you may get some very unwelcome visits from inspectors and neighbors.

Water leaving the excavation cannot be too dirty. It is necessary to apply minimum controls like filtering or barriers to meet sediment control requirements. A common method now is to filter the water removed from the excavation through a geotextile fabric bag. The requirements in your area may be stricter, so check before you create a problem for yourself.

The other consideration is where the water is coming from as it enters the excavation. If the groundwater is generally above the bottom of the excavation, it will be entering from all sides. This means you will have to use wells to keep the excavation dry.

Extra precautions

Remember that OSHA standards identify these excavations with water entering as Type C soils. If the water is entering at the bottom of the excavation, the entire depth is considered a Type C, and you must follow the required trench safety precautions. This means either using shoring boxes or sloping the sides of the excavation at a ratio of 1.5 to 1. This becomes a very large excavation very quickly!

Water entering the excavation also means taking extra precautions in installing the tanks. These include laying down a coarse drainable base, testing the tanks in place for watertightness, and using curbs and tie-downs or some other method to keep the tank from floating when it is emptied. These are all critical for the long-term performance of the system.