Calculating desiccation shrinkage

[iandig]

I have been asked by a German Design and Build contractor to review and comment on their procedure for predicting the potential settlement due to desiccation. The site is in the UK and the underlying soil is a Glacial Till/Boulder Clay, LL 43-45, PL 17-19, PI 24-26.
The equation they have used for their calculations in Germany is based on a research paper known as 'Heft 152' which [based upon my translation of it] relates to issues with pipes and water pipes. The calcualtion of potential settlement is as follows:
s ~ 1/3 x {[(w1 – w2) x ?d/?w x h0 ]/ V0}
mit:
s Setzung [mm][Settlement]
Vo Volumen des Bodenkörpers (h0 x b x l) [m³] [Volume under consideration]
h0 Höhe der betrachteten Schicht [m] [Depth/height of layer]
wi Wassergehalte [%] [Water/moisture content]
?d Wichte des getrockneten Bodens [KN/m³] [Dry Unit Weight of soil]
?w Wichte des Wasser [KN/m³] [Unit Weight of Water]
The analysis assumes that any loss in water is directly related to a loss in volume, and as this can occur in three axis, it divides the answer by 3 to predict settlement. The way the units work and cancel out, it results in the change in moisture [as a %] x the depth of layer [in m] then divided by 3 to give an answer in mm.
The results provided by the German contractor predict very small settlements due to shrinkage, whcih are unrealistic for the soils we have in the UK, i.e. with a moisture change from 35% to 23%, it only predicts 6.4mm over 1.0m
My questions are:
1. Has anyone used this equation before?
2. Has anyone used similar equations to this?
3. Does anyone have an alternative method of calculating predicted settlement?
4. What are your thoughts on this approach?
The soil is proposed to be left in-situ below a floor slab which may [in about 15 to 20 yrs] be subject to desiccation due to the presence of trees outside of the site boundary. This is NOT in accordance with what the engineer wants, but because of their refusal to remove the material prior to now, the structural steelwork and cladding are already in place. Moisture contents within the soil at present are around 19 to 21% with a localised maximum moisutre of 24%, and the soil is an overconsolidated clay.

 

[MRM]

I'm not familiar with that equation. I'd be more inclined to estimate settlement due to dessication by using phase diagrams for your soils instead. Perhaps compare their equation with the result obtained using a phase diagram method. By the way, our glacial tills (of the lodgement variety anyways) are typically very sandy with about 25% LBW, very dense, with low void ratios. The settlement due to skrinkage of those is probably much lower than yours...interesting.

Under the circumstances that you mentioned, I'd caution your client about the potential problems in not following the engineer's recommendations about not having water-sucking trees located near the building. Perhaps vapor barriers or some other elements are also being left out of the design too.

I'd also explain to them (as you're giving them your opinion of the equation) that estimating settlement of this type is dependent on many variables and that it is very difficult to do so accurately due to the unknowns involved with your site.

 

[iandig]

Thanks for your thoughts, they are very much in line with our own. Because of the way The Client's Engineer has been appointed, he is only advising The Client on what The Contractor is doing. Because of this, if its not in the standard its not OK, therefore he is not allowing the use of root-barriers etc...
Due to this, the Contractor was instructed to remove any cohesive soils which could be influenced by desiccation, the Contractor decided not to and argue their case using a procedure for determining potential shrinkage used in Germany.
When I have tried to follow their logic [i.e. from the German paper], I get 10 times the level of shrinkage, when I queried the format of their equation with them, I was TOLD [in no uncertain terms] that they are correct and the equation returns settlements in mm and not cm's. The theory assumes the material will shrink equally in three dimensions, therefore any loss in water by volume can be divided by three to provide the vertical settlement. It does not take into account any exisiting conditions within the soil, loading criteria, drainainage pathways, exisiting or future effective stress condiitons etc...and as such we have stated it is only a 'theoretical' assessment, and cannot show any evidience that what their equation says will be the settlement can be proven either by experimentation or observations of actual shrinkage.
Going back to the soil type, the Glacial Till/Boulder Clay in question has some sand and fine gravel, although the % passing the 63µm is around 85%, with the clay content in excess of 50%. It is over-consolidated OCR of around 2-3 basd on the previous SI.
We have tried to tell them that the only way to get the Engineers's approval is to do what he says, as throughout the entire project that has been the case, they just don't like being told what to do!

 

[jdonville]

iandig,

Unfortunate that you seem to be caught in the crossfire between a strong-minded Contractor and a literal-minded Client's Engineer. Hope it doesn't come to litigation.

Good luck with this situation.

Jeff

 

[BigH]

Ian: For a rough idea, you might read the first 4 or so pages of Malcolm Bolton's book "A Guide to Soil Mechanic". He talks about shrinkage of a soil below a boiler. Also see the attached list - although I can' access some of the papers - your firm might have the subscription.

http://www.atypon-link.com/ITELF/doi/abs/10.1680/igeng.1995.27274?journalCode=igeng

http://products.ihs.com/cis/Doc.aspx?AuthCode=&DocNum=87181

http://www.duke.edu/~hueckel/DesiccationPapers/GeoDenver2007.pdf

http://users.ictp.it/~pub_off/lectures/lns018/39Taboada1.pdf

http://www.nat-hazards-earth-syst-sci.net/6/825/2006/nhess-6-825-2006.pdf

Good one -

 

[iandig]

The first paper on the list I am very familiar with as it was written at the time I worked for the company where Richard and Phil worked. Their assessment looked at desiccation indicators, and from memory they did not actually go into predicted shrinkage. I have a couple of calls out for Richard and Phil, but with the takeover of the Comapny a few years back, we all 'seem' to have moved on.
I will check out the others though, any info really helps, even if it is to point out to the Cntractor that their approach is difficult[to say the least] to prove to their own adavantage.

 

[BigH]

Ian: Any chance to get a copy of the paper?

 

[BigH]

Ian - tried to send you the excerpts from Bolton's book - but the email I had for you came back no-good. Can you send me an email with yours? friesh at inco dot com is a good address to send to and bohica dot fries at bigfoot dot com.

 

[iandig]

Howard, new e-mail address is as follows:
ian_dot_gardner_at_xplor_dot_co_dot_uk
I will try and get a copy of the paper which can be sent, but I need to go back through some of my archieved cds. Please bear with me on this [just come back from leave!]
Many thanks
Ian

 

[iandig]

Hi Howard
I now have a copy of the paper for you, but in my own move have misplaced your address, can you let me know the best place to send it. I also have more info on the problem now, so if anyone else needs some data on calculating settlement due to desiccation, please let me know and I can forward it to them [plus the German paper if anyone can read it!].

 

[oldestguy]

andig:

For what it is worth, you can check out a simple computation of volume of water lost for each percent and per a unit of depth, one dimension. It is unlikely that you will get an even loss of moisture, so about all you can summarize is an estimate of differential settlement.

In my experience it is the differential settlement that causes the grief, not the total.

Root barrier? You mean to say loss of moisture exists only at the roots? No lateral migration from moist to dry?

Get 'em in the ball park anyhow.