Earth Pressure From Inclined Select Backfill Behind Wall 4

[civilguy5931]

My background is general civil, but I have worked into doing more retaining wall design lately. Please excuse me if this is a simple geotech problem.

I am looking to design a wall to retain less desirable fill material (low phi angle and ignore any cohesion). During construction, the site will be excavated for construction of the wall. I want to replace some existing material with a select backfill (gravel or crushed stone with a phi angle between 36 and 40 degrees).

How much material will need to be excavated so the earth pressure acting on the wall comes from the select backfill and not the native soil? (See the attached sketch) One of the restrictions of Coulomb earth pressure theory is that all soil layers be horizontal.

 

[BigH]

Any restriction would be only for those soils within the zone of influence. Typically, the angle would be 45+phi/2 from the horizontal for horizontal pressure (by Coulomb straight line) - but, if it were I, I would off have a 2 ft or so offset at the base for the "angle" rather than right at the base. I would probably use 60 deg for the "angle" as a practical slope. This way you ensure that the "active plane" is wholly within the select fill. Need to be careful about compacting against the wall - no heavy equipment. Still need to check sliding, overturning, toe pressures, etc.

 

[fattdad]

I agree with BigH, but to add: If you are working on a segmental retaining wall, the zone for the select backfill that affects external stability extends from the backside of the reinforced zone.

If you have a slope above the top of the wall, I'd use a chart solution, which is available through NAVFAC DM-7.2 (maybe 7.1).

f-d

¡papá gordo ain’t no madre flaca!

 

[csd72]

civilguy,

a bit more information on the type of retaining wall would be useful.

 

[civilguy5931]

Thanks for the responses. My gut was telling me that the failure angle of the active wedge was correct, but I have never seen this published anywhere. I assume that the failure angle of the native soil, and not the select backfill, would be controlling. Is this correct?

In addition, I would like to ask a few related questions.
1. What about the situation where I have a property line or other limit on the amount of excavation that can be performed? In this case, I would only be able to replace some (but not all) of the material between an assumed failure plane and the wall (see attached sketch)? Is there some earth pressure from the native soils at the back of the excavation which would act through the select backfill and then on the wall? If so, how would I account for that in the analysis?
2. How about the case of cohesive backfills, and possibly expansive clays? Obviously, the equivalent fluid pressures or passive earth pressures I would consider using are significantly different from the Coulomb active earth pressure from select backfills. How much excavation would be required for these cases?

 

[FixedEarth]

It seems that you have a grasp of earth retaining structures, but could benefit from some targeted self study.

You need to get this publication to fill in those gaps. It is an excellent book and a must for anyone designing any kind of retaining structure.

http://www.hbapublications.com/

http://www.FoundEng.com

 

[civilguy5931]

FixedEarth - Thanks for the reference. I'm a firm believer in self study. At times I wish I had the luxury of going back to college for some more classes, but life... At this point self study is my only option.

For this case, I have searched several books, including EM-1110-2-2502, NAVFAC 7.1 and 7.2, and some basic soils texts. So far I have not found anything that addresses these specific questions. (inclined backfills, replacing clays with select soils, etc.) I would be greatful if you could point me in the right direction.

 

[fattdad]

Some things are hard to type into just words.

For your question 1. If you have select backfill (let's say phi=36 degrees) and it's located within some percentage of the retained soil zone (let's say 2/3rds) and then it's some other native soil (let's say phi=25 degrees), you'd draw a line at 63 degrees from the horizontal from the base of the wall (or reinforced zone for an MSE wall). Then when the line intercepted the native soil, you'd then change that line to 57.5 degrees to daylight the ground.

That should be the "Rankine" failure surface. Now what?

I honestly don't know. . .

Here's what I'd do however: I'd measure the length of the phi=36 line and I'd measure the length of the phi=25 line and from those line lengths, I'd make a weighted average of the line with respect to the friction angle. From that I'd get friction angle to use in my hybrid earth pressure distributions.

Not sure it's completly "right" but it makes sense to me and the method is supported in practice when using chart solutions to slope stability problems.

Then again, this is not something I've messed with in my several decades of practice.

f-d

¡papá gordo ain’t no madre flaca!

 

[FixedEarth]

Attached is a great MN manual on earth pressures. In addition to Hugh Brook's compehensive book, now in its 9th edition, the following 3 books will aid you tremendously.

http://www.amazon.com/Soils-Foundat...1381/ref=sr_1_1?ie=UTF8&qid=1315581266&sr=8-1

: A practical book in soil mechanics with lots of solved problems

http://www.amazon.com/Recommendatio...=sr_1_1?s=books&ie=UTF8&qid=1315580889&sr=1-1

: The entire book is about is about earth pressures & retaining structures.

http://www.amazon.co.uk/Engineering...=sr_1_3?s=books&ie=UTF8&qid=1315581055&sr=1-3

: Chapters 6 & 7 covers very nicely the entire earth retaining structures.

As for your questions, for 1) If you design the wall using the native soil's Equivalent active fluid pressure adjusted for inclined backfill, but then you specify crushed gravel backfill, that would cover the limited wedge.

2) For moderately to high expansive soils, as you mentioned, you need to use select backfill. I do similar to what BigH has stated. Measure about 2 ft from the base of the footing and measure 0.6H from the top of the stem and fill that wedge with crushed gravel. If space is even more restrictive, you could specify light weight Geofoam.

If you have not worked for a geotechnical consulting firm, your learning curve will be a bit longer, because this is where you learn field/lab testing, forensic evaluation of leaning retaining walls and report preparation.

If you get the 4 books I have mentioned and work with a local geotechnical firm that does many earth retaining structures, you will be ok. Good luck- I was glad to hear your interest in self study, no body is born with this information. Good luck.

http://www.FoundEng.com

 

[RFreund]

Fixed Earth you have to stop recommending books my wife is going to kill me! haha

We do a fair amount of Earth Retention and without having any experience in it and no classes, it seemed over-whelming at first. I think the hardest part is that unlike steel, concrete, timber, etc where there is a common and well defined design procedure, Lateral Earth pressures can be evaluated using different theories. I did not understand this at first so I searched through books and papers looking for an "exact" procedure, which does not exist. I now own about 7 or 8 books all for retaining or foundation design. They include the above mentioned. I think I have one for wood, 2 for concrete and 2 for steel.
Anyway good advice guys as I have always wondered about this situation as well however we almost always use the lower friction angle if we can't fully encompass the failure plane. But the contractors/suppliers will push for the use of an average value (or atleast the ones who know a thing or 2 about geotech).

EIT

 

[BigH]

RFreund: I point you to the famous quote by Karl Terzaghi as to why you may, in fact, need tens of books on soils/foundations - -

“Unfortunately, soils are made by nature and not by man, and the products of nature are always complex… As soon as we pass from steel and concrete to earth, the omnipotence of theory ceases to exist. Natural soil is never uniform. Its properties change from point to point while our knowledge of its properties are limited to those few spots at which the samples have been collected. In soil mechanics the accuracy of computed results never exceeds that of a crude estimate, and the principal function of theory consists in teaching us what and how to observe in the field.”

I also point you to a very sound and true philosophy of engineering put forth by H. Q. Golder:

“For the engineer, … , there are many possible answers, all of which are compromises of truth and time, for the engineer must have an answer now; his answer must be sufficient for a given purpose, even if not true. For this reason an engineer must make assumptions he knows to be not strictly correct – but which will enable him to arrive at an answer which is sufficiently true for the immediate purpose.” (1948)