---
title: "Septic tank with soak pit: excavation, floor, plaster"
subject: "Estimating and Costing"
chapter: 6
key: "semester_7/estimating_and_costing:septic-tank-soak-pit-f"
source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/septic-tank-soak-pit-f
site: BCT Engineering
---

# Septic tank with soak pit: excavation, floor, plaster

Estimating and Costing · Chapter 6: Detailed Estimate

- Asked in: 2080 Baisakh, 4+2+2 marks
- Video: [Estimate of Septic tank](https://www.youtube.com/watch?v=vVXtY4cj_s8) — SK Civil academy
- 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-soak-pit-f
- Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-6

## Question

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.]

## 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.

## Answer

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

#### 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.

## Check yourself

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

1. Why is the soak pit excavation diameter 1.50 m although the pit is 1000 mm?
   A. The 250 mm coarse sand casing is added on both sides
   B. The manhole is 450 mm wide
   C. Working space of 1 m is allowed
   D. It is a rule that excavation is 1.5 times diameter
   Answer: A. Excavated diameter = 1.0 + 2 x 0.25 = 1.50 m. (Key point: Soak pit excavated diameter)

2. Which expression gives the soak pit excavation of 2.121 m3?
   A. pi/4 x 1.0^2 x 1.2
   B. pi/4 x 1.5^2 x 1.2
   C. pi x 1.5 x 1.2
   D. pi/4 x 1.5^2 x 0.7
   Answer: B. Cylinder volume = (pi/4) D^2 h with D = 1.50 m, h = 1.2 m. (Key point: Cylinder volume formula)

3. What is the floor concrete volume of the tank (1.5 x 0.75 internal, 150 mm thick)?
   A. 1.125 m3
   B. 0.169 m3
   C. 0.0225 m3
   D. 0.338 m3
   Answer: B. Volume = 1.5 x 0.75 x 0.15 = 0.169 m3. (Key point: Floor concrete volume)

4. In which unit is the 20 mm floor plaster quantity given, and how?
   A. m3, area x 0.020
   B. m2, length x width = 1.125 m2
   C. m, perimeter
   D. m3, area x 0.150
   Answer: B. Plaster is measured by area: 1.5 x 0.75 = 1.125 m2; the thickness only gives mortar volume 0.0225 m3. (Key point: Plaster measured in area)

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