How long can stiff fissured clay slopes stand?

Coming back to the issue of stiff fissured clay slopes, one very important question is the standup time of a slope excavated or formed (natural erosion etc) with higher inclination than the one that would be the outcome using fully softened shear strength.

Skempton (1970) in his paper “First time slides in over consolidated clays”, provided a graph presenting the decay of strength with time to almost fully softened where failure of slopes in London clay occurred (pic 1).

Strength changes with time for London Clay (from Skempton, 1970)

Mesri et al (2003) in his paper provides a graph (pic 2) compiled from numerous case studies of failed slopes in stiff fissured clays in which the vertical axis presents the percent of failure surface length (Lr) in residual condition to the total observed failure surface length (Lt)  and on the horizontal axis the age when the slopes failed. The far left points are of unidentified time. Time to failure range from a couple of years to decades.

Ratio of slip surface segment assumed at residual condition to total slip surface length ploted against age of slope

VandenBerge et al (2013), mention that “The time required to reach fully softened strength might be as little as ten years in some cases, and as long as 60 years in others”.

Potts et al (1997) in the paper “Delayed collapse of cut slopes in stiff clay” presented complex coupled finite element analyses assuming strain softening soil behavior capable of swelling. The analysis requires among other parameters the coefficient of permeability which varies with depth, and coefficient of earth pressure at rest. With this complex parametric analysis they conclude that 3:1 (H:V) slopes produced failures between 11 and 45 years. And steeper slopes fail in less than a decade. 15m high slopes failed after 145 years. All these slopes produced deep seated progressive failure.

It is very interesting to note that they evaluated deep seated failures although the problem of investigation was recent shallow failures in highway cuts and embankments.

Another attempt from Kovacevic et al (2007) presented in the paper “Predicting the stand up time of temporary London Clay slopes at Terminal 5, Heathrow Airport” in which again a complex finite element analysis with swelling behavior and differing Ko was executed. The investigation was for both shallow and deep seated failure. The conclusion of the study was that shallow failures occurred earlier than deep seated and the time to failure was influenced most by the assumptions of permeability and the effect of suction.

Cuts in stiff fissured clays will stand up for an undetermined time before failure occurs when inclined steeper than fully softened strength requires. The time may be from six months to hundred or more years. Investigating the stand up time is very complex and requires sophisticated numerical analysis which also utilizes assumptions in many of the parameters used. Very important parameter is the permeability of the clay in the development of swelling and softening.

This is captured in the classical Terzaghi, Peck and Mesri, 1996 book in which it is stated that “If the surfaces of weakness subdivide the clay into fragments smaller than about 25mm, a slope may become unstable during construction or shortly thereafter. On the other hand, if the spacing of the joints is greater, failure may not occur until many years after the cut is made.”

In my opinion the spacing and orientation of joints is the controlling parameter of permeability in such materials. So highly fissured materials with very close spacing may develop shallow slope failures very quickly. More widely spaced  stiff fissured clays with fewer joint systems can provide significant delay time to failure.

A second important issue is the evaluation of shallow or deep seated failure and how a deep seated failure can be defined. This is something very important that will be covered in another entry.

About Chrys Steiakakis

Chrys Steiakakis is a practicing geotechnical engineer with more than fifteen years of experience in the field of geotechnical engineering. He earned his bachelor and master in mining engineering from the Technical University of Crete, Greece and a second master’s degree in Civil Engineering from Virginia Polytechnic Institute and State University, USA. He has been the technical director of engineering department of General Consulting ISTRIA for four years and now he is a partner and also provides his own consultancy services via Geosysta ltd. He has been involved in numerous highway, railway and mining projects. Chrys with his long term collaboration with the Technical University of Crete has participated in numerous research projects in the field of geotechnical engineering and rock mechanics and has provided self sustained seminars of geotechnical engineering in related areas for the Industry. His main field of experience covers all aspects of tunnel design, earthworks design and monitoring (slope stability, embankment in difficult ground, reinforced embankments and retaining walls), landslide investigation and mitigation, foundations for bridges and structures, risk assessment in geotechnical projects and value engineering in large projects.

One thought on “How long can stiff fissured clay slopes stand?

  1. Chrys,
    Very interesting post. Thanks, I enjoyed reading it.

    When thinking of this sort of problems I often like to develop the stress paths required for such mechanisms. Obviously this won’t say anything about stand-up times as it is dependent on the hydraulic conductivity as you pointed out. Anyway, at least it helps me understanding better the soil behaviour required for such a mechanism. I had a go at developing such a stress path (hopefully right) which I copied on this link:

    Looking at such graph we understand the concept of the “fully softened” strength by Skempton. Although not shown, at large deformations the soil will Eventually reach the residual strength upon formation of the slip planes causing the clay grains to slip over one another significalty reducing the strength below critical state strength as indicated in the Skempton graph you present.
    It is important to note the assumption in the graph of an elastic isotropic soil which causes the effective stress path (ESP) during the undrained excavation to be vertical but not necessarily the most appropriate or strictly correct assumption for fissured clays. In addition, I have not considered the several cycles of wetting/drying involved so it it a simplified case.

    Having said that, there is a good paper on such a topic, and you’ll probably like having a look:

    Take WA and Bolton MD (2011) Seasonal ratcheting and softening in clay slopes, leading to first-time failure. Geotechnique 61(9): 757–769.

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