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Shortlisted firms for US landslide risk assessment revealed

Washington State governor Jay Inslee has named four consultancies under consideration to undertake independent assessment of the Rattlesnake Ridge landslide.

Geotechnical Resources, Golder Associates, Hart Crower and Shannon and Wilson are on the shortlist to assess the risk presented by ground movement in Rattlesnake Ridge above Anderson Quarry in Yakima, which is feared to be on a similar scale to the 2014 Oso landslide.

rattlesnake ridge

Concerns have been raised about material reaching the I-82 highway or blocking the Yakima River

Anderson Quarry ‘s owner has been taking advice from its own consultant Cornforth Consultants but public concern about the potential landslide has led the governor to appoint an independent firm to confirm the reports.

Residents in the area to the south of Yakima have been evacuated as a precaution.

According to Cornforth, the ground movement covers around 3M.m3 of material but is slow moving and unlikely to result in the same kind of catastrophic failure seen at nearby Oso. Local people have called for the I-82 highway to be closed by Cornforth has said that there is minimal risk of a rockfall reaching the road or blocking the Yakima River.

Cornforth has also said that the rate of movement has decreased in acceleration and the firm has changed its prediction of failure occurring in mid-January to later this month or early in February.

rattlesnake ridge 2

Tension cracks started to open up in Rattlesnake Ridge above an active quarry in October 2017

Landslide specialist and University of Sheffield pro-vice chancellor (research and innovation) Dave Petley said: “The implication is that this is a rotational slip but the tension cracks I have seen in images show a complex structure with some evidence of a development of a graben structure.

“Combined with the potential for weakening the materials controlling the deformation, this makes forecasting the likely future behaviour of this slope quite challenging, but of course it is the geologists on the ground who are best placed to make a judgement. In the short to medium term high resolution monitoring is the right approach.”

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