Temporary trench excavation 8

[NorthCivil]

Hi all. A bit of background.

I'm an EIT working for a company that does structural engineering for wood framed houses. As EOR on these projects we get roped into doing a bit of Geo - we aren't experts, but we sign off on bearing capacity if we can be sure of it. I'm in an area that is mostly glacial til. 2000psf bearing is what we usually allow for, and most soil in this neighborhood far exceeds it. When things get tricky, we call in a geotech.

In the past few years, during construction, permit departments have started to require an engineers letter signing off on "safe excavation", ie. it is safe for the workers to work in the excavated area. Good practice is followed wherever possible - sloping, benching where possible, tarping the edges so its protected from rainwater, setting back overburden. However, due to the nature of the sites, big footprint houses on small lots, straight cut excavations are often required, 10ft deep or so. once we get down ~2ft, is when the stiff clay layer normally begins. We will sign off on the excavation for a duration of 4-8 weeks. Due to the economics, shoring is not feasible.

From what i understand, it is normal for structural engineers in my area to sign off on it, and so far, ive heard of no disasters. It is done fully on a gut-feel basis. Being new to this business, At I'm feeling uneasy about it.

Welcoming thoughts, criticism. Are there any guidelines out there on the safety of these vertical cut excavations?

 

[darthsoilsguy2]

Slope stability, excavation rules for local (canadian) worker safety regulations, and the ability to accurately perform field soil classifications (a skill best developed by performing many lab classifications to calibrate yourself to) are the references that will best get you on your way, after that shoring design, and if you like all that but want more risk and adventure... then you study rigging and make friends with lots of people with big toys.

there were a couple of comments in your last post that make me want to take another stab at explaining a potential hazard..... the cohesive value to the clay requires an understanding of how thick the soil bank is behind the open face. cohesion could even be considered a function of that thickness. imagine a direct shear test where you run it on two identical samples of undisturbed clay, but the second time you cut the clay sample in half vertically, and remove the back half the sample and stuff it with cotton (you could put sand in as filler but we're testing cohesion and we don't want sandy internal friction angle data corrupting the test since that sand won't help the bank) ... performing the test this way, you should expect wildly different soil cohesion results. the clay (maybe a silt) looks fine on the surface and the concern about slip planes are not inside the clay but the vertical cut behind it. to be fair, i would believe the undisturbed soil likely goes pretty close to the existing adjacent bldgs since those excavations got there first and would likely have stayed pretty tight too, but as you know "likely" isn't good for reports. the concern i raised is not hypothetical. this condition falls many times and you will see it fall too. Fortunately, the vast majority of the events will occur during excavation, overnight, during rains, or when nobody is right there in the bad spot. The most common scenario for this classic failure is when the contractor is trenching in a new utility adjacent and parallel to an existing utility trench and the bank sloughs off back to the old vertical cut from the other trench wall.... and like i said... usually happens during excavation when nobody is down there and likewise goes unnoticed.

 

[BigH]

I believe that OSHA permits, for residential "basement" excavations a 2 ft offset for each 5 ft depth. You would then be able to dig "vertically" 5 ft, make a 2 ft bench and then down 5 ft to the bottom level.

You might want to consider the information given in Peck Hanson & Thornburn which I have attached.

 

[PEinc]

File does not open for me, BigH.

http://www.PeirceEngineering.com

 

[cvg]

this type of excavation is typical in our area, and has yet to fail.

this is what has me concerned. I can guarantee, this type of excavation has failed before and will fail again. This is precisely why OSHA came up with excavation safety rules

https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10931

https://www.osha.gov/Publications/osha2226.pdf

 

[Canuck65]

NorthCivil,

I think you have asked a good question and there are many very good responses above. I have a couple of other items to add, for your consideration. I also work in Canada.

1. When I operated my own small structural engineering firm, my insurance policy (from a nation wide Canadian insurer) specifically excluded two areas of practice, A) geotechnical and B) environmental. If I had wanted to provide geotechnical opinions, such as you have described, I would have had to amend my insurance policy to include geotechnical engineering and my premium would have increased accordingly. I understand these are common exclusions and from working at other firms can confirm that surprisingly many principals of small to medium sized firms are unaware of these exclusions and have opened themselves up to uninsured liability by taking on such projects similar to what you describe, i.e. providing minor (sometimes major) geotech opinions on their structural projects.

2. I have worked hard on developing professional relationships with some smaller geotech firms in my area. They are a phone call away and I often recommend them to Contractors/clients when a simple geotech opinion is required. For things like you describe, they would drop by the site and provide an immediate verbal recommendation and written recommendation within a day or two for $500 to $1000 (normally towards the lower end of the range). I consider it part of my job as a structural engineer to convince/sell the client of the benefits of paying this small amount of additional money for a geotechnical engineer, which is easy because I truly believe this is the right way to proceed. Please note that the geotech opinion could impact your projects in one of two ways, if could be status quo as they agree with past practices, or they could find that what has been done in the past is inadequate and recommend shoring, etc.... I think at a minimum it would be worth recommending contacting a few local geotech's to discuss the projects then testing out a geotech on a few projects and evaluate from there.

Good luck.

 

[ATSE]

Big H - Same as PEinc, can't open your attachment.

 

[BigH]

Sorry, putting forth the discussion and figure regarding Figure 8.4:

 

[chicopee]

Being in Canada, I would expect that the excavated areas would freeze solidly for a good portion of the year. If that is the case when the exposed surface are solidly frozen, there may not need temporary supports since these exposed surfaces would be as hard as rock. Just a thought and Canadian safety laws may allow. In our case this point was made clear to OSHA which agreed with us when they did an inspection of our site. Sheet piles could be an option or a bracing system of H beams which can be placed where the workers are located and which can be quickly dismantled and moved around by equipment.