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Temporary trench excavation

Temporary trench excavation

(OP)
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?

RE: Temporary trench excavation

4
Doing life safety work on a "gut-feel basis" isn't a recipe for long term employment and will likely end when someone dies followed by the engineer who signed off on the excavation going to jail. Read the OHSA rules, and any additional state requirements, on excavations. It isn't rocket science but there are specific requirements.

Lastly, "economic reasons" is the very reason OSHA implemented these rules to protect workers.

Mike Lambert

RE: Temporary trench excavation

Good answer, Mike. Economics should have nothing to do with safety. As structural engineers, NorthCivil's company should probably tell its clients to go to a geotech engineer for the sign-off letter. You never know when that 10' cut in "stiff clay" will decide to fall over and kill someone. It happens.

www.PeirceEngineering.com

RE: Temporary trench excavation

GeoPave and PEinc,
For temporary excavation of a small site and say, 10ft deep cut, what would you recommend regarding site evaluation, monitoring, and/or exploration to prescribe a maximum cut slope and/or determine when a shoring system is required, that was above and beyond the OSHA 1926 Subpart P provisions? That is, besides refusing to be engineer of record on a high-risk, low-fee project.

My understanding of 1926 is that a "competent person", not necessarily (and not usually) a civil engineer but instead a field supervisor, can classify the soil during excavation and proceed with construction if they follow the cookbook. Get shear strength reading using a pocket penetrometer. Look up a table. Dig. No engineer involved unless the depths or slopes from 1926 are exceeded.

It is quite practical approach for general excavation and works 99% of the time, but a little surprising that OSHA allows this where the water content in the soil may change quite a bit in 2 weeks time. Dry cohesion of clay will provide extremely high values for cohesion / shear strength, as compared with undrained shear for a saturated silty clay (false sense of stability). OSHA warns against water that is "freely seeping" into an excavation in the 22 page field pocket guide, but the moisture content of the soil can change quite a bit even if no seepage is observed.

RE: Temporary trench excavation

ATSE,

I think we are in general agreement. All site safety concerns should be the responsibility of the contractor, period. OSHA doesn't require and engineer, only a competent person as you note.

As for changes in site conditions with time, the competent person needs to be evaluating conditions as they change. When conditions change, the excavation may need to be modified. Inconvenient, yes; but that is the way it is.

Mike Lambert

RE: Temporary trench excavation

design engineers design for the betterment of the finished project. Temporary construction bracing is unrelated to the betterment of the work. Temporary support is the onus of the contractor. It should be the contractor's engineer signing off on this work. I'm not sure why I'd step into the shoes of the contractor's engineer unless the contractor paid me to follow generally-accepted practice.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Temporary trench excavation

(OP)
Hi all, thanks for the responses. a bit more information

-I am in Canada, not USA. In our jurisdiction, it is required that an engineer review excavations more than 4' deep for worker safety. I wish that site safety should be the responsibility of the contractor. The excavator operator definitely has a better feel for the stability than I do. But it is not the case, by city requirements, they must receive a letter from an engineer.

- I have read our own version of OHSA rules. Good practice regarding benching, sloping, diverting rainwater. Though at times, this is not possible due to site/construction requirements.

-it is common practice in our area for structural engineers to sign off on this. I'm in a major center and there are thousands of similar projects ongoing. My research shows 2 failures in the last 6 years, on different types of projects, both with narrow trench excavations in sandy soil conditions. If failures were happening on projects of this nature, I'm sure hiring a geotech would be common practice.

-I appreciate your comments that my company is in the wrong to be engaged in this. Though as an EIT I am in no position to tell my P.Eng supervisors, some with 40 years experience, they are in the wrong. They are very knowledgeable and I have much to learn from them. I am lucky to have this position, especially in todays tough employment market. A lot of my school friends have only been able to find minimum wage jobs at fast food joints.

I have been trying to make the best of this situation, digging for any resources I can to try to bring some science to this. I havent been able to find much. The closest thing I've been able to find is resources on standup time, but it mostly is related to tunnel excavation.

RE: Temporary trench excavation

(OP)


This is an image of a not uncommon excavation we provide safety letters on.

RE: Temporary trench excavation

I would follow all OSHA requirements (or canadian equivalent) for trenching and excavation.

https://www.osha.gov/dts/osta/otm/otm_v/otm_v_2.ht...

Any excavations that violate these requirements, should, in my opinion, be shored with temporary means consisting of stitch piers, soldier beams, soil nails, etc. I personally, would never sign off on anything exceeding these requirements. We frequently do residential basements in close vicinity to adjacent properties. Anytime that the excavation cannot follow OSHA requirements we provided soldier beams will wood lagging.

RE: Temporary trench excavation

You wrote, "In our jurisdiction, it is required that an engineer review excavations more than 4' deep for worker safety."
The key here is 'an engineer." That does not say who that engineer is or should be. Your signing of the letter gives you great risk and responsibility for very little or no compensation. Push the letter requirement back to the contractor who is doing the excavation.

www.PeirceEngineering.com

RE: Temporary trench excavation

I think the biggest concern is that structural engineers are performing work that is clearly not within their area of expertise. I am sure the agency having jurisdiction is not requiring the structural design engineer to sign off. But rather is requiring a qualified, licensed engineer with experience in slope stability and shoring design to do it. Probably a geotechnical engineer would be a much better choice. That engineer generally should be hired by the Contractor and not by the Owner.

