Laboratory Soil Tests For Shear Strength And Geotechnical Properties

Importance of Soil Tests

Laboratory sampling is one of the engineering approaches used in determining the suitability of the engineering-related product. Thus, it informs their proper application in providing solutions to challenges requiring engineering solutions. Civil engineers, geologists, and geotechnics always refer to laboratory soil tests to determine and evaluate the various technical properties of the soil to inform their suitable use on the site (Gundersen, et al 2018). For instance, the shear strength of a soil is vital in construction as it informs the materials and type of foundations to be used in a given structural development.

A soil test is essential for various reasons; to evaluate the different properties of the soil, thus inform its proper use, optimize the utilization of the soil; for instance, knowing the properties of the soil enables farmers to optimize their agricultural productions. Soil tests represent an essential aspect of geotechnical analysis that enable experts to understand a specific site’s mechanical properties and condition, thus informing the engineering applications and utilization of the site besides agricultural activities (Yin, et al 2021). Other advantages of carrying out soil tests are; to help is a diagnosis of various plants and crops culture problems, protection and conservation of environment from contamination due to excessive use of fertilizers. Therefore, this report presents a laboratory test analysis of the three soil tests to determine the shear strength among other geotechnical properties of the silt type of soil. The tests covered in the report include undrained triaxial, shear box, and oedometer tests        

Technicians widely use triaxial tests to investigate the behavior of the ground and understand the nature of the given site, which helps in deciding the appropriate engineering applications on the site or type of foundation to be applied in construction developments. The undrained triaxial test is often conducted to evaluate the shear strength of the soil sample within the triaxial compression mechanisms (Zheng, et al 2021). The primary purpose of the undrained triaxial test on the silt sample is to evaluate and analyze the undrained shear-stength parameters on the soil sample and establish the correlation of the shear-strain of the soil sample through triaxial test systems. after that, use the obtained experimental data to analyze the sample’s response to different field conditions, for example, when subjected to structural load

The main soil parameters under investigation in a quick undrained triaxial soil test are

Undrained shear strength

Soil cohesion

The angle of shearing resistance

Shear stress, shear load, and normal stress

The advantage of a quick undrained triaxial test is that it can be conducted on all three drainage conditions with complete control of the test parameters. With the undrained tests, it is possible to measure the soil’s pore pressure during the test (Gundersen et al. 2019). The stress distribution obtained by the  test is always uniform and enables the evaluation of the state of stress on the sample at any given experimentation stage.

However, the test suffers a few limitations; For instance, setting up the systems for the experiment is complicated compared to other shear-strength test methods; therefore, conducting an undrained triaxial test requires specialized skills for the set-up and handling of the test apparatus.

Undrained Triaxial Test

The shear box test is a geotechnical field or laboratory test used by geotechnicians to determine and analyze the shear strength property of rock or soil. The direct shear test can also measure the discontinuities in soil samples or rock masses. The laboratory shear box tests are conducted in accordance with the procedures and guidelines defined under BS 1377-7 and ASTM 3080 (Wang et al 2021). The test is strictly conditioned to rock masses with low shear strengths, while for the soil samples, the test has to include at least three samples from undistributed soil.

The shear box test procedure involved placing a specimen in a shear box with two stacked rings holding the soil sample, and the distance difference between the stack rings is the place at mid-height (Shariatmadari et al. 2018). After placing the samples, confining stress is applied vertically, and the upper ring is laterally retracted until the sample fails. The applied load and induced strain are recorded at frequent intervals, and the recorded data are used to calculate and analyze the stress strain and draw the stress-strain curve for the confining stress.

Soil parameters tested under the shear box test

Shear strength

The angle of internal friction (friction angle) 

Soil cohesion

The advantage of the shear box test over other tests methods is that it is simple since it involves simple equipment; therefore, setting up the system for the experiment is simple and easy. It also allows tests under different drainage, saturation, and consolidation conditions, unlike other shear test methods (Sadeghi, et al. 2019). Shear tests on soil samples fast and take shoreter time considerably compared to the time taken for a similar test in the undrained triaxial test. It is considered economical because it does not require complex systems to conduct.

