Consolidation settlement

1. Consolidation Settlement

2. When a saturated soil is loaded externally, What is Consolidation? saturated clay GL the water is squeezed out of the soil and the soil shrinks, over a long time may be up to several years depending upon the permeability of the soil this whole phenomena is called consolidation.

3. As the time passes the water comes out of the soil voids and so the volume of soil decreases, this is the cause of the settlement. Concept of Consolidation The settlement is directly proportional to the decrease in volume of soil= The volume of squeezed out water

4. Settlement is the vertically downward movement of structure due to the shrinkage in volume of soil due to consolidation. Settlement due to consolidation

5. This leads to settlements occurring over a long time, Settlement vs. Time which could be several years, depending upon the permiability of the soil.

6. It occurs due to the process of consolidation. Clay and Organic soil are most prone to consolidation settlement. Consolidation is the process of reduction in volume due to expulsion of water under an increased load. It is a time related process occurring in saturated soil by draining water from void. It is often confused with Compaction. Consolidation theory is required to predict both rate and magnitude of settlement. Consolidation Settlement

7. Since water flows out in any direction, the process is three dimensional. But, soil is confined laterally. Hence, vertical one dimensional consolidation theory is acceptable. Spring analogy explains consolidation settlement. Permeability of soil influences consolidation.

8. Equating the two expressions for average vertical strain, Settlement-Void ratio Relation consolidation settlement initial thickness of clay layer initial void ratio change in void ratio

9. Settlement computations  =q q kPa H Two different ways to estimate the consolidation settlement: (a) using m v (b) using e-log  v ’ plot settlement = m v  H

10. Settlement by M v Method ~co- eff of volume change=m v ~ volumetric strain in a soil element per unit increase in stress i.e., no units kPa or MPa kPa -1 or MPa -1

11. Virgin Consolidation Line log  v ’ void ratio virgin consolidation line  p ’ original state  vo ’ e o

12. Settlement computations ~ computing e using e-log  v ’ plot initial  vo ’ e o  vo ’+   e If the soil is normally consolidated, the entire loading path is along the virgin compression line.

13. Settlement computations  vo ’ initial e o  vo ’+  If an overconsolidated clay becomes normally consolidated by the end of consolidation, VCL  p ’  e

14. Practical Applications Prefabricated Vertical Drains to Accelerate Consolidation Installation Cross section of PVD

15. Consolidation Test Increment of load Topcap porous stone sample water confining ring settlement dial gauge At any degree of consolidation, comparison in time taken for consolidation between laboratory & field with respect to drainage path is         t = Time taken d = Drainage path

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