Gallies & Drywells
Gallies (also called galleries or galleys), along with drywells and plastic/proprietary leaching units, are alternatives to leaching trenches. They're helpful where space is a constraint, and work best where the soil type is appropriate.
Gallies are hollow, rectangular, concrete box-like structures without bottoms that are placed
end-to-end in a level excavated trench to form a row. The top of each gallie is solid concrete with an 18" clean-out lid like those on a septic tank. The sides of each gallie are perforated with small rectangular holes. They're laid in a level trench end to end and are surrounded by 12" of clean, crushed stone that's covered with approved filter fabric.
Concrete gallies are generally four feet wide, so the excavation for a row of gallies is six feet wide to allow for the stone.
Galleries are available in heights ranging from 12" to 48". The most common are the 12", the 24",
and the 48" high versions. The 48" gallies are often called "4X4's" and the 24" gallies used to be called "lowboys".
Drywells, like galleries, are hollow concrete structures with perforated sides and no bottom that are surrounded by clean, crushed stone. But they are installed separately, as leaching pits. They penetrate the soil more deeply than any other leaching structure. Due to their depth they are no longer used very often for septic systems, as the bottom of any leaching unit must be 18" above the highest seasonal water and 4' above ledge rock.
Drywells range in height from six to ten feet and are made up of two or three vertically-stacked sections that interlock. They, and 48" high gallies, should only be used in the best draining soils.
In the past, sometimes drywells were constructed on-site with concrete blocks laid on their sides in a circular pattern and topped with a concrete slab-top.
Drywells are still used commonly for storm-water runoff retention, capturing water from roofs and paved areas, as often required by Conservation or Engineering Departments. H-20's (made to be driven over) are often specified in this case, as opposed to regular H-10's (made for under lawns).
Below is a drawing of a 48" high gallie.