Janbu Tangent Modulus Theory for Clay

[Melzar]

The AASHTO LRFD Bridge manual states (C10.6.2.4.3) "In practice footings on cohesive soils are most likely founded on overconsolidated clays, and settlements can be estimated using elastic theory (Baguelin et al., 1978) or the tangent mdodulus method (Janbu, 1963, 1967). Settlements of footings on overconsolidated clay usually occur at approximately one order of magnitude faster than soils without preconsolidation, and it is reasonable to assume that they take place as rapidly as the loads are applied".

We have mostly overconsolidated clays here in West Michgan, and my colleagues typically consider such clay to be incompressible. I have never felt comfortable discounting the clay consolidation entirely, especially in areas of deep fills where the stress will act very deeply. I have always thought of these clays in the context of a recompression index and consolidation theory, but perhaps Janbu's work explains this better.

Do any of you use the tangent modulus method to estimate settlement in overconsolidated clay? Has it been your experience that such settlements occur very rapidly? It doesn't seem intuitive to me. Is the tangent modulus method more applicable for higher overconsolidation ratios?

 

[escrowe]

Based on running numerous 1d lab consolidation tests,
I'd say that settlement in OC clay tends to be quite rapid, until the applied load exceeds the maximum past pressure.

The rate of loading also tends to effect the rate and degree of settlement.

For projects in clay, you should certainly estimate maximum past pressure;
if the proposed bearing pressure significantly exceeds the maximum past pressure, then a plastic flow condition could result
and elastic theory will likely underestimate settlement.

Janbu can be applied to etimate the OCR and settlement, and may prove useful for muli-layered soil conditions.

Here are a couple of links:

http://ecow.engr.wisc.edu/cgi-bin/get/cee/532/christense/studyguide/532studyguidepp51-57.pdf

http://ecow.engr.wisc.edu/cgi-bin/get/cee/532/christense/tangentmodulusplot-hiltonhotel.xls

 

[fattdad]

1-d recompression and elastic theory are not that different. The tangent modulus (i.e., elastic theory) is consistent with work by others. Duncan developed hyperbolic paremters to model strain softening response to loading.

I would use elastic theory for stiff, overconsolidated clay.

f-d

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