Mixing the soil and moving the lamp

By Tony Heath, Project Engineer

This post covers the garden’s Phase I: demo/prep progress for the past two days, August 13th and 14th.


TUE, AUG 13: Yesterday, work progressed in the garden along two fronts. While Josh and Stephanie continued their soil excavation of the south cell, the Facilities and Services Electrical Shop (Shop 25) worked on the garden’s north side for the lamp post relocation. The Electricians began by using a jet-vac to blow high pressure water down into the hole, which cleared the dirt away without damaging any of the utility lines or roots underground. 

Facilities & Services Shop 25 Electrician using a jet-vac to clear dirt around the lamp post. Photo by Tony Heath.

This took a while because the foundation for the lamp post extends 54″ underground and is packed in with road base stone. They did uncover a pretty sizable root which had grown between the two sidewalks while they were digging and were able to leave it in place. Once they had the light post out, they initially thought that they would be able to leave the adjoining sidewalk and pull a conduit underneath it to run the cables to the new post location. Unfortunately, the stone was packed too tightly underneath the sidewalk and the crew ended up needing to pull the sidewalk panel out entirely. They will replace it when they come back to expand the sidewalk. The Electricians finished up around 1:30 p.m. and closed off that section of sidewalk until it can be replaced.

The sidewalk panel needed to be removed to run the cables to the new lamp post location. Photo by Tony Heath.

The south cell’s excavation continues — with an end in sight. We would have finished excavation yesterday afternoon, but discovered some unmarked conduit near a lamp post in the east corner and needed to work with the electricians to determine a solution. While a lot of the conduit was abandoned, some was connected to the exposed cable that Josh and Stephanie found last week, so they connected it to the conduit installed that morning and pulled out the abandoned conduit. Excavation can commence (and conclude!).

After speaking with Paul at the Landscape Recycling Center, we found out they now have plenty of our topsoil for the bioretention soil mix. With that in mind, we stopped sending topsoil to them and are stockpiling it on-site.

WED, AUG 14: Today was a bit slower of a day at the rain garden as we prepared to backfill the south cell with our bioretention soil mix tomorrow. When I arrived on site this morning, Facilities and Services’ Josh and Stephanie were working to finish excavating the east corner of the south cell beyond the lamp post where they had stopped yesterday. With the conduit running through this area and everything narrowing overall, we opted not to excavate to the full depth and instead excavated down to the top of conduit elevation, leaving a small shelf. In order to get to the far eastern corner of the garden we began removing the bike lane that extended to the McKinley Parking Lot and once excavation was complete we finished tearing out that section and backfilled it with topsoil. 

Once they were done with this, Stephanie then used a toothed bucket on the back hoe to scarify the sub-grade of the south cell. Doing this breaks up the contact layer and provides additional surface area for infiltration, while preserving large clumps of soil. Unfortunately, I was at LRC while they did this but the results look good.

The south cell post-scarification. Photo by Tony Heath.

I went by the Landscape Recycling Center (LRC) to take a look at the bioretention soil mix they produced for us. The mix follows the Minnesota Stormwater Manual design specification, which calls for 60-70% sand, 15-25% topsoil, and 15-25% compost. LRC performed the soil mixing using a large sifter which they normally used for compost. First, they screened the sand to filter out rubble, clay, and other scrap which made up about 6 cubic yards of what had been delivered. Then, they mixed the soil and sand according to our specified ratio. And finally, they mixed that sand-soil blend with compost to arrive at our final mix. The soil looks good, it has the texture of a rich sandy loam with large clumps of compost scattered throughout. At first, the compost clumps look like clay but they break up easily and you can see the rich material inside. 

They’ll start delivering the material tomorrow morning and we’ll backfill the cell by spreading the soil in 8″ lifts to achieve our 24″ depth throughout. Once we’ve got the appropriate depth throughout the cell, then we can come back and do fine grading to shape the cell and tie into the surrounding areas.

In the afternoon, Josh and Stephanie began removing the old bike path. They started at the bike racks near the Allen Hall parking lot and moved east towards the London Planetree. A bit of compaction occurred as a result of the backhoe pulling the concrete out, but nothing too severe. We’ll just have to make sure to break this compaction up before final stabilization. We finished up around 3:00 p.m. today with only a small section of bike path still to be removed east of the London Planetree. 

Stephanie removing the bike path.

Tomorrow will mostly be spent backfilling the south cell with the freshly made bioretention soil mix. Afterwards, we’ll demolish the section of sidewalk where the 2″ conduit and 12″ storm pipe will be going so that we can install those pieces and begin working on final grading.

Tomorrow should be an exciting day!

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