This paper describes investigation, testing, analysis, and slope history used to determine the two-phase failure mechanism involved in the 2014 landslide near Oso, Washington. The first phase involves a slide mass located above the frequent landslides in the lower portion of the slope and extends to near the slope crest. This slide mass had a large potential energy, which moved downslope, and pushed the water-filled colluvium that had accumulated along the slope toe across the valley, resulting in it flowing almost 1.5 km. Evacuation of the Phase I slide mass left the upper portion of the slope unbuttressed and oversteepened, causing a second landslide (Phase II) but it primarily remained on the source slope because the back edge of the Phase I slide mass prevented further movement and the dense and unsaturated upper soils did not undergo a significant strength loss like the water-filled colluvium.
This paper describes and explains the spectacular mobility of the 2014 Oso landslide, which was the cause of its fatal consequences. A geomorphic interpretation of the site conditions is used to reconstruct the landslide failure mechanism. Two numerical models are used to conduct an inverse runout analysis. The models implement a newly defined rheology appropriate for liquefied soils. It is shown that this landslide occurred in two phases, characterized by different material strengths. Although the temporal sequencing of the two phases remains somewhat ambiguous, it is clear that the distal phase underwent significant undrained strength loss (liquefaction) and travelled more than 1.4 km over a nearly horizontal surface. The proximal phase underwent brittle failure, with much less strength loss than the first phase. The parent material forming the slide mass was composed of insensitive, overconsolidated glaciolacustrine silt and clay, material not traditionally recognized as liquefiable. It is hypothesized that a substantial volume of liquefaction-prone soil was formed by colluvial softening of the parent material during the process of slope development prior to 2014.
Standard penetration tests (SPTs) have been used to estimate strength parameters of soils and weak rocks when it is difficult to obtain high-quality samples for laboratory shear testing. SPTs require 45 cm (18 in.) of split-spoon sampler penetration to determine the blowcounts per 0.3 m (1 ft), which is difficult to impossible to obtain in weak rock, that is, intermediate geomaterials. As a result, a modified SPT is presented here for sampler penetrations less than 45 cm (18 in.) in weak rocks. This new procedure is termed the modified standard penetration test (MSPT) and uses the penetration rate, not the sum of penetration blowcounts per 0.3 m, to estimate the unconfined compressive strength for the design of drilled shafts in weak fine-grained rocks. The penetration rate is the inverse of the linear slope of the penetration depth versus blowcount relationship. With this new test and interpretation procedure, 45 cm (18 in.) of sampler penetration is no longer required to estimate the unconfined compressive strength of weak rocks. An empirical correlation between MSPT penetration rate and laboratory-measured unconfined compressive strength is presented here for weak Illinois shale. This correlation could be used to estimate the unconfined compressive strength for the design of drilled shafts in weak rocks.
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