However, these advantages are weighed against test limitations like the difficulties experienced in measuring pore pressures, especially when testing undrained conditions. It is also associated with spuriously high results due to the forced failure plane in some samples. In some set-ups, the test gives unreliable results because the soil is forced to fail rather than unconditionally allowing the sample to fail along its weakest plane. The shear stress distribution on the samples is often not uniform

In tests, a cubic box measuring 100x100mm shear box was used to investigate the parameters mentioned above of silt type of soil, and the experiment ring constant for the test was at 3.15N/division. The vertical deflection dial and horizontal movement dial both were at 1div = 0.01 mm. a total of three tests were completed and tabulated as follows

Trend-lines for the loose and dense samples used in calculation of the angle of friction.

Oedometer tests are the consolidation test to determine the magnitude and rate of the volume decrease the laterally confined soil samples undergo when subjected to vertical pressure. It is the most preferred in quantifying the critical geotechnical properties of soil associated with consolidation. The primary purpose of the oedometer test was to measure the vertical displacement of radially constrained soil samples subjected to vertical loading (Pande et al 2021). The consolidation parameters obtained through oedometer tests evaluate the rate and magnitude of primary and secondary consolidation settlement of earth fill or structures.

Shear Box Test

The objective of this test was to evaluate the swelling and consolidation process of the sample soil through the use of simpe oedometer and collect experimental data to calculate the soil paremeters like constrained modulus, void ratio permeability, and consolidation coefficient of the samples. Estimates of this type are of key importance in the design of engineered structures and the evaluation of their performance

The Oedometer test is commonly used in goetechnical engineering because it accommodates many soil tests with elaborate results on various soil parameters. Odometer tests give accurate results under different drainage, saturation, and consolidation conditions, unlike other consolidation test methods. Consolidation test on soil samples is very fast and takes considerably shoreter time than the time taken for a similar test in the undrained triaxial test. However, the limitation is that the preparation of test samples takes time and requires specialized instruments to mold the samples into cylindrical shapes.

For the lab test, silt was the type of soil under investigation, and the test was completed in three stages with different load conditions. the results are tabulated as follow.

At the start of the test, w = 32% and G_s = 2.67

The evaluation graphs

Presure = ½ tsf

Insitu and laboratory tests are methods used by geotchnicians and civil engineers to analyze and determine different soil parameters vital to their engineering applications. The choice of testing approach depends on several factors, including the available resources to conduct the desired tests on rock masses or soil samples. In situ geotechnical tests covers all the tests conducted on-site or on the ground, in trial pits, boreholes, or tunnels, unlike laboratory test, where samples are taken from the field. Then the investigation is conducted in the laboratory. Dynamic penetration and vane tests are key in situ soil tests of relevance to geotechnical engineering.

For the three tests, the laboratory tests approach was considered to obtain the results that reflect the test’s aims and objective. However, in situ tests would be regarded as due to their benefits or advantages over laboratory tests. For instance, there are no site exclusion issues with in situ tests as the test is not confined to limited soil samples like the laboratory tests. It provides the technician administering tests with an extensive soil mass for characterization, thus providing more accurate results or finding than laboratory tests. Another advantage of in-site tests is that testing sare conducted on soil within their environment with no change in the stress states. For this reason, only in situ tests permit the evaluation of the at-rest stress state of soil samples.

References

Gundersen, A., Hansen, R., Lunne, T., L Heureux, J. S., & Strandvik, S. O. (2019). Characterization and engineering properties of the NGTS Onsøy soft clay site.

Karumanchi, M., & Nerella, R. (2022). Shear strength parameters from digital tri-axial test and soils stabilization with extracted nanosilica. Nanotechnology for Environmental Engineering, 7(1), 307-318.

Krishnan, J., Sharma, P., & Shukla, S. (2021). Experimental investigations on the mechanical properties of sand stabilized with colloidal silica. