---
title: "Underground RCC tank: brickwork 1:4, M20 PCC, steel"
subject: "Estimating and Costing"
chapter: 6
key: "semester_7/estimating_and_costing:ug-tank-e-fig1"
source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/ug-tank-e-fig1
site: BCT Engineering
---

# Underground RCC tank: brickwork 1:4, M20 PCC, steel

Estimating and Costing · Chapter 6: Detailed Estimate

- Asked in: 2080 Bhadra, 4+3+1 marks
- Video: [Underground Water Tank Design | RCC and Brick Underground Water Tank with Cost Estimate](https://www.youtube.com/watch?v=myGOpFPUa-8) — Civil Gyan
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/estimating_and_costing/ug-tank-e-fig1
- Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-6

## Question

Drawing of an Underground RCC water tank is given below (Fig. 1). Prepare detailed estimate for the following items of works:
i) Brickwork (1:4)
ii) PCC for M20 RCC work
iii) Steel reinforcement for M20 RCC work (assuming 1% steel)
[Figure 1: plan 3000 mm × 2300 mm outside; brick wall 230 mm thick at top stepping to 380 mm at base with 150 mm offset; clear inside 1990 mm × 1290 mm; M20 RCC walls/floor 125 mm; 125 mm thick M25 RCC slab at top; section: 800 mm and 475 mm wall heights, overall 1450 mm, clear height 1150 mm; brickwork (1:4); PCC (1:3:6) base with flat brick soling. All dimensions in mm.]

## Short answer

Brickwork in two steps by centre-line method:3.199 m³. M20 concrete for floor and lining wall totals1.446 m³. Steel at1% gives113.5 kg.

## Answer

#### Reading of Fig. 1

- Outside size at base $3.00 \times 2.30$ m; clear inside $1.99 \times 1.29$ m. Wall at the base is $0.38$ m of brick plus $0.125$ m of M20 RCC, so $1.99 + 2 \times (0.38 + 0.125) = 3.00$ m and $1.29 + 2 \times 0.505 = 2.30$ m (check).
- Brick wall: $0.38$ m thick for the lower $0.475$ m, stepping in by $0.15$ m on the outer face to $0.23$ m for the upper $0.80$ m (total brick height $1.275$ m = clear height $1.15$ m + floor $0.125$ m).
- Inside the brick: 125 mm M20 RCC lining wall, $1.15$ m high, and 125 mm M20 RCC floor. Top slab 125 mm M25 RCC (not part of the M20 quantity).

#### i) Brickwork (1:4), centre-line method

| Part | Centre-line length (m) | Thickness (m) | Height (m) | Volume (m³) |
|---|---:|---:|---:|---:|
| Lower | $2\,[(3.00-0.38)+(2.30-0.38)] = 9.08$ | 0.38 | 0.475 | 1.639 |
| Upper | $2\,[(3.00-0.53)+(2.30-0.53)] = 8.48$ | 0.23 | 0.800 | 1.560 |
| **Total** | | | | **3.199** |

The upper wall's outer face is set in by $0.15$ m on each side, so its centre-line rectangle is $(3.00 - 0.30 - 0.23) \times (2.30 - 0.30 - 0.23) = 2.47 \times 1.77$ m.

#### ii) Concrete for M20 RCC work

| Member | Working | Volume (m³) |
|---|---|---:|
| Floor | $(1.99 + 0.25)(1.29 + 0.25) \times 0.125$ | 0.431 |
| Lining wall | $2\,[(1.99+0.125)+(1.29+0.125)] \times 0.125 \times 1.15 = 7.06 \times 0.125 \times 1.15$ | 1.015 |
| **Total M20** | | **1.446** |

(For information, the M25 top slab is $2.70 \times 2.00 \times 0.125 = 0.675$ m³.)

#### iii) Steel in M20 RCC (1%)

$W = 0.01 \times 1.446 \times 7850 = 113.5$ kg

**Answer: brickwork (1:4) = 3.199 m³; M20 concrete = 1.446 m³; steel = 113.5 kg.**

## Check yourself

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

1. Why is the upper brick wall's centre-line length shorter than the lower's?
   A. Its height is smaller
   B. It is built with 1:6 mortar
   C. It uses inside dimensions
   D. Its outer face is set in by 0.15 m and it is thinner (0.23 m), so the centre line moves inward
   Answer: D. Centre-line rectangle = outer size minus the set-in minus half thickness, 2.47 x 1.77 m, smaller than the lower wall's. (Key point: Upper wall centre-line)

2. Why is the top slab not counted in the M20 quantity?
   A. It is M25 RCC, a different grade, measured separately
   B. It has no steel
   C. It is part of the brickwork
   D. It is excluded because it is thin
   Answer: A. The M20 item covers only the M20 floor and lining wall. (Key point: Which concrete is M20)

3. How is the steel of 113.5 kg found?
   A. 0.01 x 3.199 x 7850
   B. 0.01 x 1.446 m3 x 7850 kg/m3
   C. 1.446 x 7850 / 100 x 2
   D. 1.446 x 78.5 x 2
   Answer: B. 1% of the M20 volume times steel density. (Key point: Steel weight)

4. Which check confirms the reading of the figure?
   A. 1.99 + 2 x 0.38 = 3.00 m
   B. 1.99 + 0.505 = 3.00 m
   C. 1.99 + 2 x (0.38 + 0.125) = 3.00 m
   D. 1.99 + 2 x 0.125 = 3.00 m
   Answer: C. The base wall is 0.38 m of brick plus 0.125 m of RCC on each side of the clear size. (Key point: Check of outside size)

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