Drawing Notes - Person to verify soils design data 1

[RFreund]

Typically on earth retention projects there is a note on the earth retention drawings that states the soils data used in the design (typically stated on the plans) should be verified in the field during construction. My question is do you typically refer to "the contractor's competent person" or a "testing laboratory" or to a "licensed geotechnical engineer". Do you change this based on project?

EIT

http://www.HowToEngineer.com

 

[FixedEarth]

Usually it is the geotechnical firm of record that issued the soils report.

http://www.soilstructure.com/

 

[Ron]

The contractor is responsible for verify site information during construction. This should be done by requiring in the specifications/contract that the contractor hire a competent geotechnical engineer to verify conditions supplemental to the design geotechnical report.

If it is a testing function, the owner should hire the testing lab, with failing results backchargeable to the contractor.

 

[PEinc]

I agree that a contractor usually has the responsibility to report any discovered problems or changed conditions but that does not make the contractor responsible for verifying the "conditions supplemental to the design geotechnical report." For example, if the design is incorrect or if the site conditions are different than what was shown on the contract documents, the contractor is not responsible. It's not the contractor's responsibility to verify the engineer's design, assumptions, or interpretations of the site conditions or geotech report. Many engineers, CM's, owners, etc. try to make the contractor responsible for everything but that doesn't make it right. Most contractors are not capable of checking or verifying the engineer's work. Also, for many projects, the specifications say that the geotech report is for informational purposes only, is not guaranteed, and is not a part of the contract documents. So, if the contractor is not allowed to rely on the geotech report, how can the contractor be responsible for its verification?

http://www.PeirceEngineering.com

 

[RFreund]

Thanks for the replies!

A little bit of follow up:

Normally for building foundations we have a note saying something along the lines of: the bearing capacity is Xpsf based on the soils report and this should be verified in the field by the Owner's testing agency.

For earth retention I recall seeing different things from other engineers but normally the soil design parameters are presented. Either a description of the soil and/or the soil properties used in the design (unit weight, friction angle, cohesion) based on the soil borings / report.

Then there is the next part about verifying these conditions during construction. I recall seeing this a few different ways:

"The contractor's competent person"
"The contractor's testing laboratory (or geotechnical engineer)"
"The owner's testing laboratory (or geotechnical engineer)"
"A testing laboratory (or geotechnical engineer)"

I suppose whether it is the contractor or owner could also depend on the contract documents but I'm wondering how often that is specified when it comes to temporary shoring, etc.



EIT

http://www.HowToEngineer.com

 

[PEinc]

Asking the contractor to verify the project engineer's soil properties for the earth retention design is done, in my opinion, just to get the engineer off the hook if there is a shoring problem or if the soils are different than recommended. If the engineer really wants to be shielded from a potential problem, he should not be recommending the soil properties in the first place. In my experience, the engineer-recommended soil properties are usually very conservative which can cost the project owner a lot of extra money. If the engineer wants to recommend soil properties, he should accept the consequences of being wrong. If not, he should leave it to the design-build specialty contractor.

http://www.PeirceEngineering.com

 

[RFreund]

What about the case when you're the engineering for the design-build contractor?

I mean someone on site should be verifying the site conditions during construction, no? Is this usually the contractor's testing service or the owner's or I suppose it needs to be agreed upon before hand, but is one more predominant than the other?

EIT

http://www.HowToEngineer.com

 

[jgailla]

Normally the owner contracts the testing company so that there is no conflict of interest between the contractor and the testing company.

 

[dcarr82775]

RFreund,

If it is a design/build contractor, than I would say it is the design/build contractor's responsibility to note conditions differing from their design. So maybe the design/build contractor's competent person.

