---
title: "Brickwork of a septic tank"
subject: "Estimating and Costing"
chapter: 6
key: "semester_7/estimating_and_costing:septic-tank-brickwork-m"
source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/septic-tank-brickwork-m
site: BCT Engineering
---

# Brickwork of a septic tank

Estimating and Costing · Chapter 6: Detailed Estimate

- Asked in: 2074 Chaitra, 5 marks
- Video: [Estimating and Costing 2 | Chapter 9 | Part 5 | Septic Tank](https://www.youtube.com/watch?v=huveXB_NSds) — Sigma Classroom
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/estimating_and_costing/septic-tank-brickwork-m
- Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-6

## Question

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.]

## 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.

## Answer

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**.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the septic tank brickwork volume found in this answer?
   A. Centre-line method with lower 350 mm and upper 230 mm walls taken separately
   B. Outer area of tank x total depth
   C. Internal volume of tank minus liquid volume
   D. One wall thickness for the whole depth
   Answer: A. Because the wall changes thickness, the lower and upper walls are each taken by the centre-line method. (Key point: Method)

2. What is the centre-line length of the 350 mm lower wall (internal 2.0 x 1.0 m)?
   A. 2[(2.0 + 0.35) + (1.0 + 0.35)] = 7.40 m
   B. 2[(2.0 + 0.23) + (1.0 + 0.23)] = 6.92 m
   C. 2(2.0 + 1.0) = 6.00 m
   D. 2[(2.0 + 0.70) + (1.0 + 0.70)] = 8.80 m
   Answer: A. Centre-line side = internal length plus one wall thickness, here 0.35 m. (Key point: Lower wall centre-line)

3. Which parts are NOT counted as brickwork?
   A. Upper 230 mm wall
   B. Precast RCC cover, RCC baffle and base slab
   C. Lower 350 mm wall
   D. Wall below the offset
   Answer: B. The cover and baffle are RCC and the base slab is concrete; they are measured under their own items. (Key point: Items excluded from brickwork)

4. What is the total brickwork (2.331 + 1.432)?
   A. 3.76 m3
   B. 2.33 m3
   C. 4.65 m3
   D. 7.40 m3
   Answer: A. Lower wall 2.331 m3 plus upper wall 1.432 m3 = 3.763 m3. (Key point: Total brickwork)

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