Foundation - Types & Design

Introduction :-

  Foundation placed below the ground level and are known as substructure. The superstructure is placed over the substructure all elements of superstructure like slab, beam, column,
staircase, transfer the loads and moments to the substructure foundation. Substructure transfer all the load come from superstructure to the underlying soil or rock. The substructure effectively supports the superstructure. However, all types of soil get compressed significantly and cause the structure to settle.

Foundation structures should be able to carry the applied loads, moments, forces and induced reactions without exceeding the safe bearing capacity of the soil.

The settlement of the structure should be as uniform as possible as and it should be within the tolerable limits.

Type of Foundation:-

 1.Shallow foundations:-

 Shallow foundations are used when the soil has sufficient strength within a short depth below the ground level. They need sufficient plan area to transfer the heavy loads to the base soil. Shallow foundation are usually loaded no more than 6ft below the lowest finised door. The different types of shallow foundations or footings are discussed below.

• Plain Concrete Pedestal Footing:-

Plain concrete pedestal footing are very economical. Plain concrete pedestral footing carry small loads from columns.

• Isolated Footing:-

Isolated footing under individual columns, these can be square, rectangular, or circular.

• Wall Footing:-

Wall footing is a continuous slab strip along the length of wall.

• Combined Footing:-

Combined footing support two or more columns. These can be rectangular or trapezoidal plan.

• Strap Footing:-

Strap footing are similar to combined footing when two isolated footing are combined by a beam with a view to sharing loads of both the column by the footing is known as strap footing and beam is called as strap beam.

• Mat & Raft Footing:-

This is a large continous footing supporting all the columns of the structure. This is used when soil conditions are poor but piles are not used.

 2. Deep Foundation:-

The shallow foundation need more plan area and shallow foundation may not be economical or even possible when the soil bearing capacity near the surface is too low. In those cases deep foundation are used to transfer loads to a stronger layer of rock strata. 

Design Of Foundation:-

Safe Bearing Capacity of Soil:-

The safe bearing capacity qc of soil is the permissible soil pressure considering safety factors in the range of 2 to 6 depending on the type of soil, approximations and assumptions and uncertainties. This is applicable under service load condition and, therefore, the partial safety factors λ f for different load combinations are to be taken from those under limit state of serviceability (vide Table 18 of IS 456). Normally, the acceptable value of qc is supplied by the geotechnical consultant to the structural engineer after proper soil investigations. The safe bearing stress on soil is also related to corresponding permissible displacement / settlement. Gross and net bearing capacities are the two terms used in the design. Gross bearing capacity is the total safe bearing pressure just below the footing due to the load of the superstructure, self weight of the footing and the weight of earth lying over the footing. On the other hand, net bearing capacity is the net pressure in excess of the existing overburden pressure. Thus, we can write Net bearing capacity = Gross bearing capacity - Pressure due to overburden soil (11.1) While calculating the maximum soil pressure q, we should consider all the loads of superstructure along with the weight of foundation and the weight of the backfill. During preliminary calculations, however, the weight of the foundation and backfill may be taken as 10 to 15 per cent of the total axial load on the footing, subjected to verification afterwards.

Depth of Foundation:-

All types of foundation should have a minimum depth of 50 cm as per IS 1080-1962. This minimum depth is required to ensure the availability of soil having the safe bearing capacity assumed in the design. Moreover, the foundation should be placed well below the level which will not be affected by seasonal change of weather to cause swelling and shrinking of the soil. Further, frost also may endanger the foundation if placed at a very shallow depth. Rankine formula gives a preliminary estimate of the minimum depth of foundation and is expressed as d = (qc/λ ){(1 - sinφ )/(1 + sinφ )}2 (11.2) where d = minimum depth of foundation qc = gross bearing capacity of soil λ = density of soil φ = angle of repose of soil Though Rankine formula considers three major soil properties qc, λ and φ , it does not consider the load applied to the foundation. However, this may be a guideline for an initial estimate of the minimum depth which shall be checked subsequently for other requirements of the design.

Design Considerations:- 

(a)       Minimum nominal cover (cl. 26.4.2.2 of IS 456)

(b)       Thickness at the edge of footings (cls. 34.1.2 and 34.1.3 of IS 456)

(c)       Bending moments (cl. 34.2 of IS 456)

(d)       Shear force (cl. 31.6 and 34.2.4 of IS 456)

1.  One-way shear (cl. 34.2.4 of IS 456)

2.  Two-way or punching shear (cls.31.6 and 34.2.4)

(e)       Bond (cl.34.2.4.3 of IS 456)

(f)         Tensile reinforcement (cl.34.3 of IS 456)

(g)       Transfer of load at the base of column (cl.34.4 of IS 456)

(h)       Nominal reinforcement (cl. 34.5 of IS 456)

Distribution of Base Pressure:-

i.            Concentrically loaded footings

ii.           Eccentrically loaded footings

:- Case (A): when | e | ≤ L/6

:- Case (B): when | e | > L/6

iii.         Unsymmetrical footings

= qa(x,y) = P/A ± {(MyIx - MxIxy)(x)/(IxIy - )} + {(M 2 xy I xIy - MyIxy)(y)/(IxIy - I2 xy)}

where, Mx = moment about x axis,

            My = moment about y axis,

            Ix = moment of inertia about x axis,

            Iy = moment of inertia about y axis,

            Ixy = product of inertia