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EAT 212 SOIL MECHANICS CONSOLIDATION THEORY

EAT 212 SOIL MECHANICS CONSOLIDATION THEORY. LECTURER : MR. ZUHAYR BIN MD. GHAZALY H/P : 014-7170838. TOPICS. Fundamentals of Consolidation. Normally Consolidated and Overconsolidated Clays. Calculation of Settlement from One-Dimensional Primary Consolidation .

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EAT 212 SOIL MECHANICS CONSOLIDATION THEORY

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  1. EAT 212SOIL MECHANICSCONSOLIDATION THEORY LECTURER : MR. ZUHAYR BIN MD. GHAZALY H/P : 014-7170838

  2. TOPICS • Fundamentals of Consolidation. • Normally Consolidated and Overconsolidated Clays. • Calculation of Settlement from One-Dimensional Primary Consolidation. • Compression and Swell Index.

  3. FUNDAMENTALS OF CONSOLIDATION • Elastic settlement - caused by elastic deformation of dry soil and of moist and saturated soils without any change in moisture content. • Primary consolidation settlement - results of a volume change in saturated cohesive soils because of expulsion of the water that occupies the void spaces. • Secondary consolidation settlement - observed in saturated cohesive soils and is the result of the plastic adjustment of soil fabrics. It is an additional form of compression that occurs at constant effective stress.

  4. When saturated soil layer subjected to a stress increase - pore water pressure is increased. • In sandy soils (highly permeable), drainage caused by increase in the pore water pressure is completed immediately. • Pore water drainage - accompanied by a reduction in the volume of soil mass (settlement). • Rapid drainage of pore water in sandy soils - elastic settlement and consolidation occur simultaneously. • When saturated compressible clay layer subjected to a stress increase, elastic settlement occurs immediately.

  5. Hydraulic conductivity of clay is significantly smaller than that of sand, the excess pore water pressure generated by loading gradually dissipates over a long period - associated volume change in the clay may continue long after the elastic settlement.

  6. NORMALLY CONSOLIDATED AND OVERCONSOLIDATED CLAYS • Normally consolidated - whose present effective overburden pressure is the maximum pressure that the soil was subjected to in the past. • Overconsolidated - whose present effective overburden pressure is less than that which the soil experienced in the past. The max effective past pressure is called the preconsolidation pressure. • A soil in the field at some depth has been subjected to a certain maximum effective past pressure in its geological history. • The maximum effective pressure - equal to or less than the existing effective overburden pressure at the time of sampling.

  7. The reduction of effective pressure in the field may be caused by natural geological processes or human processes. • During sampling, existing effective overburden pressure is released, results in some expansion. • When specimen subjected to consolidation test, small amount of compression (small change in void ratio) will occur when the effective pressure applied is less than the max effective overburden pressure in the field to which the soil has subjected in the past. • When the effective pressure greater than the max effective past pressure, change in void ratio larger.

  8. CALCULATION OF SETTLEMENT FROM ONE-DIMENSIONAL PRIMARY CONSOLIDATION

  9. COMPRESSION INDEX (CC) • Skempton (1944) suggested the following empirical expression for the compression index for undisturbed clays.

  10. SWELL INDEX (CS) • Appreciably smaller in magnitude than the compression index and generally determined from laboratory tests.

  11. EXAMPLE A soil profile is shown in Figure 11.18. If a uniformly distributed load, Δσ, is applied at the ground surface, what is the settlement of the clay layer caused by primary consolidation if • The clay is normally consolidated. • The preconsolidation pressure (σ’c) = 190 kN/m2 • σ’c = 170 kN/m2

  12. CALCULATION OF CONSOLIDATION SETTLEMENT UNDER A FOUNDATION • Increase in the vertical stress in soil caused by a load applied over a limited area decreases with depth, z measured from the ground surface downward. • Use of Simpson’s rule to estimate average increase in the pressure below the center of the foundation.

  13. EXAMPLE Calculate the settlement of the 10-ft-thick clay layer in figure below that will result from the load carried by a 5-ft-square footing. The clay is normally consolidated. Use the weighted average method to calculate the average increase of effective pressure in the clay layer.

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