Raft Foundations: Types, Design, and Applications

What is a Raft Foundation?

A raft foundation, also known as a raft footing foundation, is essentially a continuous slab resting on the soil that extends over the entire footprint of the building, thereby supporting the building and transferring its weight to the ground. 

When determining which foundation is the most economical (foundation), the engineer should consider the superstructure load, the subsoil conditions, as well as the desirable tolerable settlement.

Types of Foundation

In raft in civil engineering, the foundation of building foundations of bridges and buildings might be divided into two major categories as below.

1. Shallow foundations
2. Deep foundations.
   1. Spread Foundation (Isolated Footings),
   2. Wall Foundation, 
   3. Raft Foundation.

1. Shallow Foundation:

A typical shallow foundation is depicted in the raft foundation diagram as per below figure (a).

Shallow Foundation  & Deep Pile Foundation

If D1/B < 1, the foundations are called shallow foundations

1. In which pr = depth of foundation below ground level, &
2. B = width of foundation (last dimension).

Common types of shallow foundations include continuous wall footing, spread footing, grillage foundation, strap footing, combined footing, raft foundation, and so on.

All these are shown in per below figure. Analysis & all design and considerations of shallow foundations are discussed as per structure design.

Shallow foundations are thus used to spread the load/pressure coming from the column or superstructure (That is several times the safe bearing pressure of supporting soil) horizontally so it is transmitted at a level that the soil can safely support.

These are used when the natural soil at the site has a reasonable safe bearing capacity, acceptable compressibility, and the column loads are not very high.

2. Deep Foundations.

A typical deep foundation is shown in the above figure (b). If Df /B 1, the foundations are called deep foundations such as piles, drilled piers/caissons, well foundations, large-diameter piers, and pile raft systems.

Deep foundations are much like shallow foundations except the load coming from columns or superstructure is transferred into the soil vertically.

These are used when column loads are extremely large, the topsoils are weak, and the soils that have good strength and compressibility characteristics are in a reasonable depth below ground level.

Further, earth-retaining structures can also be classified under heavy foundations. Foundations can be categorized in terms of the materials used for their construction and/or fabrication, including raft foundation reinforcement details.

Usually, reinforced concrete (RCC) can be used for the construction of foundations. Brick Plain, stone, and concrete pieces can also be used for wall footing once the loads transmitted into the soil are relatively small.

Engineers also use other materials like steel beams and sections (like in grillage foundations and pile foundations), wood as piles (Such as temporary structures), steel sheets (Such as temporary retaining structures and cofferdams), and other composite materials.

Sometimes, these can also be encased in concrete depending on the load and strength requirements (Bowles, 1996; Tomlinson, 2001)

2.1. Spread Foundation (Isolated Footings),

All these are also known as spread footings. An isolated footing might be of a square, rectangular, or circular shape.

It might be axially loaded or eccentrically loaded. The design of square footings amounts to calculating the size and depth of the footing and the amount of main reinforcement and dowels.

The bending moments and shear forces are got in the critical sections, as discussed above. Rectangular footings could be used where space is restricted, and it might be impossible to provide a square footing.

The rectangular footing is also provided for rectangular columns or pedestals. The design of rectangular footings is equal to that of a square footing, except it needs to be designed in both directions separately.

Thus, bending moment and one-way shear actions have to be considered in both directions.

When the total area needed for the footing was determined, the dimensions of the footing has to be chosen such that the maximum bending moment on the equivalent of the two adjacent projections is equivalent, that is, the projections on all sides of this column should be equal.

For the design of a circular footing, that supports a circular column or pedestal, the expressions applicable to your circular slabs have to be used.

These are given in 1. The shape of the structures is circular in plan. 2. The floor of circular water tanks or towers. Alternatively, the circular (round) footing is replaced by an equal square footing that could be inscribed in its perimeter.

Then the design process of the footing is very similar to that of a square footing.

2.2. Wall Foundation

Wall footings carrying direct vertical loads might be designed either in plain concrete or in reinforced concrete. Since a wall footing deflects essentially in one way, it is analyzed by considering a strip of unit width and its length.

The critical sections for calculating maximum bending moments in different types of wall footing are given in according to the below figure.

Rafting Foundation

2.3. Raft Foundation

In the raft foundation section, under normal conditions, square and rectangular footings are economical for supporting walls and columns.

But, under certain conditions, it can be desired to construct a footing that supports a line of two or more columns.

All these footings are known as combined footings. When more than one (1) line of columns is supported by a concrete slab, it’s referred to as a raft foundation. also calculation mat calculation/ Raft calculation

Combined footings could be classified generally under the following categories:(Types of Raft Foundation)

1. Rectangular Combined Foundation
2. Trapezoidal Combined Foundation
3. Strap Foundation
4. Common Types of Raft Foundations

raft foundations are usually used with soil that has a low bearing capacity. A brief overview of the principles of combined footings is given in as per the below figure, followed by a more detailed discussion on raft foundations.

2.3.1. Raft Foundation: Rectangular Combined Foundation

In several instances, the load must be carried by means of a column and the soil bearing capacity will be such a typical spread footing design will need an expansion of the column foundation beyond the property line.

In that case, a few columns could be supported on a single rectangular foundation, as shown in as per below figure.

