Settlement of foundation

Components of settlement of foundation Immediate settlement (or) initial settlement (or) elastic compression (or) distortion settlement(Si) Consolidation settlement (or) primary compression(Sc) Secondary consolidation settlement (or) secondary compression(Ss) Total settlement (S) = Si + Sc + Ss

Sources of settlement of foundation The load causing elastic compression of the foundation soil, giving rise to immediate or distortion settlement. The load causing plastic compression(giving rise to consolidation settlement, both primary and secondary). Ground water lowering Structural collapse of some soils such as saline non-cohesive soils, gypsum, silts and clays and loess, caused due to dissolution of materials responsible for intergranular bond of grains.

continued Vibrations (pile driving, blasting and oscillating machinery) Seasonal swelling and shrinkage of expansive clays. Surface erosion, creep or landslides in earth slopes. Miscellaneous sources (adjacent excavation, mining subsidence and underground erosion) The settlements from the first two sources only may be predicted with a fair degree of accuracy. The settlements due to all other sources cannot be predicted and only suitable measures need be take to reduce the settlement due to these sources.

Load for settlement analysis Dead loads weight of columns, walls, footings, foundations, the overlying fill but do not include the weight of the displaced soil Live loads depend on the use of structure and these loads may be taken from IS: 875-1964, ”Indian Standard Code of Practice For Structural Safety of Buildings, Loading Standards”.

continued Wind loads and seismic loads It should be considered wherever applicable however, if wind or seismic load is less than 25% of the combined DL and LL, it may be neglected in design and only DL and LL only considered. If wind or seismic load is more than 25% of the combined DL and LL, the foundation is designed such that pressure due to combination of DL, LL and wind(or seismic) loads does not exceed the ‘q’ by more than 25%.

General points For foundations resting on coarse-grained soils, the settlements should be estimated corresponding to the full DL, LL, wind (or seismic) Load. In such soils settlement occurs in a short time. For foundations resting on fine-grained soils, the settlements are estimated corresponding to permanent loads. All DL and the loads due to fixed equipment are taken as permanent loads. Generally, one half of the LL is also taken as permanent load. However engineering judgement is required to ascertain the permanent loads.

Immediate settlement of cohesive soils If a saturated clay is loaded rapidly, excess pore pressures are induced; the soil gets deformed with virtually no volume change and due to low permeability of the clay little water is squeezed out of voids. The vertical deformation due to change in shape is the immediate settlement. The immediate settlement of a flexible foundation, according to Terzaghi (1943) is given by:

continued -Es is determined from the stress-strain curve obtained from a triaxial consolidated-undrained test, with the consolidation pressure equal to the effective pressure at the depth from which the sample was taken. it is generally taken as the initial tangent modulus or the secant modulus. For normally consolidated clays, its value varies from 250c to 500c, and for over-consolidated clays it varies from 750c to 100c, where c is undrained cohesion of the foundation soil.

continued The value of the influence factor I for a saturated clay layer of semi-infinite extent can be obtained from below table.

continued If an incompressible layer exists at a limited depth below the footing, the actual settlement is less than that given by previous eqn. For such a case steinbrenner (1936) has given a soln. However, if the depth of the clay layer is more than 2B, the actual settlement would not change much. If the foundation is rigid such as heavy beam and slab raft, the settlement is about 0.8 times the settlement at the centre of the corresponding flexible foundation. It is approximately equal to the average settlement.

continued Eqn. Is applicable for the footing located at the surface. For the footings embedded in the soil, the settlement would be less than the computed values, and hence a reduction factor may be applied. According to Fox (1948) if Df=B, the reduction factor is about 0.75 and it is 0.5 for deep foundations (Df>B). However, most foundations are shallow. Further in the case of foundations located at large depth, the computed settlements are, in general, small and the reduction factor is usually not applied.

Immediate settlement of cohesionless soils It takes place rather quickly after the application of the load (due to of their high permeability's, the elastic as well as the primary compression effects occur more or less together, and the resulting settlement is ‘immediate settlement’). Because of the difficulty of sampling cohesionless soils, it is not possible to obtain the stress-strain characteristics of the in-situ soils As such in the case of cohesionless soils the settlements are generally determined indirectly by using SCPT or by using the charts developed by the SPT.

