Consolidation

1
Consolidation

• Introduction
• and
• Terminology

2
This is some crazy bell tower in Italy

• Although consolidation was not the culprit here,
  this is what happens when theres differential
  settlement under a structure

• Oh yeah, and when that structures almost 900
  years old!

3
So, what is consolidation?

• gradual volume reduction of
• saturated clay soil of
• low permeability
• change in pressure (load)
• creates excess pore water pressure

• water is squeezed out of the soil very slowly due
  to low permeability
• settlement occurs over long time periods

4
One-dimensional consolidation

• simplest case
• load applied over a small portion of total clay
  layer area
• zero lateral strain

• swelling?
• opposite of consolidation
• Gradual volume increase
• Negative excess pore water pressure

5
Analogy (Craig, Figure 3.2)

• heres a spring (soil skeleton) housed in a
  sealed chamber filled with water
• the valve in the seal is closed

• an extra load (pressure) is applied to the seal
• the water takes on this extra pressure, the
  spring (soil skelton) feels nothing

• valve opens, water drains out
• spring (soil skeleton) starts to compress as it
  takes on the extra load

• finally, the soil skeleton has taken on all the
  extra load
• drainage stops

6
in terms of pore water pressure
q
q
?H
si
si
us
us ue
si q
us

• the pore water pressure is in its initial state
  ie., static ? us
• the effective stress on the soil is si

• a surcharge pressure, q is applied
• the water takes on this extra pressure, u us
  ue (ue q)

• as water slowly drains away, excess pore water
  pressure, ue gradually dissipates and effective
  stress on soil, s increases

• finally, soil skeleton has taken on all the extra
  load and the effective stress on the soil is sf
  si q
• excess pore water pressure is dissipated and u
  us again

• Oh yeah, one last thing...
• The only thing we really notice is that at the
  end of this the soil has settled by ?H

7
Stress History

• Normally Consolidated Clay

• If the present effective stress is the maximum to
  which the clay has ever been subjected

Today
1,000 years ago
5,000 years ago
10,000 years ago
30,000 years ago
80,000 years ago
CLAY
100,000 years ago
8
Stress History
Overconsolidated Clay

• If the effective stress at some time in the past
  has been greater than the present value

18,000 years ago
ICE AGE
Today
5,000 years ago
20,000 years ago
15,000 years ago
30,000 years ago
80,000 years ago
CLAY
100,000 years ago
9
Preconsolidation Pressure, sc

• The maximum effective stress that has acted on
  the clay in the past
• Obtained through analysis of laboratory test data
  (Casagrandes graphical method)
• sc should not be exceeded during construction
  to ensure minimum compression

10
Overconsolidation Ratio, OCR

• How can OCR gt 1?
• Erosion of overburden
• Recession of glacial ice sheets
• Permanent rise of water table

• Normally Consolidated OCR 1
• Overconsolidated OCR gt 1

11
Whats next?
Get out your oedometers!