Cohesive or Cohesionless?

Suggest to repost on the "geotechnical engineering" forum to draw meaningful responses.

What spacing are the bored piles? How are you analyzing the wall? Typically I would use wall frictions values of 0.6 on the active side and 0.4 on the passive side for a drained analysis.

There is good advice in "Guidance on embedded retaining wall design (C760)" by CIRIA

Thank you gents.
I am using 1.8 meters spacing without anchors at the moment.
I use wallap together with hand check (just strength for hand check though)
So you think use 'adhesive' is okay for this given soil profile? Does this wall friction value apply to all soil profiles?

Those Cu values are an "undrained" shear strength. I would not use an undrained analysis for any material described as silt. The drained cohesion value would be close to zero for that material

Is there any relationship between the undrained Cu and the drained one?
If I don't misundertand what you mean, I can design it as cohesive but the shear strength need to reduce to the drained one?
If I design it as cohesionless, is 40mm lateral defection too much for 4m deep retaining wall? It is a normal apartment? Thanks.

You should ask the geotech engineer for the drained cohesion. It is likely to be about 1kPa, and you should probably just assume is it zero.

In WALLAP whether the soil is cohesive/cohesionless, or drained/undrained are two different things. So the soil could be cohesive but the analysis could be drained or undrained. You should read the sections on "drained and undrained cohesion" and "drained or undrained soil type" in the WALLAP manual.

From my limit knowledge and research on this matter, you may follow the steps to find the corresponding adhesion value:

1. Correlate Cu with SPT.
2. Determine cohesion using relationship below.


3. Determine adhesion from chart below.

Okay. Thank you very much gents.

Note that the data posted by retired13 is only applicable to an undrained analysis.

Actually you shall get the number directly from the lab. Use the suggest method only when there is no option.

My interpretation of the geotech report

Silty Clay / extremely weathered basalt will behave as a clay, which will have an undrained failure but need to consider the long term drained failure mode as well.
Highly and moderately weathered basalt will act as bedrock, the CU is probably the compressive strength and you should be looking at the drained failure path.

The very high Cu number below 6m indicates a very stiff soil. Even it is quite "hard" between 2m and 6m below ground.

The chart below does not agree my interpretation above (stiff vs hard)

retired13 et. al. - I've read T&P (1967) many times and don't remember these SPT values (sadly I do not have my book witth me. However, I have seen other sources citing T&P and the standards of using Geocon (established 1954 in Canada (M.A.J. Matich, Lec Brezenski) and Golder (who via Vic Milligan, John Seychuk, Finn Heffernan, and others broke off of Geocon)

SPT N qu Su (where qu is unconfined compressive strength and Su is undrained shear strength)
0-2 < 25 kPa < 12.5 kPa very soft
2-4 25 - 50 12/5 - 25 soft
4-8 50 - 100 25 - 50 medium, medium stiff
8-15 100 - 200 50 - 100 stiff
15-30 200 - 400 100 - 200 very stiff
> 30 > 400 > 200 hard

Be aware that in Canada, many firms (including Golder) uses the term "firm" instead of stiff or very stiff. Importannt when reading Canadian Geotechnical Journal. Also N is uncorrected for Overburden pressure.

Also be aware that in British Standards - BS 5930 uses:

very soft < 20 kPa
soft 20 - 40
firm 40 - 75
stiff 75 - 150
very stiff 150 - 300
hard > 300

And to continue - for cohesionless soils:

Descriptor US/Canada EN 1997-2:2000
very loose 0 - 4 0 - 3
loose 4 - 10 3 - 8
medium 10 - 30 8 - 25 (compact in Canada practice)
dense 30 - 50 25 - 42
very dense > 50 42 - 58 What happens after 58, I DNK

What has "ticked me off" over the years is how things keep changing - and for what purpose. In the dawn of soil mechanics, fairly well set out standards of terms were established by the Geotechnical Icons (GeoLegends). Since, with the advent of the love of codes, these established terms have been changed and for what purpose. Just glancing though EN, the definition of boulders have been changed; it has gotten to a point that one, in order to appreciate and understand the "history" of projects will be utterly confused. Think of the young engineer, say post year 2010, who has been given an assignment to review geotechnical reports and foundation design of a building constructed in 1975. He picks up the borehole log and sees stiff. He applies the EN codes to stiff to get an idea of the undrained shear strength; yet at the time, stiff represented something, at times, quite a bit different. Will he appreciate ths subtle changes?

Most of what is seen - makes little real differnce. In my view, stay with the GeoIcons but they haven't. I guess there is always a need to needless tweek (in order to find something to get paid for).

To all - I am sorry - in that the "tables" sure don't look like how I typed. But you should be able to figure it all out.

What is the relationship between Su & Cu? I don't think it is one to one though.

The table was taking out from a paper co-issued by:
Exp Services Inc., Brampton, Ontario, Canada
Jinyuan Liu Ryerson University, Toronto, Ontario, Canada
Laifa Cao & Scott Peaker WSP Consultants Limited, Toronto, Ontario, Canada

And you can read here Link, if interested. Maybe a well thought through conclusion can be drawn after reading.