Drained vs undrained calculations

[Woody1515]

Hello everyone,

When designing a foundation in a normally consolidated clay, is the foundation designed for both the undrained (short term) and drained (long term) conditions? I typically see geotechnical reports that just state the undrained shear strength, with no mention of a drained value. Also, if a drained friction angle is given for the clay, would the drained case govern due to the typically low friction angle of a clay?

Thanks in advance!

 

[EireChch]

As is typical for normally consolidated clays, the undrained strength is lower than the drained strength. This is due to the fact that the pore pressure increases and the effective stress decreases during undrained shear. For very heavily overconsolidated clays, the reverse is true: The undrained strength is greater than the drained strength, because pore pressure decreases and effective stress increases during undrained shear Duncan and Wright, Soil strength and Slope Stability (This book is a great reference, for soil mechanics in general)

You should check both conditions until the point in time when you have enough experience to know which will govern, even then...check both

 

[Woody1515]

Thanks for the response. So it is common practice to calculate the undrained capacity using the clay shear strength, then calculate the drained capacity using the clay friction angle? Most geotechnical reports I have come across just state the clay shear strength, they typically don’t give a friction angle for the clays.

 

[fattdad]

In the work that I oversee, we'd require both especially in slope stability. When designing a foundation on normally-consolidated clay, we'd first have to manage settlement and with that settlement you'd get an increase in both undrained shear strength and friction angle. The former, turns out, is more relevant. So, we'd have to chase that first - how to increase the undrained shear strength! Use wicks, surcharge, preloads, etc. Then it's how to instrument and manage secondary compression. All these can be addressed to make a spread footing work, but. . . We'd just use piles for structure and wicks for embankments.

Things get a bit more complicated in slope stability.

f-d

ípapß gordo ainÆt no madre flaca!

 

[EireChch]

Eurocode 7 Part 1 and and BS8004 Code of Practice for Foundations both stipulate that bearing capacity for fine grained soil should be based on both short term (undrained) and long term (drained) conditions.

In reality, when you read most text books they only consider the undrained condition. Most text books are generally assuming a normally consolidated, saturated clay. When you get away from NC clay and unsaturated clay things need a bit more assessment.

For normally to lightly over consolidated (OCR 1-2), your undrained shear strength (c_u)is generally in the between 25-100kPa and as such, your ultimate bearing capacity will be governed by the undrained shear strength.

In highly over consolidated clays, when you get c_u of well over 100kPa, and drained friction angles of 20-25 degrees, you drained strength will govern.

I would say you are correct in saying that most reports are based on c_u only. I think this is down to the logic of that the load is applied rapidly (compared to time to dissipate pore pressure) and therefore it is an undrained loading. This is true, however OC clay dilates and creates negative pore pressure which gives an "apparent" higher strength which is lost after dissipation of pressure. This is why OC clay is scary material to work withn when assessing slope stability. A new cut may be stable for months / years, however when negative pore pressure dissipates, the slope may fail.

Having said all that, your allowable bearing pressure should be the lesser of Ult/3 and bearing pressure resulting in a settlement within your criteria (25mm is common limit, for small foundations). The "mistake" of not considering drained strength is (possibly) due to the bearing pressure for settlement is lower than your drained bearing capacity. Foundations almost never fail in shear, excessive settlement is however one version of failure.

Disclaimer : strength parameters above are indicative, there are NC clays with c_u of >100Kpa, and drained phi outside of 20-25 deg for OCR clays.

 

[Woody1515]

Thanks for the replies everyone! I agree, consolidated soils do appear to be difficult to design foundations on. It appears most work I have seen has been completed on normally consolidated soils.