Walsh and Logan (1989) showing two of the mines underlying
Bellingham with the downtown area on the lower right.
A much bigger mine underlies the northwest portion of the city.
I previously did a write up on investigating the Sehome mine beneath downtown Bellingham patch-over-bellinghams-void. Every now and again the coal mine issues in Bellingham come up. Last week I was asked three time what I might know. The last question was regarding a property that the City of Bellingham owns along Cornwall Avenue that they may be selling. I am fairly sure that the site was investigated for the potential that a mine was under the site using a drill rig several years ago. I have heard second hand that the mine did not extend under the site, but I have never seen the drilling records. The Sehome mine has had subsidence problems in the past and reports of new subsidence would not be a huge surprise.
A property developer asked me last week about potential problems from the Bellingham Mine underlying northwest Bellingham. This mine was evaluated by Tetra Tech in the 1984. The mine had fairly good records as to shafts and drifts. No retreat mining took place. That is they left coal pillars to support the mined areas.
Collapses can and will take place as the voids left by the mine eventually collapse. The question is how will that impact the surface and structures on the surface. Knowing that areas of coal were left (pillars) and the width and height of voids, rock type, depth to voids, angle of the void, thickness of overlying bedrock, thickness properties of unconsolidated materials over the bedrock, a mathematical model can be used to calculate how the surface may be impacted. Various models have been developed based on areas where mine collapse have taken place. I used a model developed from collapses in Poland for a mine subsidence project several years ago.
Tetra Tech (1984) evaluated the Bellingham Mine and did produce a map of areas where there was a low risk of subsidence - mostly along limited areas near Northwest Drive not far from the former mine entrance where the mine is shallower.
For the Bellingham Mine the biggest risk areas are associated with ventilation shafts and locations where the coal "daylighted" into unconsolidated (non rock) sediments. In these locations the settlement models do not work so well because the loose over burden sediments can pipe (flow) into the mine voids. The Bellingham Mine records were good enough that the potential daylight areas can be identified and I would say in those areas there is a very modest risk to higher risk dependent on the depth to the mine. As for vents, they were not well mapped. I have heard second hand as did Tetra Tech of vent collapses associated with the mine. These could have been vents or piping of sediment into daylighted areas. I did see a bit of a low spot on a property that I speculated may have been a settlement feature associated with a ventalation shaft.
My company does environmental due diligence investigations, and while mine subsidence is outside the scope of that work we will say something if we think there could be an issue. There are other areas around Washington State where mining has taken place and subsidence from old mines has taken place particularly in the eastern King County area east of Seattle such as near the aptly named Black Diamond.
One final note, there is a national insurance program for damages caused by coal mine subsidence. The program is funded by a tax on coal. But a subsidence must take place first. It is generally a reactive versus proactive program.