Settlement of foundation

1.1: Settlement

           It is a vertical displacement of soil from its original level of placement. It may be uniform or non-uniform. When a part of the structure settles leaving the other part of structure then it is called differential settlement.

1.2: Causes of settlement

      Following are the causes of the settlement of foundation:-

1)Static load on soil.

2)Dynamic forces from vibration excited by masonry, traffic, pile driving operation, earthquake and various impact of soil.

3)Low bearing capacity of the soil.

4)Effect of the groundwater table.

5)Non-uniform structure.

6)Milling and tunnelling operation

1.3: Type of settlement

         Structure rigidity, soft soil condition and the load to be transmitted by the foundation affect settlement, the settlement is categorised into three main branches they are:-

1)Uniform settlement

2)Tilt settlement

3)Angular distortion

Image result for uniform settlement

 1)Uniform settlement

            A settlement is a uniform if all parts of the structure undergo equal settlement. Uniform settlement occurs under all structure supported by a very rigid raft foundation. If the occurred settlement is uniform; the structural failure will not take place. However, if the settlement is very excessive its function is impaired.

2)Tilt settlement       

               When an entire structure rotates due to non-uniform settlement, Tilt settlement takes place. If tilt exceeds a certain limit the structure could fail in several structures.

3)Angular distortion/ Non-uniform settlement

             When two foundation supporting columns wall settles unequally, the structure will be subjected to angular distortion. If angular distortion exceeds a certain limit the structure will fail in several ways. If (δ) is the difference between two foundations separated by a distance L, the angular distortion is given by

                         angular distortion =Pmax-Pmin/L = δ/L

1.4:Process of settlement

           Settlement process generally occurs in three ways:-

1)Immediate settlement

         This type of settlement takes place in a short time (about a week) after application of load and due to elastic distortion of the soil. As the settlement is experienced in a short time,  there will not be enough time for soil mass for changing its water content. Compare to consolidation settlement particularly in saturated clay. This type of settlement is very small as often neglected unless the structure is very important.

2)Consolidation 

        This type of settlement is caused due to the expulsion of pore water from the voids of a saturated soil mass under the prevailing excess hydrostatic pressure induced by load imposed on a foundation.

3)Secondary settlement

       This type of settlement takes place under constant effective stress. When consolidation is completed the absorbed water is drained out and plastic re-adjustment of soil occurs. Consequently, the soil mass is settlement is also negligible as compared to consolidation.

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