If you plan to construct a drainage bed, or drainage basin, it is important to know the absorption rate of the site's soil. This rate will let you know how much area in square feet the bed needs to cover for draining water to infiltrate, or absorb into, the soil rather than pooling on or running off the area. Pooling can attract insects and cause odors, and runoff erodes soil and can spread contaminates. Soil's absorption rate depends on the amount of clay, sand, loam or gravel the soil contains.
1
Choose at least three locations to dig a hole in the prospective drainage bed area, with the holes spaced evenly across that area. If the area is large area, then digging more than three holes will produce more accurate soil absorption rate results. For example, plan to dig one hole at each corner and one in the center of the future drainage bed area, or dig holes in a grid pattern every 20 feet if the bed's area will be very large.
2
Dig a vertical hole with a 4- to 12-inch circumference in each location chosen for a hole, using an augur or spade for the task. Each hole's sides must be vertical, and each hole must be the same depth water will enter the drainage bed, which usually is 6 to 36 inches below the soil surface.
3
Roughen the walls of each hole if necessary to keep the absorption conditions natural. If a hole's sides are smooth, solid surface rather than how soil appears naturally in the yard, then that condition will not result in an absorption rate that is accurate for the yard.
4
Place about a 2-inch-deep layer of 1/2-inch gravel at the bottom of each hole.
5
Fill each hole with at least a 12-inch-depth of water, measuring from the top of the gravel. If a hole is fewer than 12 inches deep, then put at least a 6-inch-depth of water in the hole. Allow the water to saturate the soil overnight or for a minimum of four hours, adding water to each hole as necessary during that time to keep the water level at the 12-inch or 6-inch depth above the gravel, using whichever water depth you used originally. If, however, you add water to a hole twice and both times the water drains in fewer than 10 minutes, then you do not need to add water to the hole again. Also, if the soil is mostly clay and a hole's water level does not seem to move much, then continue to maintain the hole's water level for three to five days. The time you maintain a certain water level in each hole is the saturation period.
6
Place a yardstick or ruler in each hole the day after the saturation period is over. The yardstick's or ruler's end displaying the 1-inch mark must be at the hole's bottom, above the layer of gravel. The yardstick or ruler needs to reach the top of the hole, and so use a ruler only in a hole that is no more than 12 inches deep. The water does not have to have drained completely from the hole, but it must be fewer than 6 inches deep.
7
Add enough water to each hole so that the water depth is 6 inches. Refill each hole to the 6-inch water depth every 30 minutes for four hours as water seeps from the hole. Write down the water level in each hole at the four-hour mark, but do not add water to the holes. Subtract one hole's current level of water from six to determine how many inches of water were absorbed. Repeat that task for each hole. If your yard has sandy soil and the holes' water is gone before 30 minutes pass, then refill each hole to the 6-inch water depth every 10 minutes for a period of one hour. Take the water level measurement of the holes at the one-hour mark.
8
Divide the elapsed time by the number of inches the water level dropped from the 6-inch level in a hole during the last measurement interval. If, for example, 4 inches of water remained in a hole after 30 minutes, then divide 30 by 2 to get an absorption rate of 15 minutes per inch for that hole.
9
Add the absorption rates of each hole. Divide that total by the number of holes to get the average absorption rate for the entire prospective drainage bed area. If you found more than a 20-minute per inch difference in the absorption rates of the fastest-draining and slowest-draining holes, then use the slowest-draining hole's absorption rate as the absorption rate for the drainage bed area.