What is Raft foundation?
Raft foundation is the monolithic RCC slab of uniform thickness provided throughout the entire area below the superstructure, which supports all arrangement of wall and column. Thus, it acts as a structural mat, which supports the above structure, therefore, popularly also known as Mat foundation.
Raft foundation is a type of shallow foundation, so they are not installed to the higher depth. One can simply analyze it as an extension of individual footing as; the area of individual footing is increased for decreasing the bearing capacity.
This mat foundation is formed by integrating two or more individual foundations. Due to the very large width, the bearing capacity of such a foundation is very high so that shear fails or bearing failure does not occur.
Why and where to use mat foundation?
As explained, they are extensional of individual footing, these foundation are generally economical to provide when the isolated footing area is more than 50% of the total area of structure. The structure where the wall and column are so close footing touches each other, mat foundation is prefer.
In addition, from the point of geotechnical-engineering such foundation should be used where the allowable bearing capacity of the soil is low. So that with the help of a large base area of such foundation, stress to the ground is reduced compared to a small area of the individual footing and pile footing.
The mat foundation is moreover very useful for equal settlement and prevents differential settlement. Thus, Raft foundation is used where there is Marshy land, softened soil made due to sanitary fill, debris, unconsolidated soil, and sandy soil.
Raft foundation are extremely useful to create basement apartment in structure as, the whole area, which cover the structure, is excavated.
Features of Raft foundation
The subsoil layer of the earth, which does not have adequate bearing capacity is excavated for the construction of the Raft foundation. Thus, the overburdened mass over the stable soil is reduced and net stress over soil strata is decreased. Raft foundation thus generally has a feature of partially compensated or fully compensated foundation.
The compensated foundation also known as a floating foundation is that foundation in which the weight of the building is approximately equal to the full weight of soil excavated for the foundation.
This generally only happens in the densely compacted soil, as the mass of such excavated soil is very high. Due to this, the structure can contribute the stress without any action to the bottom strata of soil as it was already applied due to overburden excavated subsoil.
The pressure bulb of such foundation is deep compare to the other as its width is very large. Thus, the loose soil mass under the raft is acted as a monolithic evenly distributed mass. Due to this only smaller differential settlement occurs.
The settlement up to 50mm is allowed along with consideration of permissible limit of differential settlement up to 19mm.
Types of Raft foundation
The raft foundation is generally divided on the basis of two categories. They are;
On the basis of support on
Raft on soil: The raft foundation is directly erected over the top layer of excavated and compacted layer of soil.
On pile: In this, the raft foundation is constructed over the cap of the pile foundation. The soil which is very fragile on top and over which massive structure is needed to be built, at such conditions raft is built over the pile.
On the basis of structural component
Flat plate: It is a single slab foundation of uniform thickness provided throughout the area covered by the structure. This kind of mat foundation is used when loads from the superstructure are comparatively low. Also, used when the spacing column is small.
Flat plate slab thickened under column: the slab thickness under the column base is provided more in order to reduce the negative bending moment and diagonal shear in the foundation.
Two-way beam and slab: in such mat foundation, beams are provided along the joining the base of the column in all directions, and above the beam, the slab of uniform thickness is provided as a mat.
Such a beam is generally called inverted beams, and the column is located at their intersection. Such foundation provides enough strength when column spacing is large and suitable when the column is not equally loaded eccentric loading over the structure.
The rigid frame (Box structure frame): In such a foundation, the box structure is provided along with the basement wall as stiffener forming a hollow compartment in the mat.
The boxes are cellular or like a rigid frame having slabs, basement shear walls. Such kind of mat foundation is used for high bending stress reduction. And also such a foundation reduces settlement and helps to control upthrust.
Bearing capacity and settlement of mat foundation
The bearing capacity and settlement for Mat foundation are different for the cohesionless soil like sandy loam soil and cohesive soil like clay. So they are briefly described as;
1) In Cohesion less soil
Bearing capacity of foundation in cohesion less soil depends upon width of foundation. As width of mat foundation is very large, bearing capacity due to shear strength is high. And can be find out by Terzaghi theory. So for the mat foundation, the safe settlement governs the design. The safe bearing capacity can be find out by Teing’s method which is depended on SPT(Standard proctor test) number and valid only for 5<N<50
If N is smaller than 5 mat foundation cannot bear the load. i.e. Bearing capacity is so small the we need deep foundation
qns = 0.22*N²*B*w2+0.67(N²+100)Df*w1———Eq 2
qns is net safe pressure,
N is SPT number,
Df is depth of foundation,
And w1 & w2 is water table correction factor.
