Causes of settlement, foundation loading and computation

Causes of Settlement, Foundation Loading and Computation of Design Loads

SETTLEMENT UNDER LOADS

Occurs immediately after the construction of structure.

Also known as ‘Distortion Settlement’, it occurs due to distortion in foundation soil.

Although its not truly elastic, it is computed using elasticity theory. It is denoted by Si.

Occurs due to gradual dissipation of pore pressure induced by external loading and consequently expulsion of water from the soil mass, hence volume change.

Important for Inorganic clays. This component is determined using Terzaghi’s theory of consolidation.

It is denoted as Sc.

This settlement occurs after completion of the primary consolidationOccurs at constant settlement effective stress with volume change due to rearrangement of particles.This component of the settlement is due to secondary consolidation. It is significant for organic soils.

Determined from the coefficient of secondary consolidation. Denoted as Ss

For any of the mentioned settlement calculations, we first need vertical stress increase in soil mass due to net load applied on the foundation.

The total settlement (S) is given as

S = Ss + Sc + Si

Underground erosion may cause formation of cavities in the subsoil which when collapse cause settlement.

Structural collapse of soil may cause to dissolution of material responsible for the inter-granular bond of grains.

Temperature change causes shrinkage in expansive soils due to which settlement occurs.

Frost heave occurs if the structure is not founded below the depth of frost penetration. When thaw occurs, settlement occurs.

Vibrations and Shocks cause large settlement, especially in loose and cohesionless soils.

Mining subsidence of ground may occur due to removal of minerals and other materials from mines below.

Land slides occur on unstable slopes, there may be serious settlement problem.

If there are changes due to construction of a new building near the existing foundation, the settlement may occur due to increase in the stress.

Dead loadLive load Wind loadSnow loadEarth pressureWater pressureEarthquake load

Dead load : The dead load include the weight of materials permanently fixed to the structure, such as beam, column, floors,walls and fixed service equipment.

Can be calculated if sizes and types of structural materials are known.

There is a problem in estimating the self weight of the structure because self weight is initially assumed and the structure is designed.

Then weight is calculated from the designed dimension and compared with the assumed weight.

Live load :The live loads are the movable loads that are not permanently attached to the structure.

These loads are applied during a part of its useful life.

Loads due to people, goods, furniture, equipment etc. are considered in live loads.

It is difficult to estimate the live load accurately. These are specified by local building codes.

Wind load :Wind loads acts on all exposed surfaces of the structure.This loads depends on the velocity of wind and type of the structure.Wind loads are specified by building codes.

Snow loads :Snow loads are occur due to accumulation of snow on roofs and exterior flat surfaces in cold climates.The unit weight of snow load is usually taken as 1kN/sq m

Earth pressure :Earth pressure produce lateral force against the structure below the ground surface or fill surface.The earth pressure is normally treated as dead load.

Water pressure :Like earth pressure water, water pressure also produces a lateral force against the structure below the water level.

Earthquake pressure :The force due to earthquake may be vertical, lateral or torsional on a structure in any direction.

Dead load and live load are computed by tributary area method.In tributary area method, it is assumed that a column carries all the load in the floor area enclosed by lines equidistant from its adjacent columns.Live loads are temporary and transient. A part of it may act for a duration that may induced settlement, especially for cohesive soil.Live loads are referred from IS-875.

It is assumed that wind load and earthquake load do not act simultaneously.

According to IS 1904-1978 foundation should be proportioned for the combination

dead load + live load anddead load + live load + wind load or

seismic load

If the wind load is less than 25% of the total Dead load + Live Load, then it may be neglected and it is designed for the combination of Dead load and live load only.

The Foundation pressure should not the exceed the safe bearing capacity by more than 25% for the combination of dead load, live load and wind load.

Settlement under loadsSettlement due to other causes