Underground Detention Systems: Modular Stormwater Modules for Every Site

Q: What is void ratio and why does it matter?

Void ratio is the percentage of a module volume available for water storage. Our modules deliver 95% net void. Gravel achieves 30%. Less excavation means lower cost smaller footprint and faster project completion.

95% void ratio. Designed for AASHTO H-20 traffic applications. ASTM F2418 resin tested. No crane needed — two workers, one rubber mallet.

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Why Engineers Specify Modular Underground Detention

The underground detention system that replaces gravel trenches and concrete vaults — at half the install time.

Designed for AASHTO H-20 / HS-25 Applications

Engineered for 40,000 lbs single-axle loads (H-20). XD variants are designed for HS-25 fire lane and highway applications. Manufacturer compression tested to 108 t/m² on a single module.

AASHTO HS-20 load rated underground stormwater detention modules for parking lots

95% Void Ratio (Max Storage)

Each cubic foot you excavate stores 95% water. Gravel holds 30%. On a 10,000 cf project, that means excavating 350 cubic yards instead of 1,100. At $15-25/yd³ disposal, the math closes fast.

High void ratio (95%) modular stormwater tanks compared to gravel trenches

ASTM F2418 Tested — US Compliance Ready

Resin independently tested per ASTM F2418 for flexural modulus and tensile yield strength (TRI Environmental, Austin TX). Qualifies as a structural BMP under EPA NPDES guidelines. Meets Florida first-flush, California LID, and Texas flood detention requirements.

Stackable stormwater storage modules for flexible underground basin design

100% Recycled Polypropylene

100% industrial-grade recycled polypropylene with proprietary performance additives. Resists road salts, diesel fuel, motor oils, and soil acids. Operating range: -22 °F to 248 °F. Contributes to LEED Sustainable Sites credits.

Recycled polypropylene (PP) material for sustainable LID stormwater systems

Rapid “Snap-Lock” Installation

18.7 lbs per module. Two workers assemble 4,000 cubic feet per day with a 2×4 and a dead-blow hammer. No crane rental, no formwork, no 28-day cure. A typical system installs in 8 hours with a 3-person crew.

Snap-lock modular stormwater detention module assembly without crane

Inspectable & Maintainable

Compatible with standard isolator row configurations. Wrap one row separately in geotextile to trap sediment upstream. Inspection ports and cleanout risers allow periodic Jet-Vac maintenance without excavating the system.

Stormwater system inspection ports and sediment traps for easy maintenance

One Subsurface Detention System, Two Configurations

Same stormwater modules. Different liner. Your municipal stormwater ordinance determines which configuration you need.

Detention — Flood Control & Peak Flow Reduction

The underground detention pond configuration. Captures stormwater runoff and releases it slowly to a downstream storm sewer at municipal permit rates.

Seal the module array with HDPE or LLDPE impermeable geomembrane, protected by non-woven fleece on both faces
Install a flow-control manhole with orifice plate or weir to restrict discharge to pre-development rates
Required for Texas/Houston flood control zones, sites with non-draining clay soils, and any municipality that caps post-development peak discharge
Qualifies as a structural BMP under EPA NPDES stormwater guidelines

Upstream pretreatment required: silt traps, hydrodynamic separators, or inlet leaf baskets to prevent sediment from entering the module array.

Infiltration — Groundwater Recharge & LID Compliance

The subsurface detention basin that lets water soak into the ground instead of leaving the site. Same modules, no membrane.

Wrap the module array in AASHTO Class 2 non-woven permeable geotextile — water passes through into the subsoil
Place on a 6 in. leveled layer of clean angular crushed stone for drainage and load distribution
Designed for California LID compliance, dry wells, septic drain field replacement, and groundwater recharge mandates
No flow-control manhole needed — water exfiltrates freely into permeable subsoil at the site’s natural infiltration rate

Some jurisdictions require a hybrid: infiltration on the base, impermeable liner on the sides. Confirm with your local stormwater authority.

