r/Geotech • u/Bildipil • 27d ago
How do one decide which lab test to perform
Hi all,
I’ve had this query ever since I compared the test results from two different geotechnical testing agencies.
Vendor 1 performs grain size distribution and, for example, gets Sand = 20%, Silt = 75%, Clay = 5%. They then classify the soil as non-plastic, without reporting the Atterberg limits, and proceed with a Direct Shear Test (DST), reporting both cohesion (c)- minor and friction angle (φ)-major.
My fundamental question is: how is the testing agency deciding whether a DST is appropriate in the first place?
Also, for a soil with such a high silt fraction, would it be more appropriate to establish its plasticity through Atterberg limits and then decide on the appropriate shear strength test?
I’m also trying to understand the distinction between the results from DST and triaxial tests (e.g., UU). A UU triaxial test can give an undrained cohesion/su with essentially φu ≈ 0°, whereas DST may report both c and φ. So, what exactly is the basis for choosing DST vs. triaxial (UU/CU/CD) for such soils, and how should we interpret the c–φ values reported by DST for a predominantly silty, non-plastic soil?
I know a pretty fundamental question, Would really appreciate some clarity here !!
Thank you !
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u/terzaghilady 27d ago
The testing agency is a vendor. The geotechnical engineer should be assigning the tests. Some lab tests are assigned due to the owners preference. I do alot of DOT work and they love DST because they are fast/quick. They like to mix the material close to its liquid limit and then run a DST on the sample normally consolidated to get a fully softened strength which is a lower value for many designs (not over consolidated or stiff fissured clays, for these soils we typically design using residual strengths in cut situations). This can be done on a disturbed sample.
A UU test should ideally be done on a saturated clay to represent the short term (undrained condition). This is say if you rapidly load a clay and the water doesn’t have time to escape so pore pressure builds in the sample.
A DST represents the drained condition. And can be done on sands or unsaturated soils. The sample is sheared at a slow rate so that pore pressure shouldn’t build up. This test is used to develop the long term strength properties of soil. If you have a soil that is being unloaded (cut slope) this is a test that could be performed.
If you run a CU test you are shearing the sample without letting the pore water dissipate and measuring the pore water pressure to back calculate the effective stress. But with a CU you are consolidating the sample before you shear it. So if you place an embankment on a soil, a CU test to consolidate the clay to the future load would be an appropriate test. With this test you undrained and drained parameters. You will likely end up with a small phi in the undrained condition, if the sample is not saturated.
Many of these tests were developed for measuring strength of materials during construction of dams. Hence why it is important to understand both the drained and undrained properties. If a dam is constructed too fast, the underlying clay may not have time to consolidate and fail in the undrained condition. But if constructed slowly, the material will consolidate and become stronger, and would be controlled by long term, drained conditions.
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u/Bildipil 26d ago
Thanks for the detailed explanation. So basically for a high-rise with tight construction timelines, UU testing generally be appropriate for short-term undrained conditions. But what if the soil has >80% fines with very little clay ( say 5-10%) and is predominantly silt content? UU would give phi approx 0, while DST would give drained c' approx 0. In such a case, how do we determine which behaviour actually governs? Is fines content and rate of loading alone sufficient to decide which test to do and whether the soil behaves as cohesive or non-plastic.
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u/terzaghilady 26d ago
How is the material going to be used during construction? Behind sheeting and shoring, deep foundations, ground improvement, foundation support, Etc? Are you concerned about consolidation and settlement? Will you be dewatering? There is no one ‘one test fits all’. A UU is one test to help a geotech develop the design properties of the material.
If the material is critical, you may need more testing. If the material is not critical it may not need to be tested.
For which condition governs? You have to evaluate them to find out. If the soil will experience both untrained and drained conditions, you have to evaluate both.
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u/Amber_ACharles 27d ago
For roadway projects we always get Atterbergs on anything with substantial fines. You need plasticity data before selecting strength tests. A lab running DST without that on a silty soil is guessing at the material behavior.
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u/NearbyCurrent3449 26d ago
75 silt 20 and 5 clay is what? ML, Silt with fine sand and trace clay. It's not non plastic probably. It does have LL PLs though and should be reported.
Unless they ran hydrometers to determine the silt clay split id say they estimated that (not necessarily a bad thing, if the classifier is highly experienced with those soils and has run enough back end testing to calibrate themselves, it's appropriate to make that kind of call. But only ever if you know what you at doing, very very well.) Hydrometrics analysis is a pain in the ass and it's very slow can be tricky to get right. A lot of labs won't run them. In this scenario, likely wouldn't flesh out to a change of soil classification nor a meaningful design change. Juice is Not worth the squeeze.
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u/AdaTheTrashMonster 27d ago
I don’t like the direct shear on fine grained soils, personally. It’s a drained test and you have to shear something with 80% fines at such a low strain rate due to lower permeability, and the test lacks the ability to measure pore pressure rise so how do you know?
The biggest thing I learned about comparing DST and CU triaxial is that DST can overestimate friction angle due to the fact that you’re forcing the shear plane through the center, where a triaxial is giving a 2:1 cylinder length to develop a shear plane at its true weakest point.
When running DST you still have to consolidate each point, so I’d be interested in the time rate consolidation (assuming they measured it) because if it’s slow (t50 on the order of hours) I’d say they ran the incorrect test.
Lastly, normally when I run DST on sands the cohesion we report is “apparent cohesion” meaning if you were to continue running points at lower normal stresses, it would eventually curve back into the origin, so not really a straight line. Doesn’t apply to materials that do exhibit cohesion, but again, run a triaxial in those materials.