Shoring sewer trench 5

[beameupscotty]

I recently had a client ask me if I would approve using timber shoring in place of metal cage to lay sewer 10 feet deep in type C soil. I told my client that this was not an area of my expertise and that I would need to consult a geotechnical engineer for an alternative design. While browsing OSHA site I found a table which described the dimension of cross braces, etc., however, many disclaimers regarding soil and construction conditions were sited. My question is should I recommend a design be made by geotech or is this usually more expensive then continuing to use metal cage. The job is small (<2acres) the line is relatively short (300 lf).

By the way, the client was suggesting 2" x 4" braces and 1-1/4" plywood, which to me sounds way under designed. I would not get in the trench with less than at least 6" x 6" braces.

 

[dirtsqueezer]

I've never heard of timber shoring for a trench. It sounds like you've created a good opportunity for yourself with that OSHA reference, though. My main concern would be liability, balanced on the other hand by being helpful to my client, which is always good business. It seems like if you were to pass along the reference to OSHA as is, you would be passing along the disclaimers, too. I personally wouldn't touch the situation with a 10-foot pole, but if you can do so by simply forwarding an OSHA reference without any of your own input, you'd be helping your client without having lost any sleep or professional leverage. I am just a sprout, but that's how I'd do it. Best of luck!

 

[PEinc]

Timber shoring is commonly used by sewer contractors, mostly in urban locations, to shore pipe trenches. The system usually consists of vertical 2 inch to 4 inch full cut timber sheeting boards supported by 2 or more levels of horizontal timber wales approximately 6"x6" up to 12"x12". The wales are then cross braced with timber struts similar in size to the wales. The vertical boards are advanced downward as the trench is dug. Design-wise, you can't go very deep before the timber sizes start to get too large. In addition, the locations of the cross braces and wales have to be planned so that they do not interfere with the excavation and the new pipe. The advantage of this type of sheeting is that you can work around underground obstructions such as other pipes that cross the trench.

Timber shoring, when designed and installed properly, protects the workers in the trench and adjacent minor structures. A metal cage (I assume you are refering to a trench box or shield) protects the workers in the trench but does not protect or support adjacent structures very well. This is due to the large amount of open excavation required to initially set the cage and to advance the cage as the pipe is installed and backfilled.

Designing and installing timber trench sheeting is a dying art. The old timers really haven't passed down enough of their tricks or methods. The OSHA manual probably has as much information on the subject as you will find anywhere.

Years ago, many sewer contractors just built the timber shoring without engineered designs. They knew what worked (or thought they did). However, today, what they did years ago does not work anymore - on PAPER, that is. Nowadays, everyone wants a design submission for approval and what worked for many years no longer meets newer criteria, codes, specs, etc.

 

[cvg]

unless you have a signed contract with this contractor, I would not approve his shoring design. This is usually not done, and if you do you will be assuming liability. My response would be to require the contractor to submit a design, stamped by a structural or geotechnical engineer.

 

[Focht3]

I agree with [blue]PEinc[/blue]. Timber shoring can provide a safe working environment under the right site and excavation conditions. But it requires an experienced timber shoring designer - a disappearing breed.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[BigH]

So - if PEinc and Focht3 know any of the dying breed - perhaps they can push the guy into putting down a "how to" book of timber shoring design! With pictures, tricks, etc. - a tome for the ages. Only comment I would have is that the cross-braces can be of steel if practical timber sizes get too large.

 

[PEinc]

BigH is right about steel being a good option to the heavy timber wales and braces. However, my experiences indicate that a contractor who usually builds with timber shoring usually does not like to mix in the steel bracing. Also, in some unionized locales, a different trade union may be required to install the steel members. The contractor does not usually like to add another trade union member to his historicallly stable crew. Sewer crews like to use chain saws. They usually do not bring welding machines to a trench sheeting job.

 

[beameupscotty]

Thank you all for the responses. I have spoken to the client and convinced him to proceed with the steel cage. Thanks to PEinc for the approximate dimensions and cvg for reinforcing what I believe about assuming liability, also, if I don't feel certain about it I let some one who does feel certain about it stamp and sign it.