A common mistake on Richmond Hill jobsites is assuming that a stiff clay during summer excavation will behave the same way after a wet October. The difference comes down to Atterberg limits. When the water content crosses the plastic limit, even a competent-looking till loses bearing capacity fast. We run the full suite—liquid limit with the Casagrande cup, plastic limit by the 3 mm thread method, and the plasticity index—to give the geotechnical engineer the numbers needed before footing design. The grain-size distribution and the Atterberg limits together define the Unified Soil Classification, and without both, the NBCC Part 4 bearing pressure assumptions become a gamble. In the Halton Till that underlies much of Richmond Hill, we routinely see intermediate-plasticity clays where a small shift in moisture can trigger serviceability problems in shallow foundations.
A plasticity index above 25 in Richmond Hill glaciolacustrine clay almost always means we need to control moisture during compaction or switch to a deeper bearing stratum.
