For the satisfactory design of foundations, the following information is necessary:
a) Type and condition of the soil or rock to which the foundation transfers the loads;
b) General layout of the columns and load-bearing walls showing the estimated loads, including moments and torques due to various loads (dead load, imposed load, wind load, seismic load) coming on the foundation units;
c) Allowable bearing pressure of the soils;
d) Changes in ground water level, drainage and flooding conditions and also the chemical conditions of the subsoil water, particularly with respect to its sulphate content;
e) Behaviour of the buildings, topography and environment/surroundings adjacent to the site, the type and depths of foundations and the bearing pressure assumed; and
f) Seismic zone of the region.
7.2Design Considerations
7.2.1 Design Loads
The foundation shall be proportioned for the following combination of loads:
a) Dead load + Imposed load; and
b) Dead load + Imposed load + Wind load or seismic loads, whichever is critical.
For details, reference shall be made to Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code.
NOTE — For shallow foundations on coarse grained soils, settlements shall be estimated corresponding to 7.2.1 (b) and for foundations on fine grained soils, the settlement shall be estimated corresponding to permanent loads only. Permanent loads shall be in accordance with good practice [6-2(10)].
7.2.2 Allowable Bearing Pressure
The allowable bearing pressure shall be taken as either of the following, whichever is less:
a) The safe bearing capacity on the basis of shear strength characteristics of soil, or
b) The bearing pressure that the soil can take without exceeding the permissible settlement (see 7.2.3).
7.2.2.1 Bearing capacity by calculation
Where the engineering properties of the soil are available, that is, cohesion, angle of internal friction, density, etc, the bearing capacity shall be calculated from stability considerations of shear; factor of safety of 2.5 shall be adopted for safe bearing capacity. The effect of interference of different foundations should be taken into account. The procedure for determining the ultimate bearing capacity and bearing pressure of shallow foundations based on shear and allowable settlement criteria shall be in accordance with good practice [6-2(11)]. Depth factor correction is to be applied only when backfilling is done with proper compaction.
7.2.2.2 Field method for determining allowable bearing pressure
Where appropriate, plate load tests can be performed and allowable pressure determined as per accepted standard [6-2(12)]. The allowable bearing pressure for sandy soils may also be obtained by loading tests. When such tests cannot be done, the allowable bearing pressure for sands may be determined using penetration test.
7.2.2.3 Where the bearing materials directly under a foundation overlie a stratum having smaller safe bearing capacity, these smaller values shall not be exceeded at the level of such stratum.
7.2.2.4 Effect of wind and seismic force
7.2.2.4.1 Where the bearing pressure due to wind is less than 25 percent of that due to dead and imposed loads, it may be neglected in design. Where this exceeds 25 percent, foundations may be so proportioned that the pressure due to combined dead, imposed and wind loads does not exceed the allowable bearing pressure by more than 25 percent.
7.2.2.4.2 When earthquake forces are considered for the computation of design loads, the permissible
increase in bearing pressure of pertaining soil shall be as given in 5.3.3.5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code, depending upon the type of foundation of the structure and the type of soil.
7.2.2.5 Bearing capacity of buried strata
If the base of a foundation is close enough to a strata of lower bearing capacity, the latter may fail due to excess pressure transmitted to it from above. Care should be taken to see that the pressure transmitted to the lower strata is within the prescribed safe limits. When the footings are closely spaced, the pressure transmitted to the underlying soil will overlap. In such cases, the pressure in the overlapped zones will have to be considered. With normal foundations, it is sufficiently accurate to estimate the bearing pressure on the underlying layers by assuming the load to be spread at a slope of 2 (vertical) to 1 (horizontal).
7.2.3 Settlement
The permissible values of total and differential settlement for a given type of structure may be taken as given in Table 3. Total settlements of foundation due to net imposed loads shall be estimated in accordance with good practice [6-2(13)]. The following causes responsible for producing the settlement shall be investigated and taken into account.
a) Causes of settlement
Elastic compression of the foundation and the underlying soil,
Consolidation including secondary compression,
Ground water lowering — Specially repeated lowering and raising of water level in loose granular soils tend to compact the soil and cause settlement of the footings. Prolonged lowering of the water table in fine grained soils may introduce settlement because of the extrusion of water from the voids. Pumping water or draining water by tiles or pipes from granular soils without an adequate mat of filter material as protection may, in a period of time, carry a sufficient amount of fine particles away from the soil and cause settlement.