 

[PEinc]

I've been the design-build contractor. If I design-build it, it's my job to make sure it works and at my cost, unless there is obviously a changed condition more significant than a few varying soil properties.

http://www.PeirceEngineering.com

 

[Doctormo]

RFreund - Some of my observations are below which some may agree with and some may not:

Quality assurance provisions and Owner specification/testing requirements for retaining wall systems often conflict and/or are routinely ignored in practice when not convenient. All the design notes and specifications are meaningless if not "enforced" during the construction process but become extremely important when the lawyers get involved later on when there is a problem thus the "CYA" nature of notes and specs. The problems with "retaining wall systems" is significantly reduced when the quality assurance provision and specification requirements are fully observed by the Owner/Engineer/Contractor or "Design-Build"contractor.

To answer your original question, a retaining wall system's only purpose is to retain soil thus not having the geotechnical engineer involved is as negligent as having the geotechnical engineer inspect the post-tensioning installation of a parking deck. There is a case to be made that a concrete wall is probably best inspected by a structural engineer but "retaining wall systems" tend to be much more geotechnical in nature and thus their involvement is important in the QA process. In simple terms, engineers with the proper background should do what they best qualified to do when it comes to providing inspection and testing for quality assurance purposes.

Most engineers can observe assembly of "retaining wall systems" per approved shop drawings but the soil related issues are best done by the geotechnical engineer for the project. This is especially important for MSE wall systems since the soils involved are the most important structural component and other items are of secondary importance. However, the application and site conditions have to be considered. A 6' wall in a sand formation probably requires little in the way of geotechnical involvement but a 10' wall supporting a drive lane on a CH - WOH foundation probably requires more geotechnical involvement than any owner could imagine and a very expensive retaining wall system when the smoke clears.

The QA provisions should require all the important soil design criteria be verified by the site geotechnical engineer and all soil inspection and testing be under the geotechnical engineer's supervision while other construction monitoring could be done by other engineers if it makes sense. If the project is a design-build arrangement, all the same provisions apply except that many times the design-build contractor will have to hire the geotechnical company to evaluate and monitor the soil work. Sometimes this is a joint arrangement as the geotechical engineer was hired by the Owner's design team originally and a conflict of interest can occur. Many times the contractor will retain a different geotechnical/testing engineer for these function which can result in some responsibility issues where there are two geotechnical engineers involved on the project.

Regardless of contractual arrangements, the QA provisions and specifications should be reviewed by all parties prior to the "wall system" construction (at a pre-con meeting?) and an agreement of who is going to do what should be determined. Items should "not be done" just because no one can figure out responsibility although there are times when some standard QA items can be minimized based on application and location (ie rock foundation probably eliminates settlement concerns unless it is missing in areas).

Just my opinions during morning coffee.

 

[Ron]

A geotechnical investigation for design purposes is done with limitations. The geotechnical report should provide those limitations, one of which is that assumptions have been made about soil conditions between the respective borings. It is usually cautioned that the contractor is responsible for verifying the soil conditions between borings and to bring variations from the design assumptions to the attention of the geotech before proceeding. This means that the contractor should hire a geotech firm to do such verification or assume the risk otherwise.

Contractors always assume some level of risk in construction. They should be compensated for conditions that are different than a reasonable person could anticipate from the information provided during the bidding process. If they are warned that conditions are likely to vary between borings and they do nothing to attempt to mitigate that condition then they assume more risk. Extraordinary effort to determine such conditions either prior to bidding or after the bid should be negotiated with the owner.

 

[Doctormo]

Ron - I tend to agree with your description of risk above but feel I should add a couple things.

1) Anytime the concept of "reasonable person" is brought into the discussion we are clearly not talking about the adversarial construction business that I am familiar with. While there are many reasonable people in the industry, the vast majority of owners and contractors are not reasonable when it comes to money issues and changes. The burden of providing overwhelming proof and evidence falls to the person experiencing the problem which is typically a subcontractor who is at the end of the cash flow chain. Many times people agree to compensate the contractor to keep the job going but later will not honor the claim in a timely manner. Of course, the contractors inflate the change costs considerably knowing that they will not be accepted as submitted and negotiated down if settled at some point. If all else fails, litigation and/or liens are the typical result when the problems are significant.

