Storm-Water Drain Estimate Against a Fixed Sanction
A junior engineer prepares an estimate for a 1,250 m long open storm-water drain. Internal width is 0.60 m and internal depth is 0.90 m. Bed concrete is 0.10 m thick across the full 0.90 m outer width; side walls are 0.15 m thick and 0.90 m high above the bed. Excavation is in ordinary soil to the outer dimensions with vertical sides, and the bed concrete is laid on the excavated base. The sanctioned amount is ₹40.00 lakh. Schedule rates are ₹250/m³ for excavation and ₹7,400/m³ for concrete; concrete wastage is 5% and no wastage is allowed for excavation. A 10% overhead and 1.5% cess on the gross cost are to be added; the cess is calculated after adding overhead. The contractor proposes to reduce the bed concrete from 100 mm to 75 mm to bring the estimate within sanction.
1. Compute the excavation and concrete quantities and the total estimate.
Read the worked answer
Excavation: outer width = 0.60 + 2 × 0.15 = 0.90 m; depth = 0.90 + 0.10 = 1.00 m. Volume = 0.90 × 1.00 × 1,250 = 1,125 m³.
Concrete per metre: bed = 0.90 × 0.10 = 0.09 m³; two walls = 2 × 0.15 × 0.90 = 0.27 m³; total = 0.36 m³/m. For 1,250 m = 450 m³. With 5% wastage = 472.5 m³.
Cost: excavation = 1,125 × 250 = ₹2,81,250. Concrete = 472.5 × 7,400 = ₹34,96,500. Subtotal = ₹37,77,750. Overhead at 10% = ₹3,77,775; gross = ₹41,55,525. Cess at 1.5% = ₹62,332.88. Total = ₹42,17,857.88, say ₹42.18 lakh.
2. Does the estimate fit the sanction? Evaluate the contractor's 75 mm bed proposal.
Read the worked answer
Sanction = ₹40,00,000. Estimate = ₹42,17,858. Excess = ₹2,17,858, about 5.4%.
The 75 mm proposal saves 0.025 × 0.90 = 0.0225 m³/m, or 28.125 m³ over 1,250 m. With 5% wastage that is 29.53 m³; at ₹7,400/m³ the saving is ₹2,18,531 before overhead and cess. That appears to close the gap, but it is not a sound basis. The bed thickness is a design element: reducing it changes cover, structural capacity and excavation depth. A 25 mm reduction without redesign and re-approval is a variation, not a saving.
Legitimate options are to reduce the length to the sanctioned amount, adopt a technically approved alternative section, or obtain a revised sanction after a proper design check. Do not certify the thinner bed merely to fit the budget.
3. If the sanction is fixed and no design change is permitted, what maximum length can be built at the same rates?
Read the worked answer
Use the accepted estimate as the cost basis. Cost per metre = ₹42,17,857.88 ÷ 1,250 = ₹3,374.29/m.
Maximum length = ₹40,00,000 ÷ ₹3,374.29 = 1,185.4 m.
So about 1,185 m can be built within ₹40 lakh at the same rates and section, leaving a small rounding margin. If the full 1,250 m must be built, the department needs an additional ₹2.18 lakh or a revised, technically vetted section. It cannot simply direct the contractor to reduce the bed thickness: that would be an unauthorised design change, and it would shift the risk of a structural or cover failure onto the department.