After the area and the traffic, the first number we ask for on every road or yard is the soil CBR, the California Bearing Ratio of the ground the road will sit on. The grid carries the wheels at the surface; the soil carries everything below. So the soil sets how thick the layers under the grid must be, and that is a large part of the cost.
To show how much, we ran one truck yard through the GridMats Design Engine three times and changed only the soil.
The yard
- 10,000 m² (about 1.08 lakh sq ft) of truck parking and marshalling, the same yard as in our truck-yard comparison
- 150 trucks a day, multi-axle trucks and tippers of 36–49 t gross; 15 years of design traffic
- Hyderabad rates, ex-GST, indicative
- Soil CBR 3%, 5% and 10%: soft clay, an average soil and a good gravelly soil
Three soils, three sections
Each section is GridMats S1, cement-treated gravel: the 40 mm OC-HD 40 grid filled with 6–12 mm chips treated with 5% cement, compacted and brushed flush, on 25 mm of bedding and a granular base sized for the soil, over a non-woven geotextile separation layer on the subgrade.
| Soil CBR | Layers below the grid | Total depth | To build |
|---|---|---|---|
| 3% | 150 mm WMM, 200 mm GSB, a geotextile, and the top 200 mm of the subgrade stabilised with cement to CBR 15 | 615 mm | ₹211 per sq ft (₹2,269 per m²) ₹2.27 Cr for the yard |
| 5% | 150 mm WMM, 200 mm GSB, a geotextile | 415 mm | ₹186 per sq ft (₹2,003 per m²) ₹2.00 Cr |
| 10% | 150 mm WMM, 100 mm GSB, a geotextile | 315 mm | ₹173 per sq ft (₹1,863 per m²) ₹1.86 Cr |
The geotextile separation layer under the base is a separate BoQ item and is not included in the GridMats figures above.
For comparison, an IRC:58-2015 concrete road for the same trucks barely moves with the soil: 580 mm at CBR 5% and 10% (a 280 mm slab on 150 mm of dry lean concrete and 150 mm of GSB), and 590 mm at CBR 3%. It costs ₹330–335 per sq ft (₹3,548–3,608 per m²). A concrete slab carries the load by bending, and the soil enters its design only through the support under the lean concrete.
What the numbers say
- From CBR 10% to CBR 3%, the S1 section grows by 300 mm and the cost by ₹38 per sq ft (₹406 per m²): about ₹41 lakh on this yard.
- Below CBR 3%, we stabilise the subgrade rather than pile on more granular base. A stiffer grid does not change this: the grid works at the surface, and the soil is carried by the layers below it.
- Against the concrete road, S1 costs 37% less to build on CBR 3%, 44% less on CBR 5% and 48% less on CBR 10%.
Why a guessed CBR is expensive
- Guess high, and the base is too thin for the ground. The yard ruts and needs repair long before its design life.
- Guess low, and you pay for layers the ground did not need: up to ₹38 per sq ft on this yard.
- Most sites have more than one soil. A yard that crosses filled ground and natural ground may need two sections, and only tests show where the line falls.
What to ask your lab for
The soaked CBR (IS 2720 Part 16) on samples taken at the level the road will sit on, from each kind of soil on the site, with the location of each sample on the plan. If the earthwork will bring in fill, test the fill too.
No test yet? Tell us the soil type and we design the first cut on a typical value for that soil and mark it for checking. The design is confirmed when the test arrives, before anything is ordered.
How we design
The GridMats Design Engine uses layered-elastic analysis, with the IRC:37-2018 checks, including its rutting criterion, for the layers under the grid, and the IRC:58-2015 method for the concrete baseline. An engineer reviews every design before it is issued.
Your site will differ
These are indicative figures for one typical yard. Traffic, soil, rainfall, local rates, freight and the base design 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.