Ever wondered what those metal baskets with rock in them that line some streets and rivers were called (come on you know you do)? Gabion Baskets! What are gabion baskets? Read about gabion wall systems here.
The logic behind gabion style retaining walls is that they are good to use in areas where there is a lot of water in the ground. The leading cause of retaining wall is not the weight of the dirt behind the wall but the extra weight and pressure of water in the soil. A gabion style wall allows water to freely flow through and it relieves pressure. Look closely and you will see concrete or stone retaining walls will have small clay tiles or drain holes near the bottom to allow water to flow out. I don't have a picture but the next time you are driving along Route 9 in Parkville just look for them.
I'm not a fan of gabions except in certain situations. The installation is very costly. It takes a lot of labor and machine time to assemble and place the metal baskets. After being placed, the baskets must be filled with small rocks before another basket can be placed on top. Over time, tree roots can grow into the baskets and expand and break the metal and ruin the wall. Also, they kind of look ugly. They aren't forgiving if one brushes up against them. They sometimes are the best option when a retaining wall doesn't have a good base to use for installation. The Turkey Creek Trail used them because other options were not feasible. The section of Barry Road from the Coves to Platte Purchase has them all over.
That being said, I do like to use them to establish the foundation for stream bank protection but only at the stream level. I feel they provide the best base and flexibility for unstable soil. As a creek moves or erodes, the baskets can hang or settle without impacting the entire system.
The Shoal Creek Trail has the best illustration of when they should be used in my opinion. In this case, the gabions sit in the creek and provide the anchor for the rip-rap which was installed on the eroding slope. The picture below was taken one year after the trail was completed and vegetation has already started coming back and growing. The gabions kept the stream bank in place allowing nature to re-create itself.
In this location the stream in the picture is a tributary of Shoal Creek. I assume the water shooting out of that 48" storm pipe is fast enough to cause the 90 degree bend as shown in the aerial below.
In this location, I was concerned that the water would keep eroding that stream bank and would was out the trail. With the gabions and limestone in place, the bank has re-vegetated which prevents erosion during big rain events. I could have done that with limestone but it would not have stayed in place as well as the gabion baskets. Over time tree growth will overtake the gabions and no one will even notice they are there nor will they be needed from an engineering perspective.
The gabions act as a block for the water in the tributary. As the water in Shoal Creek rises, the water in the tributary does also and slows down. As both streams fill up, the gabions aren't needed as the water isn't eroding and the rip rap protects the bank.
Over time I will check this out and see if my design holds up. If it does, I'll use it elsewhere. If it doesn't, I'll learn from it and make the next one better. Due to the cost and issues I outlined however, don't expect to see them around much on projects I design.
Maybe my dislike of gabions stems from too many late childhood nights watching things like this.....
Clip credit and one can watch the end of the match here at the official WWE website.
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