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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