edge effects in propagation tests
DESCRIPTION
Edge effects in propagation tests Edward (Ned) H. Bair 1,2* , Ron Simenhois 3 , Alec van Herwijnen 4 , and Karl Birkeland 5 1 US Army Corps of Engineers Cold Regions Research and Engineering Laboratory, Hanover, NH, USA - PowerPoint PPT PresentationTRANSCRIPT
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Edge effects in propagation tests
Edward (Ned) H. Bair1,2*, Ron Simenhois3, Alec van Herwijnen4, and Karl Birkeland5
1 US Army Corps of Engineers Cold Regions Research and Engineering Laboratory, Hanover, NH, USA2 Earth Research Institute, University of California, Santa Barbara, CA, USA3 Southeast Alaska Avalanche Center, Juneau, AK, USA4 WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland5 US Forest Service National Avalanche Center, Bozeman, MT, USA
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Introduction, propagation test length guidelinesExtended Column Test (ECT), l = 90 cm
Propagation Saw Test (PST),l = 100 cm or slab depth; whichever is greater
Greene, E., et al., 2010: Snow, Weather, and Avalanches: Observational Guidelines for Avalanche Programs in the United States, 136.
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Introduction, stress concentration at edges
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• G – energy release rate of the slab• G∞ – asymptotic energy release rate (no far
edge effect)• F(r/l) – correction factor• r is crack length• l is test length
G and G∞
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• 163 PSTs and ECTs on beams with l = 1-10 m, including some centered ECTs and PSTs
• Particle tracking on markers inserted into beams
• Finite element modeling for 3 profiles from pits done at tests
Overview
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1. Full crack propagation decreased with test length
2. Greater collapse occurred at the trigger and far end of the beam than in the center
3. Wavelengths were ~ 3 m, 2x what is predicted by the anticrack model
4. The far edge effect is significant for tests up to ~ 2 m
Major findings for ECTs and PSTs
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Field test summary
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• Relative to the middle of the beam, the trigger edge collapsed 160% and the far edge collapsed 128%.
Particle tracking, collapse amplitude
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Particle tracking, wavelength
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FE profiles
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• 3 profiles (a,b,c)• fixed crack length
(critical length) for each profile
Finite element (FE) results
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Correction factor from FE
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• We experimented with centered PSTs (CPSTs) and found that the critical length doubled.
• CPSTs require 2x as much digging for the same crack propagation length as a standard PST, so we find them less practical.
Centered tests
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• We were able to track the collapse amplitude over time for some of the tests that failed on the same layer.
• The average decrease was 0.65 mm/day.
Cool aside
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• PST and ECT results are sensitive to test length• Longer tests could decrease false unstable results,
*BUT* we need to test this hypothesis.• To test our hypothesis, we will examine the
accuracy of 2 m tests vs. standard 1 m tests on slope stability.
Implications
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