drainage pit - digging hole and trench

Off-The-Grid Plumbing: Build Septic Drainage Pit With Concrete Blocks!

ByDino Pelle

If you want to live off the grid, you’re going to need to build a drainage pit (also called a seepage pit), a hole in the ground hole in the ground that collects water and distributes it slowly into the soil.

These pits can be built with stone, brick, wood, or concrete. In this article, I’m going to give you step-by-step instructions for building a drainage pit with concrete blocks.

How to Build a Drainage Pit with Concrete Blocks

1. Planning the Drainage Pit

The drawings below show the layout of any cylinder-like concrete block pit, whatever its size.

Seen from above, the base of the drainage pit consists of a footing of blocks laid like wheel spokes. The wall has courses of blocks set end to end like sausage links. The corners of the blocks touch each other on the inner circumference of the circle.

concrete block drainage pit - view from above
This is the view of the drainage pit construction from above.

Seen from the side as shown below, the blocks that form the wall stand crosswise to the block that form the footing. A pipe enters near the top. Slanting slightly downward, the pipe brings waste into the pit.

A 6-inch-deep bed of gravel fills the bottom of the ring. A 6-inch thick sheath of gravel fills the spaces between the blocks and the surrounding earth.

concrete block drainage pit - view from side
This is a view of the drainage pit construction from the side.

2. Building the Lining

Dig a circular hole about 2 feet deeper and 3 feet wider than the inner dimensions of the drain pit. Level the bottom and remove any loose dirt and rocks.

To form the footing for the pit lining, lay a ring of concrete blocks with each block arrange lengthwise toward the center and with the hollow cores facing upward. In the circle formed by the inner edges of the blocks, pour a bed of coarse gravel 6 inches deep.

To build up the wall, lay a row of blocks on top of the footing, this time with the blocks set end to end around the drainage pit’s circumference and with the hollow corres running horizontally.The blocks should touch along their inner perimeter. Their outer edges should lie about 6 inches from the edge of the hole.

Stagger the second course so that each block rests across the two below it and, in succeeding courses, alternate the placement. Stop when you have come within 6 inches of ground level.

concrete block drainage pit -building the lining
One mistake people often make is to place mortar between the concrete blocks. No mortar should be used to join the blocks.

3. Connecting the Drain

Stake out a straight line from the point on the house foundation where the drainpipe exits to the point where it will enter the pit. Along that line, dig a trench about a foot wide and at least a foot deep. The trench must slope at least 1 inch for every 50 feet.

Using a star drill and a hammer, drill a 4-inch hole through teh house foundation. Put it as near as possible to the clothes washer, and high enough so the water coming from it will flow downward to the trench. Then assemble lencths of 4-inch pipe (plastic pipe is generally used) in the trench.

Insert one end through the hole in the house foundation and the other through the pit lining. The pipe should stick out about 6 inches into the drainage pit. The pipe end can be passed through a hollow core in the topmost course of blocks, or one block can be removed to make room for it.

concrete block drainage pit - connecting the drain
As you’re digging the trench, take the utmost care in making sure it slopes downward, away from the house to the pit, at least 1 inch for every 50 feet.

4. Covering the Pit

Have a building-supply firm deliver and install a concrete pit cover that fits over the outer diameter of the drainage pit lining.

With the cover securely in place, fill the remainder of the hole and the drain trench with soil to grade, and tamp down firmly. There should be at least 6 inches of soil above the pit cover and the trench.

Assemble the clothes washer as you would a sewer-connected unit. Connect the drain to the pipe that goes to the drainage pit. Finally, run the washer through a complete cycle to be sure the drain works properly.

concrete block drainage pit - covering
You can create your own drainage pit cover if you’d like. But it takes some metalworking skills to do so. Instead, get a cover from a company. It will be much easier and far less time-consuming.

How Many Concrete Blocks & How Much Gravel?

To determine how much gravel you need for the 6-inch-thick bed of a septic system drainage pit, you must compute the area of the bottom. Here’s where it might get a bit complex if you don’t like math.

The mathematical formula for the area of a circle is “pi” (a number roughly equal to 3.14) times the square of the radius. In a simple example, the area of an 8-feet-wide pit is 3.14 times 4 times 4. For each square goot of bottom area, you will need two 50-pound bags of gravel.

To determine how much gravel you need for the wall filler, first calculate the pit’s circumference (it is 3.14 times the diamter of the pit). Multiply the circumference by the depth to get the wall area. Again, use two 50-pound bags of gravel for each square foot.

drainage pit - gravel
Drainage pit gravel should be natural and rounded for the best results.

Final Thoughts

It’s not an easy job but creating your own drainage pit with concrete blocks will be satisfying. The bonus? You’ll build muscles and character from the hard work.

Call 1-Tom-Plumber

Don’t hesitate to contact us here or call us at 1-Tom-Plumber (1-866-758-6237) if you need help with off-the-grid plumbing projects.

1-Tom-Plumber’s certified team of plumbers and drain technicians respond immediately to any emergency plumbing, drain cleaning, or water damage problem. We also handle the excavation of underground water lines and sewer main lines. Our immediate-response team is available every day and night of the year, even on holidays.

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