Cohesion Values in Design 1

[AFCbuilder]

I am designing a double wall retaining system and I am wondering what values of cohesion I can use for a marine project consisting of soft to firm silty clay, clay, hard clay till and silt till layers. I found a graph in the geotechnical investigation report that plots cohesion against depth from DMT testing. These values varied between 0 near surface and over 125 kPa. However, the contract drawings had a summary table which showed effective cohesion intercept to be 0 kPa for all layers except for the bottom till layer, which showed a value of 5 kPa. Seems like the summary table is very conservative?

I have done a preliminary design for the critical section based on the 5kPa value for the till layer only but also did another design to see how some cohesion (3kPa) for the silty clay layers and found that the cohesion helps significantly. But I am not sure if I can rely on any cohesion. Does anyone have any recommendations? Is it possible to correlate blow counts to cohesion for either clay or silty clay soils? Thank you.

 

[Doctormo]

Soli cohesion is a time and moisture dependant There are also considerations for tension cracks forming and water filling the cracks that come into play when assessing earth pressures. Basic rule of thumb is to ignore cohesion for the long term design and to use the appropriate long term design values that typically do not include cohesion. A marine project would be a good place to not consider cohesion.

 

[AFCbuilder]

Thank you Doctormo. I have seen the rule of thumb to ignore cohesion for the long term design but I was under the impression that was due to drainage conditions (i.e. loss of cohesion as the soil drains). But since this is a marine project (the retaining system is being installed into the seabed), I believed that long term drainage would not pose a problem. I have re-run my analysis without considering any cohesion now but I am still curious about using cohesion values.

 

[BigH]

My take - I would not use cohesion as a general rule. In reality cohesion and the "phi" values reported from a triaxial test or direct shear are really just a mathematical expression. Many envelopes are curved so the shear strength (as opposed to the cohesion/phi reported by a straight line) will vary.

 

[Panars]

In clays, the dilatometer is measuring the undrained shear strength. Effective cohesion is a drained strength parameter and would be determined by triaxial testing with pore water pressure measurements. So what does the summary table say about undrained conditions? I would expect that you would need to analyze the wall for both undrained and drained conditions.

 

[fattdad]

I'd be careful about using cohesion for long-term design also. I'd question the use of cohesion that's derived from a DMT also. (I like the DMT though.) I'd consider stability for undrained and drained conditons.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Terratek]

What about using residual cohesion and friction angles?

 

[Doctormo]

Terratek - I think that for most retaining wall design, the retained soil is not expected to move sufficiently to enter the residual state which would include active earth pressure design (it is being restrained from movement by definition of a retaining wall). Therefore, the peak values would generally be appropriate in my opinion but I have heard justification for using residual values from time to time though. However, a reasonable assessment of soil strength should always be used from those results based on soil type as taking the peak strength of a shear or triaxial result (including cohesion?) and using the results for an earth pressure design is a good way to underestimate potential loading and have a failure down the road.

As a side note, earth pressure theory is quite unreliable in reality as soil and soil conditions are highly unpredictable over time (unless using free draining granular backfill and even this can exhibit apparent cohesion properties which initially reduces load). The intent is to come up with a reasonable magnitude of thrust that might be expected over the life of the structure based on reasonable assessment of the retained soil type. This means you take the tested results and compare to expected ranges for similar materials and personal experience then pick a number you are comfortable with based on what the structure is expected to accomplish. It is the "art" part of soil engineering and varies by individual.

 

[Terratek]

I agree with your points Doctormo. In cases where an individual or a firm has a lot of experience, the art of the profession is much easier and maybe even unofficially codified (within an organization). I don't have much experience with underwater structures, but I think a good place to start with unique projects would be to look at peak and residual values for both undisturbed and remolded samples. Plug any/all of those values into the analysis and see if the final design approaches established rules of thumb. From there, increase or decrease certain assumptions based on the circumstances of the project. I feel that if you use residual values though, you don't run much risk of overestimating strengths, while still allowing some credit for material properties, rather than just neglecting when in doubt.

Boss says, "All we have budgeted for are UC tests so just assume worst case loading, neglect unknowns and use most conservative values".