Assumption for Under Ground Water Tank (UGWT)-
>> Clear length: 30 feet,
>> Clear width: 6 feet,
>> Clear height: 7 feet.
Clear length, width and height means the inside dimension of water tank.
Wall thickness of water tank: 10 inches.
All right, now we have a drawing sheet showing all the above dimension of water tank and reinforcement details. Now, we can start earth excavation for UGWT.
Dimension Calculation for earth excavation
Total water tank length including water wall is 31 feet and 8 inches (31 feet + 10 inches + 10 inches). We need one feet clear space around water tank for easily moving labors during shuttering work. So the excavated length will be 33 feet and 8 inches (water tank length + both sides clear space).
Similarly, the excavated width will be 7 feet and 8 inches.
Now lets calculate the excavated height of water tank. We have 7 feet clear height of water tank. So the excavated height will be 8 feet and 9 inches (clear height + top slab thickness + bottom slab thickness + PCC thickness + soling thickness). Here we take top slab thickness is 6 inches, bottom slab thickness is 10 inches, PCC is 2 inches and soling is 3 inches (all these dimensions are shown in the drawing). During calculation of excavated height of water tank, one thing you should take into account that is ground level. In this example, we assume water tank’s top-level is on ground level. If water tank’s top-level is 1 feet above the ground level (+1) then the excavated height will be 1 feet less.
During excavation, maintain a slope in side soil because straight cut soil can easily break down. Also give support to side soil to prevent from break down. Be careful during excavation so that the excavated level doesn’t go below footing. It can be harmful for building. And also keep a Mud Pump ready because there may be a possibility to come up water during excavation.
After finishing excavation check whether the base soil hard enough. If the soil is loose then use macadam and compact the soil. The rest of the work of constructing water tank should be started as soon as the excavation done. Because there is a possibility to break down the side soil.
Avoid the water tank work during rainy season. It is better to start the water tank work simultaneously with footing work. It can save you from extra earth excavation and from some hassle.