Yesterday, my only thinking regarding above is perhaps they wanted to stabilize the site with a denser type of soil or something. I don't know of any underground water courses nearby but, I guess the soil maybe a bit silty here and needs to be reinforced for this use.
From Thursday..the big lane and sidewalk closure here is a massive pain in the rear end, and must be a big traffic bottleneck in rush hour...its already real bad in normal rush hour.
I would read the deep dig as largely to support the stormwater catchment system, which has interim on site storage capability (of excess water) in the material itself.
From the Storm Water Report:
They are also tapping into watermains and sewers here, not for the field, but for the washrooms located at the rear/south of the site.