Discussion of “Shear Strength in Preexisting Landslides” by Timothy D. Stark and Manzoor Hussain

The authors are to be commended for (1) reviewing shear strength concepts for existing landslides and (2) presenting and analyzing the results of meticulous laboratory research on shear strength recovery along failure surfaces allowed to rest for time periods up to 300 days. Many practitioners, in the discussers’ region of the Appalachian Plateau (and perhaps elsewhere), still do not recognize the presence of existing failure surfaces in active or dormant landslides, let alone the implications of these failure surfaces regarding residual-level shear strengths. To the extents that some practitioners in the discussers’ region recognize these issues, they tend to rely on “faith healing” for strength gain along existing landslide failure surfaces, justifying their reasoning by pointing to their (inappropriately) back-calculated shear strength values. The authors investigated strength recovery with both torsional ring shear tests and direct shear tests, but the paper focused on ring shear results because of space constraints (p. 959). We strongly encourage them to prepare another paper on the direct shear test results and their comparison to the ring shear test results. Much of the accumulated experience, both published and unpublished, in residual strength tests on landslide failure surface materials, including the discussers’ own (Hamel 1969; Hamel and Flint 1969, 1972; Hamel and Adams 1981; Hamel 2004), has been with multiple reversal direct shear tests. And, at least in the discussers’ region, the latter tests are much more commonly performed in practice than torsional ring shear tests on landslide failure surface materials. We have some questions regarding the authors’ soil samples and comments on some of their conclusions as well as long-term strength changes along the failure surfaces of colluvial landslides common in many regions, including the discussers’ own Appalachian Plateau. The authors state, “The strength recovery tests utilize remolded specimens from shear surface samples” (p. 959). We would appreciate considerably more detail on the provenance of these “shear surface samples” and the remolding process. In the discussers’ experience, identification and sampling of soils from landslide failure surfaces is the key to determining realistic strength parameters. Only if the appropriate materials are identified, sampled, and tested are the test results relevant to actual landslides (though test results from other materials may have significant research implications). Regarding the provenance of the “shear surface samples”: 1. Were they obtained from actual in situ landslide failure surfaces for the four soils tested as described in the paper? 2. If so, a. Who identified the failure surfaces in the field and obtained the samples? b. How thick were the failure surfaces (or shear zones) and how were they described? c. How were the samples obtained, e.g., by drilling or excavation? 3. If not, how and where were they obtained? It is unclear from information presented on pp. 958–959 of the paper exactly what size fraction (minus No. 40 sieve? minus No. 200 sieve? or other?) was used for the remolded test specimens. This should be clarified, along with any other pretreatment, e.g., ball-milling, of soils used in the test specimens. We are unable to find in the paper the dimensions of the torsional ring shear specimens tested. These dimensions would enable readers to better visualize and understand the tests. In their investigation of strength recovery, the authors ran ring shear tests with applied effective normal stresses of 100 to 600 kPa and rest times of 1 to 90 days and with applied effective normal stresses of 100 kPa and rest times of 300 days. They found noticeable strength recovery with effective normal stresses of 100 kPa but essentially negligible strength gain with effective normal stresses greater than 100 kPa, as shown in Figs. 3 and 4 on p. 961 of the paper. Inspection of the legend boxes in these figures indicates that only limited testing was done, or at least reported, with effective normal stresses greater than 100 kPa. Specifically, only the Madisette clay with effective normal stresses of 200, 300, and 600 kPa and the Esperanza clay with effective normal stresses of 300 and 600 kPa had effective normal stresses exceeding 100 kPa. There are no test data with effective normal stresses between 100 and 200 kPa and only the Madisette clay data at 200 kPa with effective normal stresses between 100 and 300 kPa. The paucity of data here between 100 and 300 kPa makes it hard to accept the author’s conclusion “Strength gain is essentially negligible at effective normal stresses greater than 100 kPa” (p. 962). Common sense indicates there is some effective normal stress level or range (probably specific to each individual soil, or at least specific to certain ranges of soil plasticity) where the time-dependent strength gain transitions from significant to negligible. Some experience in the Appalachian Plateau indicates this is the case, as discussed subsequently. This is certainly an area for further research. The authors do not recommend use of the recovered strength for landslide remedial measures at effective normal stresses of 100 kPa or less because the strength gain is lost with small shear displacements (p. 962). We agree with the authors on this, but note that it is not uncommon for practitioners in the discussers’ region to rely on such strength gains. We strongly agree with the authors that residual-level strength be used in all analyses of active and recent landslides and old to ancient landslides for assessment of slope stability and for design of remedial measures. Whether the residual-level strength is determined from appropriate laboratory tests on actual failure surface materials, from regional databases and experience (e.g., Fleming et al. 1970; Hamel 1980, 2004), or from back-calculation of limiting equilibrium of existing landslides is a matter of judgment in each case. Prudent engineers should consider all three of these strength assessment procedures in selection of shear strength parameters for slope analyses and design of remedial measures. Prudent engineers should also recognize that field