Geoscience Reference
In-Depth Information
Table 19.6
Wall movements required to develop the active and passive pressures
(GEO, 1993).
Soil
State of stress
Type of movement
Necessary displacement
Sand
Active
Parallel to wall
0.001 H
0.1% H
Rotation about base
Passive
Parallel to wall
0.05 H
5% H
Rotation about base
0.10 H
10% H
>
>
Clay
Active
Parallel to wall
0.004 H
0.4% H
Rotation about base
Passive
-
-
•
Due to the relative difference in displacements required for the active and passive
states for the one wall the passive force should be suitable factored or downgraded
to maintain movement compatibility.
•
Above is for rigid walls, other wall types have other displacement criteria. Refer
Chapter 23.
•
Soil nail walls deform at the top.
•
Reinforced soil walls deform at the base.
19.7 Active and passive earth pressures
•
Active and passive earth pressures are based on some movement occurring.
•
Rankine and Coulomb developed the earth pressure theories with updates by
Caquot and Kerisel.
•
Assumptions and relationship provided below.
Table 19.7
Earth pressure theories.
Theory
Rankine
Coulomb
Caquot and Kerisel
Based on
Equilibrium of an element
Wedge of soil
Failure surface
Planar
Planar
Log spiral
Wall friction
i:i
0 when ground
δ
δ =
=
δ
surface is horizontal
Pressure
Increases linearly with
Provides limiting forces on the wall, but no
distribution
depth
explicit equivalent pressure distribution
Resultant active
At horizontal. At i when
to normal to back of wall
δ
force
ground surface is sloping
to horizontal (wall with a vertical back).
δ
Active pressure
Rankine similar to Coulomb and Caquot only at
0. As
1 then 10%
δ =
δ
/
φ →
higher at
φ
<
35
◦
, but approximately similar at higher
φ
values
Resultant passive
At horizontal. At i when
δ
to horizontal. At
φ
>
35
◦
δ
to horizontal
force
ground surface is sloping
passive force and pressure
overestimated. Too high for
δ
>
0.5
φ
Passive pressure
Similar only at
0 :Varies significantly for
30
◦
δ =
φ
>
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