Hand excavated french drains work to vent groundwater
The use of a hand excavated drywell is one of the oldest forms of groundwater venting. Groundwater drainage. The same science that explains why a septic tank works, also applies to a drywell. The concept is called “perk” technology.
The hand excavated drywell stores up groundwater and/or roof water from the rain drain discharges and creates hydrostatic pressure by the weight of the groundwater or roof water from the rain drain discharges and french drains, applying hydrostatic pressure to the bottom of the hand excavated drywell.
Cracks form on the bottom of the drywell as groundwater is soaked into the ground. When a drywell is hand excavated, it is free of all loose dirt, which has a tendency to fill in cracks made on the bottom of the drywell when water soaks out, or “perks”. As the drywell becomes full of water and periodically dries out, those cracks enlarge, and the next amount of groundwater stored in the drywell soaks out quicker than the first groundwater did.
When the amount of groundwater being collected in the drywell is likely to exceed the size of the drywell until it becomes broken in and the cracks on the bottom of the drywell are formed, an overflow french drain is sometimes the best idea. The overflow functions like the leach field in a septic system that distributes groundwater from a full septic tank, out to a system of perforated pipes not far underground so the water can be soaked into the ground in a new location. This is also called a drain field.
Most areas do “perk”. Some building sites perk faster than others.
Many older homes now updating to new rain drain discharge venting systems and the installation of hand excavated french drains, were built in the 1900-1950 era. Many of those neighborhoods did not have a sanitary sewer system at that time. They were given a building permit subject to a satisfactory “perk” test, just like todays practice of making new construction in rural or urban areas without sewer go through the same “perk” test.
A large hole is dug with a backhoe about the size of the average drywell or larger, and the hole is filled with water. The sanitary inspector visits the site in 2-3 weeks after the hole is filled with water. If the water is gone, or has substantially drained out through the earth, the permit is issued for a standard septic system with a drain field. If there is a problem with the site “perking”, the building permit is denied, or a sand filter septic system is mandated. The sand filter system involves an extra sand septic tank installation for sand filtering the septic overflow water prior to venting it back to the earth through a drain field.
Since most older homes once were approved for, and used, a standard septic system for their household sewage and roof water disposal, it is logical to assume that these neighborhoods still “perk” today, and that these sites are logical candidates for the use of a drywell. As a practical matter, the sites that do not “perk” today, are likely the sites that never perked. Which means the home would not have been built on that site to begin with.
Sometimes a sump well with a sump pump wired in the exterior drywell is necessary due to rock formations or areas that are in flood planes. The sites in flood planes are not likely to have a home constructed there at all however. Building permits are almost never issued for such sites.
To make a long story short, your building site is likely to perk. If the development is new, the home is on sewer naturally, and if for some geological reason it does not perk, a daylighted vent for groundwater can be used if the site has a slope that works to gravity flow groundwater away from the home.
The daylighted vent approval is subject to the groundwater vented away from the home not flooding a neighbors home site and creating problems for them.
Hand excavated french drains and hand excavated drywells are clean of all debris and loose dirt. Hand excavated drywells last much longer, and work much better, than machine dug drywells and french drain imitations.