Bridge footings

by Tony Heath, Project Engineer

This post covers Wednesday, September 11th.


With only 3 days of construction left before planting, things are really heating up at the rain garden! There were two crews working at the garden today. While Wayne and Tristan from Grounds spent the day doing some final clean-up of the site including clearing out rocks and clay clods before beginning to spread mulch in the north cell, Josh and Mark spent the day boring the holes for the bridge footings so that the concrete finishers can come tomorrow to get those poured.

The first thing that we did to prepare for boring the holes this morning was to run a string-line from the bridge abutment that had already been poured to mark out the locations of each footing. The footings are spaced 8′ on-center and the total bridge width is 9′. Once we’d marked these spots with white paint, Mark brought the excavator in and mounted the 24″ auger to bore the holes.  

When bringing the excavator in we made sure to stay out from underneath the drip-line of the trees and to only move along a single path beneath the bridge in order to minimize compaction. The borings were 3′-6″ deep and we expected that we would run into clay but to our surprise it was mostly black dirt that came out. The location of the bridge is where some of the longest-lasting shrubs from the original rain garden were located and I suspect that the high quality soil is their doing. The reason that we made these borings so deep is that 36″ is typically the frost depth in the Illinois winter. By digging down below that line we’ll have less heave over time by avoiding the freeze/thaw cycle. The extra 6″ at the bottom are for a stone base that the concrete will be poured on. Once all of the post footings were bored, Josh and Mark began digging out the abutment footing at the north end of the bridge. This will be a thicker piece of concrete that the bridge end rests on and seeing the soil layers underneath the new concrete reminded me of a chocolate and peanut butter cake.

The grounds crew made a lot of great progress today as well. They started the morning by prepping and cleaning up the site, removing stones, raking up the sod from yesterday, removing clay clods from the topsoil that was brought in, and anything else that needed to be done before the mulch covered it all up. While they worked on that, I took the opportunity to spread mychorrizal on the soil before the mulch went in. I spread 2.5 bags across the entirety of the north cell except for the areas still covered by cardboard around the base of the Red Oak tree. We still have a whole bag left, so once we’re ready to put down mulch in that area, we’ll pull the cardboard out, lay down mychorrizal, and at that point we’ll be ready to plant!

A lot got done at the garden today and even more is planned for tomorrow. Wayne and Tristan will be back out there to continue spreading mulch, Josh and Mark will excavate and prepare the sub-grade for the south bridge abutment and lay stone for the footings, and once they’re done Eric Knisely and the concrete finishers will be out to pour the footings. It’s going to be an exciting day! Can’t wait!

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