If your residence is not close to a community sewer system, you may need a septic tank and a lateral drainage system to handle your household sewage. Pre-cast concrete septic tanks are readily available, but delivery and setup are expensive. If your local building codes allow it, you may pour your own septic tank on site.
Determine the location and depth of your septic tank. Your local building codes will establish the fall of the sewage pipe that runs from the house to the intake outlet on the septic tank. In addition, comply with the required fall of the water discharge pipe from the septic tank to the lateral leech fields. A survey crew will assess your property grade to determine the location.
Excavate the pit where you will pour the concrete tank. A backhoe will remove the soil from the pit and will dig trenches to install the piping to and from the septic tank.
Fill the bottom of the pit with at least 6 inches of sand or gravel. This will stabilize the base beneath the septic tank to reduce the possibility of shifting or cracking.
Form and pour the floor of the tank first, inserting steel reinforcement to meet or exceed your local codes. During the floor pour, install the vertical steel rebar that will reinforce the tank walls. By inserting the bars into the wet concrete floor, you provide a strong structural connection between the walls and the floor.
Install horizontal rebar rods and use rebar ties to fasten them. Building code is essential here, but standard rebar spacing is between 12 inches and 16 inches.
Order concrete after the building inspector checks the tank framing. Most communities are very strict about septic tank construction because a leaking tank can pollute streams and water tables. In addition, make provisions for the intake pipe and the drainage pipe before you pour the walls.
Form the tank cap separately on a bed of level sand. The cap should match the dimensions of the septic tank and you will position a manhole in the form before pouring. You'll also need steel reinforcement and four large steel hooks positioned at each corner of the cap and extending all the way through the concrete.
Lift the cap from the sand bed with a crane by hooking onto the four steel hooks and position it carefully on the tank before covering the cap with soil.