Rocks, rocks, and more rocks (and also filter fabric)

By Tony Heath, Project Engineer

Today was another productive day at the garden.

When I arrived this morning, the Facilities & Services crew was continuing with rock removal in the north cell. I noticed that they were using a smooth-bladed bucket for the excavator. It was more effective at pulling the rock out without bringing lots of soil with it. However, it was leaving the “ice cream” effect — closing the pores of the soil behind it, which you don’t want for a rain garden that needs to infiltrate. After a break, I asked the crew to switch buckets on the excavator to a toothed version to see if it would accomplish rock removal without closing the soil pores. It didn’t take long to see why they were doing it the other way, though. While using a toothed bucket removed a lot of soil without causing the “ice cream” effect, it left a lot of rock behind. Once I saw that, I asked them to switch back to the smooth-bladed bucket for the rock removal and we’ll just have to scarify and/or soil rip these areas before back-filling. In the images below, you can see that using a toothed bucket breaks up the surface layer of soil while big chunks of soil remain. Lesson learned.

We spent the rest of the morning working on rock removal in the western corner of the north cell near the London Plane Tree. The rock went pretty deep in this location. Interestingly there were a number of places where the rock had been buried beneath ~6″ of soil since it had been installed. I suspect that this is largely silt runoff from contributing areas mixed with dropped bark and twigs from the London Plane Tree. We ran into a few feeder roots from the tree which had worked their way into the stone but nothing substantial. We finished in the north cell around 1:30 p.m. and then moved to the south cell for the rest of the afternoon. 

Removing the rock in the south cell went much quicker than the north cell, as there were no tree roots to worry about and a wider space to work in. This rock also had much less silt fouling it up than the rock in the north cell, which had been under the trees for the last 13 years. There was still compaction underneath it where the filter fabric had been, but that is less of an issue since this soil will be removed and mixed w/ sand & compost before back-filling. Below you’ll find a video of the excavator pulling some of the rock away. 

The last thing we did today was pick up some of the decorative boulders in the south cell and move them to the area around the London Plane Tree. Once we’ve back-filled, we can put those rocks back or move them somewhere else in the garden as we see fit. 

Check back in for more updates!

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