Rip-Rap and Siltsoxx and Clay (Oh My!)

By Tony Heath, Project Engineer

This blog post covers Tuesday, August 27th.

Howdy!

After missing out on Friday due to scheduling and Monday due to rain, today was a big day at the garden as we removed the section of sidewalk between the north and south cells in order to remove the old cracked drain pipe and to install the new 12″ HDPE (high-density polyethylene) culvert. 

After the yesterday’s rain, things were pretty wet this morning at the rain garden. While the south cell is still acting like a sponge, the north cell is holding water where we’ve dug out the rock and excavated down to clay. Most of the water has receded and you can see flow tracks where it had pooled along the edge of the sidewalk before flowing down into the bottom of the swale. 

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Although the south cell had sopped it up, you can see that the water level had gotten high enough in the north cell to flow back through the old drain pipe and into the south cell, washing out the mulch around the pipe opening. This type of washout is very common around concentrated flow areas like pipe openings. To prevent washout in our finished garden, we held onto some of the larger stones from the original build so that we can place them in an apron at each pipe end. This breaks up the concentrated flow’s impact and reduces erosion.

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As we were excavating the trench to lay the new pipe, we uncovered the electrical conduit that we had just buried during the initial excavation. It didn’t occur to us then, but the conduit was installed right where the pipe needed to go. Fortunately, the conduit is pretty flexible and to resolve this issue, we raked the mulch back and lowered the conduit so we can lay the pipe on top of it. I should have caught this when they were initially installing it, but the bioretention mix is easy to work with and we were able to get things squared away. 

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Once we’d dug the trench, Stephanie used the hydraulic jack on her back-hoe to break up and remove the sidewalk between the two cells. This sidewalk was a little thinner than elsewhere on the site so it broke up pretty easily to load and haul away.

Once the sidewalk was removed, we refocused on the preparing the ground to accommodate pipe installation. As mentioned, we’re placing rocks at the ends of the pipe to prevent washout. So, Stephanie excavated out additional soil near each pipe end for the rip-rap stones. We lined the bottom and sides of these areas with landscape fabric to prevent soil migration into the stones and minimize fouling over time. While Josh and Stephanie dug the trench where the sidewalk had been, I began placing the rip-rap stones. Once the pipe is installed, we will add more stones to fill out the hole and will replace the soil and mulch around it to cover the landscape fabric.

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This afternoon, we had a guest come by the rain garden to ask if he could borrow some of our clay! Dr. Ryan Shosted from the Linguistics Department teaches a class on the Hittite language, which was spoken in the Anatolian region of Europe and the Middle East during the Bronze Age. Hittite was written using cuneiform script on clay tablets and is one of the oldest written languages in the world. Ryan was collecting a clay sample to bring to his class, which he was sending out later that afternoon to look for clay so they can practice writing cuneiform on clay tablets of their own.

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Tomorrow promises to be a busy day at the rain garden as well. We’ll finish installing rip-rap stones at the north end of the pipe and then prepare the bed with sand to make sure our slope is correct before installing the 12″ pipe itself. We’ll also be installing a section of conduit, which will run alongside the pipe and be used to run electrical cabling for the monitoring equipment. We also received our delivery of Filtrexx Siltsoxx this morning, which we’ll begin installing in key areas tomorrow as well. Siltsoxx is an erosion control product recommended by Dr. Rabin Bhattarai, Assistant Professor in the Agriculture & Biological Engineering department. Erosion control is an essential, yet often overlooked, part of rain garden construction. But not for us! We will lay the Siltsoxx around the perimeter of the rain garden to prevent erosion and sedimentation of the exposed areas of soil before we can get the mulch installed and ground cover established. Many thanks to Filtrexx’s Dr. Britt Faucette for helping us with this product. Check back for more information about our experience using Filtrexx Siltsoxx!