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ZZZMAN (Civil/Environmental) (OP)
23 Apr 09 15:24
hello everyone, if an earth dam is to be constructed. The soil on the site is clay. The initial water level is at the ground surface. During the construction, the water level has to be lowered in a short time, so that construction can proceed. This condition is commonly known as "sudden drawdown". What type of triaxial tests would you recommend to carry out, and why?

Helpful Member!(2)  dgillette (Geotechnical)
23 Apr 09 18:49
This is for short-term stability of excavation slopes?  I probably wouldn't do much in the way of triax testing for a small project.  I would lean more heavily to cone and vane to get undrained shear strengths.  I might also get some oedometer consolidation tests to help tie the strength data together.

The term "sudden drawdown" is usually reserved for rapid lowering of a reservoir after the dam is in operation, rather than dewatering for an exc.  Clay generally does not allow for sudden dewatering.

It would make it easier to give good advice if you would explain a little bit more about the site and what you intend to analyze.  In geotechnical engineering, even the easy questions are complicated by issues that are specific to the site and what is being sought.
ZZZMAN (Civil/Environmental) (OP)
24 Apr 09 16:40
actually, that is design problem. i.e. if you design to construct a dam on clay soil, cd test or cu test would you like to carry out? why? since it may happens immediate settlement during construction period.
dgillette (Geotechnical)
24 Apr 09 16:51
For starters, you generally would not do a CD test on saturated clay.  Why?  It takes too long.  For long-term stability, use CU tests with pore-pressure measurement so you can get effective-stress strength parameters.

If you want the immediate strength that governs the strength of the foundation for stability during construction, you could use UU triax tests, but I very well might put my money into cone penetrometer tests, field vane shear tests, and oedometer consolidation tests instead (depending on details of the situation, which you have not provided).
ZZZMAN (Civil/Environmental) (OP)
25 Apr 09 9:58
thanks for your kind attention, actually, the question is not real project, just a concept i want to clarify.
moreover,if in first priority just focus on  traxial test. i think result can found from both types of triaxial test, but except the time & money concerning, would you mind telling me the reason & difference on your decision, also, what 's the different on the result of triaxial test,  cone penetrometer tests, field vane shear tests?
dgillette (Geotechnical)
25 Apr 09 13:32
CPT:  Very cost effective, gives estimated stratigraphy (layering and material types) and large number of strength data over the whole site in very little time.  (Time is money.)  Requires estimating undrained shear strength using semi-empirical factors that aren't as precisely determined as we would like.  Estimated material types need to be validated by a few drill holes for index properties (grain sizes, Atterberg limits, water content, visual description).

VST:  Still requires empirical adjustment, but regarded by many as more reliable than CPT.  Gives sensitivity of clay directly.  Not as fast or cheap as CPT, but still much cheaper than undisturbed samples and triaxial tests.  The strength results can be used to verify or calibrate the CPT strength results.  In each drill hole, VST can be alternated with undisturbed samples for oedometer tests and index properties, or for a few triaxial tests if still needed.

Oedometer tests:  Usually need them because preconsolidation pressure governs almost everything with clay behavior, and you generally need to know whether the strengths you measure by CPT and VST are from over-consolidated clays.  Besides, you need oedometers to estimate settlement of the embankment.

This is as much time as I will put into this discussion.  Like most of the people who use eng-tips, I am happy to help with specific questions, but this is a very general discussion of a complicated topic.  Before any further discussion, you should read C.C. Ladd's Terzaghi Lecture on clay strengths, which appeared in the ASCE Journal of Geotechnical Engineering in August of 1991.  reading  I don't know what country you are writing from, but I believe that almost any university library will have it.  You can probably purchase it for download from www.asce.org.

 
palmahouse (Geotechnical)
23 Jul 09 22:28
For rapid drawdown conditions, you would likely model the clay behavior as undrained, because, it is weakest when undrained, and, is saturated and cannot drain quickly during rapid drawdown conditions.  So, CU may be best, because it allows you to saturate the sample and then shear it by undrained conditions - which, is similar to what would happen in the field for rapid drawdown.
iandig (Civil/Environmental)
24 Jul 09 12:34
There has been a lot of research work done on this in the UK by Andrew Charles when he worked for the BRE (now retired but still very active, and he also wrote BR 424 which I seem to keep refering to in other posts). I know the work was published as a seroes of research papers and ultimately in a BRE, but at the moment I have not sourced the specific document which covers this. Best advice is do a quick google search for papers by Charles and BRE. He presented the work as a paper last year as the Rankin lecture. I think the work was reported and included within the following publication "Water resources and reservoir engineering " published by the British Dam Association. On-site at the moment so no acces to the technical library
KVgeo (Geotechnical)
27 Jul 09 17:21
In a rapid drawdown condition the dam is in a state of intermediate stability... So generally speaking conservative undrained shear strengths should be adopted for the low permeable materials and contractive granular materials.

However, there are some additional factors to consider including moisture conditions in the clay during and after rapid drawdown which will affect undrained shear strength and whether to rely on negative pore pressures in overconsolidated clays.

I think a reasonable approach may be to adopt a effective stress/total stress envelope for low confining stress zones of the dam/foundation.

Everybody has an opinion on rapid drawdown and how to treat it.

 

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