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Civil Engineering : Underground Water Tank


Design of Underground Water Tank



The Underground tanks are used to store water, liquid petroleum, petroleum products and similar liquids. The force analysis of the containers is about the same irrespective of the chemical nature of the product. All tanks are designed as crack free structures to eliminate any leakage.

This project gives in brief, the theory behind the design of liquid retaining structure i.e. rectangular underground water tank.

This report includes the requirement of the water tank, survey, excavation methods, Reduced Levels, Average Depth of GWT, Soil on which it is formed, Depth of water Table, Type of Mix Design and Capacity of the tank.



S. N.  Title
1        Introduction
2        Requirement
3        Survey
4        Excavation
5        Design requirement of concrete
6        General design requirement
7        PCC and Water Proofing
8        Conclusion


1. Introduction

In this project, an attempt has been made to design a water tank for storing drinking and utility water to serve daily requirement of the entire society.

The project work consisted of the following –
1. Requirement
2. Survey
3. Excavation
4. Design Requirements
5. Approval from RCC Consultant
6. PCC and Water Proofing


2. Requirement

Water is one of the basic importantly for a human.An absolute “minimum water requirement” for humans is independent of lifestyle and culture, can be defined only for maintaining human survival.

Different sectors of society use water for various purposes i.e. drinking, cooking food, bathing and washing clothes, sanitation and many other daily use utilities. The water required for each of these activities varies with climatic conditions, lifestyle, culture, tradition, diet, technology and wealth.

Water requirements for a society vary upon various factors i.e. No of buildings in the nation, No of floors in each building, No of flats on each floor, etc. Moreover if society consists of row houses, then water requirement for organization increases as per gardening needs. Amenities like a typical garden, swimming pool, gymnasium, playground, jogging park and clubhouse have become additional factors of increased water requirement of society nowadays.


3. Survey

A detailed survey of the location was carried out for one day using the Dumpy Level Instrument at the site. Dumpy Level is an instrument which can be used to perform all the steps involved in the survey work such as simple leveling and height measurements etc., with an average level of accuracy.

Thus in the course of the survey, essential points of the tank were recorded.
After the survey work, the data in the form of points and their coordinates were transferred on the paper, and further calculation has been done to using the recorded reduced levels.


Excavation is the preliminary activity of the construction project. It starts from the pits for the building foundations and continues up to the handing over of the plan.

The scope of the work:
• Setting out of corner benchmarks.
• Study for ground levels.
• Survey for top levels
• Excavation to approved depth.
• The dressing of loose soil.
• Making up to cut off level
• Constructing under watering wells and interconnecting trenches.
• Marking boundaries of the building.
• Constructing protection bunds and drains


Working Procedure:
• The extent of soil and rock strata is found by making trial pits in the construction site. The excavation and depth are decided & approved according to the concerned site’s R.C.C consultant.Then, digging is carried out as follows-
• Setting out or ground tracing is the process of laying down the excavation lines and center lines etc. on the ground before the excavation is started. The center line of the most extended outer wall of the building is marked on the ground by stretching a string of wooden or mild steel pegs. Each peg may be extended about 25 to 50 mm from the ground level and 2m from the edge of the excavation. The boundary is marked with the lime powder. The center lines of other walls are marked perpendicular to the more extended walls & the excavation work is started with the help of JCB machine. Digging is done up to a depth(Approved by R.C.C consultant) of 3.5 meters


Removal of Excess Soil
• Estimate the excavated stuff to be re-utilized in filling, gardening, preparing roads, etc.As far as possible try to carry excavation and filling simultaneously to avoid double handling. Select and stack the required material in such a place that it should not obstruct other construction activities. The excess or unwanted material should immediately be carried away and disposed of by employing any of the following methods.
• Departmental labor.
• Tractor.
• Trucks.


In water retaining structure, a dense impermeable concrete is required. Therefore, the proportion of fine and coarse aggregates to cement should be such as to give high-quality concrete.

Concrete mix weaker than M20 is not used. The minimum quantity of cement in the real mix shall be not less than 30 kN/m3.

