Floating foundation – Advantage, Disadvantages, Design formula

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Whats a floating foundation?

A foundation is considered to be floating when the total weight of the building structure (super and substructure) is approximately equal to the weight of the soil and water removed from the site before the construction begins.

Floating foundation - Advantage, Disadvantages, Design
Floating Foundation

This implies that the pressure/stress developed at the bottom of the foundation is approximately equal before and after the construction of the building structure. Hence there is zero net pressure acting at the bottom of the foundation resulting in zero settlement of soil below. This can be formulated as follows

i.e. Weight of the building = Weight of the soil excavated including water

It’s an analogy to the Archimedes’ principle of floatation has resulted in such a foundation being called a floating foundation, in which a foundation structure acts as a boat floating in the water. It is also known as a compensating foundation or balancing raft.

Such a balance of weights is only approximate so it is really tough to obtain fully compensating footing. One of the reasons is that when excavating is continued below the groundwater table there may occur seepage of water which lowers the level of groundwater, thus effecting the effective strength parameter of the soil.

This concludes that for a fully compensated foundation to exist we must be able to maintain the groundwater table to its initial level which really a tough challenge. Thus a partially compensated footing may exist with the characteristics of minimum settlement and nearly equal weights.

Types of soil in which the floating foundation can be possible are classified as follows:

Type I

Type I soil are those which are safe under shear failure but yields higher settlement value especially differential settlement for the given load of building structure. A balancing raft may be implemented to reduce the settlement of soil.

Type II

Type II soil has lower shearing strength values which may result in rupture of soil if building is to be founded on ground level. In absence of a hard and rigid strata at an acceptable depth it may require a compensating footing to reduce the shear stress applied by the building load on the soil and yield acceptable values of settlement.

The concept of floating foundation can be implemented mainly in integration with raft foundation such as simple, boxed raft foundation and piled raft foundation. A floating foundation is implemented only when a simple shallow raft or other forms of the shallow foundation are not able to fulfill the settlement and shear strength requirements. They are used especially in case of very soft but dense clay or in a swap or marshy lands.

Difficulties arises to be considered during the construction of the Floating foundation

Many problems arise during the construction of the floating foundation which are required to be handled with proper techniques

Excavation:

For balancing the weight of the soil and structure, excavation of soil is carried out up to deeper depth. It results in unstable sides of the trenches which are required to be stabilized using supports like sheet piles, wooden planks, soldier piles, struts etc.

De-watering

Dewatering needs to be carried out when excavation extents below the GWT to lower the ground table and remove the excess water. It should be carried out without hampering the adjoining structures and soil structure.

Bottom heave

The pressure at the bottom of the excavation pit is reduced when the soil is removed from the trenches which result in bottom heaving. It may be due to the expansion of soil due to the release of overburden pressure, or absorption of water or due to stress from adjoining soil masses.

It can be minimized by lowering GWT or by phasing out excavation in narrow trenches and immediately placing the foundation structure.

Advantages of floating foundation

  • It can be used in soil with a low bearing capacity
  • It can be used to minimize settlement of the foundation
  • It can be used to reduce stress exerted to the founding soil
  • It can be used in soil having high moisture content
  • The foundation part can be used as basement floors
  • It can be used as an alternative for piles or other deep foundations for building structures

Disadvantages of floating foundation

  • The repair of foundation slabs is challenging.
  • The superstructure load is restricted to smaller loads
  • No. of stories and material choices are restricted to limited options like lightweight materials.
  • Requires extra care and highly skilled manpower and supervision

Design of Floating foundation

Floating foundations are designed by integrating the concept of floating with various types of raft foundations especially box type rafts.

Critical depth Dc of the foundation can be determined by using the Terzaghi equation for critical depth for depth greater than the width

Where,

Dc = Critical depth

Df = depth of foundation

s =undrained shear strength of soil = qu/2 =half of unconfined compressive strength determined from test

B, L dimension of foundation

γ = unit weight of soil

p= surcharge load

Nc = Bearing capacity factor by skempton

=5[1+0.2(Dt/B)]*[1+0.2(B/L)]

Above equation can be used to determine the maximum depth of excavation.

Happy Learning – Civil Concept

Contributed by,

Civil Engineer – Jenish Shakya

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