--- title: "Underground water tank: excavation, brick, plaster" subject: "Estimating and Costing" chapter: 6 key: "semester_7/estimating_and_costing:ug-tank-g-brick-plaster" source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/ug-tank-g-brick-plaster site: BCT Engineering --- # Underground water tank: excavation, brick, plaster Estimating and Costing · Chapter 6: Detailed Estimate - Asked in: 2079 Bhadra, 2+4+2 marks - Video: [Quantity Survey: Underground water tank.](https://www.youtube.com/watch?v=fCuGzkwlqTE) — Engr Suliman Khan - 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-g-brick-plaster - Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-6 ## Question The plan and section of the under-ground water tank which is fully constructed below the ground level. Find the quantities of a) Earthwork in excavation for construction. b) Brickwork in 1:6 cement sand mortar. c) Plastering work for inner part of tank. [Figure: Underground water tank. Top plan: outer 550 cm × 250 cm plus 30 cm wall all round (30 | 550 | 30 and 30 | 250 | 30). Sectional elevation: wall stepped with offsets of 30, 40, 50, 60 and 70 cm widths in 40 cm high steps, total height 190 cm, base footing 85 cm wide with 30 cm thick lime concrete, 3 cm thick C.C. flooring, outer plaster 1.2 cm thick 1:6 C:S, inner plaster 1.5 cm thick 1:4 C:S.] ## Short answer Excavation:7.05×4.05×2.20 =62.82 m³. Brickwork in five stepped sections:17.644 m³. Inner plaster 1.5 cm thick:29.92 m² (3.28 bags cement,0.456 m³ sand). ## Answer #### Reading of the figure and assumptions - Clear internal plan $5.50 \times 2.50$ m. The wall is stepped on the outside only, inside face vertical. Widths $0.30, 0.40, 0.50, 0.60, 0.70$ m; heights $0.30$ m for the top step and $0.40$ m for each of the other four (total wall height $1.90$ m). - Base footing: lime concrete $0.30$ m thick, $0.85$ m wide (0.075 m projection on each face of the 0.70 m base). The tank is fully below ground, so the top of the wall is at ground level and excavation depth is $1.90 + 0.30 = 2.20$ m. - Floor: 3 cm C.C. flooring on the lime concrete; inner plaster 1.5 cm in 1:4 from floor to top of wall. #### a) Earthwork in excavation Footing outer size: length $= 5.50 + 2 \times (0.70 + 0.075) = 7.05$ m; width $= 2.50 + 2 \times 0.775 = 4.05$ m. $V = 7.05 \times 4.05 \times 2.20 = 62.82$ m³ (no working space or side slope allowed). #### b) Brickwork in 1:6 mortar (centre-line method for each step) | Thickness (m) | Height (m) | Centre-line length (m) | Volume (m³) | |---:|---:|---|---:| | 0.30 | 0.30 | $2\,[(5.50+0.30)+(2.50+0.30)] = 17.20$ | 1.548 | | 0.40 | 0.40 | $2\,[(5.50+0.40)+(2.50+0.40)] = 17.60$ | 2.816 | | 0.50 | 0.40 | $2\,[(5.50+0.50)+(2.50+0.50)] = 18.00$ | 3.600 | | 0.60 | 0.40 | $2\,[(5.50+0.60)+(2.50+0.60)] = 18.40$ | 4.416 | | 0.70 | 0.40 | $2\,[(5.50+0.70)+(2.50+0.70)] = 18.80$ | 5.264 | | **Total** | | | **17.644** | #### c) Inner plaster 1.5 cm (1:4) - Inner perimeter $= 2\,(5.50 + 2.50) = 16.00$ m; plastered height $= 1.90 - 0.03 = 1.87$ m (above the floor). - Area $= 16.00 \times 1.87 = 29.92$ m² - Wet mortar $= 29.92 \times 0.015 = 0.4488$ m³; dry $= 1.27 \times 0.4488 = 0.5700$ m³; cement $= 0.1140$ m³ $= 3.28$ bags; sand $= 0.4560$ m³ **Answer: excavation = 62.82 m³; brickwork = 17.644 m³; inner plaster = 29.92 m² (3.28 bags cement, 0.456 m³ sand).** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The tank is fully below ground. Why is the excavation depth 2.20 m? A. Wall height 1.90 m only B. Wall height + 3 cm flooring C. Plaster thickness + wall height D. Wall height 1.90 m + lime concrete 0.30 m Answer: D. Excavation goes down to the bottom of the footing concrete, which lies 0.30 m below the wall base. (Key point: Excavation depth) 2. Excavation plan is 7.05 x 4.05 m because A. It is the footing's outer size: 5.50 + 2 x (0.70 + 0.075) by 2.50 + 2 x 0.775 B. It equals the clear internal size C. It is the top-wall outer size D. Working space of 1 m is added Answer: A. The footing is 0.85 m wide and extends 0.075 m beyond the base step on each face. (Key point: Excavation plan size) 3. Why is the brickwork worked out step by step? A. Each step uses a different mortar B. Each step has a different thickness and so a different centre-line length C. Steps are deducted from the plaster D. The height changes continuously Answer: B. Each step's centre-line = 2[(L+t)+(B+t)] with its own thickness t. (Key point: Stepped wall brickwork) 4. From wet mortar 0.4488 m3, dry mortar is found by A. Dividing by 1.27 B. Multiplying by 1.5% of plaster C. Multiplying by 1.27 D. Multiplying by 0.015 Answer: C. Dry volume is about 1.27 times the wet volume to allow for voids and wastage. 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