Geoscience Reference
In-Depth Information
•
This effect is accounted for by applying a higher factor of safety to overturning as
the force is not applied one-third up from the base.
19.3 Coefficients of earth pressure at rest
•
The coefficient of at rest earth pressure (K
o
) is based on negligible wall movement.
•
∼
For lightly overconsolidated clays K
o
1.0.
•
For highly overconsolidated (OC) and swelling clays K
o
>>
1.
•
As plastic clays may have high swelling pressures, this material should be avoided
where possible.
•
The OC formula shown for granular soils and clays produce the same at rest
value values for
30
◦
. Below this friction value the clay K
o (OC)
value is higher,
especially for low friction angles.
φ =
Table 19.3
Relationships for at rest earth pressure coefficients (part from
Brooker and Ireland, 1965).
Soil type
Relationship
Normally consolidated
K
o (NC)
1 - sin
(Granular soils)
=
φ
K
o (NC)
0.95 - sin
(Clays)
=
φ
K
o (NC)
0.4
0.007 PI (PI
0-40%)
=
+
=
K
o (NC)
=
0.64
+
0.001 PI (PI
=
40-80%)
Overconsolidated
K
o (OC)
(1 - sin
) OCR
sin
φ
(Granular soils)
=
φ
K
o (OC)
(1 - sin
) OCR
1
/
2
(Clays)
=
φ
Elastic
K
o
= ν
/(1 -
ν
)
-
- angle of wall friction.
- NC - normally consolidated.
- OC - overconsolidated.
-
φ
ν
- Poisson ratio.
- PI - plasticity index.
- Values applied in above relationship presented below.
19.4 Variation of at rest earth pressure with OCR
•
The at-rest earth pressure varies with the plasticity index and the overconsolidation
ratio (OCR).
•
The formulae in Table 19.3 are used to produce Table 19.4.
•
The table illustrates that the at rest pressure coefficient value can change
significantly with change of OCR.
•
* Approximate “Equivalent'' Friction angle from cross calibration of elastic and
friction angle formula to obtain K
o
. Note the slight difference in friction angle
using this method as compared to that presented in Chapter 5.
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