The lid of your septic tank must support the weight of the soil as well as any other objects that may be placed over it, or pass over it. If the septic tank lid is damaged, it could allow dirt, debris or surface water to enter the tank. Any of these elements can cause the tank's efficiency to drop or could cause the tank to fail entirely. Repairs or replacement of the lid should be completed as quickly as possible.
Remove the dirt over the septic tank. This can be done with a shovel, but a backhoe will be much faster and easier. The backhoe can also be used to lower the new lid into the hole. Most septic tanks are buried at a maximum depth of 24 inches. Remove enough soil to expose the entire lid and up to one foot on each side of the tank. Dig carefully, so as to not further damage the existing lid.
Remove any broken or loose pieces of concrete from the damaged lid. Once all large pieces are removed, sweep the area clean to remove all dirt and debris. Once the lid is clean, take careful measurements of its overall dimensions. If there is an access hatch/port on the lid, you will need to make careful measurements of its location as well. Transfer these measurements onto the piece of plywood. Mark out the outline of the tank and the location of the access hatch.
Screw the 2-by-4s around the outside of the outline you have made on the plywood. Install several screws in each board for sturdiness. Place additional 2-by-4s around the markings for the hatch opening. The outside of these boards should on the inside of the lines marking the hatch opening.
Lay rebar inside the 2-by-4 frame. For added strength, you can intertwine the rebar in a criss-cross pattern. Use the hammer to bend two short pieces of the rebar into semi circles. Stand these up about one foot from each end of the frame so they will extend above the top of the 2-by-4s. Once the rebar is in place, begin mixing the cement in the wheelbarrow. Pour cement into the form until it is level with the top of the frame. Smooth the cement and allow it to harden for at least 48 hours before removing the frame.
Attach chains to the two rebar loops exposed above the concrete. Use the backhoe to move the new lid into place. Before setting it down, apply a generous amount of caulking to the perimeter of the old lid to help form a watertight seal with the new lid. Carefully line up the new lid and lower it into place. Apply more caulk around the seam to ensure a good seal.
Build a frame of 2-by-4s with inside dimensions that match the opening in the new lid. Attach additional 2-by-4s on the outside of the frame so that 2 inches of the second board extend above the first board. Fill this form with concrete and smooth. When you remove the frame, it will leave a block that will fit into the hatch opening and will rest on the 2-inch lip created by the second set of 2-by-4s. Place the lid over the opening. Re-cover the tank with soil and the tank is ready for operation.
While the tank is exposed, it would be advisable to have it pumped out. This will give you a fresh start when the new lid is put in place.
Do not enter the septic tank. The low oxygen levels and fumes can cause suffocation and even death.