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Septic tank with soak pit: excavation, floor, plaster

Short answer

Soak pit excavation for 1.5 m excavated diameter and 1.2 m depth is 2.12 m3; septic tank floor concrete is 0.169 m3; 20 mm floor plaster area is 1.125 m2.

Prepare a detailed estimate of the following items from the septic tank with soak pit from the given fig 1. a) Earth work in excavation for soak pit b) Cement concrete flooring for septic tank c) Cement plaster (1:3) of 20 mm for septic tank floor Dimensions of septic tank: width = 750 mm, length = 1500 mm. Soak pit is of 1000 mm diameter. [Figure 1: septic tank with soak pit; 100 dia vent pipe, 450 dia manhole cover with frame, 100 thk R.C.C. slab, 375 mm brick walls, 125 mm walls, 250 mm offsets, heights 300, 850, 1200, 150, 100 mm; soak pit with stone/brick aggregate filling, 250 mm thick outer casing with coarse sand, overflow outlet, depth 1200 mm and 700 mm.]

Items are taken from the dimensions stated: tank 1500 mm × 750 mm (internal), soak pit 1000 mm diameter.

Assumptions (from the figure)

  • Soak pit: 1000 mm diameter (internal) with a 250 mm coarse sand/aggregate casing around it, so the excavated diameter = 1.0 + 2 × 0.25 = 1.50 m.
  • Depth of soak pit = 1.2 m (the 1200 mm marked); the 700 mm dimension is the filling/cover zone inside this depth.
  • Floor of septic tank: cement concrete 150 mm thick over the internal area (the 150 mm marked).

a) Earthwork in excavation for soak pit

V=π4D2 h=π4×(1.50)2×1.20=2.121 m3V=\frac{\pi}{4}D^2\,h=\frac{\pi}{4}\times(1.50)^2\times 1.20=2.121\ \text{m}^3

b) Cement concrete flooring of septic tank

Volume = length × width × thickness = 1.5 × 0.75 × 0.15 = 0.169 m³.

c) Cement plaster (1:3), 20 mm, on septic tank floor

Area = 1.5 × 0.75 = 1.125 m². Mortar volume = 1.125 × 0.020 = 0.0225 m³.

Answer: Excavation for soak pit = 2.12 m³; floor concrete = 0.169 m³; floor plaster = 1.125 m².

✓ Answer checked by Claude Opus 5.5 (high effort)

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