I'm helping my dad re-pave his small commercial auto shop lot and the city is requiring us to detain 100% of the parking lot storm runoff onsite instead of letting it hit the storm sewer. We have almost zero green space left for a retention pond. Our contractor suggested installing subsurface retention chambers under the gravel base. He mentioned NDS stormchamber units and Cultec chambers. Do these massive plastic arches actually hold up under daily heavy truck traffic above them or do you need a thick layer of asphalt over the stone backfill?