Installation of a caisson at subgrade depth of 65 ft (19.8 m) and adjacent to support for heritage building Monitoring plan Forces due to frost behind and beneath any exposed ground surfaces can be many times greater than the normal forces an excavation shoring system is typically designed to withstand. According to Environment Canada, frost can occur at any time between Frost Impact on Shoring Forces due to frost behind and beneath any exposed ground surfaces can be many times greater than the normal forces an excavation shoring system is typically designed to withstand. According to Environment Canada, frost can occur at any time between November 1 and May 15 and has been known to heave entire buildings several inches. At local excavations, November 1 and May 15 and has been known to heave entire buildings several inches. DEEP FOUNDATIONS * JAN/FEB 2020 * 91