A drywell is used to quickly transfer excess surface water deeper into the subsoil. It can be as simple a hole dug with a post hole digger and filled with gravel and sand, or as complex as a pre-cast concrete sleeve lowered into a large hole and fed by drainage pipes.
Before putting in a drywell be sure to do a percolation test, or “perc test”, to see if a drywell will work in your soil conditions. Using a pothole digger or soil auger dig a small diameter hole four feet deep. I realize that this is not easy to do, but it is easier in putting in drainage features that end up not working with your soil type.
Take note of the soil types coming up out of the hole – How deep is the subsoil? What is the soil texture? Soil texture determines how quickly water will be absorbed into the subsoil. The ability of soil to absorb water is known as soil percolation. Soils containing a balance of coarse and fine particles are the best types for drainage, or percolation, of water. Soil containing a high amount of clay is not a good choice for drainage and you may have to dig deeper to get past the clay layer.
To perform a perc test, pick a spot along the path of your planned drainage system where you hope to locate a drywell anyway (see Step 3). Using a pothole digger or soil auger dig a hole four feet deep. If groundwater immediately fills in the hole then you have a high water table and a different drainage design must be considered.
Summer months and periods of dry weather are times when the groundwater table is at its lowest level. Test results done during these periods will not necessarily reflect year-round conditions.
After digging the hole, pour 5 or more gallons of water into the hole. Make note of the time it takes for the water to drain from the hole. A one-inch drop in water level in three minutes is considered very good. If the water does drain very quickly, immediately add another 5 gallons of water to check that it is not just a case of dry thirsty soil.
If the water drains very slowly or remains in the hole with no drop in level by the next morning, the soil percolation is considered bad and drywells should not be a part of your drainage plan. You will have to design your drainage system to drain to a ditch, curb, or other downhill location.
How to construct a small simple drywell
Often a simple drywell can eliminate a small soggy spot in a lawn, or a string of them in the bottom of a French drain can eliminate the need for a long drainage system opening into a ditch or other drainway.
Fortunately they are also simple to construct. Here is how to make a small simple drywell to drain a single soggy spot in a lawn:
Obtain posthole digger or auger and set it to dig a 6 to 8 inch diameter hole. Dig a hole at least 4 foot deep measured form the ground surface
Obtain a roll of the fabric drainage pipe sleeve commonly sold in hardware and building supply stores. The 4 inch sleeve works if you can keep your hole diameter not much over 6 inches, otherwise the 6 inch sleeve is best. Here is a link to see typical fabric drainage sleeve
Cut off a length of the fabric drain sleeve about 8 feet long. Tie a knot in one end.
Load one shovel full of pea gravel into the fabric tube. Lower the gravel- weighted knotted end of the tube down to the bottom of the drywell hole.
With a helper holding open the end of the fabric tube begin filling it with pea gravel. Occasionally wiggle the fabric around to make sure the tubing is stretching to fill the hole and the gravel is settling.
Continue filling to within about 8 inches of the ground level. Tie a knot in the top of the sleeving and cut off the excess material.
Fill the remainder of the hole to the top with the pea gravel.
Here is an example from someone who used small simple drywells to dry a soggy yard. It would have been even better had he used the fabric drain sleeves.
When combining a string a drywells with a French drain you can use our method of constructing simple drywells by digging down from the bottom of the French drain to a combined depth of four feet. *See Below.
How to construct a large drywell
Before constructing a large drywell be sure that your soil has the percolation capacity to handle it. A large drywell can be used as a central collecting location for drainage lines feeding from multiple areas.
There are many ways to construct large drywells. The following online articles will help you get started, though you may find that this is best left to a drainage contractor.