For safe settlement pressure of settlement 25mm is considered,
qnp = 17.5*(N-3) w2———-Eq 2
To design mat foundation allowable bearing capacity is used to the smaller value from the Eq 1 and Eq 2.
Another equation to find out safe settlement pressure which is known as Bowel’s equation
qnp = 12.2N(B+0.3/B)²* Rd*w2————-Eq4
Rd =1+ 0.33 Df/B
In general qnp=12.2N Rd w2 (s/25)———Eq 5
As B is so large, the term (B+0.3/B) is nearly equal to 1.
Where s is settlement.
2) In Cohesive soil
The net ultimate bearing capacity of foundation is cohesive soil is determined by Skempton’s equation
qnu = Nc[1+0.2(Dt/B)][1+0.2(B/L)]*Cu – – – – – – – – – Eq 6
where Nc is taken as 5
So, (Safe bearing capacity) qns = qnu/FOS
Minimum FOS is taken as 2.5
Settlement on the clay are generally consolidate settlement except on over consolidated clay. Generally, the design of mat foundation should be equal to allowable settlement pressure generally 25mm of settlement
Design of the Raft foundation
Mat foundation must be designed sufficiently safe to shear failure. The settlement should be within the allowable limits. Different methods and approaches are there in order to design the raft foundation on the basis of the assumption made and accuracy. They are,
1) Rigid beam method (conventional method)
In this method, the slab is assumed infinitely rigid on comparison with sub soil. The flexural deflection are not supposed to do any effect on contact pressure distribution of mat. And pressure distribution coincide with line of action of the resultant force of all loading acting on it.
2) Simplified Elastic method
This method is devised on the assumption that soil behaves as an infinite number of individual independent elastic springs, which absorb tension as well as compression. This method was created by Winkler.
Thus the method accounts for the elasticity property of footing and the limitations of this method are that actual soil doesn’t act as it was hypothetically assumed.
3) Elastic method
In this method, the assumption made is that soil is considered as homogeneous, isotropic, and linearly elastic half-space. The method is based on the elasticity theory of soil.
As we know elasticity theory is an ideal theory, thus this method also gives only the approximate result. Further, the method is more complicated and liable to design.
4) Non-linear Elastic method
Here the soil is considered as a non-linear elastic material. This method is based on actual behavior and property of soil.
Different numerical and computational techniques and approaches such as the Finite Element Method are used. Thus it can be easily and directly implemented on computer software for design. For learning purposes, we generally adopt a conventional design approach.
The general procedure for it is as explained below :
2. Determine the contact pressure
a. If resultant force passes through center of foundation,
q = Q/A
or, qavg = Q/A
c. Divide raft into the number of beams. Each beam should contain the column load
d. Draw SFD and BMD for each stripe
e. Determine the modified column load.
h. Modify every Coulomb’s load modification factor. And find out the bearing capacity of the soil. The modification of column load is done by multiplying q1, q2, q3, etc by Fc for respective stripe.
Simple Numerical of Raft design
A square footing is to be designed to carry a load of 500KN, if the depth of foundation is (Df)1.5 m, then for the suitable size of mat foundation with factor safety of 3 can be determined if all the necessary data of field condition are provided
(assuming water table is at base of foundation)
If, ф = 25,
ϒ = 16KN/m3
Advantages of raft foundation
- The raft foundation is shallow so it doesn’t require more excavation.
- Raft foundation can provide the provision of a basement compartment in the building.
- Raft foundation is extremely advantageous to the soil which has less bearing capacity.
- Raft foundation has a larger base area so reduces the pressure intensity to ground strata.
- Raft foundations are economic in the sense that the slab of raft can be directly used without any other concrete work.
- Raft foundation reduces differential settlement.
Disadvantages of raft foundation
- It is costlier than individual foundation
- The unnecessary area where there is no significant stress, there also the reinforcement is used.
- There is a probability of uplift of structure when the groundwater table rises making nearby soil with more pore water pressure.
- Waterproofing and proper management of water in the basement is difficult.
- Edge erosion can occur.
I hope this article remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha