Most people would agree that the analysis of failures is important. The basic reasoning is that each such case history represents a situation where the designed factor-of-safety value has fallen below unity. Conversely, each case of nonfailure represents a factor-of-safety value of one or higher. How much higher is, of course, not known. Thus failures (which are indeed unpleasant for everyone involved) are instances where “lessons learned” should be meaningfully investigated.
The above said, we often hear of geosynthetic failures and are sometimes asked to participate in forensic analyses. Such failures are sometimes one-of-a-kind; however there are several common geosynthetic application areas where they occur somewhat frequently. Invariably the importance of laboratory test results plays a critical role in the design and subsequent performance. In this regard, we have grouped four application areas that have similar failures and we have written about them in various publications. They are:
- mechanically stabilized earth (MSE) wall failures.
- geotextile filter failures.
- solid-waste landfill failures.
- veneer cover soil failures.
In all of these cases, design using both conventional (limit equilibrium) and emerging (load reduction and factor design) methods requires a resistance value for a factor-of-safety calculation and it must be obtained experimentally. The necessary laboratory experiments fall under the category of performance tests. There are five tests in these failure categories that have major significance in arriving at a reliable factor-of-safety value. They are indicated in `s Table 1~ and are contrasted to the application areas just mentioned. The perceived relative importance of the test method results to the particular application is also included.
|Application Area||Interface Shear Test ASTM D5321 or ISO 12957||Wide Width Tensile Test ASTM D4595 or ISO 10319||Transmissivity Test ASTM D4716 or ISO 12958||Permittivity ASTM D4491 and AOS ASTM D4751 Tests or
ISO 11058 and ISO 12956
Unfortunately, these five test methods have the highest statistical variation of all geosynthetic tests. Using the Geosynthetic Accreditation Institute’s-Laboratory Accreditation Program (GAI-LAP) Proficiency Test Program, the uncertainty in this regard indicates the following `s (Table 2)~.
|ASTM/ISO Test Number||Description of Tests||Proficiency Test Uncertainty Values|
|D4595/10319||wide width tension||12%|
|D4751/12956||apparent opening size||24%|
These values of uncertainty include both reproducibility and repeatability per the following equation:
U= (Sr2 + SR2 )(1/2)
where (according to ASTM terminology):
U = combined uncertainty—an indication of the variability associated with a measured value that takes into account two major components of error: (1) bias, and (2) the random error attributed to the imprecision of the measurement process.
Sr = repeatability—an established value below which the absolute difference between two “within-laboratory” or “within test-site” test results may be expected to lie with a specified probability.
SR = reproducibility—the precision of a test method expressed in terms of agreement expected between measurements made in different laboratories using similar apparatus and the same procedure.
As a single example of how values of uncertainty affect the resistance value used in design, assume that a laboratory submits to a client a 30° interface shear value. The range of this value taking the combined uncertainty of this test into consideration is as follows:
±0.16 (30°) = ±4.8° or 25.27° to 34.8°
Such a range will lead to considerable factor-of-safety values in any given application.
Considering this variation of possible values (aka “the combined uncertainty”) leads to the purpose of this column—to bring awareness to all involved in geosynthetic design that the reliability of a given factor-of-safety value can be greatly influenced by its laboratory procedures and testing protocol. The variation in such experimental values can easily dominate the design model accuracy, live load estimates, geometric characteristics, and other engineering properties, etc.
What can be done to control or mitigate this situation is in the hands of standards setting agencies (such as ASTM, ISO, and GSI) and the testing laboratories themselves. We need to sharpen our standards and craft clearer and more succinct test methods where the procedures and equipment are unequivocal and straightforward to implement. It then follows to have high quality laboratories with trained technicians performing the requisite testing.