With soil samples from your project site now in tubes, jars, and bags, the next step is to determine the specific tests that will yield the most useful data for your design. The laboratory can conduct any tests you assign, but selecting the right ones is crucial—not only to obtain the insights needed for a sound design but also to stay within budget. The goal is to test sufficiently but avoid excess. Effective testing is a critical part of managing risk, and risk management is essential to achieving efficient and safe designs.
The first goal in testing assignments is to come up with a soil profile. First, you assign index tests for classification and strength to differentiate between the layers you have encountered on your project site. From the drillers’ logs, you can get a sense of the characteristics of these underground layers, but is the clay at 20 ft the same as the clay at 50 ft? Blow counts and color can only go so far to separate clays into layers. And what about those pesky transitional layers? How important will those be in your design calculations?
The basics of soil mechanics and foundation engineering as learned in college courses help us identify different strength tests for different soil types and are dependent on the timeline of your construction project, the intermediate phases of the construction timeline, and the end goal for structure stability.
Let’s start with the basic strength tests.
Unconfined Compression (UC) and Unconsolidated Undrained Triaxial (UU)
Historically, Unconfined Compression (UC) and Unconsolidated Undrained Triaxial (UU) are the two first and most simple types of strength tests performed in soil laboratories. Today, these are both considered Index tests and are not recommended to use in design but are valuable to differentiate the layers in the soil profile. Neither test accounts for the lateral in-situ pressures or the reduction in strength due to internal pore water pressures that are key to more modern soil testing methods. The UC is the most basic; just trim the sample to length, measure, weigh, and shear it. The UU gets a little more accurate in modeling the specimen, as you apply a meaningful calculated pressure to the specimen and then shear it. Using the results of these types of tests, you can start to get a sense of your strata layers underground and if the clay at 20 ft is seemingly the same as the clay at 50 ft, keeping in mind that it is at a deeper depth and that will influence the strength. These tests are not definitive and may raise more questions and warrant further testing, which will help you mitigate risks on the project.
Consolidated Undrained Triaxial (CU), Consolidated Drained Triaxial (CD), Direct Shear (DS) and Direct Simple Shear (DSS)
Moving on to strength tests intended for design. There are several of them, including Consolidated Undrained Triaxial (CU), Consolidated Drained Triaxial (CD), Direct Shear (DS, a drained test) and Direct Simple Shear (DSS, an undrained test). The reasons to choose one over the other depend on the material being tested, the time (length) of your project’s construction, the specifications for the type of structure you are working on, and the client’s preferences.
Soil Types and Testing Applications
Coarse-grained soil will have high permeability and the ability to drain under loading, therefore no internal pore-water pressures will build up inside the specimen and are usually tested under drained conditions.
Fine grained soils will have lower permeability and a sensitivity to internal pore-water pressures during loading and are typically tested under undrained conditions and for short term loading. They could also be run under drained conditions when modeling a long-term loading situation.
Encountering a mix of soil types is common and larger sizes of gravel would trigger the need for larger specimens for testing in the laboratory. The typical rule of thumb is the smallest dimension of the specimen should be from 1/6-1/10 of the largest grain size particle in the material. Often with undisturbed tube samples, this is unknown at the start of the test but could help explain an outlier data point. Assigning x-ray analysis to the tube before testing will help to see large stones, voids, or other anomalies in the tube sample. These sections of the tube can then be avoided helping to save material and time. This extra step to mitigate risk may be worth the cost as undisturbed tube samples are costly to collect.
Consolidated Undrained Triaxial (CU) and Consolidated Drained Triaxial (CD)
Triaxial tests are done on cylinders of soil specimens trimmed or reconstituted to fit the size of the equipment used. The specimen is encased in a latex membrane and placed inside a test cell. The cell is pressurized, usually with water surrounding the outside of the specimen and tubing is used at the top and bottom of the specimen that allows internal pressurizing for back saturation and/or draining from within the soil specimen. Saturation is a must in triaxial testing, and usually these specimens are nearly saturated as they come from below the natural water table under the ground. Other specimens can be compacted to the requirements of the project for material brought onto the site for specific purposes. These test specimens require a larger amount of material to test and are run in groups of three with low, medium, and high pressure to obtain the design parameters.
Direct Shear (DS) and Direct Simple Shear (DSS)
Direct Shear and Direct Simple Shear tests are done on smaller “hockey puck” sized soil samples. These soil specimens are placed in the test apparatus and given “free access” to water and will soak it in or drain it out as the soil type material wants to do. DS tests are also run in groups of three. Applications for DS tests are typically for drainage sand, fill, embankment placement, landfill cover, and other above-the-ground water applications. DSS testing on the other hand is a technically more advanced test, used to model plain strain conditions. One application is in slope stability analysis for the part at the flat portion of the slip circle – a plain strain condition. DSS tests are also run to obtain static strength compared to post cyclic strength of the same soil to see how the vibrational event in the cyclic test degraded the soil.
Many variations of advanced testing for strength can be run to more closely model the in-situ conditions. The more information you can gather, the better and safer your design will be with the lowest amount of risk and the most cost-effective to your overall project design success.
Post by: Nancy J. Hubbard, a Project Manager, Quality Assurance Technician, and Deputy Quality Manager Advanced Laboratory Geotechnical Engineer for GeoTesting Express. Nancy has been with the company for 25 years. Nancy has a BS in Civil Engineering from WPI and a ME(C) in Geotechnical Engineering and Geology from Cornell University. Nancy provides a key role in the lab working with and maintaining the automated testing equipment, quality system, and provides training, reporting, and answering technical questions for technicians in both the Acton and Atlanta GTX Labs, and for clients as well. Nancy is an active member of the ASTM D18 Committee which covers soil and rock testing standards and methods.
