FOUNDATION SELECTION CRITERIA FOR BUILDINGS

Selection criteria for foundation for buildings depend on two factors, i.e. factors related to ground (soil) conditions and factors related to loads from the structure. The performance of foundation is based on interface between the loadings from the structure and the supporting ground or strata. The nature and conditions of each of these varies, so, the selection of appropriate foundation becomes necessary for these variations depending on circumstances.

Selection of Foundation based on Ground Conditions:

The ground or soil condition is necessary for determining the type of suitable foundation. The soil on which the industrial, commercial or residential building rests may be stable, level and of uniform composition, but in some situations it may be otherwise.
Following are criteria for selecting suitable foundation based on soil condition:

• Where soil close to the surface is capable of supporting structure loads, shallow foundations can be provided.
• Where the ground close to surface is not capable of supporting structural loads, hard strata is searched for, and in some cases, it may be very deep, like in case of multi-storey buildings, where loads are very high. So, deep foundations are suitable for such cases.
• Field up ground have low bearing capacity, so deep foundation is required at that place, whereas uniform stable ground needs relatively shallow foundation.
• Level of the ground also affects foundation selection. If the ground is not levelled, and has gradient then step foundation may be preferred.

Selection of foundation based on Loads from Building:

The loading condition i.e. type and magnitude of loads, depends on the form and type of building to be constructed. In case of low rise building with large span, the extent of loading is relatively modest, so shallow foundation is preferred in this case. While high-rise building with short span has high loads. Therefore, deep foundation is required in such cases. Deep foundation is provided because ground at greater depth are highly compacted.
In case of framed structure multi-storey building, where loads are concentrated at the point of application, the use of pads and piles are common. Where, loads of the buildings are uniformly distributed, like from masonry claddings, the piles are not needed.

FOUNDATION CONSIDERATIONS BASED ON SOIL TYPES

Soil type affects selection of types of foundations, foundation depth and foundation sizes.
Following are the considerations for foundation based on soil types:

Soil Type	Foundation		Trouble areas
	Types	Reason for use
Sand	Footings	Easy to construct and economical	Bearing capacity may be a problem but in most cases it is sufficientExcessive settlement in wet and loose deposits. Confining pressure is usually low.
	Retaining Structures	Must be used since sand cannot support themselves
	Deep foundations (Piles)	Uses friction resistance but low in bearing capacity
Clay	Footings	Economic but may have problem with bearing capacity in saturated clays	Low bearing capacity.Generally low shear strength when wet. High consolidation in soft clays Swelling is possible. Over-consolidated clays may contain cracks and fissures.
	Retaining Structures	Clays are self-supportive up to a certain height (critical). Must be used if height increases beyond the critical.
	Deep foundations (Piles)	If bearing capacity is low, piles may be driven to rock. May change formation of clay.

 

Following table shows solutions to some problems in foundations based on soil types:

Soil Type	Nature of problem	Possible solution
Sand	Settlement	Loose sands must be compacted
		Lowering water table may result in sand densification
	Bearing Capacity	Compaction increases cohesion and friction thus bearing capacity increases
		Use of deep foundation
Clay	Consolidation	Lowering water table
		Pre-loading
		Drive pile to rock
	Bearing capacity	Compaction
		Use of deep foundations
	Expansion or swelling	Treat or stabilize soil
		Maintain constant water table
		Alter soil nature (similar to stabilization)
		Include swell pressure in design

18 M Long Foundation

DEPTH OF FOUNDATION ON DIFFERENT SOILS

The depth to which foundation should be taken depends on the following principal factors:
(a) Securing of adequate allowable bearing capacity,
(b) In the case of clayey soils, penetration into the soil has to be below the zone where shrinkage and swelling due to seasonal weather changes and due to trees and shrubs are likely to cause appreciable movements,
(c) In fine sands and silts penetration has to be below the zone in which trouble may be expected from frost,
(d) The maximum depth of scour whenever relevant (say, in bridge piers) should also be considered and the foundation should be located sufficiently below this depth, and
(e) It should be below the top soil, miscellaneous fill, tree roots etc.
All foundations should be taken down to a minimum depth of 0.5 m below natural ground level. In filled-up ground it may be necessary to go beyond the depth of fill or take special precautions. In such cases, it may be necessary for economic considerations to have the foundation at a higher level, and get the difference in level between the base of foundation and the level of excavation filled up with either:
(a) Concrete of allowable compressive strength not less than the allowable bearing pressure on the soil, or
(b) Incompressible fill material, for example sand, gravel etc. in which case the width of fill should be more than the width of foundation for dispersion of load on either side of the base of foundation.
In sloping grounds, the horizontal distance from the bottom edge of the footing to the ground surface shall be at least 60 cm for rock, and 90 cm for soil. A line drawn at an angle of 30″ to the base from the outer edge should not intersect the sloping surface (Figure 1).

Fig.1: Foundation on sloping ground

Foundation near Existing Building
The minimum horizontal distance between existing and new footings shall be at least equal to the width of the wider footing. In important cases, analysis of bearing capacity and settlement shall be carried out.
Foundation at Different Levels
In the case of footings on granular soil, the distance between the footings should be such that a line drawn between the lower adjacent edges of the footings shall not have a slope steeper than one vertical to two horizontal as shown in Figure 2.

Fig: Foundation on granular soils

In clayey soils, a line drawn between the lower adjacent edge of the upper footing and the upper adjacent edge of the lower footing shall not have a slope steeper than one vertical to two horizontal as shown in Figure 3.

Fig: Foundation on clayey soils