---
title: "Septic tank: excavation, brickwork, PCC"
subject: "Estimating and Costing"
chapter: 6
key: "semester_7/estimating_and_costing:septic-tank-c-three-items"
source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/septic-tank-c-three-items
site: BCT Engineering
---

# Septic tank: excavation, brickwork, PCC

Estimating and Costing · Chapter 6: Detailed Estimate

- Asked in: 2081 Bhadra, 3+4+3 marks
- Video: [Septic tank numerical problems/ Estimation of Septic Tank/ Calculate Earthworks in Excavation/Design](https://www.youtube.com/watch?v=zoCU6YXIqrY) — Ashif Azad
- 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-c-three-items
- Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-6

## Question

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]

## 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.

## Answer

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).

## Check yourself

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

1. How is the pit plan size 4.06 x 2.26 m obtained from the tank outer size 3.46 x 1.66 m?
   A. Add 0.15 m PCC projection and 0.15 m working space on each side
   B. Add 0.15 m on one side only
   C. Add 0.30 m projection only on length
   D. Subtract the wall thickness 0.23 m
   Answer: A. PCC projects 0.15 m and a further 0.15 m working space is left, so 0.30 m is added each side: 3.46 + 0.6 = 4.06 m. (Key point: Excavation size)

2. What is the depth of excavation of 1.65 m made of?
   A. Liquid depth 1.5 m plus 0.15 m PCC thickness
   B. Wall height 1.8 m minus 0.15 m
   C. Liquid depth 1.5 m plus 0.30 m above GL
   D. PCC 0.15 m plus 1.8 m wall
   Answer: A. The pit goes down to the bottom of the PCC: 0.15 + 1.5 = 1.65 m. (Key point: Excavation depth)

3. How is the outer wall brickwork 3.858 m3 calculated?
   A. Centre-line 9.32 m x 0.23 m x 1.80 m
   B. Outer perimeter x 0.23 x 1.65
   C. Internal area x 1.5 m
   D. Centre-line 9.32 m x 0.115 m x 1.80 m
   Answer: A. Outer walls use the centre-line length 9.32 m x thickness 0.23 m x height 1.80 m. (Key point: Brickwork method)

4. Why is PCC taken as 3.76 x 1.96 m?
   A. Tank outer size plus 0.15 m projection on each side
   B. Internal size 3.0 x 1.2 m
   C. Excavation size minus working space of 0.15 m on one side
   D. Outer size minus 0.15 m each side
   Answer: A. PCC projects 150 mm beyond outer wall faces: 3.46 + 0.30 = 3.76 and 1.66 + 0.30 = 1.96 m. (Key point: PCC size)

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