SCPT The soil layer is divided into small layers such that each small layer has approximately constant value of cone resistance. The avg. Value of the cone resistance of each small layer is determined. The immediate settlement of each small layer is estimated using the following eqn. Given by De Beer and Martens (1957)

The use of charts IS: 800-Part I-1976,”code of practice for calculations of settlement of shallow foundations subjected to symmetrical static vertical loading”, has provided a chart for the calculation of settlement per unit pressure (load per unit are or load intensity in kN/m2) as given next page. Thus knowing the N and the B, settlement per unit pressure may be obtained from which the settlement under any other pressure may be computed assuming that the settlement is proportional to the intensity of pressure.

Relationship between settlement per unit pressure and B for different values of N for cohesionless soils.

Consolidation settlement or primary compression It occurs in saturated clayey soils when these are subjected to sustained loads, because the excess pore water pressures cannot be dissipated immediately due to low permeability. The theory of one dimensional consolidation given by Terzaghi can be applied to determine the consolidation settlement as well as the time rate of dissipation of excess pore water pressures and hence the time rate of settlement.

The effective stress increment Δσ‾at the middle of the layer has to be obtained by using the theory of stress distribution in soil. The time rate of settlement or time factor Tv is given by the equation The time rate of settlement is dependent, in addition to other factors, on the drainage conditions of the clay layer.(single drainage or double drainage). In the case of foundation of finite dimensions, such as a footing resting on a thick bed of clay, lateral strain will occur and the consolidation is no longer 1-D. Lateral strain effects in the field may induce non-uniform pore pressures and may be one of the sources of differential settlement of foundation.

Permissible settlements It depends on the type of soil, the type of foundation and the structural framing system. IS:1904-1966 permits a max. settlement of 40 mm for isolated foundations on sand and 65 mm for isolated foundations on clay. Per. Sett. Is higher for clays because progressive settlements on clayey soils permit better strain adjustments in str. Member. In raft foundation on sand is 40 mm to 65 mm and for raft foundation on clay is 65 mm to 100 mm.

continued The per. Settlement for raft foundation > isolated foundation because the raft bridges over soft patches of the soil, and the differential sett. are reduced. The max. Permissible differential settlement is 25 mm for foundations on sand and 40mm for foundations on clay. The angular distortion in the case of large framed str. Must not exceed 1/500 normally and 1/1000 if all kinds of minor damages are also to be used prevented.

qna for cohesionless soils qna is limited either by qns or qnp . In this case it is generally governed by the qnp, because in the case of footings of usual size the qns is quite high. However, in this case of narrow footings on water-logged sands the qns may be the controlling criterion for the design. Qna is usually determined by plate load test or by using empirical relationships based on SPT For small boulders and stones use plate load test and for homogenous soil use SPT

continued As indicated below a no. of empirical relationships based on N have been given by various investigators for the qnp, which may be used as the qna for designing footings on cohesionless soils. Peck’s equation Teng’s equation Meyerhof’s equation Bowel’s equation IS:6403-1971 equation

Peck’s equation Terzaghi and peck (1967) gave charts for determining allowable bearing pressure to limit a max. Settlement of 25 mm and diff. Settlement of 19 mm for footings of different sizes on cohesionless soils. Peck (1974) revised the curves of Terzaghi and peck to take into consideration the later research, and gave the following eqn. For safe settlement pressure.

continued

IS: 6403-1971 IS: 6403-1971 has given the following eqn. For the safe settlement pressure,

The use of charts For a footing of known width and known N the allowable bearing pressure to limit a max. Settlement of 25mm and 40 mm may be obtained from below charts.

qna for cohesive soils It is generally controlled by net safe bearing capacity, although settlement criterion may be the controlling factor in the case of soft clays. For firm to stiff clays it is not necessary to compute settlements, especially when the str. are lightly loaded as the settlements are quite small. A F.O.S of 3 against shear failure would generally ensure that the settlements are within the safe limits. However it is essential the consolidation settlement in all cases of heavy str.

continued qns depends on the shear strength of the clay. The min. Value of the shear strength should be taken for the computations of qu. On sat. Clays, the time of loading is relatively rapid, and the undrained conditions usually apply. The drained cohesion c is determined from the vane-shear test. The qu is determined using the bearing capacity theories for ϕ=0 condition. For computing settlement, the size of footing is first selected considering the qns and then the settlements are checked. It may, however, be noted that the width of the footing has very little effect on the magnitude of the settlement. The settlement depends mainly on the magnitude of the load. Consequently, if settlements control the design, merely increasing the size of the footing may not help to solve the pbm. In such cases an alternative type of foundation such as a mat foundation, a pile foundation, etc, may be adopted.