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 individually, separated from each by at least 4 times the diameter, center to center.
In the code drywells are called leaching pits. If you go to buy one, it's called a drywell. It's a bit confusing, just like the term used by the state is gallery, the plural being galleries. Some manufacturers spell the singular galley, others gallerie. The plural turns up as galleys or galleries, depending on the company.
Drywells 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.
Precast concrete drywells are available in heights from three to ten feet, though they're most often six to eight feet in older septic systems. They're 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. Personally, I say the soil should perc no slower than 10 minutes per inch, but the code says 20. These deeper structures tend to seal themselves up over time, unless the soil is very well-draining.
Current code (2003) also dictates that "leaching pits shall not be more than 8' below grade," which seems to eliminate using 10' deep, or even 8' deep drywells.
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.
A Drywell's Effective Leaching Area (E.L.A.) is calculated in this way:
The excavation diameter (which with an drywell that's 8' wide at the bottom would be 10' with 12" of stone all around) X 3.14 ("pi") X the depth of the usable (wet-able) stone. You get no credit for the bottom.
Drywells are still sometimes specified by engineers for the retention of storm-water runoff - capturing water from roofs and paved areas - as is often required by town 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; below that a drawing of a 6' 5" high (precast concrete H-10 loading) drywell
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