Earth pressures - Geotechnical Investigation and Design Tables - page 247

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i = slope of backfill surface.

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Passive pressures based on Coulomb Theory can overestimate passive resistance.

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Basic Rankine pressures are based on active pressure K a = (1 - sin φ )/(1 + sin φ ).

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Rankine Passive Pressure (K p ) = 1/K a .

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Coulomb Theory includes wall friction angle, and slope of backfill.

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10 ◦ . Active pressure increases considerably for a sloping backfill i >

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10 ◦ . Passive pressure decreases considerably for a sloping backfill i >

19.8 Distribution of earth pressure
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The wall pressure depends on the wall movement. For a rigid wall on a competent
foundation the movement is reduced considerably.

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The Rankine earth pressure distribution is based on a triangular pressure dis-
tribution with the resultant force acting at 1/3 up from the base. This point of
application can vary in some cases. Therefore calculations should allow for this
possibility by either shifting the point of application or factoring the overturning
moments accordingly.

Table 19.8 Distribution of earth pressure.

Type of wall foundation material Backfill Point of application of resultant force

Wall founded on soil Horizontal, i

= 0 ◦

0.33 H above base

Sloping at i upwards 0.38 H above base

Wall founded on rock Horizontal, i

= 0 ◦

0.38 H above base

Sloping at i upwards 0.45 H above base

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The triangular earth pressure distribution is not applicable for multi-propped/
strutted walls with little movement along its full height.

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Use of FS 2.0 for overturning and 1.5 for sliding accounted for this possibility

with previous approaches. Limit state procedures factoring strength only do not
currently account for the above condition explicitly.

=

19.9 Application of at rest and active conditions
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While the concept of no wall movement suggests that the at-rest condition should
apply, the application is not as self-evident. The cases below illustrate when the
higher at rest earth pressure condition applies instead of the active case.

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Tied back walls may be considered rigid or non-rigid depending on the deflections.
If the wall movement calculations (based on section modulus) show little to no
deflections then the at rest condition should apply.

•

Walls over designed (with high factors of safety) and based on the active earth
pressure condition, may not deflect. The at rest condition must then be checked
for stability.

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Some designers use a value average between the K o and K a conditions where
uncertainty on the earth pressure condition exists.