Do trench boxes qualify as "shoring"?

[driller03]

Background: A 25' wide trench has to be constructed 14' deep in a 25' corridor. There is no room to lay back the slopes. There are buildings on shallow foundations adjacent to the trench.

If the design engineer has specified that the contractor use shoring for the construction of the trench---do trench boxes qualify as "shoring"?

 

[GeoPaveTraffic]

Sometimes yes and sometimes no.

For the conditions you specify, I would say that a trench box would not be appropriate. The reason is the adjacent buildings on shallow foundations.

If you use a trench box, there is likely to be some ground loss adjacent to the box; which is likely to result in settement of the adjacent buildings.

Shoring/trench protection serves two purposes. 1. to protect the workers in the trench. 2. to protect adjacnet improvements. Number 1 is always required, number 2 is not always required.

 

[PEinc]

GeoPaveTraffic is correct. Trench boxes protect people, not adjacent structures or utilities. They also minimize the needed amount of excavation and backfill. Frequently, the excavated ground surface behind the trench box walls is not in full and proper contact with the box walls. Therefore, the ground can collapse against the trench box. In addition, usually a significant open excavation needs to be made just to set and/or advance the trench box. During this open excavation, the adjacent structures or utilities are not supported.

 

[Mickney]

Wow, I am envisioning one heck of a trench box...

I hope that the design engineer did more than just say "use temporary shoring". What foundation loads are on the adjacent buildings? In my opinion, a "designed" temporary shoring system incorporating soil conditions, adjacent building loads, groundwater, etc. would be required in this case.

 

[DRC1]

I agree with the above posts, espcially the last, in that the engineer should have been more clear. The contractor may tell you this is commonly done, especiallly for lighter foundations. He is correct I do see it done often, but that does not make it correct. Howver, OSHA requires that a PE sign off on the shoring system if it is going to take load (i.e. within the zone of influence) of nearby footings. I have seen contractors choke the excvation betwwen the side walls and the box with crushed stone imeadiately after placing the box to prevent sidewall movement and it seems effective.

 

[PEinc]

DRC1, yes but how does the contractor move the box without making an open, unsupported cut. he can't. To move the box, you need an open cut or you need no earth contact with the box walls.

Mickney, a trench box can be designed for all of the items you list. However, what the design does not consider are the points I just made to DRC1. The box may be strong enough to support the loads but the construction method is what can get you into trouble.

 

[DRC1]

I am not advocating using a box to suppprt a foundation, but it is done on occasion. Generally trying to advance the box to continue a trnch is a problem. It is generally usedin a limited area or in conjunction with other trench boxes.Generally the boxes are dug down and extracted vertcially.

 

[dirtygeo]

A trench box is for one purpose only, safety of workers entering a trench. Trench boxes do not provide any lateral support to the excavation, especially as it relates to supporting adjacent structures. There is always a gap between the outside of the box and the excavation, and the excavation is unsupported before the box is put in place and when it is removed. Maybe in stiff cohesive soils you may not see any lateral movements, and thus vertical settlement away from the trench, but I would never want to stick my neck out like that when there are structures within the zone of influence of the excavation.

The existing structures need to be underpinned or the excavation properly supported to prevent lateral movement and coresponding settlement. Sheet piling, soldier piles and timber lagging or caisson wall depending on the soil conditions, groundwater, and type, condition and load on the adjacent foundations to be supported.

This is the kind of project that causes lawsuits when the neighbours building settles and cracks as a result of excavation. Don't fool around an get into trouble.

 

[Riggly]

I am late on this post, but I will also add that groundwater condition needs to be taken into account. Lowering the groundwater can induce settement. Still I think a lot of wild shots have been fired here. We dont, know the distance of the adjacent structures, types of soils, loads, groundwater conditions etc. How long is this excavation? Have a geotechnical engineer assess, and analyze. Have anyone of you seen a 25' wide trench box? That is pretty large. What kind of crane can access that location? (I dont think a backhow can handle that one easily).

 

[driller03]

Thank you for your responses...so here's the rest of the story. Specifications on this project called for "shoring" to be utilized in the construction of the utility trench. This was due in part to the close proximity of existing footings and the fact that separate construction contracts were ongoing and the utility contractor could not work outside of the 25' boundary.

So, 2 contractors include full blown shoring in their price (h-piles and timber lagging or sheet piling). The third contractor indicated that he was going to use trench boxes. Contractors 1 and 2 bids were $3 million, contractor 3's bid was $2 million.

So, as the owner and/or the engineer what do you do? Do you throw out the 3rd bid as non-responsive? If the owner doesn't have the backbone to stand behind the engineer's requirements to use "shoring" and not trench boxes, what should the engineer do?

 

[dgillette]

Could you give the low bidder a chance to revise, on the grounds that his other unit prices were better, but he seems to have missed the boat on the need to protect adjacent footings? (assuming that the other unit prices are better)

 

[driller03]

I'm sure a low bidder would love the opportunity to adjust his bid $999,999 and recover all of the money he left on the table. Is a public bid setting the place to allow a contractor with a non-compliant bid to adjust his bid?

It is pretty clear from the responses above that trench boxes are personnel protection devices and not shoring walls.