2) The vast majority of geotechnical reports are silent on site retaining walls and provide little in the way of borings or testing that could be extrapolated into a design without qualification and assumptions at bid time. I have seen hundreds of geotechnical reports that do not address 30' walls on sites with either borings near the wall location or any index testing of soils much less any strength testing or specific considerations for building such a structure. This problem is a result of how geotechnical proposals are solicited (taking low cost proposal regardless of scope) or the geotechnical work being done prior to the completion of site plans with the loop never being closed with the final plans. Sometimes it is because geotechnical engineers do not want to get into the liability of site retaining walls because it is too much liability for something no one wants to pay for to obtain relevant information. Lots of reasons but the information is generally lacking as result.

Many state DOT's (not necessarily counties and cities) do a much better job than the private sector since site retaining walls are a major part of whether a road works or not (private sector is mainly interested if a building works or not). Walls are addressed by both the structural and geotechnical people to see what works in a given location and the appropriate party leads the design effort with the others support (structural - concrete walls, geotechnical - wall systems). However, that does not mean the DOT projects are without incident as there are many exceptions also where items are not covered well or create unsolvable situations requiring more work on their part.

I agree that contractors should assume some level of risk and they should price that level of risk accordingly when bidding projects. However, it is very easy to add $10K to a bid to retain a geotechnical firm to do the testing that the owner did not have done but it is not so easy to price what the geotechnical engineer might come up with in his investigation. I have seen jobs require all sorts of remedial work as a result of the post bid geotechnical investigation that the "reasonable Owner" ends up paying for and the "unreasonable Owner" ends up in litigation as a result.

Too much typing this morning...

 

[RFreund]

hmmm, interesting. Thanks for all the comments!

EIT

http://www.HowToEngineer.com

 

[darthsoilsguy2]

as with most things.... it makes more sense on extreme examples and gets murky in gray areas. Nobody would really expect a wall contractor to know the difference between 5 degrees of phi-angle on the soil..... but a contractor that used on-site materials significantly more clayey and silty than design specs allowed for a wall, i don't have much sympathy for them. i expect contractors to know the materials they work with steel, concrete, soils, SFRM, or otherwise.

 

[Ron]

Well said DSG....I teach construction materials at a local university to budding contractors. I think it is extremely important for contractors (and engineers) to know construction materials. In my consulting practice I run into so many contractors and engineers who don't have a clue about construction materials beyond looking up some general materials properties. They rarely know the "how" and almost never know the "why".

 

[Doctormo]

In my experience, the problems occur when design-build becomes competitive engineering/construction where cost and not quality is the driving factor. The owners and general contractors drive most of these problems but it takes the subcontractors to complete the "circle of failure" and agree to do the work under unfavorable conditions. Most subcontractors can not see past the work they are doing on a given day and the implications of future performance problems are the last thing on their mind when the rain starts.

The essence of this is when one has a crappy pile of soil adjacent to hole to be filled in or a wall to be back-filled. Human nature is to take the nearby crummy soil and fill in the hole or backfill the wall. It is even better when it is late in the day or on Saturday when no one is around to question the decision. I have reviewed many wall installations where the contract plans, the project specifications, and the shop drawings all clearly call for granular backfill and onsite clays are used. Of course, everyone on the entire project site is surprised by this observation except the engineers involved. The contractors then submit a change request for the granular backfill since they could not read the documents.

Thus QA provisions and language that describes responsibility begins which is somewhat useless since the same people who can not read the documents are required to read the "fine print" also. I am a little jaded as a result of my experience but it happens over and over again. It just takes people to read, understand, and enforce basic construction documents in the field. If one does not know what the #200 sieve is or how to interpret it, they should ask some one who does.

'nuff said.