RE: Temporary trench excavation

(OP)
Appreciate the response to all above.

You are correct that the letter does not need to be from a structural engineer - it can be from any P.Eng. When soil conditions are not excellent, we bring in geotechnical engineers. The reality is, clients do not want to pay for another consultant. If we refused to provide this service, we would fail to land a lot of jobs, as it is normal for the structural EOR to provide it. We get roped into it as EOR on these tiny projects. We could kick it back to the contractor - this would be typical in commercial projects, but this is small scale residential. Having geotechnical consultants is the exception, not the norm.

This is not our area of expertise. My company will continue to provide this service, that is not within my control. I am simply an EIT, working under graybeard engineers, who engineer by feel and experience. I am trying to bring as much textbook science to this as I can.

Appreciate the resources you guys have provided me. I plan to keep investigating this.

RE: Temporary trench excavation

The photos that NorthCivil posted give me heartburn. Look at the depth of cut vs proximity of adjacent structures, and the moist soil. There appears to be a lot more faith and risk taking involved than actual engineering. May the odds be with you.

RE: Temporary trench excavation

@ATSE, I had the same thought when I saw those pics.

RE: Temporary trench excavation

i can't think of any engineering reference that would say how to handle that cut in the photo except with shoring. rules of thumb about excavation depths are for cuts in the middle of an open field. the soil along the sides of each home is a 4 ft wide sliver for the length of the building. you won't find a rule of thumb for that. there could even be an opportunity for hydrostatic forces to come into play based on some hidden condition with existing foundation drains.

RE: Temporary trench excavation

I'm hoping those two adjacent buildings have basements of a depth around that of the excavation, or that's terrifying.

RE: Temporary trench excavation

to put another viewpoint to the concern TLHS shared. my 1st statement assumes the canadian homes footings on both sides have a 5' to 6' of frost protection cover and won't contribute weight or potentially open the door to legal complaints about settlement due to this construction. i'm pointing out that the normal slope stability and excavation rules references which an engineer can hang their hat on don't address the scenario when there is a buried wall creating a vertical failure plane immediately behind the excavation. the soils in the open cut are native but at what point do those soils change to the foundation wall backfill...6"... 12"...18"...etc... and what do we wish to guess is an accurate description of that fill to say it will be stable. i would personally get in that hole, but i wouldn't feel like i could defend a letter assuring that a bank won't slough off while somebody is tying footing rebar. i know this was just an example, but i'm just throwing this out for the general public since identifying and communicating unseen and unknown risks is a valuable engineering habit to develop.

RE: Temporary trench excavation

Northcivil,

I have considerable sympathy for your predicament and perhaps your title is a bit misleading.

for me there is a big difference between a "trench", i.e. anything where width is less than double the depth say and an excavation like you show.

I can only assume you pepper your "sign off" with a numbe rof conditions such as sheeting the sides ( good idea- I'll have to try that - and maybe stat to include things like a minimum width to depth ratio, maximum time they can leave it open, call someone if ther eis any sign of soil cracking or falls / heavy rain etc etc

for true trenches, then the risk start to climb a lot and that's wher eI would start to get nervous and insist they use benching / trench boxes / sheeting / piles as required. Those sorts of supports are easy to find and use compared to large areas like you show.

Clearly you need to make sure the adjacent houses don't fall down / crack, but assume that would be part of the design, otherwise you would be truly negligent if you don't consider how something is going to be built.

don't forget at least two exits by ladders / steps.

It may not be correct, but domestic working / contractors work to a different cost base than industrial / commercial outfits, but I think you need to establish limits you won't go beyond e.g. depth, size, type of soil and then stick to it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Temporary trench excavation

(OP)
Hi all, thanks for the responses.

I have gently brought up some of my concerns with my supervisors, and discussed many of the items you guys have flagged.

Regarding shoring, it would make the placement of footing & foundation wall formwork difficult, as the formwork begins right next to the edge of the cut.

Littleinch, you are correct, I used improper terminology to describe what is going on. these are technically "shallow" excavations, with straight cut walls. You are correct, our "sign off" is peppered with all those clauses and more. We have our own limits as you describe.

Still haven't found many resources to guide me. Though, from what I see, this type of excavation is typical in our area, and has yet to fail. The cohesive strength of the clay is holding everything together, though like others have said, its difficult to quantify, and there is potential for unseen slip planes in the clay.

Though I am not keen on this line of work, I am learning much on the structural front. Going forward, when I have to deal with these items, I will try to act as a faithful agent of my employer as well as a guardian for public safety as best as possible.



RE: Temporary trench excavation

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.

RE: Temporary trench excavation

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.

RE: Temporary trench excavation

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.

RE: Temporary trench excavation

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

RE: Temporary trench excavation

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

RE: Temporary trench excavation

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.

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