On an asphalt or paver car park, the rain runs across the surface to a lined drain at the kerb. On the permeable GridMats systems it goes down instead: through the surface into the base, and out through pipes below it. That changes what you build at the edges, and it is why we ask for the rainfall before we design.
We ran one car park through the GridMats Design Engine to show what each system needs.
The car park
- 3,000 m² (about 32,000 sq ft), 60 m by 50 m, cars and SUVs
- Soil CBR 5%, flat ground, free-draining soil
- Moderate rainfall (750–1,500 mm a year), and the same car park again at very high rainfall (over 2,500 mm a year)
- Bengaluru rates, ex-GST, indicative
Where the rain goes, system by system
| System | The surface | Where the rain goes | Drains in the design |
|---|---|---|---|
| S1 cement-treated gravel | Semi-pervious | Across a 2% cross-fall to the edges | French drains with a perforated pipe along both edges, two outfalls |
| S2 stone chip | Permeable | Through the chips into 150 mm of open-graded GSB, which stores about 59 mm of rain | Perforated pipes along the low edges, laterals at 25 m, outfalls every 40 m |
| S3 grass and soil | Permeable | Through the grass into 175 mm of open-graded GSB, which stores about 62 mm of rain | The same sub-surface pipes as S2 |
| Asphalt or paver blocks | Sealed or near-sealed | Across the surface to the kerb | A lined surface drain with kerb outlets, and an edge drain |
On S2 and S3 the rain is managed below the surface. Edge and outfall drains go in only where the site levels need them. For the design storm on this car park, 60 mm an hour, the surface takes in water faster than it falls.
What it costs
- Drainage is a small part of the price. On this car park the drain items come to about ₹1 per sq ft on S1 and ₹3 per sq ft on S2 and S3 (₹12 and ₹32 per m²), out of ₹117–157 per sq ft for the whole surface.
- The rainfall moves it. At very high rainfall the pipes go up from 100 mm to 150 mm, the laterals close up from 25 m to 10 m, and the outfalls from every 40 m to every 25 m. The S2 and S3 drain items roughly double, to about ₹6 per sq ft (₹62 per m²).
- The base storage is checked too. At very high rainfall the Design Engine flags the 59 mm stored in the S2 base as thin for that rain and asks for a 100 mm drainage layer or closer laterals.
- The pipes need somewhere to go. Every outfall has to reach a storm drain, a soak pit or a recharge well on the site. That connection is part of the site's civil works.
What we ask you for
- The city, so we know the rainfall
- The site levels, or a survey, and where the water leaves the site today
- Whether the ground ponds or stays wet in the monsoon, and how deep the water table is
- The soil, with a soaked CBR test if you have one
A shallow water table or a clay that holds water changes the design: a geotextile and a drainage layer go in, and on a site that cannot take water into the ground, S2 and S3 drain to the outfalls through the pipes.
Green cover and permeable area
S2 and S3 support green-cover rules and permeable-area requirements, subject to local approvals.
How we design
The GridMats Design Engine sizes the layers with layered-elastic analysis and the IRC:37-2018 checks, and the drainage from the rainfall band, the design storm, the site levels and the soil. An engineer reviews every design before it is issued.
Your site will differ
These are indicative figures for one typical car park. Rainfall, levels, soil, local rates and freight change them, and every GridMats site is priced from its own design. Ask for your free estimate: our engineers follow with the design, the bill of quantities and the cost comparison against your conventional section.