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There are no undrained layers between depths x and y

Article:

643

Title:

There are no undrained layers between depths x and y

Situation:

The following error message appears sometimes when calculating base stability:

"There are no undrained layers between depths x and y"

This message is generated when there are no undrained layers between the excavation's formation level (at depth H) and a depth 0.7H below formation level. These are the relevant depths for the calculation of base stability according to Bjerrum and Eide's formula, which requires an estimate of the average undrained shear strength of the ground in order to determine a factor of safety against basal heave (see Chapter 6 of the ReWaRD Reference Manual).

ReWaRD 2.5's base stability calculation considers only undrained soils (e.g. clays) - excavations in drained soils (e.g. sand) are usually stable against base heave, except in the presences of high water pressures below formation level. ReWaRD 2.5 does not check for this condition - you should make a separate calculation.

Solution:

If cohesive soils do exist between H and 0.7H on the excavated side of the wall, check that they have been put into undrained (and not drained) layers. ReWaRD only considers soils in undrained layers as contributing to the average undrained shear strength.

Product:

ReWaRD 2.5

Operating systems(s):

Windows 95, Windows 98, Windows Me, Windows NT4, Windows 2000

Date created:

17 August 2001

Last modified:

17 August 2001

Online article:

www.geocentrix.co.uk/support/kb/643.html