- 2074 Chaitra · 5 marks
Brickwork of a septic tank
Short answer
Brickwork by centre-line method, lower 350 mm wall gives 2.331 m3 and upper 230 mm wall gives 1.432 m3; total septic tank brickwork is 3.76 m3. RCC cover, baffle and base slab are excluded.
Workout quantity of brickwork of a septic tank. [Figure: Septic tank. Plan: internal 2000 mm × 1000 mm, wall 230 mm, 450 mm and 50 mm (baffle) marked. Section: wall 230 mm at top stepping to 350 mm, 150 mm offset, total internal depth 1800 mm, 350 mm cover zone, 75 th RCC precast cover, 50 th RCC baffle, 150 mm base slab with 1:20 slope, inlet/outlet 150 mm, 300 mm.]
The brickwork volume of the septic tank walls is found by the centre-line method, treating the 350 mm lower wall and the 230 mm upper wall separately.
Assumptions (from the figure)
- Internal plan 2000 mm × 1000 mm; total internal depth 1800 mm. The wall is 350 mm thick for the lower 0.9 m and steps to 230 mm for the upper 0.9 m. The inner face is taken as vertical, so the 120 mm difference forms an offset on the outer face.
- The 75 mm precast RCC cover and the 50 mm RCC baffle are RCC items, not brickwork; the 150 mm base slab is concrete. The small inlet/outlet openings (150 mm) are ignored.
Centre-line lengths
- Lower wall: 2[(2.0 + 0.35) + (1.0 + 0.35)] = 2(2.35 + 1.35) = 7.40 m
- Upper wall: 2[(2.0 + 0.23) + (1.0 + 0.23)] = 2(2.23 + 1.23) = 6.92 m
Calculation
| Item | No. | L (m) | B (m) | H/D (m) | Qty (m³) |
|---|---|---|---|---|---|
| Lower wall 350 mm (centre-line) | 1 | 7.400 | 0.350 | 0.900 | 2.331 |
| Upper wall 230 mm (centre-line) | 1 | 6.920 | 0.230 | 0.900 | 1.432 |
| Total | 3.763 |
Answer: Brickwork in the septic tank = 3.76 m³.
✓ Answer checked by Claude Opus 5.5 (high effort)
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