Calculating desiccation shrinkage 1

[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.

 

[jdmm]

I can't tell you of an experience where the recommendations were verified but I once had to do the same thing for a heavy clay (bentonitic) where they wanted to raise the temperature to 100C to remove dry-cleaning contaminants. I took a simple approach and took samples of the material, measured them; subjected them to the prescribed temperature increase and remeasured them. There was a significant decrease not only in vertical measurement but diametrical. I assumed that due to the distribution of temperature and moisture availabilty that shrinkage in the horizontal direction would be reduced or eliminated but the vertical direction would be similar to experimental. The results indicated massive settlements compared to acceptible settlements. I'm sure that this approach could be refined but it was enough to indicate to us that their remediation plan would result in significant settlements to the existing buildings.

 

[iandig]

Thanks for the response. Since posting it on this forum, I also posted on another of the forums and have had some additional feedback. In addition to this, we have been able to source a number of other publications which reference the estimation of shrinkage settlement, and have been able to respond to the Client with a better assessment of the likley ground movement. Based on the current moisture of the ground and the moisture where no further shrinkage is likely to occur, two seperate methods returned predicted settlement of 12 and 13mm. Using the equation from the German paper it returned an initial assessment of about 6mm, however when you follow the transalted logic they used to come up with this equation, it came out over 60mm, hence the reason for posting the original thread.

 

[dgillette]

Pardon my use of "English" units, even though you are in England where metric units are used instead.

Assume (for argument's sake) that there are 100 lb of dry soil with SG=2.7. This has a solids volume of 0.59 c.f. If the water content is 35%, the water volume is 0.35*100/62.4 = 0.56 c.f., so the total volume (if no air voids) is 0.59+0.56=1.15 c.f.

Changing the water content to 23% makes the water volume 0.37, and the total volume 0.96. The new volume is 100*0.96/1.15 = 83% of the original. A 17% reduction in volume is 17% settlement if in 1D (no vertical shrinkage cracks), ~6% settlement if the strain isotropic. Therefore, you would expect settlement of at least 6% of the 1 m thickness or, drumroll please, 6 cm! Therefore, I believe iandig is more correct than the equation.

Hope this helps (and that I got my math right).

DRG

 

[iandig]

Thanks for that, its a relief to get the same answer independantly. I have a number of other equations if this subject comes up again, which appear to provide a better assessment of the 'likely' shrinkage.

 

[civilperson]

The volume of voids may not be saturated and then the loss of water represents a fraction of the available settlement.

 

[dgillette]

The calculations, both mine and the original, assume that there are no significant air voids. However, if the water content is 35% as cited by iandig, there probably isn't much in the way of air in the voids. If saturated, the dry unit weight is only about 86 lb/ft^3.

I would, in general, consider the volume change calculated that way to be an upper bound rather than a lower bound, unless there is some compaction or a new, higher overburden stress. With air in the voids, I think the actual percent shrinkage would be LESS than what's calculated here because the void space lost to removing the water is a smaller fraction of the original total volume. Also, if the soil gets close to its shrinkage limit, the change in volume for a given loss in water content gets real small.