What Is a Floating Foundation: Floating Slab Foundation

What Is a Floating Foundation?

A floating foundation is a type of shallow foundation used in construction. It consists of a concrete slab that is poured directly on the ground, without any additional footings or support structures.

The slab is typically reinforced with steel bars or wire mesh to provide additional strength and durability. Floating foundations are used in areas where the soil is stable and able to support the weight of the structure.

They are typically used for smaller buildings or structures, such as sheds or small homes. The advantages of using a floating foundation include reduced cost and construction time, as well as the ability to easily move the structure if necessary.

However, they may not be suitable for areas with unstable soil conditions or areas that are prone to flooding. It is important to consult with a structural engineer to determine the appropriate foundation type for a particular construction project.

The construction details are explained in detail in the following sections.

The main purpose or aim of the floating foundation is to balance the weight of removed (dragged) soil by a structure of the same weight which causes zero settlement to the structure. For this reason, it is also termed a balancing raft foundation.

Read More: What Is The Strongest Foundation For A House

Suitability of the Floating Foundation

The suitability of the Floating foundation is as follows

• A floating foundation is best suited for differential settlement foundation problems under heavy loads.
• A floating foundation construction in such a situation helps in reducing the settlement.
• It is mostly used in soils that have low shear strength.
• A floating foundation is also used in soils that do not contain any hard or stiff layer at a required depth. In such conditions, a floating foundation reduces the shear stress force on the structure.

How to Build a Floating House Foundation

The construction or building of a floating House Foundation requires details to start the execution of the proposed building on solid earth strata to completely support the whole weight of the superstructure

As mentioned they are more used in garages, an extension of the house, or sheds that do not necessarily require huge foundations.

These buildings are light in structure. So the floating foundation was found most suitable and economical when compared with a conventional or normal foundation.

The construction of a floating foundation involves the following steps:

1. Site excavation for floating foundation construction
2. Laying gravel base for drainage
3. Reinforcement of foundation
4. The casting of the foundation

1. Excavation For Floating Foundation

The initial stage of the construction of the floating foundation is to clear the land and excavate the area as per the plan. This is the initial preparation for the construction.

2. Laying Gravel Base Drainage

This step involves making the drainage in the sides and bottom of the foundation, for the water to drain out through the gravel base as shown below.

The trench constructed is sealed with Geo-textile and gravel as shown below, which will remove water from the concrete foundation as shown in figure 2.

3. Reinforcement for Floating Foundation

The reinforcement molds are placed for the casting procedure. To strengthen the slab, the reinforcing bars are provided as per dimensions, codes, and specifications.

4. Casting of the Foundation

The concrete is poured into the formwork. It is compacted either by labor or a mechanical vibrator depending on the area and workmanship.

Proper compaction has to be done, to make the slab fully strengthened and productive. Proper curing has to undergo for almost 2 to 3 days after placing concrete.

Read More: Types Of Foundations & Their Uses In Building Construction

Principle of Floating Foundation

The main principle of the floating foundation is to balance the weight of dragged soil by a structure of the same

which leads to zero settlement of the structure.

So, this foundation is also known as balancing the raft foundation.

• Let’s consider a ground with a water table at the top as shown in the figure. The ground or base is excavated or dragged up to a certain depth which is below the water table.
• Now in the next step, a building is constructed which is as same as the weight of the removed soil and water.
• Even if the depth of excavation is below the water table the total vertical pressure in the soil below the foundation is unchanged because of its balancing weight.
• But here one point should be kept in mind we cannot build a structure immediately after the excavation.
• At the time of construction, the effective vertical pressure below the depth of excavation may slightly increase because of the unbalanced weight. So, this type of floating foundation can also be known as a partly compensated foundation instead of a fully floating or compensated foundation.

Stability of Floating Foundations

Floating foundations are best suited for the following types of soils:

• The soils which are having good shear strength but having a problem with more settlements and differential settlements due to heavy loads. In such a case, the floating foundation can minimize the settlement values to a greater extent.
• The soils which are having lower shear strength and there is no hard layer or strata of soil at a reasonable depth. Here also, the floating foundation is desirable to decrease the shear stresses to a satisfactory level, and hence settlement is prevented.

Difficulties In the Construction of Floating Foundation

At the time of the construction of the floating foundation, some problems may arrive which can be prevented by taking proper care.

The following are floating slab foundation problems,

1. Excavation
2. Dewatering
3. Critical depth
4. Bottom heave

1. Excavation

The excavation made for a floating foundation should be done carefully. The walls of the excavated part should be supported using piles or timber piles or soldier piles etc.

2. De-watering

If the depth of the excavation is below the water table then dewatering is essential. It is better to check and establish the water table level prior to the excavation.

De-watering should be done cautiously otherwise it may cause a lowering of the water table which disturbs the adjoining structure.

Care has to be taken to see that the adjoining structures are not affected by the dewatering process of the foundation.

3. Critical Depth

If the soil has low shear strength as specified above, there is a limit to the depth of excavation which is known as critical depth.

There are many equations available to determine the critical depth of excavation among them one is given below known as Terzaghi’s equation.

   Terzaghi’s equation for critical depth is:

Where,

Dc = Critical depth

s = Shear strength of soil

B = Width of foundation made (If width is less than the length of foundation “L”)

ᵧ = Unit weight of soil

4. Bottom Heave

When the soil is excavated up to some depth, the pressure of the soil below this depth is lowered which results in the formation of a heave.

The formed heave may lead to the settlement of the structure or foundation. We cannot prevent the formation of heave but there are some methods to minimize the formation of heave.

Digging narrow trenches and filling them with concrete soon after the excavation minimizes the heave formation.

Installing friction piles before the excavation starts or after the completion of half excavation can also prevent the formation of heave under the foundation.

Lowering the water table also, it can be minimized. To prevent the heave formation in the sidewalls of the trench, lateral support should be provided.

Heaving can be reduced by phasing out excavation in narrow trenches and placing the foundation soon after excavation.

Advantages and Disadvantages of Floating Foundation

Advantages Floating Foundations

• The floating foundation can be employed in site areas with lower soil-bearing capacity (SBC) and where investing large money in soil treatment is futile.
• This system lets us use it over loose soil or in soil with varying compressibility.
• The floating foundation has the property of spreading the vertical loads or stresses coming over it to be disturbed over a large area.
• A floating foundation is a good solution when it comes to the extension of houses. It is best suited when we require a building extension without affecting the already existing foundation.
• This won’t interfere with the building structure already built.
• A floating foundation can be used in high-moisture soil.
• The floating foundation does not require the usage of footer trenches. They can be poured with the help of trenching or digging which is economical.
• It requires very little digging of soil because deep footer trenches are needed.
• It avoids seepage of water and frosting. Due to this purpose, the foundation can be insulated from frost or moisture.
• Most suitable for heavily used highways.
• The floating foundation does not cause any disturbance to the earth layer beneath neither it will interfere with the quality.
• In areas where there are possibilities of shifting in the earth layer, mainly due to high moisture content, the floating foundation is best suited.

Disadvantages of Floating Foundations

• The footing slab does not make underground land available for underground access for connection lines facing the utilities.
• It has primitive technology.
• The floating foundation has lower design resonance.

Conclusion

In conclusion, floating foundations are a viable solution for building structures on water bodies where traditional foundations may not be feasible or cost-effective.

They provide an innovative and flexible approach to construction and that can be used for a range of applications such as homes, offices, and even offshore wind turbines.

The advantages of floating foundations are numerous, including their ability to adapt to changing water levels, their minimal environmental impact, and their cost-effectiveness compared to traditional foundations.

Furthermore, floating foundations can be designed to withstand harsh weather conditions and seismic events.

Despite their benefits, floating foundations do have some limitations, such as their susceptibility to strong currents and waves, and the need for regular maintenance to ensure their structural integrity.

Overall, floating foundations represent a promising technology for coastal development, and as more research and innovation are dedicated to improving their design and performance, they have the potential to become an increasingly attractive option for sustainable building and infrastructure development on water bodies.

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