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)].