Technical Specifications — Designed for AASHTO H-20 / HS-25 Applications

Two module sizes. Four duty variants. Manufacturer compression tested to 30 tonnes (108 t/m²) on a single module.

Stormwater Module 6841 (17.7″ Height)

Stack	Size (mm)	Tank Volume (ft³)	Tank Volume (gal)	Net Water (ft³)	Net Water (gal)
Single	26.8 × 16.1 × 17.7 in (680 × 410 × 450 mm)	4.44	33.22	4.31	32.21
Double	53.5 × 16.1 × 17.7 in (1360 × 410 × 450 mm)	8.89	66.44	8.63	63.05
Triple	80.3 × 16.1 × 17.7 in (2040 × 410 × 450 mm)	12.93	97.62	12.54	93.81
Quad	107.1 × 16.1 × 17.7 in (2720 × 410 × 450 mm)	17.17	128.47	16.65	124.58
Pent	133.9 × 16.1 × 17.7 in (3400 × 410 × 450 mm)	21.42	160.21	20.78	155.41

Stormwater Module 8053 (20.9″ Height)

Stack	Size (mm)	Tank Volume (ft³)	Tank Volume (gal)	Net Water (ft³)	Net Water (gal)
Single	31.5 × 19.3 × 20.9 in (800 × 490 × 530 mm)	6.02	45.12	5.84	43.76
Double	63.0 × 19.3 × 20.9 in (1600 × 490 × 530 mm)	12.04	90.24	11.68	87.52
Triple	94.5 × 19.3 × 20.9 in (2400 × 490 × 530 mm)	18.06	135.36	17.52	131.28
Quad	126.0 × 19.3 × 20.9 in (3200 × 490 × 530 mm)	24.08	180.48	23.36	175.04
Pent	157.5 × 19.3 × 20.9 in (4000 × 490 × 530 mm)	30.10	225.60	29.20	218.80

Duty Variants & Load Ratings

Variant	ARW-6841 Rating	ARW-8053 Rating	Typical Application
LD	44,000 lbs (20 t)	44,000 lbs (20 t)	Landscaped areas, pedestrian plazas
SD	88,000 lbs (40 t)	88,000 lbs (40 t)	Residential driveways, light vehicle traffic
HD	8,200 lbs/ft² (40 t/m²)	12,300 lbs/ft² (60 t/m²)	Commercial parking lots, access roads
XD	12,300 lbs/ft² (60 t/m²)	16,400 lbs/ft² (80 t/m²)	Fire lanes, highways, industrial yards

Material & Structural Ratings

Material: 100% industrial-grade recycled polypropylene with proprietary performance additives. Virgin PP available for specific project requirements.
Load Rating: Designed for AASHTO H-20 / HS-25 applications. Engineered for 40,000 lbs single-axle loads under commercial parking lots and roadways.
Chemical Resistance: Unaffected by road salts, diesel fuel, motor oils, and naturally occurring soil acids and alkalis.
Temperature Range: -22 °F to 248 °F (-30 °C to 120 °C).
Durability: Polypropylene does not rust, rot, or degrade in buried conditions. Designed for long-term underground service.

Hydraulic Performance

Void Ratio: 95% net void space. Each cubic foot you excavate stores 95% water versus 30% for gravel.
Applications: Underground stormwater detention (flood control), infiltration basins (groundwater recharge), and rainwater harvesting.
Connections: Universal compatibility with 4 in. to 12 in. Schedule 40 PVC and standard corrugated HDPE pipe.
Flow Control: Orifice plate or weir-type manholes restrict discharge to municipal permit rates for 2-year through 100-year design storms.
Stacking: Up to 6 layers (Pent configuration) for maximum storage depth in constrained footprints.