So, as the engineer, do you demand the contractor to provide shoring in compliance with your bid documents for his bid amount of $2million?

If the owner will not back you, what do you do?

 

[PEinc]

Regarding Bidders 1 and 2: Soldier beam & lagging sheeting is a flexible system that is not usually suited for supporting adjacent structures. You risk building settlements and lost ground while installing the lagging. Also, if you have a ground water condition that is not properly dewatered, you risk additional problems. Steel sheet piling is also a flexible support system. You have the same settlement concers, less water concerns, but more vibration concerns. Bidder 2's trench box method should be rejected.

You have adjacent buildings on shallow foundations. Therefore, you should underpin the buildings unless they are a sufficient distance away from the trench. If there is ground water above subgrade, proper dewatering will be needed to underpin the buildings.

 

[dgillette]

Is it a lump sum, or are there unit prices associated with all the other work items? If it's the latter, you can fix the other unit prices, and have a basis for negotiating the price for shoring, thereby getting what you need and, with luck, saving the owner some $. Their shoring price shouldn't be any higher than the other bidders' prices. If it's a lump sum, there are still ways, but you would need to have the right staff who could do independent cost estimates, and require the bidder to show the basis for estimating all the work items.

If the low bidder is presented with the issue before the contract is signed, he may withdraw his bid. If you were to accept the bid, THEN point out the problem, there might be some legal implications for you, and a small contractor could end up being driven into the ground before the project is done - can't get blood from a turnip - and everybody loses. Riggly above mentioned the issue of a 25-foot trench box being feasible. If you and the owners accept a proposal that you know is of doubtful feasibility, seems you are asking for huge claims, and maybe worse.

It's probably the owner's call, but you'll still need to cover your own butt in case the neighbor's footings are affected by the lack of shoring. If they aren't doing what you think is correct, send them a registered letter describing the risk and your recommendation not to proceed without real shoring. Don't know if that's complete protection for you, but it would help in getting their attention.

 

[PEinc]

If the engineer called for sheeting along existing buildings, and if the buildings settle, all will be drug into the claim or lawsuit. The contractor will say that he built what the engineers called for. I repeat: sheeting (soldier beams, sheet piling, or a trench box) may not be the proper way to support the adjacent structures.

 

[cvg]

if the contractor is required to "protect the adjacent structures during his work" than the liability is his. His method of protection is up to him. If the engineer specifies a specific method of "shoring" in the bid documents, then the liability partially shifts to the engineer. If the engineer directs the contractor how to do his job, nearly all of the liability shifts to the engineer.

 

[DRC1]

Even if trench boxes are recognized as shoring by OSHA, you still a plan signed by a PE to excavate near a building, as per OSHA and most public agency specs. He may not have a stamped plan yet, but have his PE submit a letter stating he intends to use the shoring and will meet all contract specifications, or which parts of the spec he will look to change. IO assume since this is a public job it is bonded, so I would have the bonding company tell you they are copestetiic with this also. If you feel what he is doing complies with the specifications, award the job. If you don't like what you see, best bet is to rebid (with tighter specifications).
It does depend on what the situation is. If it is a slab or pile foundation and you are a few feet away and there is no water, maybe it is not a big deal. If it is a significant structure and you are below the footing or the water table or both, then it is a big deal. I suspect since the other two biddres had more substantial systems it is a big deal. I would also talk to the other bidders to get their perspective.
However, I would not allow price adjustment. The other bidders will object. Either decides it meets spec and is what you want and award or rebid clearly stating what you want.

 

[TDAA]

How adjacent are the structures? What is the depth of the bottom of footing to the depth of bottom of excavation?

It may be that all the "shoring" has to do is protect the people in the trench.

There are a lot of good opinions above, but they all guess about the information I have questioned about. DRC1 hit on it a bit with the fact it is situational.

 

[driller03]

With all due respect, many of you are doing what engineers do so often...they get too focused on the details and miss the real issue. The questions are not "what is the shear value of the soil, how close are the structures, to what decimal point did they round off their values, etc." the question is a specification and contractual question.

Many times engineers are placed in the role of being the lead individual on a project and you are charged with opening, verifying, and accepting the bids. In this instance, the engineer clearly called for shoring to be utilized. And yes, the engineer had his reasons for requiring shoring---type of soil, proximity of adjacent structures, etc. Bottom line is that the plans called for shoring. A bid was turned in that ignored the engineer's requirement and proposed to use trench boxes---which is not shoring and does not accomplish the intent of the engineer's requirement.

It's not up to you to re-evaluate your design and lower your requirements to fit the contractor's bid, it's not up to you to allow him to re-visit his bid and add money to included shoring, it's up to you to enforce the requirements on the drawings to keep the bid market fair and equitable. Had the contractor bid the shoring as specified and provided the owner with a deductive alternate to use trench boxes---fine. But as the bid was turned in, in my opinion, it should have been deemed non-responsive and thrown out.

 

[GeoPaveTraffic]

driller03,

The point of the original question is wheather a trench box can be considered shoring. The consensus of posts, and my personal opinion, is that in some cases a trench box can be considered shoring and in some cases it can't.