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?

 

[GeoPaveTraffic]

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

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[ATSE]

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.

 

[GeoPaveTraffic]

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

 

[fattdad]

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!

 

[NorthCivil]

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.

 

[NorthCivil]

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

 

[jdgengineer]

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.html

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.

 

[SlideRuleEra]

I have been... digging for any resources I can to try to bring some science to this. I haven't been able to find much.

By all means download the California DOT Trenching & Shoring Manual. It is better than many textbooks.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[cvg]

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.

 

[NorthCivil]

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.

 

[ATSE]

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.

 

[cegeoguy]

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

 

[darthsoilsguy2]

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.

 

[TLHS]

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

 

[darthsoilsguy2]

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.

 

[LittleInch]

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.

 

[NorthCivil]

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.