Drained and Undrained Passive Rankine Earth Pressures for Narrow Cohesionless Backfill on Cohesive 2

[Geotech-ing]

Assuming Rankine conditions, how would one estimate both the drained passive force and undrained passive force for cohesionless (phi') soil narrowly backfilled at an angle overlain on native cohesive soil (c'-phi'-su) like in the picture without using a trial wedge method or finite element methods?

 

We can assume that the passive failure surface strictly goes through the native stiff clay as shown by the red line and that the narrow backfill is 1H:1V/45 degrees shown by the white line.
 

The picture is edited from the research paper, Passive earth pressure of narrow cohesionless backfill against inclined rigid retaining walls under translation mode by Fu-Quan Chen et. al (2020)

 

[Geotech-ing]

The lack of answers is very concerning and leading me to think that the industry may be wasting money and time on complex situations by incorrectly idealizing the clients' problems in the field. Engineering judgment is lacking in the industry nowadays within field personnel and it shows.

I am a newcomer in this industry and it is discouraging to find a lack of technical mentors with a great deal of engineering judgment in this field of work who is willing to pass the torch. My hope in receiving answers to the OP is not just for personal knowledge but also for a reserve of information for future generations of geotechnical engineers that may face complex situations similar to this.

 

[MTNClimber]

I'm not sure what you're trying to say here. Are you saying that we suck at mentoring you or that your co-workers suck at mentoring you? Or both?

I have a question about your question: By the title of the paper, doesn't the paper answer your question?

Another question, why not use the trial wedge method?

Also, who backfills a "gravity wharf" with loose fill? If you could construct a concrete wharf, why couldn't you compact the backfill?

...So many questions!

 

[PEinc]

I believe that you are asking about something that probably has never be addressed in a geotech text book. So, before others give you possible calculation methods; what calculations, if anything, have you tried so far? You said that you are "a newcomer in this industry." As we tell students in the Student Engineer General Discussion forum, we are not here to do your work for you. Take a stab at it, show us your effort, and you may get some responses.
You might get some suggestions if you included more information on the soils and ground water behind the wall.

http://www.PeirceEngineering.com

 

[Geotech-ing]

Not this forum specifically, but just senior engineers in general, in the real world across the industry as a whole. I'm not sure if it is just me feeling this way or if other newcoming geotechnical engineers feel this way as well. I recently joined this forum because I see promise in the response time and quality of the posts.

The paper refers to finite element models for narrow backfill against rock. I do not think it addresses failure surfaces into further secondary layers such as native consolidated stiff clays, which may highly be the case assuming relatively flat Rankine passive failure angles.

We are not using the trial wedge method, finite element methods because it is an exercise to develop engineering judgment in field-back-of-the-envelope conditions.

Also, we are assuming staged construction conditions for this problem. The backfill has yet to be compacted, but it will experience wave forces causing passive conditions on the wall. Please just hypothetically assume these conditions, and do not come up with an excuse that it will be compacted sooner or later or of the likes.

Again, this is an exercise of engineering judgment, not only for myself, but for future engineers who may find this valuable regarding passive earth pressures being developed under variable conditions.

 

[Geotech-ing]

I am not sure what more information I can provide for the "soils behind the wall."

I have given the soil geometry/stratigraphy/properties in the OP. There is no groundwater table as shown by the picture. A narrow cohesionless backfill with soil typical/conservative properties. A native stiff clay cut at 45 degrees with typical/conservative soil properties. When I say typical/conservative soil properties, these are the baseline properties that any reputable geotechnical engineer should know by heart. Typical values are: cohesionless with phi=30 and stiff cohesive with phi=33, c=200psf, su=2000psf.

What information are you suggesting is missing?

Some of my efforts include calculating the passive pressures as Rankine individually with each soil type as homogeneous while disregarding the other. Then comparing the results. However, then the next question is how do I develop the judgment to inderstand which is the more reasonable/realistic result. This splitting of soil does not seem to be the right approach intuitively neither. If I assume it is all sand, then I will discount the fact that the failure surface "goes through" the stiff clay layer, especially in an undrained case.

Please advise, I am not asking for you to do my work, I am asking for thoughts. This really feels like college all over again, where egos collide. There seems to be a lack of collaboration or community within the engineering community. Engineers seems on the edge, defensive-like, compared with sportsmen that strive to be a team and advocate for everyone to be the best versions of themselves.

 

[MTNClimber]

Why must you know how to determine the "allowable wave force" in the field? No engineer would have to determine this in the field. You would always do this as a desktop study. This scenario seems like an academic problem more than a real-life problem.

Anyways, if I had to determine the allowable passive resistance of a retaining structure using back-of-the-envelop methods, I would assume all of the material is the weakest material, the backfill or the clay, and determine the allowable passive resistance for that material.

 

[MTNClimber]

Geotech-ing - I don't think you realize how odd the question is. Why wouldn't you use a trial method or finite element analysis? Is there a rogue wave coming in 3 hours and you need to determine if you need to compact the fill before it hits?

 

[EireChch]

I was looking forward to replying to this, but given the tone of your text and attitude I have decided not to

 

[PEinc]

Ditto.

http://www.PeirceEngineering.com

 

[Geotech-ing]

I'm sorry. I'm a frank and straight-to-the-point writer. I honestly don't mean to offend anyone. I give as much information as possible just like a professional would. To be honest, I am attempting to imitate the writings of Terzaghi, Peck, and other geotechnical predecessors. Again, I'm sorry if I offended anyone in these sensitive times. I hope all the best for those who cannot reply. I look forward to working with all of you on this forum in the years to come. Kind regards!

 

[geotechguy1]

Maybe in the spirit of collaboration, you could set an example by providing us with how you would approach the calculations for this problem, and post your calculations with justifications and references.

 

[EireChch]

I dont think anyone is offended but like I said its the tone that erked me and the condescending know it all attitude. I can tell you for certain that you will find much less "collaboration" in the industry if you proceed in the same manner.

Anyways, you have apologized so moving on.

To be honest, the reason why i think you have gotten so little response is that this would be a case where I would only use the trial wedge method.

To try answer your question, I would probably calculate earth pressures assuming clay and earth pressures assuming sand and take the most conservative . Yes this is overly conservative and leaves you with a costly wall but it also leaves you with design that is unchallengeable.

ps - regarding your parameters that are a give for "reputable" engineer, if you go around designing retaining walls with 200psf (10kPa) of c' you will soon find yourself in trouble.