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ditch retaining wall construction
Thanks ! I'm going to do some serious research on this.
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ditch retaining wall construction
If the water is moving, you need to get the bottom of the wall below the sediment bed of the creek, or the bed will erode out from the bottom of the wall. You also need embedment at the bottom of the wall to resolve the tie back force at the top, otherwise the bottom of the wall fails, ( pushes out ). For embedment, you should have at least as much in the soil as the height of the retaining wall.
If you are at the water table, jetting 5" cedar posts into the ground with a water jet is a simple and fast way to set the structure of the wall. For a water jet, use a 10' piece of 3/4' pipe on a 2" pump. The water jet will blow the hole as quickly as you drop the post into it. The challenge is holding the post in place until the soil sets the post after you remove the jet. Once the posts are set, use the stringers at 2' centers vertically as you described. I then use galvanized barn sheeting( about $.50 / sq ft ) and again jetted it in behind the stringers. Cap the whole thing off with treated 2 x 8's.
The setback of your deadmen is deternined by the height of the wall and the angle of repose of the backfill soil. As a rule of thumb, come back from the wall at least as far as the wall is high above the bottom of the ditch and then add as necessary to make sure your deadmen are in virgin soil. There are two types of deadmen depending on how they develop their resisitance to movement. You can use passive earth pressure, or depend on the contact area of the deadman against movement. You don't need a lot of mass, just a large contact area to resist movement. Often, a continuous ditch is dug along the setback distance and filled with concrete and 2 rows of no.4 bars. Tie the posts back with rebar or stainless wire rope. It is good to slightly pre-stess the tie backs before backfilling, otherwise the posts could move until the tie back is stressed. The other type is mass. Dig a hole the proper distance back and fill with concrete. Add a 3/4" stainless ring with 8" of embedment in the concrete.
To calculate the tie back force at the top of the wall, it can be approximated by the following:
P (force/ft of wall)=[(height x width) / 2] x 125lbs/cu.ft.
This resultant force is applied to the wall by the retained soil about 1/3 down from the top of the wall. This is the ideal elevation for the tie-backs. This is per foot of wall, so multiply this by your post spacing for your tie back force.
Height... is the height of the retained soil
Width.... is the width of the retained soil, which is dependant on the angle of repose. Assume 45 degrees conservatively, and the width is 75% of the height.
Looks like a mess at first, but you need to hold back the weight of a wedge of soil. Keep in mind that any vertical loads along the wall will result in additional horizontal reactions on the wall. Add 30% to the tie back force for any vertical loads along the wall. Add a good geo-fabric behind the wall before backfilling to allow water to pass through but not soil. You want to avoid building any hydraulic pressure behind the wall.
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ditch retaining wall construction
Exelent information ! how long will the steel sideing last ? Why cedar posts instead of treated 4x4? On the subject of steel against treated wood, Ive always been told not to do it since the metals in the treatment cause accelerated corrosion where the wood contacts the steel.
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ditch retaining wall construction
In many places, treated wood is not allowed in waterway contruction. Cedar will hold up extremely well without warping, twisting, or rotting. It will also not leak toxins into the waterway. The galvanized barn siding should last at least 20 years. We haven't replaced any for the past 15. You are right about treated wood and metal. The posts and stringers are cedar. The treated cap won't effect the siding.
This makes for a nice looking wall, and are pretty easy and cost effective to construct. At last years material prices, they were running about $9/ft.
Good luck
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ditch retaining wall construction
In many places, treated wood is not allowed in waterway contruction. Cedar will hold up extremely well without warping, twisting, or rotting. It will also not leak toxins into the waterway. The galvanized barn siding should last at least 20 years. We haven't replaced any for the past 15. You are right about treated wood and metal. The posts and stringers are cedar. The treated cap won't effect the siding.
This makes for a nice looking wall, and are pretty easy and cost effective to construct. At last years material prices, they were running about $9/ft.
Good luck
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04-27-2004, 17:48 
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