The design principles of underground tanks are same as for tanks resting on the ground. The walls of the underground tanks are subjected to internal water
pressure and outside earth pressure. The section of wall is designed for water pressure and earth pressure acting separately as well as acting simultaneously. Whenever there is possibility of water table to rise, soil becomes saturated and earth pressure exerted by saturated soil should be taken into consideration.
The design of Underground tanks can be easily understood from the numerical explained below:
Q. Design an underground reservoir 12 m. x 4 m. x 4 m. deep. The long walls will be designed as cantilevers and the top portion of the short walls will be designed as slab supported by long walls. Bottom one metre of short walls will be designed as cantilever slab.
Design of long wall
1. Pressure of saturated soil acting from outside and no water pressure from inside. Earth pressure at base will be due to water pressure plus due to submerged weight of soil.
Maximum B.M. at base of long wall
= 12,800 kg.m.
= 1,280,000 kg. cm.
Effective depth required
= 30.11 cm.
Provide overall depth of 35 cm. with effective depth of 31 cm.
= 48.2 cm2
Provide 20 mm φ bars at 6 cm. centers on outside face. Area of steel provided = 52.36 cm2.
Reinforcement is curtailed in the same manner as in the second case.
Direct compression in long walls. Direct compression is caused in long walls because of earth pressure acting on short walls which act as slab supported on long walls.
Direct compression at 1 m above base
= 7200 kg.
This will be taken by wall and the distribution steel provided.
2. Water pressure acting from inside and no earth pressure acting from outside.
Maximum water pressure at base