Settlement of Existing Fill 2

[molerat2210]

I have a site with an existing fill between 4 and 17 feet thick over a firm, residual soil. The area of the existing fill is about 4 acres. I need an intelligent method for calculating long-term settlement of the existing fill layer. The fill consists of sandy elastic SILT and micaceous silty SAND. SPT N values range from 2 to 10. The fill is clean with little debris or organics. Moisture contents around 20 to 30 percent. Existing compation about 80 to 85 percent of modified proctor. No additional surcharge is proposed. Site area will be used as a cemetery. Desire to limit future ground surface distortion. I understand that any method is probably a calculated guess at future settlement behaviour.

 

[msucog]

what kind of settlement can the "tenants" tolerate? just kidding...i'm sure i'm not the only one that thought of that...

if it were me, i'd bypass lots of fancy testing and guesstimated analysis and tell them to compact the top and grow some nice grass and call it good. maybe do a favor and not charge them lots in the hope for future work or the client having a good word to pass along to other future clients.

how long has the fill been there and was it placed or "dumped"?

 

[Dave6000]

The best method I've found for estimating settlement is the Schmertmann method. See Foundation Design, 2nd edition by Donald Coduto, Prentice Hall, 2001, page 231. The method uses instiu tests using the SPT or CPT to estimate material modulus and from that and some physical dimensions it estimates settlement. It can also be used with some simple instruments that measure modulus directly without penetrating the gound.

 

[civilperson]

The existing fill will be disturbed whenever the roads are built and again when landscaping. Scarify and compact the top 18" in three lifts under pavement and use gentle curves for slope changes in seeded areas. The settlement will be un-noticed if it occurs at all if a minimum slope of 1.5% is maintained to prevent ponds.

 

[BigH]

Basically, as I see it, long term settlement will be associated with creep of the fill as the fill is now in place (except near the roads as civilperson points out). Schmertmann gives an approach to estimate creep of sandy soils.

 

[molerat2210]

I did consult the current tenants on the settlement question. I could not even get a cryptic answer out of them (this is the best I got, Msucog).

Best guess about the age of fill is that it was placed incrementally over the last 5 to 15 years. New area was a former dump area for soil diplaced by "tenants" in other portions of the site.

I went ahead with Schertmann's method as suggested by Dave600. The selection of the modulus is basically a guess from the SPT data. This seems to work better if I had CPT data. For delta P, I divided fill into sublayers and assumed surcharge as weight of fill above each sublayer. I am coming up with about a max of about 3 inches. I actually used what I think is a watered-down Schmertmann's from NAVFAC. I am going to look around for the other reference to see if it has more content.

If the answer is up to 3 inches, then I think all can live with this. I will suggest to scarify and beat the site site with something big like CAT825.

Thanks all.

 

[GeoPaveTraffic]

5 to 15 feet of fill that has been in place for minimum of 5 years...

I would say you are safe in assuming that the minimum settlement will be less than 3 inches. The only concern I have is with the micaceous sand. Have not worked with it, but I understand it does some odd things.

 

[molerat2210]

Micaceous silty sand is cruddy stuff but makes marginal quality structural fill when used right. Basically, it's made of thin, platy particles. Low density, very moisture sensitive, very erodible.

 

[fattdad]

The first question that popped into my mind is what is the relationship between the liqiud limit and the natural moisture content? Assuming that the natural moisture content is noticably less than the liquid limit, there is a degree of pre-consolidation that has occurred since the fill was place - whether from the original compaction or from decsication (sp) over time. From your description, there will be little new loading associated with the plan for development. As such, the long term settlement is likely to relate to secondary compression over time. You could assign a C-alpha value for the soil type (correlated to the natuarl moisture content) and then look at the anticipated behavoir over the next two decades or so.

Regarding the construction of roads, I'd agree to scarifying, recompaction (to 95 percent compaction) and then construction of the appropriate pavement section.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[molerat2210]

Fattdad, this seems to be a specific method for getting creep settlement. LL ~ 50, MC ~ 25%. I think you are saying use this data to get void ratio then C alpha. I had not considered this.

Thanks Fattdad
You ain't no flaca?

 

[fattdad]

Not sure about the intermedieate step of getting void ratio. I'm saying that if the in-situ moisture content is half the value of the liquid limit, then the soils have some measure of pre-consolidation. Then it's fair to correlate that the remaining compression will occur from long term secondary compression. You said that the fill has been there for 3 years. So, you'd just correlate moisture content (i.e., 25 percent) to Calpha (about 0.3 from my graph) and run the numbers: DeltaH=Calpha*H*log(tsc/tp). For what you've told me and for 20 years, I'd set up the problem as DeltaH=0.3*H*log(23/3)

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[fattdad]

oops, this can't be right. Calpha is 0.003 (0.3 percent) and the time to the end of primary may not be the three years so far. This problem is kinda of vague as there are some unknowns. That said, using 0.003 is MUCH more reasonable. Sorry for the miss-type. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Maybe I'm wrong and am not seeing this properly - but the fill has been in place for a length of time. Settlement (as you got with Schmertman) would, in sands, have already taken place during placement and in immediately after. It undoubtedly has undergone precipitation loading - so that is really a non-issue. Again, I would see future settlement except where you are introducing structures (say a maintenance shed, road, etc) to be solely related to creep (like secondary consolidation) - and that is given by C2=1+0.2(log(t/0.1)). In your calc - take your settlement as original - say 2 inches, 3 inches, 4 inches - whatever and multiply by the above from, say 5 years to 15 years (i.e., 10 years from today as you have 5 years of creep already). If you do this - you will find, say over 10 years that the expected settlement would be in the order of 0.3 to 0.5 inches or so due to additional creep. This is hardly noticeable. So, in my view, given the 5 years of fill to date (and assuming no added loading), you wouldn't much notice any settlements.

 

[fattdad]

If you are dealing with elastic silt and it's fill, one mechanic for long-term (i.e., post-primary) consolidation is given by: DeltaHsc=Calpha*H*log(tsc/tp); where H is the thickness of the subject layer, tsc is the time-frame for evaluating secondary consolidation and tp is the time to primary.

If you are dealing with sands, then the long term compression is given by a scaler Ct. Duncan's settlement manual lists values of Ct ranging from 1.0 (for one month after initial elastic compression occurs) to 1.5 (for 30 years after initial elastic compression occurs).

Now that I've taken the time to type this, I would agree with BigH - this is not likely a problem. If there is no additional loading and the fill has been in place for several years, the remaining settlement potential is likely to be negligable (sp).

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[soildynamics]

fattdad

are you sure about the correctness of your statement:
"Assuming that the natural moisture content is noticably less than the liquid limit, there is a degree of pre-consolidation that has occurred since the fill was place"

It is the plasticity limit that , upon cpmarison with moisture content, can give you indication on the preconsolidation state of the soil and not the liquid limit (LL). The plasticity index is calculated (LL-PL) and that can give indication on the compressibility of the soil before loosing its matrix structure.

 

[fattdad]

solidynamics: Normally consolidated clays in the near surface have natural moisture contents at (or near the liquid limit). There is no doubt that a natural moisture content at the plastic limit is pre-consolidated. I am also aware of relationships between Atterberg limits and compressibility.

Now that I've said all of this, if it's a critical project, I'd do an odometer test.

f-d

¡papá gordo ain’t no madre flaca!