Underground Stormwater Systems

Detention stores runoff temporarily and releases it at a controlled rate to a sewer or outlet. Infiltration stores and distributes runoff into suitable native soil. A combined system can do both. Soil permeability, groundwater, contamination risk, hydraulic capacity, and approval criteria determine which function is appropriate for a specific site.

We install modular chamber fields, precast or cast-in-place concrete tanks, oversized pipe systems, infiltration galleries, manifolds, flow-control structures, pumps, and inspection access. Harrowgate also installs pretreatment and outlet connections. The approved hydraulic, structural, geotechnical, and product drawings determine the exact system and construction sequence.

The engineer calculates effective volume from chamber or tank geometry, connected pipe, stone voids where credited, outlet level, and maximum design water level. Harrowgate verifies excavated footprint, base elevation, row spacing, unit count, manifold configuration, stone limits, and controlling inverts, then provides surveyed as-built information for comparison with the design.

Sediment, trash, and oil can consume storage, clog infiltration surfaces, block outlets, and make maintenance difficult. Pretreatment can include catch-basin sumps, oil grit separators, filters, or chamber isolator rows. Harrowgate protects these components during construction and cleans them before commissioning so permanent capacity is not lost to site sediment.

We maintain required spacing, place approved washed stone evenly, keep heavy equipment outside exclusion zones, use controlled lifts, preserve geotextile, and build minimum cover before larger equipment crosses the field. The construction-loading plan governs machine size and travel. Damaged units or displaced rows are corrected before they are buried.

Only where site investigation and design demonstrate suitable infiltration or an engineered outlet addresses the limitation. Native-soil permeability, groundwater, bedrock, contamination, and setback conditions are evaluated by the project team. Harrowgate protects the approved infiltration interface from compaction and sediment, but construction technique cannot make unsuitable native soil freely draining.

Commissioning confirms clean pretreatment, unobstructed inlet and outlet piping, correct flow controls, accessible inspection ports, operating pumps or valves where used, surveyed inverts, and no visible construction damage. Pipe or tank testing is completed as specified. Harrowgate removes accumulated sediment and records the buried configuration before final handover.