Tag Archives: fully softened shear strength

Slope stability and scale effects

In previous entries the issue of stiff fissured clays and the time to failure was briefly touched. The design of such slopes is not a trivial matter and requires significant knowledge of soil mechanics, geology, hydrogeology etc. One additional issue mentioned (one that sometimes is neglected) is the scale effect. This was presented in the previous entry for a very deep mine in rock. This issue of scale effect in relation to stress field will be briefly presented for the case of stiff fissured clays and hard soils.

In the following picture a large highway cut of about 30m is shown. For a civil engineering project this is a significantly high cut. The effective stress filed in this cut can range from of 50 – 500kPa which is the normal range for laboratory testing.

Highway cut

In the second picture a large excavation for a lignite mine is presented. The depth of excavation of this multi bench cut is around 135m. The excavation of this type needs to consider bench stability of slopes with heights of around 18m and also overall slope stability for highs above 135m. In the second case a large part of a possible failure surface could be in a stress field of around 1500-2000kPa or even more.

 Coal mine slopes

In the following figure the two types of cuts are compared and one can easily understand the significance of scale effects in the design of the different cuts.

Scale difference of coal mine and highway slopes

The scale of the mine excavation is such that even in one cross section, one has to consider besides the stress field, differing geology (pic 4), presence of faults, ground water locations and pore pressures etc. We will focus on the stress dependency at this point.

mine slopes

According to Stark et al, (2005) both fully softened and residual failure envelopes are stress dependent. In this work Stark et al provides an empirical graph regarding the stress dependency until 700kPa of normal stress for residual friction angle and 400kPa for fully softened friction angle.

Shear strength information for higher effective stresses >1MPa are not readily available. Furthermore execution of such tests in very high effective loads is not easy for most commercial laboratories. It may even be very difficult to execute ring shear tests in very high loads due to sample thickness and squeezing out from the sides.

In such high slopes the failure surface can pass from a number of soil layers with different shear strength properties. It is not easy to evaluate the “average” shear strength of layers involved in a possible failure surface. Unfortunately a rule of thumb for selecting shear strength parameters for such slopes cannot be provided. Engineering judgment is required in selecting such parameters and the stress conditions must not be ignored. Shear strength tests should be evaluated in relation to the expected stress field.

Slope design in stiff fissured clays

A very difficult issue is the design of slopes or cuts in stiff fissured clays (pic 1). The difficulty lies in the evaluation of shear strength for stability calculations. Much work has been done on this issue especially by Skempton (1964) with his excellent Forth Ranking Lecture titled: “Long-term stability of clay slopes” and many others have contributed significantly on this issue.stiff fissured clay

In the classical Terzaghi, Peck and Mesri, 1996 book the following is mentioned regarding this issue: “Almost every stiff clay is weakened by a network of hair cracks or slickensides.” (pic.2). “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.”Stiff fissured clay exposed next to a sliding soil mass

The reduction in strength with time, due to the presence of fissures, has been attributed to swelling and softening due to water infiltration in this hairline cracks especially when stress relaxation and crack opening occurs in excavated slopes.

Laboratory shear strength evaluation of such stiff fissured clays is difficult because large samples are required in order to include significant number of hairline cracks and even if such samples can be tested, the long term swelling and softening cannot be fully developed in the laboratory.

Duncan and Wright (2005) propose, based also on the work of Skempton (1970) to use the fully softened strength for long term slope stability evaluation of stiff fissured clays that have not undergone any prior movement or failure. This fully softened strength can be correlated to the peak strength of normally consolidated clays. In the laboratory the fully softened strength is evaluated on remolded samples of stiff fissured clays.

A very recent paper by VandenBerge, Duncan and Brandon, 2013 presents the outcome of a workshop that took place in 2011 at Virginia Tech, regarding the fully softened shear strength for stability of slopes in highly plastic clays.

In this paper the most recent views regarding the softening process, the way to measure or estimate the fully softened strength and how and when to use it in stability analysis are presented. Together with the paper of Vanderberge et al, it is worth reading the Lade paper in Engineering Geology (2010), titled “The mechanics of surficial failure in soil slopes” where a power function failure model is proposed for the shear strength of clay for shallow stability evaluation. This criterion is mentioned also in the paper by VandenBerge et al (2013).

I would like to bring attention on some issues which are mentioned also in the papers but relate more to practical issues of the subject:

  1. Great care should be given when site investigation is executed and evaluated in such stiff fissured caly materialsFissured clay. If core samples are not collected then it is very difficult to distinguish between stiff clay and stiff fissured clay. Even when samples are collected, great care should be given to break up some core samples because the fissuring will not be observed as can be seen in the picture 3.
  2. In situ tests such as SPT will produce high values, misleading the investigator to think that a very strong (even cemented) material is found.
  3. Shear strength tests on intact samples will produce high values of cohesion, again misleading the investigator to believe that a very strong material is present. In the laboratory the fissuring may not be reported during sample preparation due to the small sizes required.
  4. The additional difficulty comes on how to persuade the Owner or Contractor about the problems (failures) that Steep stable excavation in stiff fissured clay  may be formed after the slope has been excavated (maybe after very long time). They will evaluate the data from the investigation which show high values and if they are not fully aware about the behavior of stiff fissured clays they will push for a more optimistic design in order to reduce excavation volumes. The situation becomes even more difficult in design – built projects where during excavation the contractor may need to use hydraulic hammers to break up the material and steep slopes are  stable (pic. 4). In such situation everybody “blames” the designer for a very conservative design if fully softened shear strength has been used.
  5. The evaluation of existing stable slopes not designed with fully softened shear strength is another difficult situation. If the fully softened shear strength is used the FS may be found to be even below unity FS<1.0 but the slope is stable for a couple of years after excavation. It is difficult to persuade the Owner of such slopes (usually highway or railway) that in the future stability problems may occur.
  6. Finally it is very difficult to evaluate in what part of the slope and how deep you will use fully softened values especially for high slopes.
  7. Another critical issue is the evaluation of the long term pore pressures to be used in the analysis. In my opinion this is the most difficult issue but I will get back on this in another entry because much could be said.

As a closing remark I would like to state the final conclusion of the Vanderberge et al, 2013 paper: “Consideration of local experience with regard to slope performance, recognition of the possible consequence of slope failures, and application of sound engineering judgment are all essential elements of a comprehensive approach to geotechnical engineering of slopes.”