Comments about reporting of soil lab tests
In this blog we have reviewed the purpose of geotechnical laboratory tests, the application of the International System of Units, and the importance of keeping equipment calibrations up to date. Now, we are going to review a no less important topic: how to present the results of the tests in a clear way. Do you want to know more? Keep reading...
Content
Recording of data in the soil laboratory
One of the fundamental aspects in any laboratory (not only in Soil Mechanics), is the recording of data. This premise may seem obvious, and perhaps laughable to some laboratorians reading this post. However, it is a point that I consider important to address, especially because of the economic and technical implications of an inadequate treatment of the data.
I will start with the economic aspect. In general, obtaining samples for laboratory testing is a complicated and costly process. Even though a routine geotechnical study does not have a major impact on the overall costs of a project, it does for the company performing the study. Consider all the activities required to take a soil sample: transportation to the site of the equipment and personnel, topographic location of the borehole, installation of the drilling equipment at the point, availability of water for drilling, support vehicle, technical personnel (drillers and supervisor), logistics for field work (which may include room for personnel, obtaining work permits, availability of fuel for vehicles and equipment, food for personnel, among other things)... To this should be added, of course, the technical difficulties inherent to drilling and the process of obtaining a soil sample. What am I getting at with this? Well, it is important to be clear about how difficult and expensive it is to obtain a soil sample. That is the first thing we must assimilate.
The second important aspect to consider is the technical aspect. Given the difficulties and the cost associated with each sample that arrives at the laboratory, it is essential to ensure order and care in the treatment of the samples and in the data obtained from each test. Why? For a very simple reason: in most cases, a certain sample may be destined to one and only one test, which implies that, if the data obtained is lost, there may not be more samples to repeat the test and obtain the information required by the Geotechnical Engineer.
All this leads to the need to: 1) to have forms (preferably on paper) for each laboratory test, where the data associated with each test are clearly and neatly written down. This obviously implies that no data or results will be written down on paper or in notebooks that could later be lost; 2) to have file folders for each project that is being executed, duly organized.
Figure 1 Traitment of lab data
In addition, a healthy practice is to have personnel in the laboratory to transcribe all the data into digital format, generating the final logs that will be delivered to the client.
Of course, if the laboratory has modern equipment for special tests (direct shear, unconfined compression, triaxial, permeability, CBR, etc.), the data and test results will be immediately available on the computer. In this case, the important thing is to store the file properly, to avoid accidental deletion or confusion and assignment to a different project.
Standards applied in laboratory tests
In most laboratories in the United States, Latin America and Asia, ASTM (American Society for Testing and Materials) and AASHTO (American Association for State Highway and Transportation Officials) standards are usually used, even though most countries have their own standards (derived, in most cases, from these American standards). In Europe it is more common to follow EUROCODE. In any case, the content of the standards is practically the same, regardless of the generating entity.
Table 1 shows the AASTHO and ASTM standard designations for laboratory tests required in routine geotechnical studies.
Table 1 Some AASTHO and ASTM standards usually used in routine geotechnical studies (Source: Das, 2002).
Test | Code AASHTO | Code ASTM |
Moisture content | T-265 | D-2216 |
Specific gravity | T-100 | D-854 |
Sieve analysis | T-87; T-88 | D-421 |
Hydrometer analysis | T-87; T-88 | D-422 |
Liquid limit | T-89 | D-4318 |
Plastic limit | T-90 | D-4318 |
Shrinkage limit | T-92 | D-427 |
Standar Proctor compaction test | T-99 | D-698 |
Modified Proctor compaction test | T-180 | D-1557 |
Sand cone | T-191 | D-1556 |
Permeability | T-215 | D-2434 |
Consolidation | T-216 | D-2435 |
Simple direct shear | T-236 | D-3080 |
Unconfined compression | T-208 | D-2166 |
Triaxial | T-234 | D-2850 |
AASHTO Soil Classification System | M-145 | D-3282 |
Unified Soil Classification System | — | D-2487 |
The important thing to take into account, especially on the part of laboratorians, is that the standards being followed in the test must be clearly stated and indicated in the test form, not only to perform the procedure according to this or that standard, but also so that the end user knows the criteria under which the test was developed.
Graphs and tables
The graphs and tables related to each laboratory test should be prepared as simply as possible, so that they can be easily interpreted by the Geotechnical Engineer.
Although it may be obvious to mention, I consider it important to emphasize that the axes of the graphs must be properly identified, including the units of the parameters represented on each axis. Figure 2, taken from Das (2002), illustrates the above.
Figure 2 Examples of laboratory test charts: (a) incomplete chart, drawn incorrectly and unclearly; (b) correct chart (Source: modified from Das, 2002).
In the case of tables, they must have their rows and columns properly identified, including the units associated with the parameters represented in them.
Finally, it is important to note that test results should be expressed according to the International System of Units. This is a very good practice, which brings clarity to the understanding of the physics associated with each test.
References
- Das, B. (2002) “Soil Mechanics Laboratory Manual”. Fifth Edition. Engineering Press, Inc. California, USA.
- Head, K. (1980) “Manual of Soil Laboratory Testing – Volume 1: Soil Classification and Compaction Tests”. Pentech Press. London, UK.
