by Tony Heath, Project Engineer
Howdy!
Friday was a busy day, capping off a very busy week at the rain garden. With rock removal finally complete, the Facilities and Services crew could begin excavating the south cell in earnest. There was quite the mix of soils in there with a layer of black topsoil sitting over top of a layer of clayey fill. The soil profile is pretty clearly artificial with large sections of contrasting soils sitting right next to each other. In addition to the clay and topsoil, Josh and Stephanie found a large amount of bricks, rubble, and concrete, which were either a remnant of an earlier foundation or structural fill that had been dumped some time in the past.
At around 8:30 a.m., Josh and Stephanie uncovered an underground electric line buried about 18″ below existing grade between 7-10 ft inside of the sidewalk running east to west. At first, we tried to simply dig under the cable to get down to our sub-grade elevation, but we soon discovered that underneath the cable was a concrete-encased duct bank marked by sand. Josh explained that when they bury ducts or conduit, they will often back-fill with sand so that when you dig down in the future and see sand, you’ll know that something is below. At this point, we decided it would be best to leave that soil in place – especially since we plan to back-fill with mostly sand anyway – and dig down to our 2.5′ sub-grade depth beyond that. We made good progress the rest of the afternoon and got about half of the south cell excavated. We shipped the topsoil to Landscape Recycling Center, where it could be mixed with sand and compost and used as back-fill. The clay was sent to the “straw pile” to be used as fill material elsewhere on campus. On Monday, we’ll continue digging out the rest of the cell, making sure to use a toothed bucket on the final pass so that the ground is scarified and the pores are opened up before we begin to back-fill on Tuesday.
The buried electric lines. Photo by Tony Heath. 
The excavated south cell. Photo by Tony Heath.
Meanwhile, Ryan Pankau, Extension Horticulturalist, and Greg Smith, Arborsmith Certified Arborist, met onsite at 6:30 a.m. Friday and used an airspade to perform a root survey of the area around the red oak. They took 18″ borings, pictured to the right, along multiple transects around the tree to develop a map of the root network. The roots of the red oak were pretty extensive, only tapering off when they got within 10 ft of the sidewalks. They even discovered some roots from the London Planetree, which had made their way over.
We also had a visit from Diane Plewa, Plant Diagnostic Outreach Specialist at the University of Illinois Extension Plant Clinic, to talk about the health of the red oak tree.
Based on their findings, Greg recommended that we take steps to care for the tree to offset any damage we’ll do by excavating. He recommended mulching as well as the use of a growth regulator to discourage growth of the crown, allowing the tree to allocate more of its resources to root growth and defense after the disturbance of construction. Greg also pointed out how anthropogenic the soil profile was with huge variance in soil character from spot to spot, especially when you got near the sidewalks. As follow-up, Ryan is preparing a diagram of the borings they took to map the soil density and we will revise the swale locations to shift them closer to the sidewalks to minimize the potential impact to the trees.
Finally, we say goodbye to Eric Green, Construction Manager, whose last day was Friday. On Monday, he starts a new position. Happily, we’ll still be working with him, just in a different role. Thanks for all your good work, Eric! We could not have done this work without you. Best of luck in your new role as SSC Coordinator!
All in all, it was a good week and we are looking forward to getting back out there on Monday.
redoakraingarden.org 