A drainage pit, often called a dry well, is an underground structure designed to manage excess surface water or runoff from impervious areas like roofs, driveways, or saturated lawns. It functions by temporarily storing the collected water and allowing it to slowly infiltrate the surrounding soil strata. This engineered solution relies on the natural permeability of the earth to safely disperse collected water back into the groundwater system.
Site Assessment and Planning
The initial phase involves determining the optimal placement for the dry well to maximize its effectiveness and ensure compliance with local building codes. The pit must be situated downhill from the water source to ensure gravity assists the flow. It should also be positioned a minimum of 10 feet away from any structure’s foundation to prevent hydrostatic pressure from damaging the walls. Furthermore, maintaining a distance of at least 50 feet from septic fields or property boundary lines is generally recommended.
Before any excavation can begin, contacting the national utility locating service by dialing 811 is an absolute requirement to prevent striking underground infrastructure. This free service marks the precise locations of buried electric, gas, water, and communication lines. Waiting the mandated period for utility marking ensures the chosen location is safe for digging to the planned depth and diameter.
Proper sizing of the pit depends directly on the volume of water it must manage, which is typically calculated based on the square footage of the drainage area and the local rainfall intensity. A common guideline suggests that a pit approximately one cubic yard in volume (roughly three feet deep, wide, and long) can handle runoff from about 500 square feet of impervious surface in moderately draining soil. The required dimensions will increase significantly in areas with heavy clay or poorly permeable subsoil that naturally slows infiltration rates.
Materials Required
Gathering the necessary components ensures a fluid construction process once the digging commences. The most important material is clean, coarse aggregate, like washed gravel or crushed stone ranging from three-quarters to one-and-a-half inches in diameter, which provides maximum void space for water storage. This aggregate must be paired with high-quality, non-woven geotextile filter fabric, which prevents fine soil particles from migrating into the stone and clogging the system over time. Other tools required include a shovel, a hand tamper for settling the aggregate, and the appropriate perforated inlet pipe.
Constructing the Drainage Pit
Excavation of the hole is the first physical step, requiring digging down to the pre-determined depth and diameter established during the planning phase. It is beneficial to keep the excavated soil nearby, as this material will be used later for backfilling and grading the surface area. The walls of the pit should be kept as vertical as possible, but maintaining the structural integrity of the surrounding soil is more important than achieving perfect geometric alignment.
Once the hole is dug, the entire cavity must be lined with the non-woven geotextile filter fabric. The fabric acts as a permeable barrier, allowing water to pass freely into the gravel reservoir while strictly barring fine sediment from the surrounding soil that could rapidly reduce the storage capacity of the pit. Ensure the fabric is draped deeply enough so that sufficient material remains at the surface to completely wrap over the eventual aggregate fill.
A base layer of approximately six inches of the coarse aggregate should be placed at the bottom of the fabric-lined hole to establish a stable foundation. This foundational layer prevents the incoming drainage pipe from resting directly on the soil, which could lead to settling or shifting over time. The rest of the drainage material, whether loose stone or a dedicated plastic dry well barrel, is then carefully lowered into the cavity.
The connection of the incoming drainage line, such as a four-inch perforated pipe from a downspout, is the next step. The pipe should enter the pit near the top and extend into the aggregate, ensuring a slight downhill slope of about one-eighth to one-quarter inch per linear foot from the water source to the pit. This calculated pitch guarantees that water flows efficiently by gravity without pooling in the pipe itself.
With the inlet pipe securely positioned, the remaining coarse aggregate is poured around it until the stone fills the pit to approximately six inches below the surface grade. The stone should be spread evenly to eliminate large air pockets. Excessive tamping should be avoided, as it can compact the surrounding soil and reduce the pit’s infiltration rate.
Finally, the excess filter fabric that was left draped over the sides must be folded completely over the top of the stone fill. This action creates a sealed envelope around the drainage material, preventing any soil or debris from the immediate surface layer from migrating down and clogging the system.
Finishing and Maintenance
The completion of the dry well construction involves carefully backfilling the remaining six inches of the pit above the wrapped filter fabric with the excavated soil or fresh topsoil. The final surface must be graded so that it slopes subtly away from the pit’s perimeter in all directions. This prevents surface water from pooling directly over the drainage system and encourages runoff to travel toward the intended inlet point.
Routine upkeep is necessary to preserve the dry well’s long-term functionality and infiltration rate. Homeowners should periodically inspect the inlet point, such as the catch basin or the end of the downspout pipe, for accumulating debris like leaves, silt, or pine needles. Clearing this blockage is a proactive measure that prevents sediment from being carried into the underground system where it is difficult to remove.
If the drainage pit appears to be holding water longer than expected or the flow rate significantly diminishes, the system may require troubleshooting. Often, a slow flow indicates a partially blocked inlet pipe, which can sometimes be cleared by carefully flushing the line with a standard garden hose or professional jetting equipment. If the issue persists, it may indicate a build-up of fine sediment within the stone aggregate itself, necessitating an inspection to determine if the system needs replacement or professional remediation.