SETTLEMENT of FOUNDATIONS.The vertical downward movement of the base of a structure is called settlement and its effect upon the structure depends on its magnitude, its uniformity, the length of the time over which it takes place, and the nature of the structure itself.

Foundation settlement may be caused by some or a combination of the following reasons:

1.Elastic compressionof the foundation and the underlying soil.

2.Inelastic (or plastic) compressionof the underlying soils, which is much larger than the elastic compression. The inelastic compression can be predicted by the theory of consolidation.

3.Ground water lowering.Repeated lowering and rising of water level in loose granular soil tends to compact the soil and cause settlement of the ground surface. Lowering of water level in fine grained soils cause consolidation settlement. The major settlements in the city of Mexico has been due to ground water lowering, and due to this, the city has been called as the sinking city of Mexico.

4.Vibrationsdue to pile driving, blasting and oscillating machineries may cause settlement in deposits of granular soils.

5.Seasonal swelling and shrinkageof expansive clays.

6.Ground movement on earth slopes, such as surface erosion, slow creep or landslide.

7.Other causessuch as adjacent excavation, mining subsidence, underground erosion, etc.

A certain amount of elastic and inelastic settlement of foundations is unavoidable, and it should be taken into account in design. Provided the settlement is uniform over the whole area of the building and is not excessive, it does little damage. If, however, the amount of settlement varies at different points under the building, giving rise to what is known as relative or differential settlement; stresses will be set up in the structure.

It is suggested that the allowable pressure should be selected such that the maximum settlement of any individual foundation is 2.5 cm. It has also been suggested that the differential settlement of uniformly loaded continuous foundation and of equally loaded spread foundations of approximately the same size, is unlikely to exceed half the maximum settlement, and that normal structures such as office buildings and flats can satisfactorily withstand differential settlements of about 18 mm between adjacent columns spaced 6 to 8 m apart.

The recommendations of American Codes are based upon the simple logic that if the maximum total settlement is kept within a reasonable limit, the differential settlement will be only a fraction (generally about three-quarters of this limit), depending upon the type of structure and pattern of Ioading. The allowable maximum settlement values are given below:

According to Polshin and Tokar, brick masonry will crack (due to differential settlement) when the unit elongation amounts to 0.0005. Based on this criterion, the permissible differential settlement of brick walls is shown in Fig. 2.25, and is as follows

Where L is the wall length an H si the height of wall measured above the base of footing. The rate of differential settlement is defined as the slope or the relative settlement between two points divided by the horizontal distance.

FIG. 2.25 PERMISSIBLE DIFFERENTIAL SETTLEMENT FO BRICK WALLS.