Soils Report Language 4

[haynewp]

Do Geotech's ever put wording in the soils report that deep foundations or some type of soil modifications are "required" or "recommended".

The reason I am asking is I have a report where a building is going on about 13ft of existing unknown fill. The report is worded in such a way that it neither "requires" nor "recommends" a soil reinforcing system be placed instead of typical spread footings. It just says "as a lower risk option to spread footings" a soil reinforcing system may be used. The reason it gives is more that normal settlement might occur if typical footings are used.

I have done deep foundations before, but I don't remember the report being worded in such a way. But I don't remember it saying piles were "required" either. If you know that some soil modifications must be done, what is the standard way of conveying this in the report?

 

[eric1037]

We use similar types of language. The reason is that the owner decides (with guidance from the structural engineer, general contractor, and geotechnical engineer) how much they want to spend on a structure and how much risk of poor performance they can tolerate.

If the site has 13 ft. of existing, inorganic fill that looks decent, but does not have documentation regarding method of placement, compaction, etc., there is always a risk of poor performance.

The structural engineer, the general contractor, the geotechnical engineer, and the owner should have a meeting to discuss the budget issues for each alternative and discuss the owner's tolerance of risk of settlement.

If possible, it is probably worth it to try to find any available documentation of the existing fill.

 

[GeoPaveTraffic]

The language that I have used in the past is "... that the site is not suitable for the proposed construction on shallow foundations with significant site modifications and as a result deep foundations or ... are recommended."

If an owner or engineer asked for specific recommendations for a type of foundation that I did not believe was suitable for the site, then I simply would not give them the recommendations.

 

[eric1037]

I have used language like "Considering the information from the soil borings performed, the construction of deep foundation may be more economical than the construction of shallow foundations due to...."

The problem is that there may be a hungry earth work contractor that has their own gravel pit or sand mine that could do a remove and replace significantly cheaper than a deep foundation. The same is true in the other direction.

The only one that really knows the economics of each alternative is the general contractor and his subcontractors.

The architect and the structural engineer are the only ones who know what magnitude of total and differential settlement the structure can tolerate.

The owner is the one paying the bill and living with the final product.

Therefore, I try to come to some kind of consensus among the project team prior to completing a report. Then if things go bad, they are not all pointing a finger at me. They are all involved in the decision. This also reduces the questions and revisions after the report is complete.

 

[BigH]

I have usually "recommended" a foundation support system to the client (normally industrial clients of long-standing). If shallow foundations are not suitable, I say so. There are times that shallow foundations might work given various ground treatment conditions (subexcavate and backfill, preload, etc.); I would say so but at the same time given that there is a risk involved, I would state that for a more positive support system, deep foundations can be considered. Below are two wordings akin to the original post that I have used. The point is, too, that we can recommend; client - given other alternatives (contractor with cheap gravel pit, as mentioned) can always come back to us and say, "How about this?". This is one reason that we would always say that we wish to review the foundation system selected. To be frank, though, I've gone the way of the other posts as well - depends on client, job, your continued involvement, etc.

1) "Based on the variable, non-engineered nature of the fill, the thickness of the fill and the presence of the loose silty sand to sandy silt below, spread foundations are not considered suitable for the support of the roof or head frame structures. It is recommended that foundation support be derived from deep foundations driven to end-bearing in the hard glacial till, the surface of which is approximately 60 ft below existing grade."

2) "From a geotechnical engineering point of view, the most important feature influencing the selection of the foundation system to support the proposed structure is the presence of an extensive deposit of generally loose, heterogeneous fill which extended to depths of about 18 to 22 ft below existing grade. The site is not judged suitable for using spread foundations and it is recommended that deep foundations be utilized to bypass the fill deposits in a positive fashion with a view towards supporting the Addition within the underlying clayey silt till."

 

[fndn]

Recently I have noticed that clients are shopping around for cheap solutions to the point where they show up during drilling and ask for a solution before any lab work or analysis have been done. On the other hand, the well financed clients will usually say I know I need piles but would rather limit to drilled shafts.

If the site is not suitable for shallow footings, I say so in couple of areas then give them choice of deep foundation types, depths and geometry.