Geoscience Reference
In-Depth Information
7.10 Overconsolidation ratio
•
The Overconsolidation ratio (OCR) provides an indication of the stress history of
the soil. This is the ratio of its maximum past overburden pressure to its current
overburden pressure.
•
Material may have experienced higher previous stresses due to water table
fluctuations or previous overburden being removed during erosion.
Table 7.10
Overconsolidation ratio.
Overconsolidation ratio (OCR)
OCR
P
c
/
P
o
=
Preconsolidation pressure
Maximum stress ever placed on soil
P
c
=
Present effective overburden
P
o
=
γ
z
Depth of overlying soil
z
Effective unit weight
γ
Normally consolidated
OCR
1 but
1.5
∼
<
Lightly overconsolidated
OCR
1.5-4
=
Heavily overconsolidated
OCR
>
4
•
For aged glacial clays OCR
=
1.5 - 2.0 for PI
>
20% (Bjerrum, 1972).
•
Normally consolidated soils can strengthen with time when loaded.
•
Overconsolidated soils can have strength loss with time when unloaded (a cutting
or excavation) or when high strains apply.
7.11 Preconsolidation stress from cone penetration testing
•
The Preconsolidation stress is the maximum stress that has been experienced in
its previous history.
•
Current strength would have been based on its past and current overburden.
Table 7.11
Preconsolidation pressure from net cone tip resistance (from Mayne et al., 2002).
Net cone stress
q
T
P
o
kPa
100
200
500
1000
1500
3000
5000
−
Preconsolidation pressure
P
c
kPa
33
67
167
333
500
1000
1667
Excess pore water pressure
u
1
kPa
67
133
333
667
1000
2000
3333
•
For intact clays only.
•
For fissured clays P
c
=
2000 to 6000 with
u
1
=
600 to 3000 kPa.
•
The electric piezocone (CPTu) only is accurate for this type of measurement. The
mechanical CPT is inappropriate.
7.12 Preconsolidation stress from Dilatometer
•
The Dilatometer should theoretically be more accurate than the CPTu inmeasuring
the stress history. However, currently the CPTu is backed by greater data history
with a resulting greater prediction accuracy.
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