The tank is fully below ground. Why is the excavation depth 2.20 m?

Wall height 1.90 m + lime concrete 0.30 m

Plaster thickness + wall height

Excavation plan is 7.05 x 4.05 m because

It is the footing's outer size: 5.50 + 2 x (0.70 + 0.075) by 2.50 + 2 x 0.775

It equals the clear internal size

Why is the brickwork worked out step by step?

Each step has a different thickness and so a different centre-line length

Each step uses a different mortar

Steps are deducted from the plaster

The height changes continuously

2079 Bhadra · 2+4+2 marks

Underground water tank: excavation, brick, plaster

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).

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.]

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).

✓ Answer checked by Claude Opus 5.5 (high effort)

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Underground water tank: excavation, brick, plaster

Reading of the figure and assumptions

a) Earthwork in excavation

b) Brickwork in 1:6 mortar (centre-line method for each step)

c) Inner plaster 1.5 cm (1:4)

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