By Louis Marcil
I frequently receive questions about when and how to use pressuremeters and which model will be the most suitable for the project. This series of posts is designed to shed some light on this confusing matter, starting with an introduction to pressuremeters. I hope this proves useful and when questions arise do not hesitate to contact me. I am always glad to talk about pressuremeters.
But first, a few words about myself.
Trained as a civil engineer, I have been working for Roctest for over 20 years now in Montreal, Canada. I was lucky enough to be introduced to the pressuremeter test (PMT) at a very young age, quickly developing a great passion for it. Working with Roctest gave me the chance to gain solid experience in all aspects of pressuremeters, from the design and manufacture of the equipment to the testing and use of the results.
The importance of characterizing the ground before building.
Proper characterization of mechanical properties of the ground where structures will be built is, of course, very important. The lack of knowledge about these properties can have catastrophic effects. A very old example of that is the ‘bent’ pyramid in Egypt, which is among the first large pyramids ever built. Due to serious problems during construction as a result of large ground movements, it was decided to decrease the slope of the pyramid faces in order to reduce the overall weight of the structure. This problem could have been prevented with better information about the ground properties.
Over the years, many testing methods have been developed to avoid such problems. These tests can be done using samples (laboratory tests) or directly on-site (in-situ testing). The pressuremeter test is included in this second category.
It consists of placing a cylindrical probe in the ground and expanding it to pressurize the ground. The aim of this in-situ test is to obtain information on the stiffness and on the strength of the ground by measuring the relationship between the radial pressure applied to the ground and the resulting deformation. The main benefit of this test is the fact that the parameters it yields can be highly representative of the future foundation behavior. For instance, settlement and pile lateral deflection predictions based on PMT parameters, normally prove to be highly representative of real movements. Another advantage is that it can be performed in almost all types of soils and rocks.
Unfortunately, these advantages do not come without some disadvantages. Thus, carrying out the test requires a certain amount of know-how. A potential user should plan to devote some resources to the learning and practice of this test. Also certain soil types will be more difficult to test. Finally, there are many types pressuremeter equipment, tests and interpretation methods to choose from, adding to the confusion.
Overall, when properly conducted, the PMT test yields valuable information, making it a useful tool for various applications,. This is especially true where undisturbed samples cannot be obtained, making other conventional in situ tests difficult to perform (e.g. in hard soils and rock), and on large projects where obtaining more and better information on the soil properties is justified.
Finally, as mentioned above, different types of pressuremeters are available. Knowing a bit more about these different types is useful for selecting a model properly suited to the user’s needs, and for better understanding differences regarding their operational characteristics and the results they generate. In my next blog I will clarify the differences between these types. Until then, I hope you can take advantage of this beginning of summer to gradually get out (where possible) of these last months of confinement!
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