Settlement vs. Time

[abusementpark]

Do you soils guys have any good rules of thumb for building settlement vs. time?

For instance, after about how many years can you confidently assert that the vast majority of a building's settlement has occurred assuming it is on clayey soils?

 

[Ron]

abusementpark...not really an estimable parameter without some specifics of the soil. At least need some classification and PI info to make a reasonable guess. Clays vary all over the place with regard to time rate of settlement. The drainage path makes a big difference...can it drain to the top and bottom? The clay origin makes a difference, and obviously the thickness of the clay layer or lense makes a difference.

Most geotechs can give a rough idea if they have lots of data from the area, but in terms of conservatism...geotechs lead the pack!!

 

[abusementpark]

I understand there are a lot different parameters with degrees of influence. However, there must some rational upper bound. 10 years? 50 years? 100 years? etc.

 

[VAD]

amusement park:

As a structural you should realize that your loads are transmitted eventually to the ground where it can bear on a variety of soils. In your case it is a clayey soil. Lets forget that there may be a variety of so called clayey soils as Ron alluded to. In order for the ground to settle, the load transmitted to the ground has to be of a certain magnitude to cause settlement to occur. For most buildings with loads when converted to applied pressure on a footing would generalls be in the realm of 150-200 kPa say 3 floors. You would likely have better numbers as a structural. The ground if well behaved no surprises below and within the influence of the applied pressure would not be expected to settle more than 25 mm. Taking the term clayey as being your accurate description I would say that this settlement of 25 mm or less would be expected to occur within a year after your full unfactored structural load becomes effective.
Ron, we are all conservative depending on the risk involved. I ofen wonde as well how the structural engineer obtains his load factors. To me there is a large amount of fiction to those values which may never be seen in some situations.

The interesing aspect of the question asked by amusement park further reinforces the need for structural and geotechs to work together bu that seems to be a pipe dream. Structurals get very cagey when one asks for thier loads and similarly geotechs seem to relish making their recommendations a black box.

I like to looks at the issue as one of soil structure interaction and hence loads and reactions ( soil )are paramount rather that who is or is not conservative. We all are at times and moreso when we play "gods" ar each end

 

[fattdad]

Just to point out there is an implied assumption: The "Gravity turn on Method." We usually look at settlement that results from the application of structural load (or earth fill placement). The tacit condition is that the load is applied at time zero.

How does that work? It's interesting to consider the buildup of loads and the tracking of settlement (or excess pore pressures) as loads or applied or the structure is assembled.

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

VAD...I agree. I've spent most of my career working side-by-side with Geotechs. Have done quite a few geotechnical investigations myself and ran a geotechnical lab for quite a while. The need for geotechs and structurals to work together is immense. I've seen structurals ask for parameters but not understand why they needed them (modulus of subgrade reaction for mats as compared to pavements for instance). I've seen geotechs give results and recommendations without understanding their impact on the structure. Unfortunately, the two don't even talk to each other much less understand each other!!

 

[abusementpark]

VAD,

The impression I am getting from your post is that most of the settlement would be expected to occur within a few years. Is this correct?

 

[VAD]

Yes

 

[DRC1]

I disagree. With out knowing a lot of parameters, you just don't know. Although, to a degree, they are important, loads are the big driver in clay settlement. It is drainage, which is a function of permiability and draiange path distance. Permiability in soils vary for gravelly soils from E+03 to tight clays with E-o9. this is a range in magnitude of E11. This is comperable to the amount of change in your pocket to the size of the national debt. Another problem is that basic consolidation analysis assumes homogenous and isotropic pemiability. That some may exist somewhere, but is pertty rare. Thus if there is a small lens of E-05 silty caly in E-08 clay, one part of the building may settle considerably faster(though the total magnitude would probably be as perdicted) than another part of the building.
Soil labs do testing, including consolidation testing for a reason. And no it not just to have paper to stick in the back of the report to make it look thicker. Eart materials do not have certified test reports and industry standard yield strengths. If you want to know you need to test.

 

[VAD]

DCR1:

There is alot being done without consolidation testing as well. Perhaps amusementpark should provide the loads and the subsurface conditions - boring log, and depth of footing etc so that one can determine if testing is required. I guess your diasgreement is based on your thoughts and my comments are based on my assessment as well. We can both be incorrect. Unfortunately, many questions posed beg interpretation.

 

[fattdad]

The answer is about 10 to 100 years give or take an order of magnitude.

If the loads never exceed the past consolidation pressure (i.e., if the behavoir is elastic), you may be looking at 2 to 20 months.

If you exceed the past consolidation pressure (as previously referenced) the time factor depends on the soil's permeability and the distance a drop of water has to travel to excape the clay layer. In a "tight clay" (whatever that means), this could be decades and decades.

The risk warrants a geotechnical study where somebody with local knowledge can share their experiences.

f-d

¡papá gordo ain’t no madre flaca!

 

[Mickney]

Site specific laboratory testing or insitu specialty testing (pressuremeter, etc.) to more accurately estimate the potential settlement is far cheaper than dealing with a magnitude of settlement after construction that is larger than predicted based solely on soil test borings. In my humble opinion and how I approach "settlement sensitive" projects, do not predict settlement without a reasonable amount of laboratory testing or other in-situ testing (pressuremeter, etc.).

 

[iandig]

I have only briefly read through the replies so far, so apologies if I replicate a previous response.
In consideration of settlement, please note that there are a number of separate mechanisms which will induce movement (settlement), and these include:
1. Elastic settlement;
2. immediate settlement;
3. primary settlement;
4. secondary settlement (peat and alluvial soils in particular)
5. consolidation settlement;
6. Self-weight settlement (of fill materials);
7. creep settlement;
8. others (for others to comment!!)
Some of the above are different names for the same principal action, some are quite different, some are not present with certain ground conditions and all will be happening concurrently. Hence, predictions of settlement and the associated time for settlement to occur are difficult to predict with any level of accuracy, however there are some ‘rules of thumb’ which can help the process.
As would be expected, elastic/immediate settlement happens very quickly (during construction). Primary settlement will also happen in a relative short period of time, however this will be wholly dependant upon the actual material. Secondary settlement occurs over a much longer period of time, and is most critical to assess when considering an organic alluvial and peat soils. With thee materials, and change or increase in loading after construction will replicate the same process of settlement, therefore by overlaying a failed road over peat (without removal of the failed layer) will end up mirroring the failed surface.
Consolidation settlement takes time, directly associated with the permeability of the soil, the length of the drainage path and the direction or directions of the drainage path. 5 m of clay could take 10 to 15 years, on-going settlement could be on-going for many years. This needs to be modelled by a competent geotechnical engineer. Different layers of soil will be affected in different ways, have different settlement characteristics and different times for it to occur. In general when I assess this, I talk about total settlement, residual settlement after construction is completed and any on-going
Self-weight and creep settlement are generally assessed using the factor ? over a log 10 cycle. Therefore the level of settlement determined by using ? between 1 and 10 years, will be the same as between 10 and 100 years. Only relevant where fill/deep fill/deep made ground underlie the site.
There are also a number of different techniques to use in assessing the settlement characteristics of the ground, from consolidation testing, flat plate dilatometer, dutch cone CPT testing, elastic modulus, etc… and each of these can give different assessment to the level of settlement. My advise is, get a good geotech engineer who you trust, pay him some money and get a good assessment for each site on an individual basis (of course I would say that!) when it is critical to the overall design.