Subject:- soil mechanics learning about pre-consolidation pressure

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

2. PRECONSOLIDATION PRESSURE
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 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
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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.

4. 4

5. MECHANISMS CAUSING PRECONSOLIDATION
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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
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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).

7. THANK YOU !
ANY QUESTIONS ?