Water from Small Dams

6.8 Bill of Quantities (BQ) and costsIn order to work out the cost of survey, design, tools, equipment, materials and labourwhich make up the overall Bill of Quantities it is first necessary to calculate the amountof material which needs to be excavated. The box shows how this can be calculated.The formula for estimating the volume of soil works for a dam wall is:-V = 0.216HL (2C + HS)Where:-V is the volume of soil in cubic metres.H is the maximum height of the dam wall in metres - before settling = 7.7 mL is the length of the crest in metres= 41.6 mC is the width of the crest in metres= 3 mS is the sum of the upstream and downstream slope = 5.5(Upstream slope 3:1 + Downstream slope 2.5:1 = 5.5)Therefore:-V, the volume of soil needed for the dam wall, can be found as follows:V = 0.216 H L x (2C + HS)V = 0.216 x 7.70 x 41.6 x (2C + HS)V = 69.19 x (2C + HS)V = 69.19 x (2 x 3 + 7.7 x 5.5)V = 69.19 x (6 + 42.35)V = 69.19 x 48.35 = 3,345 m 3The soil for the dam wall will come from a borrow pit dug in the reservoir and theexcavation of the spillway. The quantity from each source can now be worked out asshown below.Soil to be excavated from a 1m deep, 10 m wide and 60m long spillwayWidth x mean Depth x Length10m x 1m x 60 m = 600 m 3Soil to be excavated from a borrow pit = 3,345m 3 - 600m 3 = 2,745m 340