The design of the concrete mix shall be such that the resultant concrete is sufficiently impervious. Efficient compaction preferably by vibration is essential. The permeability of the thoroughly compacted cement is
dependent on the water-cement ratio. Increase in water-cement ratio increases permeability, while concrete with the low water-cement rate is difficult to compact. Other causes of leakage in concrete are defects such as segregation and honeycombing. All joints should be made water-tight as these are potential sources of leakage. Design of liquid retaining structure is different from ordinary R.C.C, structures, as it requires that concrete should not crack and hence tensile stresses in cement, should be within permissible limits.



5.1.1 MOVEMENT JOINTS. There are three types of movement joints.

(i)Contraction Joint.It is a movement joint with deliberate discontinuity without an initial gap between the concrete on either s0ide of the joint. The purpose of this joint is to accommodate contraction of the concrete.


A contraction joint may be either complete contraction joint or partial contraction joint. A total contraction joint is one in which both steel and concrete are interrupted and a partial contraction joint is one in which only the concrete is interrupted, the reinforcing steel running through.


(ii)Expansion Joint.It is joint with the complete discontinuity in both reinforcing steel and concrete and it is to accommodate either expansion or contraction of the structure.


This joint requires the provision of an initial gap between the adjoining parts of a structure which by closing or opening accommodates the expansion or contraction of the structure.

(iii) Sliding Joint. It is a joint with a complete discontinuity in both reinforcement and concrete and with special provision to facilitate movement in the plane of the joint.


This type of joint is provided between wall and floor in some cylindrical
tank designs.

This type of joint is equipped for convenience in construction. Arrangement is made to achieve subsequent continuity without relative movement. One application of these joints is between successive lifts in a reservoir wall.

The number of joints should be as small as possible, and these bones should be kept from the possibility of percolation of water.


A gap is sometimes left temporarily between the concrete of adjoining parts of a structure which after a suitable interval and before the structure is put to use, is filled with mortar or concrete completely. With suitable jointing materials. In the first case width of the gap should be sufficient to allow the sides to be prepared before filling.







6.1 Plain Concrete Structures.

Everyday concrete member of reinforced practical liquid retaining structure may be designed against structural
failure by allowing tension in plain concrete as per the permissible limits for stress in bending. This will automatically take care of failure due to cracking. However, nominal reinforcement shall be provided, for plain concrete structural members.


6.2. Permissible Stresses in Concrete.

(a) For resistance to cracking. For calculations relating to the resistance of members to splitting, the allowable stresses in tension (direct and due to bending) and shear shall confirm the values specified in Table 1.

The permissible tensile stresses due to bending apply to the face of the member in contact with the liquid. In members less than 225mm. Thick and in contact with fluid on one side these permissible stresses in bending also apply to the face remote from the liquid.


(b) For strength calculations. In strength calculations, the permissible Concrete stresses shall be by Table 1. Where the calculated shear stress in concrete alone exceeds the allowable value,

reinforcement acting in conjunction with diagonal compression in the concrete shall be provided to take the whole of the shear.



7.1 Plane cement concrete (P.C.C.)-

After approving(Bearing capacity of soil) from R.C.C consultants construction of underground water tank is started. 150mm thick plane cement concrete (p.c.c) is done as per mix design M-20.

The ratio is as follows-

Cement———–50 kg
Metal ————-20mm @224 kg
Crush sand——-207 kg
Fly ash ———–224 kg
Fosroc chemical—248 ml
Water ————37.20 ml


7.2 Water Proofing-

• Waterproofing is done at the top of the P.C.C by rough shabad flooring.
• First 20 mm morter of 1:6 ratio is applied, & above this rough shabad flooring is done.
• Joints are filled with cement between the rough shabad flooring.
• After this 12mm plaster coat is applied.
• Steel reinforcement is set as per consultant drawing.



Storage of water in the form of tanks for drinking and washing purposes, swimming pools for exercise and enjoyment, and sewage sedimentation Tanks are gaining increasing importance in the present day life. For small capacities, we go for rectangular water tanks while for bigger sizes we provide circular water tanks.

Design of water tank is a very tedious method.

Particularly design of underground water tank involves lots of mathematical formulae and calculation. It is also time to consume. Hence program gives a solution to the above problems. There is a little difference between the design values of the program to that of manual calculation. The program gives the least value for the design. Hence designer should not provide less than the values we get from the program. In case of theoretical calculation designer initially, add some extra values to the obtained values to be in safer side.


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