Seasonal swelling and shrinkage of expansive clays.
Ground movement on earth slope, for example, surface erosion, slow creep or landslides.
Other causes, such as adjacent excavation, mining, subsidence and underground erosion.
PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS
Table 3Permissible Differential Settlements and Tilt (Angular Distortion) for Shallow Foundations in Soils (Clause 7.2.3)
Sl No.
Type of Structure
Isolated Foundations
Raft Foundations
Sand and Hard Clay
Plastic Clay
Sand and Hard Clay
Plastic Clay
Maximum Settlement
Differential Settlement
Angular Distortion
Maximum Settlement
Differential Settlement
Angular Distortion
Maximum Settlement
Differential Settlement
Angular Distortion
Maximum Settlement
Differential Settlement
Angular Distortion
(1)
(2)
mm (3)
mm (4)
(5)
mm (6)
mm (7)
(8)
mm (9)
mm (10)
(11)
mm (12)
mm (13)
(14)
i)
For steel structure
50
.003 3L
1/300
50
.003 3L
1/300
75
.003 3L
1/300
100
.003 3L
1/300
ii)
For reinforced concrete structures
75
.001 5L
1/500
75
.002L
1/500
75
.003 3L
1/300
125
.003 3L
1/300
iii)
For multistoreyed buildings:
a) RC or steel framed buildings with panel walls
60
.002L
1/500
75
.003L
1/300
75
.002 5L
1/300
125
.003 3L
1/300
b) For load bearing walls
1) L/H=21)
60
.000 2L
1/5 000
60
.000 2L
1/5 000
← Not likely to be encountered →
2) L/H=71)
60
.000 4L
1/2 500
60
.000 4L
1/2 500
iv)
For water towers and silos
50
.001 5L
1/666
75
.001 5L
1/666
100
.002 5L
1/400
125
.002 5L
1/400
1) For intermediate ratios of L/H, the values can be interpolated.
1 The values given in the table may be taken only as a guide and the permissible total settlement/different settlement and tilt (angular distortion) in each case should be decided as per requirements of the designer.
2 L denotes the length of deflected part of wall/raft or centre-to-centre distance between columns.
3 H denotes the height of wall from foundation footing.
b) Causes of differential settlements
Geological and physical non-uniformity or anomalies in type, structure, thickness, and density of the soil medium (pockets of sand in clay, clay lenses in sand, wedge like soil strata, that is, lenses in soil), an admixture of organic matter, peat, mud;
Non-uniform pressure distribution from foundation to the soil due to non-uniform loading and incomplete loading of the foundations;
Water regime at the construction site,
Overstressing of soil at adjacent site by heavy structures built next to light ones;
Overlap of stress distribution in soil from adjoining structures;
Unequal expansion of the soil due to excavation for footing;
Non-uniform development of extrusion settlements; and
Non-uniform structural disruptions or disturbance of soil due to freezing and thawing, swelling and softening and drying of soils.
7.2.4 Shallow Foundations on Rocks
Estimation of the safe bearing pressures of rocks for shallow foundations based on strength, allowable settlement and classification criteria; and also design and construction of shallow foundations on rocks shall be carried out in accordance with the good practice [6-2(14)].
7.3Pad or Spread and Strip Foundations
7.3.1 In such type of foundations, wherever the resultant of the load deviates from the centre line by more than 1/6 of its least dimension at the base of footing, it should be suitably reinforced.
7.3.2 For continuous wall foundations (plain or reinforced) adequate reinforcement should be provided particularly at places where there is abrupt change in magnitude of load or variation in ground support.
7.3.3 On sloping sites the foundation should have a horizontal bearing and stepped and lapped at changes
of levels for a distance at least equal to the thickness of foundation or twice the height of step, whichever is greater. The steps should not be of greater height than thickness of the foundations.
