pre consolidation pressure

Subject:- soil mechanics
learning about pre-consolidation pressure

1. PRE-CONSOLIDATION PRESSURE 5 TH SEM CIVIL SOIL MECHANICS (2150609) GOVERNMENT ENGINEERING COLLEGE, BHARUCH Presentation Done By : MITHAIALA NACHIKETA R. 180143106010

2. PRECONSOLIDATION PRESSURE 2 It is defined as the ratio of undisturbed peak undrained shear strength to totally remolded undrained shear strength. Pre-consolidation pressure is the maximum  effective  vertical overburden stress that a particular soil sample has sustained in the past . Pre-consolidation pressure cannot be measured directly, but can be estimated using a number of different strategies . If the current vertical effective stress ( σv ′) is equal to or greater than  σp ′, the soil is said to be normally consolidated.

3. METHODS 3 Using a consolidation curve :( Casagrande  1936 ):- Choose by eye the point of maximum curvature on the consolidation curve. Draw a horizontal line from this point. Draw a line tangent to the curve at the point found in part 1. Bisect the angle made from the horizontal line in part 2 and the tangent line in part 3. Extend the "straight portion" of the virgin compression curve (high effective stress, low void ratio: almost vertical on the right of the graph) up to the bisector line in part 4 . The point where the lines in part 4 and part 5 intersect is the pre-consolidation pressure.

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5. MECHANISMS CAUSING PRECONSOLIDATION 5 Various different factors can cause a soil to approach its pre-consolidation pressure : Change in total stress due to removal of overburden can cause pre-consolidation pressure in a soil . Change in pore water pressure: A change in water table elevation, Artesian pressures, deep pumping or flow into tunnels, and desiccation due to surface drying or plant life can bring soil to its pre-consolidation pressure . Change in soil structure due to aging (secondary compression): Over time, soil will consolidate even after high pressures from loading and pore water pressure have been depleted . Environmental changes: Changes in pH, temperature, and salt concentration can cause a soil to approach its pre-consolidation pressure. Chemical weathering: Different types of chemical weathering will cause pre-consolidation pressure. Precipitation, cementing agents, and ion exchange are a few examples .

6. USES 6 Pre-consolidation pressure is used in many calculations of soil properties essential for structural analysis and soil mechanics. One of the primary uses is to predict settlement of a structure after loading.  This is required for any construction project such as new buildings, bridges, large roads and railroad tracks. All of these require site evaluation before construction. Preparing a site for construction requires an initial compression of the soil to prepare for foundation to be added. It is important to know the pre-consolidation pressure because it will help to determine the amount of loading that is appropriate for the site. It will also help to determine whether recompression (after excavation ).

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