Foundation Underpinning 1

[twich]

I am the owner's representative, and an employee of the owner, on a court facility complex to be constructed adjacent and beside a historic 1896 Circuit Court building. I am a structural engineer and have some minor underpinning experience, but nothing like what is proposed. Also, the project is being designed by an architectural firm, who subed out the engineering.

My owner's contract with the Architect called out an additional fee for special foundation design for shoring and underpinning. No problem with that. Now the structural engineer is suggesting the underpinning design be included in the General's contract. Again, no problem, if my owner gets a deduct from the Architect.

The exterior walls of the old court building are a double wythe brick masonry bearing wall, bearing on limestone foundations, bearing on fine grained soils consisting of solf to hard clay and silt. The bottom of the stone foundation is 3'-6" below grade. The design calls for a basement in the new building butting against this exterior wall. There is little leaway in this design option, due to limited space in a downtown area. The excavation will be 15'-6" below and on a direct vertical plane from the stone foundation.

My question is: Should this task be "assigned" to the General, and if so, what pre-qualifications would you suggest I get before allowing any General to bid? Or should I insist the Architect hire a Structural Engineer that specializes in underpinning and shoring?

And does anyone have any tips on helpful sites I could access? Thanks in advance for you help.

TIH - twich

 

[Lutfi]

I think you should hire an engineer who is well versed in the subject matter. You mention that you have limited experience with underpinning; which is normal. We all can not be experts in all fields. Do not try to learn on the job by reading web sites or books.

I think you would look much better and serve you client better by bringing on board an expert on your side. This expert will help guide and answer your questions.

Regards

 

[jheidt2543]

twich:

A few comments:

1. Your project is very similar to one I was involved with about 15 years ago. The courthouse was somewhat older (Abe Lincoln practiced law in it, really!) and the excavation for the new basement was only 8' below the existing stone foundations. I was working for the General Contractor at the time and the Architectural/Sturctural drawings only had a note that said "Underpin existing foundations". The soils were glacial deposited sand and gravel.

2. For that project, since we had done a fair amount of underpinning in the past, we used a two pass underpinning method. We underpinned the entire length of the adjacent existing foundation wall down 4' and then retraced our steps for a second pass of 4' below that. It worked very well. We set up settlement monitoring stations before starting and read them weekly as I remember it.

3. Personally, I believe the structural engineer of record should design the underpinning. He is the one most familiar with the loads, soil conditions etc. If he isn't experianced enough to do it, he should hire an engineer who is. Ultimately, he is responsible for the structural integrity of the building.

4. If, because of the FEAR of liability, the engineer decides to STICK the contractor with the design responsibility, then the contract should CLEARLY state that it is the contractor's repsonsibility and he should be REQUIRED to have the design performed by a structural engineer and submitted to the engineer of record BEFORE starting the work.

5. The contract documents should also CLEARLY state that the contractor performing the underpinning work submit a prequalification form showing experiance with at least five similar underpinning projects. You don't your job to be the first one the contractor has done.

 

[PEinc]

I disagree with jheidt2543 on this one.

I suggest that the underpinning design and construction be included in the general contractor's scope of work under a performance specification which contains a prequalification requirement for underpinning experience on several, recent, similar projects. Then list the names of several possible underpinning specialists such as Schnabel Foundation Company, Nicholson Construction, Hayward Baker, Berkel & Company, Spencer White & Prentis, etc. These companies have significantly more experience than any engineering consultant you may hire. These contractors have extensive design/build experience in underpinning, tieback anchors, deep foundations, retaining walls, etc. Also, they will have the insurance and will be responsible for the construction and performance of the system. They also will be more familiar with the different underpinning methods available. However, the owner's engineer should provide as much information as possible to the contractors regarding construction of the existing building, soils information, building loads, footing elevations, etc.

Additionally, I would not recommend underpinning any building by making several underpinning passes as described by jheidt2543. I would not want to support an existing building on a stack of vertically unconnected concrete underpinning blocks, especially for deeper underpinning which would need lateral support from tieback anchors or raker braces. Each time you install a pass of underpinning, you risk the possibility (probability?) of settlement. It is not unusual to see some (hopefully minor) amount of settlement when you underpin a building. In my opinion, underpinning with sereral passes compounds the amount of potential settlement with each pass. From my experience, those who perform underpinning with a series of shallow passes are usually inexperienced and incapable of designing or building deep pit underpinning.

If the contractor design/builds the underpinning, make sure that the permanent lateral load is addressed by either the structural engineer's building design or by the underpinning contractor with, if necessary, permanent tieback anchors. Frequently, architects and or structural engineers assume that, because the building has been vertically underpinned by the contractor, the new building will not be subjected to lateral earth and surcharge pressures. The underpinning will permanently support the existing building vertically but the underpinning may be, and usually is, supported laterally by temporary bracing or tieback anchors. If so, eventually the lateral load will be transfered to the new structure. Someone must determine whether or not the new building will need to be designed to resist the lateral earth pressures behind the underpinning. This is a common point of confusion on many projects.

 

[BigH]

Contractual points are not really my cup of tea - but I agree with PEinc that the only ones I want doing the work would be specialty contractors/designers - his list covers whom I am aware of as the best around. They can be designated subcontractors to the general contractor - or the general contractor can nominate one. But whoever must be vetted by the Owner/client. This really is specialty work and should be handled as such. As JHeidt points out - monitoring is extremely important. For an historic building such as this - don't go 'cheap'.
and . . .

 

[twich]

Thanks to all. PEinc, I will certainly follow up with your suggestions, or go down for the count with the owner, my employer. I believe the consequences are too great, not to.
Thanks again for the help.

