- 2081 Bhadra · 3+4+3 marks
Septic tank: excavation, brickwork, PCC
Short answer
For assumed two-chamber septic tank: earthwork excavation 15.14 m3, brickwork in 1:6 mortar 4.07 m3, and PCC 1:3:6 foundation 1.11 m3. Dimensions from the drawing should be substituted in the same format.
Determine the following quantities from drawing of septic tank. i) Earthwork in excavation ii) Brickwork in (1:6) cement sand mortar iii) PCC in foundation (1:3:6) [Drawing not included in the scanned paper]
A drawing is not given, so a typical two-chamber septic tank is assumed. Excavation is taken for the plan of the PCC plus working space; brickwork by the centre-line method; PCC over the full footprint.
Assumed data
- Internal size 3.0 m × 1.2 m, liquid depth 1.5 m; outer walls 230 mm in 1:6 mortar from top of PCC to 0.30 m above GL (height 1.80 m); a 115 mm partition wall (3.0 m internal length is shared by two chambers) rising 1.5 m.
- PCC 1:3:6, 150 mm thick, projecting 150 mm beyond outer wall faces; working space 150 mm more on each side.
- Outside size of tank = 3.46 × 1.66 m; PCC size = 3.76 × 1.96 m; excavation size = 4.06 × 2.26 m; depth of excavation = 0.15 + 1.5 = 1.65 m.
- Centre-line length of walls = 2[(3.0 + 0.23) + (1.2 + 0.23)] = 9.32 m.
i) Earthwork in excavation
| Item | No. | L (m) | B (m) | H/D (m) | Qty (m³) |
|---|---|---|---|---|---|
| Pit for tank | 1 | 4.060 | 2.260 | 1.650 | 15.140 |
| Total | 15.140 |
ii) Brickwork in 1:6 cement sand mortar
| Item | No. | L (m) | B (m) | H/D (m) | Qty (m³) |
|---|---|---|---|---|---|
| Outer walls, 230 mm (centre-line) | 1 | 9.320 | 0.230 | 1.800 | 3.858 |
| Partition wall, 115 mm (to 1.5 m) | 1 | 1.200 | 0.115 | 1.500 | 0.207 |
| Total | 4.065 |
Small openings (inlet, outlet, manhole recess) are ignored in a first estimate, which slightly overstates the brickwork.
iii) PCC in foundation (1:3:6)
| Item | No. | L (m) | B (m) | H/D (m) | Qty (m³) |
|---|---|---|---|---|---|
| PCC 1:3:6 bed under tank | 1 | 3.760 | 1.960 | 0.150 | 1.105 |
| Total | 1.105 |
Answer: Excavation = 15.14 m³; brickwork 1:6 = 4.07 m³; PCC 1:3:6 = 1.11 m³ (assumed dimensions).
✓ Answer checked by Claude Opus 5.5 (high effort)
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