Foundation Types and Uses (including full structural design of raft and pile foundations)

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Foundation can be defined as that part of the structure that transmits loads directly to the underlying soil. It is the most important part of the structural engineering system. Foundation may be broadly classified into two based on depth to breadth ratio (D/B) namely:

1. Shallow foundations: For this type, the D/B ≤ 1. Shallow foundations are always preferable for any type of structure wherever it is possible to use it because it is cheaper and takes less time and stress to install. Examples are pad foundation or bases, strip footing, strap footing, raft foundation etc
2. Deep foundations: For this type, the D/B ≥ 4, where D is often taken as L_p which means length of pile. Deep foundations are only used in situations where shallow foundations are not suitable such as foundations in collapsible soils, high rise buildings, foundations in made-up ground, water or water front structures etc. Examples of this are piles, drilled piers, drilled caissons etc

Types of shallow foundations explained

Strip Footing: This is a continuous strip of concrete designed to spread load from uniformly loaded walls to sufficient area of the soil. It supports mainly the load bearing walls and can sustain up to 2-storey building. The width of the strip is usually about 450mm. The thickness of the foundation depends on the strength of the foundation materials but must not be less than the projection of the foundation and in no case less than 150mm. There are classes under strip foundation that includes ordinary strip, wide strip, deep strip etc.

Figure 1: Strip footing

Wide strip footing

This type of footing is used in places where the bearing capacity of the ground is low such as in marshy areas or soft clay, silt or made up ground. The wideness of the strip is to reduce pressure on the poor soil arising from the load by spreading the load over wide area of the soil. Since H ˂ B as usually the case, transverse rebars are provided to withstand tensile forces that arise therefrom.

Note for strip footing: On slopy sites, it is an economical procedure to use stepped foundation. This helps to reduce the amount of excavation and backfill. The stepping is done to follow the line of ground surface and the depth of each step is usually 225mm. The lap of concrete at the step must not be less than the depth of the footing.

Deep strip footing

This class of strip footing is used in shrinkable clay soil to counteract variable soil conditions at different seasons. The depth of this footing is usually 900mm or more. Usually, the deeper the foundation thickness, the greater the resistance to fracture from unequal settlement.

Pad Foundation

This is an isolated foundation to support columns. The load of the structure is transmitted through the individual columns and the area of the foundation is determined by dividing the column load (P) plus the weight of the foundation (W) by the allowable bearing pressure (q_a). That is, A = (P + W)/q_a. The minimum footing thickness is 150mm.

Combined Pad footing

This consist of 2 or 3 columns sharing the same footing. Combined footings are used only when there is need for them and the conditions where they are used are:

1. When the adjacent columns are too close that their footings overlap, then the footings should be combined.
2. When one footing is too close to the boundary of a site that a single pad footing would be eccentrically loaded, then the footing should be combined with the footing of adjacent column. The aim of this is to avoid overturning forces or uplift that could result due to eccentricity of footing.
3. Where a framed building is to be erected alongside an existing structure, it is often necessary to use a cantilever or assymetrically combined base for column next to the existing building so that pressure on the soil due to the base may not so heavily surcharge the subsoil under the foundation of the existing building so as to cause appreciable settlement.

Combined footing can be formed of any shape as far as the basic requirements are satisfied. Rectangular, trapezoidal and T-shape are usually common.

Mat or Raft foundation

This type is suited for use on soil of low bearing capacity such as soft clay or silt and foundation of multi-storey framed buildings. If in such areas, the total area of the individual footings or strip required cover more than 50% of the projected area of the building, it is no longer economical to use single pad footing. Raft foundation should be used instead. It is common to form a raft foundation for the whole building rather than separate strip or pad footing for columns. Rafts act as a single entity thus avoiding relative settlement between the foundation of columns. The thickness of raft foundation is usually based on the design but values of 300mm are common.

Raft foundation is used in structures when

1. The allowable soil pressure is low
2. There is possibility of differential settlement
3. The loading is heavy
4. Spread footing would cover more than one half of the plan area

Types of raft foundation

There are many types of raft foundation. To know the suitable one to use, it is necessary to confirm that

1. The soil is not suitable for pad footing or other shallow foundations. This can be confirmed through soil tests.
2. Raft and not pile are the suitable type of foundation.
3. The structural configuration of the building is studied with key emphasis on column spacing.
4. The column loads are determined for all columns.

With this knowledge, the following types of raft applies:

(a) represents a true raft which is a flat concrete slab of uniform thickness throughout the entire area; this is suitable for closely spaced columns, carrying small loads.

(b) represents a raft with a portion of the slab under the thickened column; this provides sufficient strength for relatively large column loads.

(c) is a raft with thickened bands provided along column lines in both directions; this provides sufficient strength, when the column spacing is large and column loads unequal.

(d) represents a raft in which pedestals are provided under each column; this alternative serves the same purpose as (b).

(e) represents a two-way grid structure made of cellular construction and of intersecting structural steel construction

(f) represents a raft wherein basement walls have been used as ribs or deep beams.

Considerations in the structural design of raft foundations

The simplest form of raft foundation consists of reinforced concrete slab that supports the columns and walls of a structure and distributes the loads to the underlying soil.

The slab is designed as a column flat-slab floor supported without upmost deflection at the columns and walls. The soil pressure acting against the slab is commonly assumed to be uniformly distributed and equal to the total of all column loads, divided by the area of the raft. The moments and shears in the slabs are determined by the use of appropriate coefficients listed in the codes for the design of flat-slab floor.

On account of erratic variations in compressibility of almost every soil deposit, there are likely to be correspondingly erratic deviations of the soil pressure from the average value. Since the moments and shears are determined on the basis of the average pressure, it is considered good practice to provide the slab more reinforcement than the theoretical requirement and to use the same percentage of steel at top and bottom

The flat slab analogy is valid only if the differential settlement between columns is small and furthermore, if the pattern of the differential settlement is erratic rather than systematic.

Also, even if deep-seated or systematic settlements are negligible, the flat-slab analogy is likely to lead to uneconomical design unless the columns are more or less equally spaced and equally loaded. Otherwise, differential settlements may lead to substantial redistribution of moments in the slab. Under such circumstances, rafts are sometimes designed on the basis of the concept of the modulus of subgrade reaction, which implies that soil is considered to be analogous to a bed of closely and equally spaced elastic springs of equal stiffness in its stress-strain behaviour.

Find HERE typical design of raft foundation

Construction of raft foundations

Raft foundations are invariably constructed of reinforced concrete. They are poured in small areas such as 10 m × 10 m to avoid excessive shrinkage cracks. Construction joints are carefully located at places of low shear stress—such as the centre lines between columns. Reinforcements should be continuous across points. If a bar is spliced, adequate lap is provided. Shear keys may be provided along joints so that the shear stress across the joint is safely transmitted. If necessary, the raft may be thickened to provide sufficient strength at the joints.

Pile Foundation

This is used where the formation is bad but there exists a firm stable soil a little below the surface of the formation level. The bored piles carry the load through the soft soil or fill material to the firm strata. It is useful in expansive soil (soils that swells or shrinks such as montmorillonite or black cotton soils).

Find HERE the structural design of pile foundation

Reference

Venkatramiah, C. Geotechnical Engineering. 3^rd edition. New Age International (P) limited, Publishers, New Delhi, India.

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