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