TIH - twich

 

[CSEllc]

My two cents is since you are going to underpin a significant amount, I woulkd recommend going with a "specialty" contractor or a contrcator who has had significant amount of underpinning experience. I would also have the contractor (either specialty or not) submit sighed and sealed details on thier approach to the underpinning and have the SER review them for conformance to the project spec's. Granted, the actual method of underpinning is a means and methods of construction, I believe the SER should have a say, or at least a look at what will be proposed.

Ultimately, SER is responsible for the project, no matter what the contrcators do.

 

[Focht3]

Hmmm,

Oddly enough, I agree with all the posts so far. I would sum up the differences between the opinions as regional variations in practice. Personally, my practice is more like [blue]jheidt2543[/blue]'s. But, then, I have some great experience with excavation retention and underpinning - within certain geographic and geologic constraints, of course. Personally, it sounds like the SER took an assignment that s/he wasn't qualified to perform. As [blue]CSEIIc[/blue] so aptly said, the SER is still responsible for the project - no matter what the contractors do.

And 'dumping' the responsibility onto a specialty contractor has clear financial implications for the owner. In my experience, the specialty contractors will only propose the technique that they are familiar with - not necessarily the best one for the site and structure. For example, [blue]PEinc[/blue] disparages [blue]jheidt2543[/blue]'s two pass approach. While it may be theoretically true that a two pass underpinning approach could result in more settlement than a single pass approach, "the proof's in the pudding." [blue]jheidt2543[/blue] indicated that settlements were monitored (a key element of these kinds of projects) and that building performance was acceptable. Remember that [blue]jheidt2543[/blue] indicated the building was supported on glacial till; it's entirely possible that a single pass approach might have been less successful in those soils. Local experience is the best measure, in my opinion.

Having said that, I would pre-qualify any contractors for this work. Get your geotechnical engineer involved in this underpinning design and pre-qualification effort - this is really his/her area of expertise. And look real hard at the lateral load issue that [blue]PEinc[/blue] so aptly described. Keep in mind that a clever geotechnical engineer and nimble specialty contractor can often come up with an underpinning system that meets multiple needs simultaneously.



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?"

 

[jheidt2543]

Focht3,

Everytime I see your Hmmmmmmmmm, I get that warm fuzzy feeling that one of your really cogent comments is coming. The regionalism comment is an important one to keep in mind with most of the responses we see in these forums. In fact, I think that is one of the most interesting aspects of reading through the forums. We are all somewhat captive to our personal experiances. Even when we study a problem and read of alternate solutions, we tend to gravitate to what has worked for us in the past, it is just "safer" that way, but not always the most economical.

I agree that if a single pass of underpinning can be used it is quicker and more cost effective. However, in the instance I related, we were underpinning a stone foundation wall set in sand & gravel soils. We were very concerned about movement of the structure and the effect vibrations would have while doing the work. So, most of the digging was done by hand, which meant men in the hole, that pretty much limits how deep you can go on one pass (read that as OSHA regulations). Trench boxes are pretty cumbersome for this type of work, but not impossible.

I also agree with PEinc's that getting people with prior experiance involved is important. However, the size and importance of the project also play a part. There is no question that the big name contractors PEinc mentioned are experts in their field, but they may not be interested in every project. There are usually good local firms that can be prequalified for projects too, both contractors and engineers.

I also still think it is the EOR's responsibilty and duty to be intimately involved in the underpinning design. If he isn't qualified or comfortable, then he should seek out and hire a consulting engineer who can. I think most full service geotechnical firms would do a good job at a reasonable price. I think the Owner gets better pricing for his project when all the design bases are covered BEFORE the bid opening and traditional separation of responsibilities is maintained (design/build is a separate topic, I'm thinking here of the traditional plan & spec, bid, build projects). All the prequalified contractors are bidding on the same thing.

 

[PEinc]

A few more thoughts or comments.

Digging by hand does not limit the depth to which underpinning pits can be excavated. Since the deep pits are shored with lumber as they are dug, OSHA requirements are met. It is not unusual to dig underpinning pits 20, 30, 40, even 60 feet deep. Expensive maybe, but not unusual. For pits this deep, it would not seem practical to use a multiple pass method. In addition, for deep pits, the lateral loads usually make it necessary for the pits to have some vertical continuity (shear and moment capacity) between rows of tieback or brace support. Stacked piers with cold joints between them will not have that continuity. Installing vertical dowels between successive layers of stacked piers is not desirable. Dowels interfere with excavation and drypacking. With continuous, deep piers, the levels of tiebacks or braces can be positioned so that the amount of tension in the unreinforced concrete piers is within allowable limits.

It seems to me that some confusion here may be associated with using backhoes to excavate the underpinning pits. In my experience, underpinning pits (especially deep pits) are usually excavated by hand, not by machine. The only time that machines are used to excavate pits is when the soils are very competent and can stand vertically unsupported, when the pits are shallow, and when no one needs to enter the excavated pit. Sometimes, a backhoe is used to start a deep pit prior to a worker installing the first level of shoring boards. After that, the pit is excavated by hand. Underpinning specialty contractors are less likely to machine excavate than are excavating contractors and concrete contractors who share the work scope required for shallow underpinning projects.

Winterkorn and Fang's Foundation Engineering Handbook and Ratay's Temporary Structures in Construction both have fairly good chapters on pit underpinning.

Focht3 mentioned that some the specialty contractors will only propose the technique that they are familiar with - not necessarily the best one for the site and structure. I agree with this statement. It applies especially to tieback contractors. Some so-called specialty contractors have only one method (Plan A) of performing a particular type of work. They have no backup plan if the first does not work. They have no Plan B, C, D, etc. that may be required due to changed field conditions. In my opinion, "specialty" contractors with only one method are one trick ponies and are not really specialty contractors. It is very important to hire a contractor who is capable of providing several different solutions to the same problem if necessary.