HomeMy WebLinkAboutBLD1999-00583 Geotechnical Report 7f.
G. W. Thorsen, Consulting Geologist
1926 Lincoln Street, Port Townsend, WA 98368 �� , �.Cr
fit
(360)385-6002
Michael Ajax, Building Official June 8, 1999
621 Sheridan St.
Port Townsend,WA 98368
Subject: Thorndyke Bay landslide area involving summer residence of Marijane Jarboe(follow up to our
phone conversation June 7)
General findings: The following observations and conclusions are based on a June 5 recon of the uplands
of the property as well as the beach and adjacent beach and bank areas.They are also based on periodic
observations of the Squamish Harbor/Thorndyke Bay shoreline geology since 1974. I did no borings or
other subsurface explorations.
• The structure is near the toe of a very large deep-seated ancient landslide.
• Undeformed old-growth stumps suggest initial failure at least 300 years ago(more probably during a
millennium of wetter climate.
• Even without significant new movement,there may be minor"adjustments"among the various slide
blocks.
• This episode of movement was probably caused by an unusual sequence(rather than a single) of wet
years.
• There is no way to know if the slide will reactivate again next year or within our lifetimes.
• Individual properties cannot be permanently"stabilized", and there is a question whether even a
community effort could guarantee future stability.
• Such slides move slowly, so hazards are largely economic, although there can be secondary human
hazards.
Site-specific findings: Each structure on this slide will likely have unique problems. Some may"raft
along"on intact slide segments with no apparent damage while others will experience damage due to
differential movement between adjacent slide segments. Here I will differentiate between potential human
hazards and"latent"hazards.
The former could include:
• Possible electrical fire
•Tree(s) falling on the house
• Injury from shattered glass
The latter could include:
• Septic system disruption
• Loss of water(either a sheared well or broken supply line)
• Disrupted access (driveway)
,6-- / ,
30
,,,i, .
J Recommendations: As the former hazards are probably of more concern to the county, I will largely
confine my recommendations to that area.
• Contact electric utility regarding options to protect against damage in service line(a possible fire
hazard).
• Ensure that any broken water line will not flow(thereby aggravating the slide hazard).
• Consider removing the large conifer on the southeast that is leaning toward the house,
• Tape the fixed windows to minimize hazard from shattered glass. Monitor sliding doors for"binding".
• Release the southtist corner of the house from the foundation to allow leveling.
• Consult an experienced contractor re a telescoping(?)buffer panel between the house and the deck.
,,.. 7-e-7,II 4) 7.Z.-,4„.„.1
Gerald W. Thorsen, C. P. G.
cc: Ma
rijane Jarboe
-mot�c.� A.
..,-= c PROFtss10\\\\1
iii
it.
rc�� 1525 B���c�0,�l1,
W H
V.• t AIPG ..y i
ll�' C .,y k i