Factor of Safety in Settlement Calculations 4

[Okiryu]

Hi, do you use any factor of safety when calculating settlements? Somebody asked me this question and I could not provide a good answer, I think. I answered that I do not use a "real FS" (i.e. a factor of 3 used for bearing capacity calculations), rather I use the most conservative data which is obtained from the field or lab. For example: consider the thickest clay layer in calculations, use values from the lower bound from test results, etc.

Also, settlements are calculated based on provided loads from the structural engineer or assumed loads based on experience with similar type of structures, which are also conservative approximations.

So, all these conservative parameters may be contributing to increase the "safety" of the analysis.

I think that settlements are based on servicilability requirements and not sure how to relate to a FS. I saw an old thread here about FS which involved discussions about LRFD vs. ASD methods in structural design, but would like to see more comments about this topic.

Thanks in advance for you opinions and Happy Holidays !!

 

[ATSE]

No, calculated foundation settlements do not have factors of safety.
However, you should report settlement as a range (high and low), not as a singular value.
The range will depend on the quality of your subsurface information, certainty in the load profile, grading, groundwater...

 

[oldestguy]

Remember that settlements depend on loading. Chances are the load you put in, as recommended for design is pressure units, is likely well below the actual loads that the footing will receive. Thus your settlement calculation is likely to be pretty conservative already.

 

[Okiryu]

Thanks. So, perhaps the "amount of safety" we put in the calculations will depend on how much conservative the parameters are. I think this make sense, but this is difficult to explain to people who are familiar to seeing FS in the calculations.

 

[Doctormo]

Serviceability (i.e: settlement) is typically a displacement consideration and is calculated with true loads (load factors = 1.0, LRFD or no safety factors, ASD) to predict true deformation under a specific loading condition. It does no good to apply a load factor to a pressure and/or a safety factor to a consolidation coefficient and then calculate some dimension that becomes inflated and will not occur in reality. The goal is to accurately predict the movement and then decide if it is OK or not for the performance of the structure then 1) adjust the design accordingly or 2) require remediation to reduce settlement or both. This becomes a discussion between Owner, structural, and geotechnical engineer since additional costs can creep in to deal with settlement problems and some assumption of risk may need to be made.

Settlement is very difficult to predict with any accuracy so geotechnical engineers usually express a range and hope they are not off by too much so the structure design can accommodate the range of movement expected. To do it correctly, the structural engineer has to furnish unfactored live and dead loads along with the proposed foundation configuration to evaluate structure settlement. Many times additional testing may have to be done to refine settlement estimates.

 

[Ron]

this is difficult to explain to people who are familiar to seeing FS in the calculations.

If they understand the concept, then they would not expect a FS. If they don't understand the concept, you shouldn't have to justify your calculations to them since they obviously don't understand those as well.

 

[Okiryu]

Thanks for all the responses. So, when structural engineers provide column loads, for static conditions, are they unfactored (service) D + L loads? Similarly, for short term loads, unfactored E and W loads?

 

[Doctormo]

In my experience, you have to always ask for unfactored loads for bridges and buildings to be sure. Short term loads may be unfactored like seismic and collision, others may not like wind loads which last more than a second or two.

 

[BigH]

Settlement computation, in my opinion, is fraught with many uncertainties.

First there is the question of what loads to use. You want the "real" loads. Ideally, for clays, you would want the dead loads and the permanent "live" loads - i.e., furniture, storage of materials and the like - live loads that will be more or less permanent. Then you will also want the transient live loads - wind loads, unbalanced water pressures (on mass concrete dams) due to reservoir level flucutations - you will also want to have an idea of any seismic loading. For sands, you want the similar loadings but realize that for transient dead loads, they can induce additional settlements - the rebound after removal may not always be the same as the loading (although, in practice, it would probably be negligible). This is not an issue as much is clays as the transient load will be taken immediately by porewater pressure build-up and then subsequent porewater pressure drop on the transient load removal - of course, there are times that the transient loadings might exceed the equilibrium porewater pressure and the transient loading can add a permanent porewater pressure load that will add to the consolidation.

Secondly, you need to determine what methods of computation you will use. For sands, some, like OldestGuy, likes Hough's method (which I believe you can also find in the WSDOT geotechnical manual). Many others, though like Burland and Burbidge (see M.J. Tomlinson's Fdn Dsng and Construction book). Others like Schmertman's Method. Many years ago, Ground Engineering had a great article that compared about 15 different methods of settlement in sands to actual tests. There conclusion was to pick three methods and then take the "average" of the three for your estimate. The problem with sands, too, is that many methods use "elastic" theory and hence the E value is a necessity. Being 20% off on the E value chosen will put you 20% off on the settlement computation. Of course, one could always look into using the theory of particulate half space (see Milton Harr's book on Mechanics of Particulate Media (1977) - he also wrote a book on Mechanics of Soils as Particulate Media Under Moving Loads (1989)) - have really never seen this referenced any any other text book that are typically used in soil mechanics. Okay - so the bottom line is you have uncertainty in E - hence uncertainty in the estimated settlements.

For clays - you will have the problem of what the overconsolidation ratio is. Using traditional oedometer results, you might think you have a normally consolidated clay - but it is much more likely that there will be some quasi-preconsolidation that you may or may not know from your oedometer tests - or estimate - like "assume" (as bad as some think) that the Pc' (preconsoldiation pressure) is 10% higher than the existing Overburden pressure. You will also have issues with Cc/1+eo (in my view, always plot strain vs log p' rather than e - as the slopes will already, then, have the 1+eo built in). Some posit that you plot log strain vs log p' to get the Pc' value - it is subjective. Personal note is that years ago Casagrande came up to the Geocon office in Toronto (Fred Matich studied under Terzaghi and Casagrandes) - and some engineer proudly showed him about 20 plots using his graphical method of determining Pc'. He looked at the plots and said - "No - this should be about here and that about there . . ." Dejectedly the engineer said "but I did it according to your method." - and he replied, well, I had to have some way to get my students to come close to the correct value . . . To add to this - do you take into account Bjerrum's pore pressure settlement coefficients? . . . and Fox's depth correction factor. There is also estimating settlement using stress paths. Som and Das (Theory and Practice of Foundation Design) compared the "traditional" method and the stress path method for the same "footing model" - they got 48.2 mm (using Skempton/Bjerrum method but only 30.8 mm using the stress path method. So, there is the rub - they differ by approximately half of the computed settlements. (by the way I do not believe in decimals of a mm !!)

Now, you also have the effects of creep - in both clays and sands - see Schmertman.

Sorry to ramble on - it is a Monday night - at the office as we have aircon and internet . . . but, the basic gist is that settlement computations provide what? - estimations!

I remember some learned types in the past say that if you are within 30% of the actual value - you've had a good day - owe yourself a beer! Steel and concrete have Coefficients of Variation of 5% say - soils are in the range of 25 to 50% depending on the parameter. So many times, you might just see soil reports simply state that settlements are expected to be less than 25 mm, say, or 40 mm . . . there is no hard number to put on it.

Safety Factor on settlements? NO. But certainly computations and judgment to come to a "close" range? Yes - but always with a needed amount of wiggle room. The estimations depend on quality of soils data and reliability of the methods of computation.

 

[Okiryu]

BigH, thanks for taking the time to reply. Your reply is very useful. Also, I was thinking that Mike Duncan's paper about reliability in geotechnical engineering can be used for settlement calculations. What do you think?