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How to build a drywell or seepage pit: this document describes the actual building (on-site construction) of a drywell or seepage pit. The specifications for drywells & seepage pits, such as pit size, depth, absorption area, site space requirements & clearances, are described separately at DRYWELL / SEEPAGE PIT SPECIFICATIONS.
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(v) Seepage pit linings may be precast concrete, cast-in-place concrete, or built in place with unmortared hollow cinder or concrete blocks.
Concrete shall have a minimum compressive strength of 2,500 psi and 3,000 psi is recommended.
Material with comparable structural strength, determined in accordance with commonly accepted sewage construction standards, principles or practices, may be allowed on an individual basis to prevent unreasonable hardship, provided public health is not prejudiced.
(vi) The separation between the outside edges of seepage pits shall be three times the effective diameter of the largest pit. This separation is measured as the undisturbed soil between pit excavations.
(vii) Pits shall be designed with sufficient structural stability to withstand lateral soil forces as well as vertical loads.
The schematic of a seepage pit (left, courtesy USDA) shows typical construction of a site built seepage pit using brick, block, or pre-cast concrete rings with open joints, surrounded by stone to improve soil absorption.
(i) Laterals leading to each seepage pit must be at least four inches in diameter with a minimum slope of 1/8 inch per foot.
(ii) Seepage pits shall not be connected in series. A distribution box shall be required where more than one seepage pit is installed.
(iii) The pit excavation is to be raked to minimize sidewall smearing that may occur and reduce infiltration capacity. If groundwater is encountered, the pit shall be backfilled with the original soil to a level at least three feet higher than maximum groundwater and adjustments made in the pit dimensions.
(iv) The linings are placed upon a concrete block, poured concrete, or precast footing and surrounded by a six inch minimum annular ring of large aggregate (2 1/12 - 4 inches in size).
(v) The rock is covered to prevent soil from filling the void spaces. Building paper, a four inch thick layer of hay or straw may be used.
(vi) The seepage pit cover shall be structurally sound and capable of supporting 300 pounds per square foot at the weakest point. Covers may be precast concrete or cast-in-place and shall be reinforced. A manhole with an opening of at least 20 inches in the shortest dimension shall be provided.
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Questions & answers & design specifications: how to design and build a drywell or seepage pit for graywater or septic effluent disposal
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Technical Reviewers & References
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Design Manuals for Septic Systems
Onsite Wastewater Disposal Books