I have a situation where we need to dig a 15' Deep trench for a 10' dia. x 60' long water tank. The trench only needs to be open for about 1 day. OSHA requires us to shore the trench or layer it back. Is there an easy and cost effective solution shoring the trench. The solution can bend, but it cannot break. Assume 1. We cannot layer back the slopes, 2. We have no room to tieback the top. Any suggestions? Thank you.
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1 day shoring to meet OSHA. 1
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It often seems like bracing a trench can cost more time and money than what you are putting in the trench. I do not need a rigid structure, like alot of the long term solutions show. The structure can bend, but it cannot break or overturn. 1) The trouble with jacks, is they get in the way of inserting the tank. 2) The trouble with having someone drive in sheet piling is that it is going to cost $40,000.
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A 15' deep trench is approximately twice the height of the room you are sitting in. Do it right. Install sheeting. Slope a couple of feet off the top and install some cantilevered sheeting.
There is no mention of soil type or water/seepage. Usually, the bottom 4' can be essentially vertical, if and only if, the top portion thereafter is properly sloped or benched (and if soil is suitable/stable). Advice from geotech is required. No short-cuts I hate to say. Why can you not slope: property line issues? You have to think about men going down there as well (I presume, to level the bottom or compact the base?) Do not take a risk!
I serously doubt you could install the 60 feet long tank with steel shoring boxes in place. Same thing with struts. tie backs, sheet piling, or a laid back trench would work. Soil stabilzation using grouting might work. I can think of no temporary methods that are cheap.
A 20' diameter water main was constructed in DuPage County, Illinois, in the 90's. It stretches a few miles long, and in the dead center of streets. It was done using the steel shoring boxes so there was ample driving space left besides the excavation. The general contractor was KENNY, I believe.
cntw1953 - pipe comes in short segments, jointed together. for large pipe these joints are usually only 8 - 10 feet long. therefor the pipe joints can be inserted between the struts on the boxes which are slightly longer. However, this is a 60 feet long tank and I assume it does not come in segments, just one piece (WDS1 - correct me if this an incorrect assumption). if so you would need 60 feet of clear space (without struts) to drop the tank in. how will you shore this 60 feet long section?
first of all the soil would have to be suitable, ie: dry sandy soil - would not work. Wet clay soil, maybe? Second - several inches deep would not provide global stability and the trench wall could still collapse. Third, it is not quick or cheap. I believe sheetpiles would be cheaper.
The process is described below and doesn't sound cheap or fast.
"Pipes would first be installed using directional drilling at an angle ... Then, super-chilled salt water would be continuously pumped and recycled back through the pipes to freeze a four-foot thick layer of the soil beneath and on the sides of the pits..."
The process is described below and doesn't sound cheap or fast.
"Pipes would first be installed using directional drilling at an angle ... Then, super-chilled salt water would be continuously pumped and recycled back through the pipes to freeze a four-foot thick layer of the soil beneath and on the sides of the pits..."
Here is an interesting article "Soil Freezing Technology is Gaining Ground" at this address :
Commercial soil freezing would most likely be extremely expensive for the original situation described by wds1. This is a very small shoring project. Again, if possible, slope a couple of feet off the top and install some cantilevered sheeting that is about 12' high.
Yes, freezing, using engineering design, can work. You need someone that knows how to do it and is willing to put their career on the line to sign off on a system that will keep people alive.
Leaving something open to the air to freeze a bit of it is ridiculous, and never an approved method that I have seen.
Leaving something open to the air to freeze a bit of it is ridiculous, and never an approved method that I have seen.
Ground Freezing involves pumping chilled prine through a network of wells. It is highly spealized and generally the most expensive option. OSHA lists some trenching details. Anything (including all steel sheet piling) that does not comply with these details must be stamped by a PE. Shoring boxes need to be certified by a PE (usually included in sales documents) and must be on site. Boxes have limitations on depths of excavation. The biggest boxes I have seen are 30 feet cclear. Those are very heavy and require a couple large backhoes or a good size crane to handle. If you have a stiff clay, you may have some options. If you have sand, I agree with PEInc that your best bet is sheeting or soldier pile and lagging. I would suggest you have a local geotech look at the site. In my opinion, $40,000 seems like a really good price ($20 per sq foot) Note that a leading cause of construction fatalities is trench and shoring collapse and it was generally in an excavation that would be open for a few hours or so.
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