Strength & Deformation tests
In a compression test, also known as an oedometer test, a soil sample is placed in a steel ring, after which a load is applied vertically to the sample.
This load is increased and decreased in a series of pre-calculated steps. Each step must be applied for at least 24 hours. If the sample is still insufficiently compressed, the duration of the step must be extended.
During execution of the full test, it is ensured that the sample is wet by installing the steel ring in cylindrical trays filled with water. The results of the test include the a,b,c-isotachs, Nen-Bjerrum, Taylor and Casagrande parameters.
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With a CRS test, a soil sample is placed in a steel ring that is located in a cell in which water can be pressurised around the sample. This ensures that the sample is completely saturated during the test. This condition cannot be guaranteed in a compression test. After the sample is completely saturated, a series of steps is also applied in a CRS test by increasing or decreasing the vertical load on the sample or a phase in which no load is applied but the height is kept constant (relaxation phase). During these steps, the load is applied while maintaining a constant settling of the sample.
The results of the test include the a,b,c-isotachs and Nen-Bjerrum parameters.
Triaxial tests are a very accurate and advanced method for determining the (undrained) shear strength of the underground. In addition to determining the (undrained) shear strength, the internal angle of friction and cohesion can also be calculated. These parameters are used in many underground models to determine, for example, the failure mechanism of flood defences.
When carrying out the triaxial tests, there are various options for performing the test. The tests can thus be performed isotropically and anisotropically. Under isotropic loading, the sample is uniformly consolidated on all sides. Thus, any increased surface load during a consolidation phase is not taken into account. Which is the case in an anisotropic test. In addition, you can choose to perform the test over-consolidated or normally consolidated. In the case of an over-consolidated test, the starting point for the surface load is the ground stress during anisotropic consolidation. In a normally consolidated triaxial test, the limit stress is maintained as the surface load during anisotropic consolidation.
At SOCOTEC’s geotechnical laboratory, we have 18 setups for performing various types of triaxial tests capable of handling samples with a diameter of 67 or 50 mm. We offer the following types of triaxial tests:
- Unconsolidated and undrained (UU) triaxial test
- Consolidated and drained (CD) triaxial test
- Consolidated and undrained (CU) triaxial test
- In the Netherlands, the UU triaxial tests are often carried out on impermeable or poorly permeable cohesive soils (clay and silt), where the test is initially loaded isotropically. Thereafter, the surface load is increased incrementally under undrained conditions until the sample fails
CD triaxial tests often use samples with low/no cohesion and good permeability. The sample is usually only consolidated isotropically, after which the surface load is increased until the sample collapses or, if desired, the test is continued until a certain elongation percentage has been achieved. During the course of the test, the soil is under drained conditions.
Cohesive soils (clay and silt) that are not or only poorly permeable are usually used for CU triaxial tests, and organic soils are also used to a far lesser extent. The sample is always first isotropically consolidated and optionally also anisotropically consolidated. After this, the surface load is increased until the sample collapses or, if desired, the test is continued until a certain elongation percentage has been achieved.
Direct Simple Shear (DSS) Test
The (undrained) shear strength can be derived under compressive conditions such as in a triaxial test, but can also be determined under conditions in which shear strain is applied. SOCOTEC’s geotechnical laboratory has 2 setups with which shear strain can be applied by means of the Direct Simple Shear (DSS) method. Both devices are located in a conditioned lab.
In this test, a cohesive sample is placed in the device and a stack of rings is placed around the sample. Use of this stack of rings allows the deformation in the sample to behave naturally without the presence of a pre-imposed shear surface as is the case in a Direct Shear (DS) test.
During execution of the test, the sample is initially consolidated in over-consolidated or normally-consolidated conditions, after which the sample is sheared until the desired shear percentage is reached. During shearing, the base of the apparatus is moved horizontally at a constant speed while maintaining the height of the sample to maintain a constant volume during shearing.
Laboratory Vane Test
In SOCOTEC’s geotechnical laboratory, we can use the so-called lab vane apparatus to determine the indicative undrained shear strength. The test resembles the Torvane test, with the difference that the fins of the lab vane are fully/deeply pressed into the sample. To determine which torsion spring and vane blade should be used, a Torvane test must be carried out prior to execution. During execution, the vane blade is placed in the sample using the apparatus. The vane blade is then rotated at a constant speed until resistance disappears. The resistance can then be read and used to calculate the indicative undrained shear strength.
The results of the lab vane test are of course much more accurate than a Torvane test and are also very well suited to organic soils. The lab vane tests are a nice addition to a survey project that also includes field vane tests.
Fall Cone Test
In SOCOTEC’s geotechnical laboratory, we can use the fall cone test to determine the indicative undrained shear strength of the soil. The method we follow for this is described in NEN-EN-ISO 17892-6.
The undrained shear strength can be determined for both disturbed and undisturbed material. Naturally, the test on disturbed material yields a different value than one on undisturbed material. The in-situ, undrained shear strength is more comparable to the results of the test on undisturbed material. Determining this indicative value is a nice addition to a test collection that also includes triaxial tests, for example.