The trucks are filled with snow from somewhere in the city and they are dumping it at the rear of the lot. Then the giant plows or whatever they are called - big machines with buckets in front (front-end loaders, perhaps?) - are pushing the piles up higher and higher. I expect to see some mountaineering by schoolchildren tomorrow.
Update: The snow-filled trucks apparently arrived all night. I think I heard one around 5 a.m. When I looked out this morning, one whole end of Lot 7 was piled high with snow.
This lot is 94 feet wide. The portion between the back fence and the driveway off Park Avenue (where it says "City of Plainfield Exempted") is about 70 feet wide. The snow looks to be about 12 feet high, so in cubic feet I'm guessing we're looking at 79,000 cubic feet of snow. If the pile is higher, so is the total cubic feet.
Here's a map that shows the dimensions of the space. Click to enlarge.
I will try to get some figures from the city on snow disposal. It looks like a terrific job by the Department of Public Works!