Typical Concrete Pit Construction

Preliminary Design

• The first step is to estimate how big your concrete pit should be. If the pit will be part of an existing structure, you may be constrained with respect to length and width. In general, round pits are less expensive to construct than square or rectangular pits. The volume you need will depend on how long the manure must remain confined and the amount you expect to generate. As a general rule, it's best to anticipate that pumping will not remove all of the manure -- typically some of the waste will remain at the bottom of the pit, even following pumping.

Sumps and Soil

• The soil should also be investigated when choosing where the pit will be situated. It's important to know the maximum height the water table will attain, for example, and how great the distance between the bedrock and the base of the concrete pit will be. If your manure has reasonably low viscosity and low solid content, you may also want to consider incorporating a sump into your design. Manure that collects in the sump can be removed by a pump when desired.

Design

• Concrete pits should always be built from high-quality, air-entrained steel-reinforced concrete to ensure both high compressive and tensile strength. The walls must be able to bear both inward pressure created by the surrounding soil and outward pressure created by your manure. Adding supports like slats, lids or beams at the top may be helpful, although in this case the rebar in the concrete should be oriented vertically. If the wall is not supported at the top, you need hoop tension steel running around the perimeter if the tank is circular, or a cantilevered design in other cases.

Footing

• The pit needs to be mounted on a broad concrete footing that can disperse the weight of the pit. How broad it must be depends on the bearing strength of the soil in your area -- in other words, how much weight it can support. Stiff concrete mixes are best; these kinds of mixes are often more difficult to pump and to work, but adding plasticizers can sometimes be helpful. Any and all joints in the walls should be vertically aligned if at all possible.