Coefficient of Consolidation (Cv) Question with Project Background 1

[Blairsy]

I am attempting to estimate the time rate of primary consolidation for a particular soil profile due to placement of a large area fill load.

We drilled several SPT borings at the site and determined that the subsurface profile generally consists of 10 feet of existing fill (mostly granular, medium dense to dense) over 3 feet of silty fibrous peat over up to 40 feet of Boston Blue Clay (BBC)(CL) to contact with a gravel layer. Based on the SPT data, the BBC ranges in consistency from stiff (top 1/3) to soft/very soft (middle 1/3) to medium stiff (lower 1/3). Out of all the borings, we were only able to walk away with one reliable tube sample (taken in the soft clay), which I submitted for a 1D incremental consolidation test. My Cc and Cr values from the consolidation curve reconstruction agree fairly well with published BBC values and my Cc value is on par with some of the liquid limit correlations. With that said, I have run through some other correlations for the peat and stiffer clay layers to get the information necessary to calculate the total (ultimate) primary consolidation settlement of each layer and all layers combined. I am now stuck at the time-rate portion of this exercise.

The lab results for the consolidation test provide a summary table of values associated with the end of primary consolidation (EOP) for each load increment relative to Log of Time and Square Root of Time space. One of those values is the coefficient of consolidation (Cv) such that there is a Cv value for each load increment (as there should be since Cv is a function of k which changes with e). I understand this is all pretty basic so far, so here is my ultimate question: is there a correct approach to selecting a correct Cv value out of the bunch for use in the time-rate calculation to determine time (t) at 90% consolidation?

I have considered doing the following and seeing what happens:
1. Averaging all the Cv values.
2. Selecting the Cv value associated with the load increment stress nearest to the proposed vertical effective stress.

Once I determine the above, would it be acceptable to apply that Cv value to the entire BBC thickness even though there are definite differences in stress history?

Any input is greatly appreciated!

 

[LRJ]

I'd say Option 2 is the correct approach. However, this implies time-dependency of the c_v parameter, so you'll want to find a way to automate the selection; generally you'll find a good trend with applied stress. You might also find the variation of c_v makes little difference to the result, in which case Option 1 is okay.

For conventional settlement analyses, the c_v parameter just controls when primary consolidation ends/secondary consolidation begins. While you would expect more primary consolidation from softer material, stiffer material may initiate secondary consolidation sooner. Nonetheless, for most applications, I'd expect applying results from a soft layer to the stiffer layer would achieve larger settlement results.

 

[fattdad]

I'd expect the Cv values in recompression will be from a similar family of values. I'd expect the same for the Cv values in virgin compression.

Don't forget to consider secondary compression in the peat! Heck, the Blue Clay too!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Blairsy]

Thank you both for your replies. I'll begin with using the lowest and highest Cv values reported from the lab data and seeing how much of a difference it makes; if this difference in time rate is negligible, this discussion is probably moot.

fattdad: in regards to secondary compression, the lab-reported C-alpha values for each load increment on the BBC test samples are all zero (reported as 0+e00). Does this make sense to you? In terms of total primary consolidation settlement, I am getting about 8 inches from the peat and less than 1/2 inch from the clay, so comparatively speaking, the clay is not really much of a concern for the proposed fill load. Do you have any good sources on secondary compression of fibrous peats? I'll need to assume some values based on moisture content, organic content, and visual classification.

 

[fattdad]

look at the last slide of this presentation. I don't know the reference for the presentation, the last slide is the Mesri correlation to water content.
.

https://slideplayer.com/slide/10533550/

Your lab reports Csub-alpha as zero because they triggered the next load to immediately apply at the end of primary. I reject those lab certificates and require all consolidation tests to run for at least 4 hrs after the end of primary or until the straight line is defined.

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoEnvGuy]

I am not that familiar with boston blue clay but based on the info posted I think your in the range as per the photo below. You should report the range of time by varying the Cv and thickness/drainage path. Taking either the min or max or two standard deviations of variation from mean of all your estimates.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[Blairsy]

fattdad - We did one 24-hour load hold in the virgin compression range for the sole purpose of getting a decent C-alpha value, which was still reported as zero. We used Geotesting Express, which seems to be a pretty reliable lab. I have also seen you reference the "expanded layer" technique in some older threads (provided in the NAVFAC manual). I am hesitant to assign a single Cv to the entire clay stratum since my soft clay is sandwiched between two stiffer clay layers and if I treat the middle soft layer as its own layer with its own Cv value, theoretically there would be no vertical drainage and the pore pressures would never dissipate in that layer. This has lead me to the expanded layer method. Do you have any other info/references on that method besides NAVFAC?