Wednesday, December 26, 2012

Cisterns, Out Houses and Erratics

I had conducted a geology hazard assessment a while back and had the opportunity to take a look at the foundation excavation at the site. Most of the excavation was into very hard glacial till, sediment deposited directly by glacial ice. The specific area was covered by a few thousand feet of ice during the last glacial period approximately 15,000 years ago.

But there was more to this foundation excavation than just the glacial till.

Note the rounded concrete patch with curved bricks
 and the pipe on the excavation wall

Close up of masonry lined basin now filled with cement

In excavating the foundation a couple features from that past were encountered. One was an old out house pit. My first experience with that was within my own yard as our home predated sewer by approximately 50 years. But the brick feature was something else. In the early days of this community water supplies were challenging as the community is on a peninsula with no major streams. One solution was to construct cisterns to store water.

From a geotechnical perspective, features such as these fall into the somewhat unexpected category, but features inconsistent with the geology should always be expected in urban environments.

Besides the unexpected anthropocene features, I enjoyed checking out the soil profiles the excavation exposed.

Dark organic rich soils overlying very compact glacial till

Closer view of till/topsoil interface

The glacial till at this site is a bit sandy. It will drain very slowly despite its extremely compact condition. The upper soil just above the till though will hold lots of water and in this case has developed into a thick organic rich soil. But with removal of the thick top soil layer also removes water storage capacity fro stormwater.

Besides the unexpected anthropocene features shown above, glacial till or drift can provide some other difficult surprises - big erratics.

This granodiorite boulder was carried from the British Columbia Coast Range

And for the excavators - not a bad view


Dan McShane said...

Not specific enough in light of 27.53.30(9) and 27.34.220 and a review of PTMC. And you made another assumption that besides code was incorrect.

Anonymous said...

It seems odd that you continue to delete these comments. The fact that you site 27.53.30(9) means nothing. Was the site that was destroyed in your pictures formally evaluated? If it was evaluated and determined not eligible for listing that is one thing. If it was not evaluated, that does not mean it is not eligible and therefore not subject to the law. I am unclear on your citation of 27.34.220, which provides the powers of the SHPO, who is also the Director of DAHP. I am not clear on what my other assumption was.

Perhaps you should contact Dr. Allyson Brooks, Washington State SHPO for her opinion on the matter. She has tirelessly fought to make Washington state a leader in the field of historic preservation and would likely be very interested in the impact on the archaeological site documented in your blog. Her email is and her telephone number is 360.586.3066. I am certain that she or her staff would be more than willing to provide their insight.

I am quite a fan of your blog, so please do not feel as if I have any malice towards you. However, if you indeed are able to see impacts happening on archaeological resources, I would hope that you could spread some of your wisdom to help preserve the past.

Dan McShane said...

Greatly appreciate the anonymous comments that brought up the concern about potential archeological resources. The post was about dealing with the engineering geology issues that old past excavations can cause, but it should be noted there could well be historic archeological resources as well. The best and easiest way to deal with that as an excavation contractor is to call the DHAP as suggested. Regrettably, I felt the initial comment was an overstatement that was premised on assumptions that were indeed not true. But that said, it was a missed opportunity on my part to talk about the archeology/historic values that so often come up when folks are digging. In this particular case I was not involved in that portion of project so it was not on my radar. Perhaps well worth a future post. Regards.

Anonymous said...

Thank you very much Dan. I appreciate your willingness to address this on your blog. Particularly as someone who is close to the soil or close to those who might be digging. You are another set of eyes on the ground and advocate for the history that lies beneath our feet even if it is relatively recent geologically speaking. Archaeological sites are endangered resources and our SHPO has worked to make our state a leader in their preservation. But, that preservation will only happen if people have the knowledge and people care. I greatly enjoy your blog and look forward to many more posts.