Standard Penetration Tests: converting N values to densities 3

[Anniepops112]

Hi

I am trying to design a retaining wall atm but I am having some problems as I can't find anyway to convert the N Values I have from borehole readings to densities. I am sure this really is very simple to do but I have unsucessfully searched the web and my core textbook, "Soil Mechanics: Concepts & Applications" by Powrie, and several books at the library I can not find the answer.
I am sure it a fairly simple procedure to convert the values over, but I really can't find it!

Any useful ideas?

Many Thanks

Anne
x

Yes I know the cone test is far more useful etc but these are results from bore holes so it's all I have to work with!!

 

[msucog]

i'm not sure it's as easy and simple to do as you mention. what are you specifically trying to do? i presume you're trying to put a % compaction to old, undocumented fill based on blow counts...is this correct? and with that, are you trying to pull out the design strength parameters for the soil?

 

[DRC1]

Try this link

http://www.usace.army.mil/publications/eng-manuals/em1110-2-2504/c-3.pdf

This is the US Corp of enineers Sheetpile design guide. They have a a table & chart for conversion.

SPT's do not provide the highst quality data. It is necessary to use a lot of judgement & common sense when devloping design parameters from SPT.

 

[msucog]

i would use extreme caution with that sort of thing. "granular"...i've seen lots of soils that would be described as sands but act like a silt (especially when there's mica). if you're testing fill or residual with rock fragments, all you've got to do is have a piece of aggregate or rock fragment in the spoon to really mess up those nice charts. if you've got geology specific charts, then i'd say you've got a better chance of not having to totally pull a number out of your arse...i emphasize "better". why not just do a little more testing to provide a more realistic assumption? or maybe just completely make up the number since there's seemlingly little info available...
i guess annie should respond before we/i continue assuming what she's trying to get to...

 

[Anniepops112]

Hi,

It's theoretically designing a retaining wall and I only have bore hole records so I can do no further testing. All I have is the N Values, ground water level, height of site above ground and where our building is going on the site. This is all very conceptual design to get us thinking about the design process. In theory I need to be able to calculate this off infomation given. All I have is the N Values and what I need is the densities so I can perform a basic analogue on the site.

Not sure if that clears anything up? or anyone has anyother ideas?

Thanks for your help

Anne

 

[msucog]

when you ask for densities, are you referring to in-place density of old fill, unit weight density of the soil, or are you trying to get to something like the strength properties (phi/c) of the soil?

 

[Anniepops112]

I need both (I think). Literally all the data I have is from a borehole, so the N Valesw, bore hole diam, dephts and thickness of each strata and description of strata.

To design the retaining wall I need the underdrained shear strenght (Cu, tau), the saturated and bulk densities and the angle of friction (phi) so I can find the k values.

Once I've got the Cu value, I can solve for short term effect and d x fs then I can solve for long term.

The actual calculations for te retaining wall look pretty straight forward, it's just trying to get the numbers to work with to get me there that's causing the problems.

 

[jdonville]

Annie,

What it sounds like you are doing is a preliminary level design/feasibility analysis.

Firstly, why isn't your geotech doing this? They should know what parameters to use for such analyses. Why they didn't report undrained shear strength or unconfined compressive strength numbers for cohesive soils is a bit of a puzzler.

Second, for a quick and dirty look like this, you should probably use presumptive unit weights and strengths, unless the gotech report contains better-quality information. Typically, 120 pcf (total unit weight) is assumed for soils in structural design codes. For cohesive soils, unless the site conditions are poor, you can safely assume phi' of about 28 degrees. Sands can be assumed to have phi' of about 30 to 32 degrees. Check your state or local building codes for presumptive undrained strength values.

Good luck,

Jeff

 

[msucog]

jdonville hit it on the head...

 

[BigH]

As usual, good advice from jdonville (although I like to use 125 as the "prescriptive" unit weight - this makes the bouyant unit weight "1/2").
Unit weights are always the conundrum for retaining wall design. We spend so much time determining Ka to the nearest 3 decimal points; yet unit weights are almost always "by the seat of the pants!".
Table 3.4 in Bowles 5th edition gives the ball-park unit weights based on N values for "cohesionless" soils; also for estimation of phi' values. For Su vs N, most texts have the "prescriptive" correlation:
N = 2 ; Su = 12.5 kPa
= 4 ; = 25 kPa
= 8 ; = 50 kPa
= 15 ; = 100 kPa
= 30 ; = 200 kPa
USE THE ABOVE WITH CAUTION - but I have found it to be reasonable (and conservative).

 

[msucog]

keep in mind auto hammer to rope and cathead differences...yet another reason to have the geotech provide recommendations regarding the soil parameters.

 

[fattdad]

Bazaraa, 1967 prepared a graphical relationship between N-value and effective overburden pressure to relative density. Gibbs and Holtz, 1957 prepared a similar relationship. Bazaraa's relationship are more conservative. Shmertmann, 1975 presents a relationship between N-value and overburden pressure to effective friction angle. The aformentioned graphs are presented in "Engineering Properties of Soils", UCLA-ENG-7652, May 1976 by Ladd and Lee.

DM-7 presents relationships for SPT n-value and unconfined compressive strength for clay. This graph compiles the various correlations by Sowers, and Terzaghi & Peck.

I may have other correlations - bear in mind that these N-value correlations often relate the N-1-60 values. This is the N-value that would be obtained under 1 TSF of confining stress with a rope-hammer combination at 60 percent effeciency. The typical "safty" hammer normally operates at 60 percent effeciency.

Good luck in your design!

f-d

¡papá gordo ain’t no madre flaca!