Sustainable Stream Protection – 30 Feet Down

The world looks different today than it did yesterday. As we face the unknown of the coronavirus, Sain Associates will continue to bring weekly content to the blog. We find comfort in providing uplifting news but also bringing a sense of routine while we navigate through this fluid environment. 

By: Darren Hamrick, P.E., LEED AP, Team Leader – Civil Engineering Division

Before the recent COVID-19 lock-down, our projects seemed to be moving at rocket speed, with tight timelines for construction to meet opening dates. The priorities revolved around expediting and obtaining all the required permits as quickly as possible. However, sometimes it can help to step back, take a deep breath, and look at the bigger picture to overcome significant obstacles by breaking them down into small pieces.

Recently we worked on an industrial project that required significant investment installing bottomless culverts over existing streams to maintain an existing stream bed while allowing construction of a large industrial project (which would create over 400 new jobs).

Most of these stream segments were very narrow and could be stepped across, but in some areas, they had pools as wide as 16 feet. We initially looked at installing a conventional, large “arch” bottomless culvert across these streams. However, due to the broader stream segments and heavy loads, the cost of the arch culverts was several million dollars.

Furthermore, we were filling significantly (greater than 30′) over these culverts, so the structural requirements were quite heavy and needed robust structural capacity to completely bridge and protect the stream while supporting the project above. We collaborated with the geotechnical and structural engineer to provide pre-cast reinforced concrete planks that could bear on the soil around the stream banks and be placed over the stream without disturbing it.

We began by obtaining a detailed field-survey of the stream alignment and widths, and we designed each bridge plank to fit together and span the varying widths of the stream. All of these individual planks were pre-cast with lifting lugs, which allowed them to be carefully set in place and pieced together like a jigsaw puzzle that meandered along the natural stream.

The final bid from the contractor had a total installed price of approximately $1.75M, which was much, much less than the $5M estimated cost for using huge bottomless arch culverts. While the straight concrete planks are not as architecturally pleasing as an arch culvert, they met the functional requirement, and they were buried beneath 30′ of fill material. The owner who was responsible for funding construction costs thought they were beautiful.

Bridge planks were pieced together like a puzzle to follow the stream alignment.
Broader segments of the stream had to be spanned with thicker concrete planks.
Some portions of the stream were much narrower and well-defined.
This plan illustrates irregular-shaped panes where the stream meanders.
Larger planks had to be handled and set in place very carefully

Leave a Reply