Expert: Arshad Khan - 12/3/2009

Question
QUESTION: The school at which I volunteer in Kathmandu has an opportunity to gain a financial grant. This will be from a European Embassy in KTM which grants funds for approved "micro-projects" such as water tanks for irrigation of crops.
We feel that we need a 100,000 litre tank which resolves into 8 metres dia and 2.2metres high.
The base will be 150 mm and the walls will be 125 mm.
I need to be able to estimate the weight of steel in this tank in order to submit a "Financial Plan" to the Embassy.
Q 1. How much steel per cubic metre of concrete in the  8 cubic metres in the tank base and 4 cubic metres in the tank walls, please?
Q2. For estimating quantities of sand, gravel and cement in the concrete in the tank, is it OK to use 700 kg/m3 sand, 1150 kg/m3 gravel and 300 kg /m3 cement??

Many thanks for your help.
Kind regards,
Robert.

ANSWER: Hi Robert,

Firstly I would recommend that you review the geometry so as to minimise total surface area, hence materials. For example, even if you are not providing a roof, a tank of 6.5m diameter and 3m height has some 6% less surface area and is also better proportioned than a 8m dia tank 2.2m high.

Secondly, if you are not going to incorporate a water-proof internal lining (such as rubber membrane of waterproof plaster), then the RC structure needs to be waterproof and durable by having sufficient cover to the rebar. For that, the minimum thickness recommended is 225 to 250mm for both walls and the base. (you will need to locally thicken up the base to probably 300-500mm under the wall to distribute the concentrated self weight of the wall to the foundation- this depends on the foundation strata.

Finally, you will need to decide how the tank walls are to be designed/detailed- either as a sliding joing at the bottom (with a flexible water-bar) or with the fixed base- if the latter, then the wall definitely has to be on the thicker side as you need designed vertical rebar in addition to the hoop rebar.

As for quantities, for rebar I would suggest around 60 kg/m3 for the base and around 100 kg/m3 for the walls as a ball-park budget estimate.

For the concrete, this should be at least grade 30 (N/mm2 at 28 days) and you can work out the quantities using this useful website:

http://concrete.union.edu/general.htm

The only other item you need to cost is the foundations excavation/blinding and temporary shuttering/formwork and of course labour.

---------- FOLLOW-UP ----------

QUESTION:   Thank you Arshad for your quick response. The information was very helpful to do the estimate.
In order to  get 100,000 litres, I am now costing a 7  m dia tank with wall 't' 250 mm and 3m height.
The foundations will need careful study with 100 tonnes water and 26 tonnes concrete loading.
The sub-soil in the terai region of Nepal is sandy. What is your opinion of a steel pile of  150 OD approx 5- 6 foot in ground filled with cement, acting as a friction pile and bearing on a concrete pad approx   1m x 1m . The pile pattern could be circles of  8 outer  piles,  4 inner  piles and one centre pile. Is there another, better design? Is there a professional engineer who could design the steel fixing for us? We would of course pay a fee for this service.
I was so happy to receive your response and I have no hesitation in rating you 10/10 !!
With sincere thanks,
Robert.

Answer
Hi Robert,

The dimensions now look fine. I have designed many such tanks and have never had to resort to using piles. Sandy material, if it is capable of supporting 75-100 kN/m3 as a safe bearing capacity, will not require any piles, especially if you locate the base slab some 0.5-0.75m below natural ground, depending on the local conditions.

For a design, the best option would be to use a local engineer who can inspect the soil conditions before designing the foundations and the structure itself.