---
title: "Well foundation, 5 m internal diameter"
subject: "Estimating and Costing"
chapter: 6
key: "semester_7/estimating_and_costing:well-foundation-5m-k"
source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/well-foundation-5m-k
site: BCT Engineering
---

# Well foundation, 5 m internal diameter

Estimating and Costing · Chapter 6: Detailed Estimate

- Asked in: 2075 Chaitra, 7 marks
- Video: [Estimation of Open Well | Open Well Estimate | Quantity Surveying | #openwell #civilengineeringedu](https://www.youtube.com/watch?v=q9hGtb2leh8) — Civil Engineering Edu
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/estimating_and_costing/well-foundation-5m-k
- Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-6

## Question

Workout the quantity of well foundation of a bridge. The well is to be circular of 5 m internal diameter with 800 mm wall in 1:6 cement and sand mortar. The well is to be founded on strata 15 m below bed of river which is dry during winter. Bottom of the well is to be plugged with 1.5 m thick cement concrete 1:4:8 and the top to be sealed with 1 m thick cement concrete 1:2:4 and central portion is to be sand filled.

## Short answer

Outer area 34.212 m², inner 19.635 m². Sinking: outer area × 15 m = 513.18 m³. Brickwork: ring area × 15 m = 218.65 m³. Bottom plug (1:4:8): inner area × 1.5 m = 29.45 m³. Top seal (1:2:4): inner area × 1.0 m = 19.63 m³. Sand filling: inner area × (15-1.5-1.0)=245.44 m³.

## Answer

**Data.** Internal dia 5.0 m, wall 0.80 m, so outer dia = 5.0 + 2 × 0.8 = 6.60 m. Founding depth = 15.0 m below river bed (dry river, so no water-table complication). Assumptions: the top of the well is at river-bed level and the steining includes the curb (a separate RCC curb would be deducted from the brickwork).

- $A_o=\frac{\pi}{4}(6.60)^2=34.212$ m²; $A_i=\frac{\pi}{4}(5)^2=19.635$ m²; ring area $=14.577$ m².

| Item | Working | Quantity |
|---|---|---|
| Excavation / sinking | 34.212 × 15.0 | 513.18 m³ |
| Brick masonry 1:6 in steining | 14.577 × 15.0 | 218.65 m³ |
| Bottom plug, CC 1:4:8 | 19.635 × 1.5 | 29.45 m³ |
| Top seal, CC 1:2:4 | 19.635 × 1.0 | 19.63 m³ |
| Sand filling (15.0 − 1.5 − 1.0 = 12.50 m) | 19.635 × 12.50 | 245.44 m³ |

**Answer:** Excavation 513.18 m³; brickwork 218.65 m³; bottom plug 29.45 m³ (1:4:8); top seal 19.63 m³ (1:2:4); sand filling 245.44 m³.

## Check yourself

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

1. With internal dia 5.0 m and 0.8 m wall, what is excavation/sinking to 15 m?
   A. 19.635 x 15 = 294.5 m3
   B. 34.212 x 15 = 513.18 m3
   C. 14.577 x 15 = 218.65 m3
   D. pi/4 x 5.8^2 x 15
   Answer: B. Outer dia = 5.0 + 1.6 = 6.6 m, area 34.212 m2. (Key point: Outer dia and sinking)

2. Which area gives the steining brickwork?
   A. Ring area 14.577 m2
   B. Inner area 19.635 m2
   C. Outer area 34.212 m2
   D. Outer area minus 14.577
   Answer: A. Masonry forms the ring between 5.0 m and 6.6 m: 14.577 x 15 = 218.65 m3. (Key point: Brickwork area)

3. What depth is sand filled?
   A. 15.0 m
   B. 15.0 - 1.5 - 1.0 = 12.5 m
   C. 15.0 - 1.5 = 13.5 m
   D. 15.0 - 1.0 = 14.0 m
   Answer: B. Sand fills between the 1.5 m bottom plug and the 1.0 m top seal. (Key point: Sand fill depth)

4. What is the volume of the 1.0 m top seal of 1:2:4 concrete?
   A. 19.635 x 1.0 = 19.63 m3
   B. 34.212 x 1.0
   C. 14.577 x 1.0
   D. 19.635 x 1.5
   Answer: A. Plugs and seals fill the internal bore, so inner area 19.635 m2 x thickness. (Key point: Plug and seal volumes)

## Close variants set in other exams

- [Well foundation, 4.5 m internal diameter](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Awell-foundation-4-5m-p) — Estimating and Costing, chapter 6, asked 2×

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