# Relationship between drained and undrained shear strength of clay

Will it make sense to analyze this problem using both drained and undrained shear strength of the clay? Just to clarify this more, please see the. [Video 9 of 12] Videos designed and presented by Declan Phillips PhD P.E. and Alan O Reilly BEng and the generous support of the Faculty of. silts and clays) experience undrained conditions. • If the rate of loading A soil with a tendency to compress during drained loading will exhibit an ratio or the initial undrained shear strength as shown in the figure above.

What if the clay under the dam becomes normally consolidated with undrained conditions being more critical? How could two portions of the clay layer overconsolidated away from the dam with drained conditions being more critical and normally consolidated clay underneath the dam with undrained conditions being more critical combined together in one analysis.

Thank you once again ntschwanz Geotechnical 29 Jan 16 I think you're assuming a scenario where the clay is heavily over consolidated prior to placing the embankment but the embankment loading is such that the post construction effective stress exceeds the preconsolidation pressure beneath the embankment but doesn't at some distance away clay is normally consolidated beneath the emb but remains heavily OC'd away.

Now you have compressive stresses tending to positive PP generation and shear stresses that cause positive PP beneath the embankment and negative PP at distance. The undrained and drained analyses still apply but the question of whether the rate of dissipation of the negative PP in the heavily OC'd clay is faster than the rate of dissipation of positive PP in the normally to lightly OC'd clay can be considered.

If you believe that's that case then you can make decisions on appropriate shear strengths to use throughout the clay or move to an advanced analysis. If we could calculate the exact PWP at any location at any point of time, there wouldn't be a need for undrained shear strength parameters. Thanks once again, I really appreciate your time and effort. Undrained and Drained Shear strength for the same clay layer ntschwanz Geotechnical 1 Feb 16 We may have the ability to calculate PWP to use in an effective stress based model, but not with the accuracy or confidence generally needed throughout the entire range of possible loading.

So in your embankment dam example you look at the end-of-construction short term undrained load case, the steady-state-seepage long term drained load case along with others. Can positive PP generation due to potential dilatant behaviour of the OC clay be good rationale for use of drained more conservative in this case rather than undrained strength for OC clay?

Undrained and Drained Shear strength for the same clay layer moe Geotechnical 26 Feb 16 Undrained and Drained Shear strength for the same clay layer marc Geotechnical 17 Mar 16 The Critical State occurs at the quasi-static strain rate. It does not allow for differences in shear strength based on different strain rates. Also at the critical state, there is no particle alignment or specific soil structure.

Almost as soon as it was first introduced, the critical state concept has been subject to much criticism --chiefly its inability to match readily available test data from testing a wide variety of soils.

This is primarily due to the theories inability to account for particle structure.

### Shear strength (soil) - Wikipedia

Further, an assumption commonly made to make the model mathematically tractable is that shear stress cannot cause volumetric strain nor volumetric stress cause shear strain. Since this is not the case in reality, it is an additional cause of the poor matches to readily available empirical test data.

Additionally, critical state elasto-plastic models assume that elastic strains drives volumetric changes. Since this too is not the case in real soils, this assumption results in poor fits to volume and pore pressure change data. Steady state dynamical systems based soil shear [ edit ] A refinement of the critical state concept is the steady state concept.

## Shear strength (soil)

The steady state strength is defined as the shear strength of the soil when it is at the steady state condition. The steady state condition is defined Poulos as "that state in which the mass is continuously deforming at constant volume, constant normal effective stress, constant shear stress, and constant velocity.

Poulosthen an Associate Professor of the Soil Mechanics Department of Harvard University, built off a hypothesis that Arthur Casagrande was formulating towards the end of his career. Poulos Steady state based soil mechanics is sometimes called "Harvard soil mechanics". The steady state condition is not the same as the "critical state" condition.

**Soil Strength Example**

The steady state occurs only after all particle breakage if any is complete and all the particles are oriented in a statistically steady state condition and so that the shear stress needed to continue deformation at a constant velocity of deformation does not change.

It applies to both the drained and the undrained case. The steady state has a slightly different value depending on the strain rate at which it is measured.

Thus the steady state shear strength at the quasi-static strain rate the strain rate at which the critical state is defined to occur at would seem to correspond to the critical state shear strength.

However, there is an additional difference between the two states.

This is that at the steady state condition the grains position themselves in the steady state structure, whereas no such structure occurs for the critical state. In the case of shearing to large strains for soils with elongated particles, this steady state structure is one where the grains are oriented perhaps even aligned in the direction of shear.