Engineering Support & Resources

Material Color: Standard Industrial Black (UV Stabilized Recycled PP).
CAD & Submittals: Full CAD details, step-by-step installation guides, and submittal packages available upon request.
Stamped Layouts: We provide project-specific stamped engineering layouts with cover depth designs and pipe penetration details.
Compliance: ASTM F2418 resin tested by TRI Environmental. Qualifies as structural BMP under EPA NPDES stormwater guidelines.
Turnaround: Typical proposal and layout delivery within 2 business days.

Underground Detention vs. Concrete Vaults vs. Surface Ponds

Installed cost per cubic foot and available land drive most detention decisions. Here is where four standard approaches compare.

System Type	Material ($/cf)	Installed ($/cf)	Install Speed	Land Footprint
Modular Geocellular	$4.50 – $6.50	$8.50 – $13.00	4,000 cf/day, 2 workers	Minimal (below grade)
Arch Chambers (HDPE)	$6.00 – $8.00	$12.50 – $17.00	Moderate, crane required	Minimal (below grade)
Gravel / Stone Beds	$3.50 – $5.00	$9.00 – $13.50	Slow, compaction required	Large (30% void only)
Surface Detention Basin	N/A	$0.30 – $1.00	Fast, earthwork only	Very large

Sources: WGA Engineering cost studies, EPA stormwater BMP cost guidance, market survey data 2025-2026.

Gravel beds store water too, but at 30% void ratio versus 95% for modular crates, you need 3x the excavation volume. On a 10,000 cf project, that is the difference between removing 350 cubic yards of soil and 1,100 cubic yards. At $15–$25 per cubic yard for excavation and disposal, the smaller dig often offsets the higher material cost entirely.

One honest note: if your site has available acreage and no traffic loading requirement, a surface detention basin is the most cost-effective choice by a wide margin. Modular underground detention earns its place on constrained sites where land value or traffic loading justifies the premium.

Need a Stamped Layout and Cover Design for Your Underground Detention System?

Request a Stamped Engineering Layout

Installation: Six Steps, Two Workers, No Crane

Snap-lock assembly with hand tools. A 4,000 cubic-foot underground detention system typically installs in 8 hours with a 3-person crew.

Excavate and level. Dig to design depth plus 6 in. for the stone base. Compact subgrade to 95% Standard Proctor density.

Place the liner. Detention: lay HDPE or LLDPE geomembrane with fleece on both faces. Infiltration: wrap with AASHTO Class 2 non-woven geotextile.

Assemble modules. Insert large pegs into small plates to form side panels. Seat with a 2×4 and dead-blow hammer. No bolts. No adhesive.

Stack and lock. For multi-layer configurations, invert the completed crate, align the next unit, tap to engage. Verify lock at every joint.

Connect pipes and flow control. Cut openings for 4-12 in. inlet/outlet pipes. Install flow-control manhole with orifice or weir plate.

Backfill and compact. Place engineered fill in 12 in. lifts. Compact each lift. Pave or grade the surface.

EQUIPMENT

Project Case Studies

Verified installations using modular geocellular underground detention systems in California.

Commercial Research Campus — San Diego, CA

Challenge. A 12-acre biotech campus needed two separate underground detention systems: one for first-flush water quality capture, the second for 100-year peak flow detention. No surface area available for ponds.

Solution. Two modular underground detention systems beneath the parking structure. Tank 1: 9,737 cubic feet (first flush). Tank 2: 17,939 cubic feet (peak flow). HS-20 rated modules stacked 3 layers deep.

Result. Both systems fit within the existing building footprint. Zero surface land consumed. Field assembly completed in under 2 weeks with a crew of 4.

Golf Course Flood Detention — Palm Desert, CA

Challenge. A Coachella Valley golf course required 99,000 cubic feet of stormwater flood control detention for monsoon storm events. The system had to fit beneath irrigated turf without disrupting play.

Solution. Modular geocellular crates installed in phases across multiple fairway corridors. Geomembrane-lined for full detention. Discharge controlled through orifice manholes.

