Buoyancy pressure is a design load to be considered for structures built below the water table. The deeper the excavation, the greater the buoyant pressure that the water exerts.
It is very important that civil engineers know how to calculate buoyancy pressure because many structures are built below the water table.
Let's see which structures we need to take buoyancy pressure into account.
- Underground tanks
- Basement slabs
- Slab foundations
- Dams
- Concrete slab
Design of underground tanks for buoyancy pressure
A tank built below the water table will float on water if we do not take into account the rising pressure of the water. Furthermore, this can lead to structural failure.
The following illustration shows a tank built underground.
As shown in the figure above, buoyant pressure is applied to the foundation.
How to calculate buoyancy force
The pressure at depth “h”; P
P = hρg
This question can be formulated as follows:
P=ϒ i H
Slab area = A
Buoyancy force = ϒ i hA
Safety factor against buoyancy pressure
In general, the safety factor against buoyancy is in the range of 1.2 to 1.5. It is normally kept at 1.2.
The following procedure can be used to check buoyancy.
- Calculate the buoyancy force according to the above equation
- Calculate the weight of the structure. The weight may not be the total weight of the structure if construction is carried out in several phases without drainage. In these situations, part of the structure that will be erected in the first phase must be taken into account when calculating the weight. If dehydration is carried out until the structure reaches strength, the total weight of the structure can be taken into account when evaluating the factor of safety against buoyancy.
- The safety factor against buoyancy pressure = structure weight/buoyancy force > 1.2
- The base plate must be designed to withstand the pressure exerted by the water and the pressure from the earth due to the container loads.
Buoyancy in basement slabs
Underground floors are typically constructed below the water table and in various phases of construction.
Furthermore, basement floors can have several levels.
Typically, these panels are designed for buoyancy, which is only applied to the bilge panel after construction. If the area to be constructed is covered with supports such as sheet piles, secant piles, etc., there will be no water in the excavation.
However, the water pressure in the basement slab must be taken into account during the work. As the basement is relatively deep, the thicker slab must be designed to withstand the forces involved.
For deeper basements, an additional anchoring system is required to absorb upward forces on the basement slab.
If the basement slab is on rock, it may be supported by the rock. However, in some structures, the basement slab and the entire structure are supported by pile foundations.
If the basement slab and superstructure rest on rock, Rock Anchor must be designed to withstand upward forces.
Furthermore, if the structure is on piles, the piles must be designed to withstand tensile forces. Piles must be designed to withstand axial tensile forces. Furthermore, the piles must be sufficiently anchored in the rock to ensure sufficient friction.
Buoyancy pressure in slab foundations
Like other structures, slab foundations are designed to provide buoyancy.
However, due to the greater thickness of the foundation slabs, buoyancy forces are not critical, especially in basements built very close to the ground.
As basement levels increase in depth, it may be necessary to consider upward pressure on the foundation.
Increased pressure in dams Increased pressure in dams
Dams are built to collect water for power generation, irrigation, drinking water production, etc. Due to the importance of these structures, their useful life is expected to last 120 years or more.
Furthermore, they are built as rigid structures that can withstand any force applied to them.
Typically, concrete structures are built on rock. However, it can also happen that they are built on solid ground.
Even if the building is built on rock and the rock has been plastered to improve its permeability, watercourses can form beneath the foundation.
Two methods are used to design these structures.
- For relatively light structures, these are anchored to the rock to avoid a tipping moment due to buoyancy pressure. However, this method presents some risks, as rods can corrode when exposed to a corrosive environment over a long period of time, even if the rods are galvanized. Most of the time, anchors designed for pulling forces are placed evenly between the soil and rock.
- The weight of the structure remains greater than the buoyancy pressure. Therefore, there are no tipping moments.
It is necessary to design the entire structure for upward water pressure. As indicated in the figure above, the thickness of the final part of the structure is comparatively smaller than the warhead area.
There are methods for determining pressure values under the base, which are not discussed in this article, and are intended to be used to verify tipping and structural designs.
Furthermore, the effect of buoyancy pressure must be considered as one of the most important aspects in the design.
Depending on the type of structure, buoyant pressure on retaining walls may also need to be taken into consideration.
Fluctuating pressure on retaining walls
When a retaining wall is built to retain fluids and the wall is taller, too much upward pressure is created on the retaining wall, which can lead to collapse.
Most people forget to take into consideration the buoyant pressure on the foundation structure during construction.
Although we consider the weight of water to contribute to the restoration moment when reviewing tipping calculations, the upward buoyant pressure applied to the base creates a tipping moment.
Therefore, we must pay attention to these aspects when designing. For more information, see the article Calculation of stability of retaining walls in other designs.
