Our intention to add a second floor to the home at 205 Crichton Street requires an extremely good understanding of how the existing building was built. In preparing as built architectrual drawings of the home, investigated the size of the footings and the bearing capacity of (what we thought would be) the soil below. After saw-cutting the concrete slab floor of the basement to expose the footing inside the building, we found shale (a type of rock) directly under the basement wall footings. To be certain of the extent of the shale (and since we could not cut up the slab in the majority of the basement since it is an occupied rental suite) an excavation was done outside the foundation wall. Here’s a video of what we found at the bottom of the hole.
Although one might intuitively believe that rock makes for a good foundation, the shale hiding below grade was of concern for two reasons. First, rock is much more expensive to excavate than earth, potentially as much as twice the cost. Given that the design calls for a significant amount of excavation over the majority of the site (to hide parking spaces and an outdoor amenity area below grade), I saw the excavation budget growing significantly. Second, I feared that the shale found was Bilings Shale, a type of rock that degrades rapidly when exposed to the atmosphere, creating the need for additional measures to prepare the site for construction and additional excavation costs.
What shale we do? As it turns out, nothing. A bit more exploratory digging and our geotechnical engineer’s (Kollaard Associates) review put my mind at ease. First, the shale is relatively easy to scrape away (without the use of an expensive hoe ram), at least as far down as we need to excavate. Second, it was determined that the shale uncovered was not of the Billings formation. Bottom line: we have solid bedrock to build on.