Result. 99,000 cubic feet of underground stormwater detention capacity for the 100-year design storm, with zero visible surface impact. Turf restoration completed within one growing season.

Frequently Asked Questions — Underground Detention Systems

Are these modules traffic rated? Can they support heavy trucks?

Yes. Our modules are designed for AASHTO H-20 traffic applications (40,000 lbs single-axle load). Each module is manufacturer compression tested to 30 tonnes (108 t/m² on the module footprint). XD variants are engineered for HS-25 fire lane and highway applications. With correct cover depth and engineered backfill, they are designed for use under commercial parking lots, fire lanes, and roadways.

Does this system meet local municipal and state stormwater codes?

Modular underground detention qualifies as a structural BMP under EPA NPDES guidelines. Module resin is tested per ASTM F2418 by TRI Environmental (Austin, TX). State-level compliance varies: California mandates LID infiltration where soils allow, Texas/Houston requires on-site detention to cap peak discharge at pre-development rates, and Florida requires first-flush volume capture. Confirm requirements with your local stormwater authority — every jurisdiction writes its own rules.

Can I use these modules for an Infiltration Basin or Dry Well?

Yes. Wrap the module array in AASHTO Class 2 non-woven geotextile (instead of an impermeable geomembrane), bed it on 6 in. of crushed stone, and the system functions as an infiltration basin. Works for LID compliance, dry wells, and drain field replacement where soil permeability allows.

Can I use this for a Septic Leach Field or Drain Field replacement?

Yes. The 95% void ratio and permeable geotextile wrap configuration allows the module array to replace conventional gravel leach fields at roughly one-third the excavation volume. Check local health department requirements for minimum setbacks and soil percolation rates.

My site has a high water table. Can I install a shallow system?

Yes. The ARW-6841 stands 17.7 in. tall, roughly 30% shorter than the 8053 series. A single-layer system requires as little as 18 in. total depth. Verify your municipality’s minimum cover requirement for the applicable load rating.

How do I prevent the system from clogging?

Install pretreatment upstream: silt traps, hydrodynamic separators (CDS units, Downstream Defenders), or inlet leaf baskets. For added protection, use an isolator row — one row of modules wrapped separately in geotextile to trap fine sediment. Inspection ports allow Jet-Vac cleaning without excavation.

Can I use this system to harvest rainwater for my garden?

Yes. Seal the system with an impermeable geomembrane liner for full containment. Add a submersible pump and draw water for irrigation. The 95% void ratio maximizes storage capacity per cubic foot of excavation.

How much does an underground detention system cost?

Installed costs for modular geocellular systems typically fall between $8.50 and $13.00 per cubic foot. For comparison: HDPE arch chambers run $12.50-$17.00, gravel beds $9.00-$13.50, and surface detention basins $0.30-$1.00 per cubic foot (but require significantly more land).

What is the difference between detention and infiltration?

Detention captures runoff and releases it slowly to a downstream system. The water leaves the site. Infiltration captures runoff and lets it percolate into the ground. The water stays. Same modules, different liner: geomembrane for detention, geotextile for infiltration. Your municipal stormwater ordinance determines which you need.

What is void ratio and why does it matter?

Void ratio is the percentage of a module’s volume available for water storage. Our modules deliver 95% net void. Gravel achieves 30%. That means you excavate roughly one-third the volume to store the same amount of water. Less excavation = lower cost, smaller footprint, faster project.

Project Gallery: Underground Detention and Infiltration Installations

See our modular systems in action—from heavy-duty parking lot detention basins to high-capacity residential dry wells.

AASHTO H-20 rated underground stormwater detention modules installed beneath commercial parking lot

Modular geocellular underground detention system installation showing snap-lock module assembly

Geomembrane-lined underground stormwater detention system with flow control manhole connection

Workers assembling modular stormwater modules for underground detention at residential development site

Completed underground infiltration system with geotextile wrap before backfill at construction site

Large-scale underground detention system installation showing multi-layer module stacking configuration

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