7.3.4 Ground Beams
The foundation can also have the ground beam for transmitting the load. The ground beam carrying a load bearing wall should be designed to act with the wall forming a composite beam, when both are of reinforced concrete and structurally connected by reinforcement. The ground beam of reinforced concrete structurally connected to reinforced brick work can also be used.
7.3.5 Dimensions of Foundation
The dimensions of the foundation in plan should be such as to support loads as given in good practice [6-2(10)]. The width of the footings shall be such that maximum stress in the concrete or masonry is within the permissible limits. The width of wall foundation (in mm) shall not be less than that given by:
B=W+300
where
B= width at base, in mm; and
W= width of supported wall, in mm.
7.3.6 In the base of foundations for masonry foundation it is preferable to have the steps in multiples of thickness of masonry unit.
7.3.7 The plan dimensions of excavation for foundations should be wide enough to ensure safe and efficient working in accordance with good practice [6-2(9)].
7.3.8 Unreinforced foundation may be of concrete or masonry (stone or brick) provided that angular spread of load from the base of column/wall or bed plate to the outer edge of the ground bearing is not more than 1 vertical to 1/2 horizontal for masonry or 1 vertical to 1 horizontal for cement concrete and 1 vertical to 2/3 horizontal for lime concrete. The minimum thickness of the foundation of the edge should not be
less than 150 mm. In case the depth to transfer the load to the ground bearing is less than the permissible angle of spread, the foundations should be reinforced.
7.3.9 If the bottom of a pier is to be belled so as to increase its load carrying capacity such bell should be at least 300 mm thick at its edge. The sides should be sloped at an angle of not less than 45∘ with the horizontal. The least dimension should be 600 mm (circular, square or rectangular). The design should allow for the vertical tilt of the pier by 1 percent of its height.
7.3.10 If the allowable bearing capacity is available only at a greater depth, the foundation can be rested at a higher level for economic considerations and the difference in level between the base of foundation and the depth at which the allowable bearing capacity occurs can be filled up with either: (a) concrete of allowable compressive strength not less than the allowable bearing pressure, (b) incompressible fill material, for example, sand, gravel, etc, in which case the width of the fill should be more than the width of the foundation by an extent of dispersion of load from the base of the foundation on either side at the rate of 2 vertical to 1 horizontal.
7.3.11 The cement concrete foundation (plain or reinforced) should be designed in accordance with Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code and masonry foundation in accordance with Part 6 ‘Structural Design, Section 4 Masonry’ of the Code.
7.3.12 Thickness of Footing
The thickness of different types of footings, if not designed according to 7.2, should be as given in Table 4.
7.3.13 Land Slip Area
On a sloping site, spread foundation shall be on a horizontal bearing and stepped. At all changes of levels, they shall be lapped at the steps for a distance at least equal to the thickness of the foundation or twice the
Table 4 Thickness of Footings(Clause 7.3.12)
SI No.(1)
Type of Footings(2)
Thickness of Footings, Min(3)
Remarks(4)
i)
Masonry
a) 250 mmb) Twice the maximum projection from the face of the wall
Select the greater of the two values
ii)
Plain concrete:
a) For normal structures
a) 200 mmb) Twice the maximum offset in a stepped footingc) 300 mm
—For footings resting on soilFor footings resting on top of the pileResting on soilResting on pile
b) For lightly loaded structures
a) 150 mmb) 200 mm
iii)
Reinforced concrete
a) 150 mmb) 300 mm
Resting on soilResting on pile
PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS
height of the step, whichever is greater. The steps shall not be of greater height than the thickness of the foundation unless special precautions are taken. On sloping ground on clay soils, there is always a tendency for the upper layers of soil to move downhill, depending on type of soil, the angle of slope, climatic conditions, etc. Special precautions are necessary to avoid such a failure.
7.3.14 In the foundations, the cover to the reinforcement shall be as prescribed in Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code for the applicable environment exposure condition.
7.3.15 For detailed information regarding preparation of ground work, reference shall be made to good practice [6-2(15)].