When the net allowable soil pressure is known, the size of this foundation ( B X L) could be determined in the following manner:

Determine the area of the foundation

A = (Q1+Q2)/qnet all———————(A)

• 1. Q1,Q2 = Column loads
  2. QNet all = Net allowable soil bearing capacity

Rectangular combined footing  /Rafting Foundation   

Determine the location of this result of the column loads. as per the above figure 

X = (Q2L3)/(Q1+Q2)———————(B)

To get a uniform distribution of soil pressure below the foundation, the resultant of the column loads must pass through the centroid of the foundation. Therefore,

L = 2(L2 + X)———————(C)

L = Lenght of the foundation

When the length L depends upon, the value can be obtained as follows:

L1 = L – L2 – L3———————(D)

Note:- that the magnitude of will be known and depends on the location of the property line

2.3.2. Raft Foundation: Trapezoidal Combined Foundation

Trapezoidal combined footing ( According to the below Figure ) is sometimes used as an isolated spread foundation of columns carrying large loads in which space is tight.

The size of this foundation that will uniformly distribute pressure on the soil can be obtained in another manner:

When the net allowable soil pressure is known, determine the area of the foundation:

A = (Q1+Q2)/qnet all———————(E)

Q1,Q2 = Column loads

qnet all = Net allowable soil bearing capacity

Trapezoidal combined footing /Rafting Foundation

A = (B1 + B2 )/2 X L———————(F)

Ascertain the location of the resultant for the column loads:

X = Q2 L3 / Q1 + Q2———————(G)

From the property of a trapezoid,

X + L2 = (B1 + 2B2) / (B1 + B2) X (L/3)———————(H)

With known values of A, L, X, and solve Eqs. (G) and (H) to obtain B1 and B2 Note that, for a trapezoid,

L/3 < X + L2 < L/2

Cantilever Footing -Strap Footing

2.3.3. Raft Foundation: Strap footing

Strap footing construction uses a strap beam to connect with an eccentrically loaded column foundation into the foundation of an interior column. (See per above figure).

Cantilever footings could be used in place of trapezoidal or rectangular combined footings if the allowable soil-bearing capacity is high and also the distances between the columns are large.

2.3.4. Raft Foundation: Common Types of Raft Foundations

The raft foundation, which is sometimes known as a raft foundation, is a combined footing that could cover the whole area under a structure supporting several walls and columns.

In raft in construction, raft foundations are sometimes preferred for soils that have low load-bearing capacities, but that is going to need to support high wall or column loads.

Under certain circumstances, spread footings would need to cover more than half of the building area, and Raft foundations may be more economical. In terms of raft foundation design, several types of raft foundations are used currently.

Some of the common ones are shown schematically in as below figure and include the following: Flat plate (As below figure, a). The mat is of uniform thickness. Flat plate thickened under columns (As below in Figure,b).

Beams and slab (As below figure,c). The beams run both ways and also the columns are located in the intersection of the beams. Flat plates using pedestals (As below in Figure,d).

Slab with basement walls as part of the mat (As below Figure,e). The walls act as stiffeners for the raft foundation 

Common Types of Rafting Foundation

Mats clouds be supported by piles, which help reduce the settlement of a structure constructed over highly compressible soil.

Comparison of isolated foundation and raft foundation (B = width, Df = depth)

In the place where the water table is high, mats are usually placed over piles to control buoyancy.

As the under figure shows the difference between also the depth Df and the width B of isolated foundations along with raft foundations. per below figure shows a flat-plate raft foundation under construction.

FAQs about Raft Foundations

What is a raft foundation?

A raft foundation, also known as a raft footing foundation, is a continuous slab that extends over the entire footprint of a building. It supports the building and transfers its weight to the ground.

When should you use a raft foundation?

Raft foundations are typically used in areas where the soil has low load-bearing capacity but needs to support high wall or column loads. They can be more economical than spread footings when spread footings would need to cover more than half of the building area.

What are the types of raft foundations?

Common types of raft foundations include:

• 1. Flat plate: Uniform thickness throughout.
  2. Flat plate thickened under columns.
  3. Beams and slab: Beams running both ways with columns at intersections.
  4. Flat plates using pedestals.
  5. Slab with basement walls as part of the mat.

How do you design a raft foundation?

The design of a raft foundation depends on factors such as soil conditions, building loads, and desired settlement tolerances. Engineers calculate the size and depth of the foundation, as well as the amount of reinforcement required.

What are the advantages of using a raft foundation?

Raft foundations distribute loads more evenly over a larger area compared to isolated footings, reducing the risk of differential settlement. They are suitable for areas with poor soil conditions and can be more cost-effective for large buildings.

Can raft foundations be used in high water table areas?

Yes, in areas with high water tables, raft foundations can be placed over piles to control buoyancy and reduce settlement. Piles help to stabilize the foundation and prevent it from floating.

How do you compare isolated footings with raft foundations?

Isolated footings support individual columns and have smaller widths compared to raft foundations, which support multiple columns and cover a larger area. Raft foundations are used when spread footings would cover more than half of the building area and are more economical in such cases.

What materials are commonly used for raft foundation construction?

Raft foundations are typically constructed using reinforced concrete (RCC). Other materials such as brick, stone, and steel may also be used depending on the design requirements and site conditions.