Menard Pressuremeter - practical limit?

[Focht3]

I would like to pose a not-so-hypothetical problem. An engineer has suggested that Menard pressuremeter tests be performed on a local site in order to better characterize the site conditions. The project could easily support the cost.

The soils are likely to be hard calcareous clays with some thin limestone ledges and discontinuous limestone gravel. Shear strengths should be pretty high - starting at about 5 ksf (240 kPa) or more at the ground surface, and may exceed 10 ksf (958 kPa) within 30 feet of the ground surface. The soils are also dense (UDW 130+ pcf - 2083 kg/m³) and pretty dry (LI < -0.1). Rough estimates of the shear modulus of these soils ranges from a low value of 250 ksf (11 MPa) to well over 5,000 ksf (240 MPa.)

And now the questions:

1. What is the practical limit of the Menard pressuremeter with respect to the maximum soil shear modulus?

2. Is this approach likely to be successful? Why or why not?

3. Any alternate recommendations for testing at this site?

Let me hear from you!

 

[touma]

I would like to share some of my experience with the pressuremeter on similar sites and hope that it will be of benefit to you:

1. In terms of strength, you could obtain a pressuremeter with an adequate probe to provide you in the neighborhood of 100 bars.

2. The major problem you will face is in the hole size produced (e.g., large hole, unstable hole...) and particularly you will face problems in the fractured limestone layer.

3. Based on just the reasons stated above, successful becomes a relative term. If you mean as compared to conventional drilling/sampling and laboratory testing, I will deinitely favor drilling and high quality samling (barrel size ~ 101mm, split double) and if you have the chance, use air as a flushing medium (generally this will improve recovery), and I am assuming here that your water table is not shallow. The use of a samller size bit with such formation has proven to be inadequate especially if water is used for flushing

4. SInce your budget allows it, it would not hurt to have few pressuremeter boreholes, properly controlled, along side the larger size boreholes. The shape of the curve is a good indicator as to the validity of the test.

Good Luck

 

[touma]

I just remembered anotherthing:

* What are you looking for? is it a modulus? if that is the case, the range of values we have obtained from various site ranged between 650 and 1500 kg/cm2. The 650 value was obtained in marly formations (U.C ~ 50 to 100 kg/cm2)

 

[Focht3]

Touma:

Thanks for your comments - they closely mirror my opinions as well. The project structural engineer is pushing the use of the pressuremeter; I'm not sure why, but suspect that he had a good experience on another project. (By 'good experience', I mean that the pressuremeter results were sufficient to give the project geotechnical engineer a basis for reducing his/her immediate settlement estimates - or increasing the allowable bearing stress.)

I co-authored a paper in 1982 that looked at the actual &quot;immediate settlement&quot; performance of various buildings (about 33, as I recall) in the Houston area. The foundation types were all footings of some type - no piles / piers / caissons. The buildings ranged from a few stories in height to 40+ levels. We found that the Menard gave reasonable results (when interpreted by an experienced engineer) but that a self-boring pressuremeter gave results that were too stiff. Cyclically loaded CU triaxial tests gave the best correlation with building performance.

I appreciate your response and welcome any additional comments.

 

[02111933]

My question is: what is your purpose for going to the expense of using the Menard Pressuremeter?

If it is solely for the purpose of analysing the behavior of a laterally loaded pile within the cohesionless soil layer, you can use the principles of the Pressuremeter Theory authored by Baguelin, Shields, et al which are based on correlations between N values and subgarde reactions of various types of such soils (based on Menard's Pressuremeter results) and analyse the pile as an elastic beam and calculate bending moments from the horizontal force applied on the pile utilising constants C and l (Greek lamda) derived from the soil / pile interaction (subgrade reaction), calculate steel reinforcement required and calculate deflection if required using Tomlinson's proposed formulae for such deflections .. also depending on soil characteristics and pile loading.

I would very much, by the way, discourage the use of computer software to analyse single pile behavior since no two situations are alike and software generally tends to generalise solutions to suit time saving intentions - not to produce analytically accepted results.

 

[Focht3]

02111933:

Thanks for your comments. My problem is axial loading, not lateral. And the idea is to support the use of spread footings, strip footings, etc. - rather than drilled piers.

Personally, I think the tool (pressuremeter) has been oversold - to the detriment of good engineering.

Regarding your statement:

I would very much, by the way, discourage the use of computer software to analyse single pile behavior since no two situations are alike and software generally tends to generalise solutions to suit time saving intentions - not to produce analytically accepted results.

I both agree and disagree. Not all computer code is alike; but the software, after all, is only a tool. The art is in the engineer's judgement and experience.

Thanks for your comments!

 

[VAD]

Hello Focht3:

Interesting questions and concern. I presume that the Engineer may not be familar with the local practice and performance of structures in the locale in question. If he does, perhaps he wants to undertake some research as the job can afford it. As such his belief may be that he can find some useful correlations, as well as determining some parameters that are not readily obtained through laboratory testing.

It would appear that it is possible to obtain good samples as you have provided some strength information. Touma's response suggests that drilling can allow good samples to be obtained as he has experience in the area.

I am sure you know the debate of lab testing versus insitu testing and the advantages and disadvantages of both. As Touma and you have indicated there is a need to be able to assess the results obtained from the pressuremeter testing. If this type of testing is not done frequently then the preparation of the borehole would be an important issue that could influence results, there may be other issues. Generally, the shape of the curve is important as well as pointed out by Touma.

With the characteristics of the site there is no problem in determining that footings will perform well and hence no need to undertake pressuremeter testing unless there has been some concern about performance or some question of uneconomic designs etc, but I doubt it.

I hate to disagree with the intent of the engineer and would want first to find out from the Engineer his/her reason for proposing the testing. Generally, there is always something that bugs each and everyone of us in the geotech business and we constantly search for answers. This could be the case, there could be others as well.

Assuming that the tests will be done then one would expect that the results are put to good use and would be a positive contribution to the local knowledge base so that there is confidence in using the conventional approach.

Too often, we tend to undertake work and not use results if they do not fit our preconceived ideas. The client pays and often finds that he has gained nothing from the exercise. I am a firm believer that if you are going to undertake a variety of tests then you must as well report those results and discuss the findings in relation to the results of the other tests. In other words, the Client must receive the benefit of what he has paid for irrespective of whether or not it demonstrates that the coventional practice is sound. It is a professional responsibility. For the one who has proposed the test, this event could result in him/her gaining valuable experience. There is pain associated as well with doing and reporting on different types of tests epecially if the results are dependent on characteristics of the instrument and operator's experience.

With respect to the limit of the limit pressure with respect to the the shear modulus, values of 6 to 10 times the undrained shear strength have been reported.

With respect to recommendations for testing at the site, careful monitoring of the drilling progress and soil cuttings, and SPT testing would be sufficient if monitored by your experienced technologist whom you have groomed to be your eyes and ears. It is always good as well to stand at the hole yourself and dirty your hands. Ninety percent of your evaluation comes from this effort.

I am sure that the above is not new information to many.