95% vs 100% MDD - calculating difference in self weight settlement?
95% vs 100% MDD - calculating difference in self weight settlement?
(OP)
Situation - I have been recommending 95% compaction for controlled fills and using the rule of thumb that you may see as much as 1% of settlement from the controlled fill due to self weight (assuming a silty sand). Additionally, I understood that this settlement predominately occured during the construction of the fill.
I am with a new firm and they want to recommend 100% MDD with the belief that this will eliminate the self weight settlement. They also do not believe the self weight settlement occurs during construction.
I have spent a few hours trying to calcualte this behavior without success. Has anyone been through this exercise?
Thanks,
jth88
I am with a new firm and they want to recommend 100% MDD with the belief that this will eliminate the self weight settlement. They also do not believe the self weight settlement occurs during construction.
I have spent a few hours trying to calcualte this behavior without success. Has anyone been through this exercise?
Thanks,
jth88





RE: 95% vs 100% MDD - calculating difference in self weight settlement?
One other thought, if the fill is being placed on a compressible foundation, the extra fill weight will create additional foundation settlement.
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
Settlement of the foundation soils is an additional source of settlement that was calculated, both quantity and time rate.
Thanks,
jth88
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
If you have clayey or silty materials, more movement will take place.
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
BigH - you are correct, I am interested in the self weight settlement of the fill. Or how can I mathematicaly show that an incresing from 95% to 100% MDD does or does not reduce self weight settlement. For this exercise I am not worried about the settlement of the underlying foundation soil.
BigH - not sure what you mean by "given that 95% would be less stressful to the foundation soil than 100%."
Ron- do you have any references on the energy increase required to go from 85 to 90, 90 to 95 and 95 to 100 percent compaction.
Another problem I have with requiring 100% MDD is the difficulty it will cause the contractor. He will have to watch the moisture content very closely or he will have to use larger equipment to make sure he is above 100% MMD based on Standard Proctor.
Thanks,
jth88
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
In Central Virginia (i.e., in the Piedmont), there are many projects that use fine-grained earth fill. When the earth fill exceeds 10 or 15 ft (thicker?), I recommend settement plates.
Regarding 100 percent Modified v. 95 percent Standard. I always reference 95 percent Standard - never had a problem. I'm also very critical of the reference Proctors being used and insist on confirmation one-points and full curves on a regular basis to make sure we gauge that the reference density that we're using is accurate.
Regarding 100 MDD v. 95 SDD, yeah it may be "better", but why not require 105 MDD, that would beat 100! Somewhere there has to be reason included into this discussion. I mean what would be the contract value of 100 MDD - ALOT!
f-d
¡papá gordo ain't no madre flaca!
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
As for calculating the effects, start with a layer of x01 thickness and assume an MDD of, say 110 pcf and a moisture content of 10% and assume an E value of E01. Put another layer on top of x thickness. Calculate the stress in the first layer x01. Estimate the settlement due to x02 as delta p x 0.7/E (assuming poisson's ratio of 0.3). Then put on another layer x03. Compute the stress increase in x01 and x02. (neglect, for now the increase in E). compuate the settlements of x01 and x02 due to x03. Continue. Then do the same for 100% MDD.
The fallacy of the exercise is that (1) you are assuming uniform loading in an elastic medium (not a particulate medium as put forth by M.E. Harr), (2), the compaction is exactly 95% MDD and that in placing the layer and compactin, you do not get any increase in compaction of the underlying layers, (3) E remains the same throughout the compaction process where, in fact, it would slightly increase due to additional confining pressures, etc. . . .
I suppose you could try it for 10 layers or so and see what difference you get - for academic exercise.
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
Poorly compacted fill or fat clay may compress more. Fat clay, of course, may swell, especially under small overburden pressure.
Some research on "Hydrocompression Setttlement of Deep Fills" was reported by Brandon, Duncan, and Gardner in the Journal of Geotechnical Engineering Vol 116, No. 10, October, 1990. See also "Collapse of Compacted Clayey Sand" by Lawton, Fragaszy, and Hardcastle in the same Journal, Vol 115, No. 9, Sept, 1989.
Some earth dam designers use a 1%-2% rule of thumb for settlement of the embankment, expecting half of this during and soon after construction and the other half as seepage develops. I have generally found this to be conservative if 95% compaction was specified and enforced.
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
95% Standard has worked for a long time... i'm hesitant to believe that any "raising the bar" has been based on 95% STD actually failing in the field (as opposed to be perceived as failing). seems to me like the bar is just being raised to get a Safety Factor on getting plain old 95% STD.
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
Based on my own experience and the soils I normally come across, then there is a case for distinguishing between 95% and 100% of the 'standard' [2.5kg] proctor. Most of the earthworks I get involved with have a high proportion of cohesive soils. Following current standards [SHW & BS EN] National Guidance [BRE] and best practice, we do distinguish between the two levels of compactive effort. This is of note because we find that in many instances, 100% of the 2.5kg rammer is close to 95% of the 4.5kg. Please note this reflects the cohesive soils from my region and would not want this to be a 'global' generalisation. Where we are placing cohesive soils as 'general fill' then typically we would look at the 95% of the 2.5kg rammer, which in turn 'typically' returns shear strength of around 50 kPa and/or CBR in the order of 2.5% on the wet side of the curve. 2.5% CBR is the minimum allowed for sub-grade to pavement foundations [IAN 73/06], below this you "have to do something else..." and this corresponds well with the min shear strength for trafficability. Where the fill is placed below more sensitive structures, i.e. buildings, we are reccomended to use the 4.5 kg rammer, as this should have a higher bearing capacity and less suceptible to long-term settlement. Tyically we would find CBR's in the order of 5% /shear strengths of around 80-100 kPa at 95% MDD of the 4.5kg compaction on the wet side, which in turn fits well with the expected performance of the material at 100% of the 2.5kg rammer. We do, however, also place great emphasis on the air void content, with a maximum of 5% specified for cohesive fill.
Now going back to the original post:-
in my experience for cohesive soils, 100% MDD of the 2.5 kg rammer will settle less than 95% of the same MDD, but will not remove it completely. Cohesive soils tend to settle less during construction and more long-term due to consolidation settlement. Granular soils will probably experience most if not all of their settlement during construction. The general split between granular and cohesive, based on the SHW, is the amount passing the 0.063mm sieve, more than 15% its cohesive, less than 15% its granular. This is NOT the same assessment we make when describing the soils geologically as part of the SI.
So in summary it depends on the soil type and nature of the structure being constructed. I have tested fine grained 'cohesive' soils [sandy SILTS] where the MDD using the 2.5 and 4.5 were very similar, but were unable to achieve less than 8% air voids.
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
BigH - I hope to have time this weekend to try the academic exercise.
I was also going to look at the change in air voids from the 95% and 100% MDD Std Proctor states.
I will review the paper listed. I fear that these papers are for larger >> 20 foot. Not sure of the applacability for smaller fills.
Thanks,
jth88
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
RE: 95% vs 100% MDD - calculating difference in self weight settlement?
RE: 95% vs 100% MDD - calculating difference in self weight settlement?