www.sefindia.org

[kaidjohars]

do we need to consider the buoyancy effect due to Submerged condition on isolated foundation, what shall be criteria to calculate the buoyant force on isolated foundation, as onec it is submerged it will have water effect all around and above foundation unlike hollow structure (sump pit, UG water tank)...please help

[kaidjohars]

PLEASE HELP

[es_jayakumar]

The effect of submergence on isolated footings is that the load on them will get reduced by a margin equal to the weight of the water displaced by the footing, pedestal and column base, up to the GWT. But  this reduction in load cannot be taken advantage of in the footing design, in case the submergence  recedes during summer.
Submergence of soil can affect the SBC of it.

E S Jayakumar

[Dr. N. Subramanian]

Dear Er.

If the foundation is submerged, you need to take submerged weight of soil in the calculations. In most of the cases the buoyant force need not be taken, as the vertical load will be much higher than the buoyant force.

In liquid retaining structures we consider buoyant load because of the possibility of tank empty condition.

Best wishes,
NS

[kaidjohars]

For the Conveyor gallery foundation  governing case is Uplift Condition for  DL- wind (Suppose 260kn of uplift and 40kn horizontal), for that do we need to consider the buoyant force  or not ????

[Dr. N. Subramanian]

We were thinking that you are referring to the normal case of a column with compressive force. For uplift force, the buoyant force will act in the same direction of the tension force in the column and hence should be considered.

Best wishes,
NS

[nimish.khanolkar]

I think here a discussion of what exactly this buoyant force is, is warranted. There are two ways of looking at this which would give you the exact same result -
1) A body immersed in a liquid(in this case groundwater) experiences a force equal to volume of displaced liquid. This is the famed Archimedes Principle.

2) The principle of hydrostatic pressure - namely, pressure in a static fluid is normal to surfaces and is a function of depth only.

1) is in-fact derivable from 2) or vice versa.

Lets look now at a foundation at depth z, of thickness t. Lets work in KN. So water density is 10kN/m3.
By 2) There would be hydro-static force on the top of this foundation pointing downwards equal to 10*z*A.  A is plan area of footing.
There would be hydro-static force on bottom of this foundation pointing upwards equal to 10*(z+d)*A.
Net force = 10*A*(z+d) - 10*A*(d) = 10*A*d  upwards
But wait--what have we got? 10*A*d is actually 10 * V where V is volume of the foundation . This is the mass of water that would be contained in a volume equal to the foundation, or as the term is called - mass of displaced water.
So we began from 2) and reached 1). Net uplift equals mass of displaced water.
For an isolated footing, only uplift of 10*V is there as derived earlier- where V is volume of the isolated footing itself. This is seldom significant.

For tanks, the hydrostatic pressure acts only from down, in empty condition , as the top surface of tank base is exposed to atmosphere- and so this results in a force equal to 10*A*(z+d) which is much higher and hence is more critical.
Looking at it another way, the tank displaces much much more water volume, infact equal to it's entire volume including thickness of base.. which is 10*A*(z+d) making things critical.

Hope this clears up your understanding of buoyancy....

Based on

[kaidjohars]

its clear now thanks

[PREETI_SARKAR]

when calculating the buoyant force on the foundation, what density should we use to multiply the volume of the soil displaced by the foundation? Unit weight of water? Unit weight of saturated soil? Unit weight of submerged soil?

[vikram.jeet]

UPLIFT PRESSURE

Best way is to take as under :
Df = Depth of Foundation
Dw = The Ht of water level above Footing base submerged soil zone
Dd  = ( Df - Dw)  taking as  dry soil zone

Uplift pressure U = Vw *Dw
Vw= is unit wt of water = 1000 kg/m3
Vs = Saturated soil unit weight
Vd = Dry unit weight of soil above water table
A= plan area of footing

Wt of soil & Footing Minus Uplift = ( A*T * Vc )+ (A*Vd*Dd) + { A*Vs*(Dw-T) }  -   (A*Vw*Dw)

IDEAS IS
TAKE FULL WEIGHTS OF ALL COMPONENTS AND DEDUCT UPLIFT PRESSURE FORCE. IE TAKE SATURATED WEIGHT OF SOIL UPTO WATER TABLE PLUS DRY WT. ABOVE IT MINUS UPLIFT FORCE
THIS WILL AMMOUNT TO TAKING BUYONT WT OF COMPONENTS SUBMERGED

Modified to clarify dated 25.03.2025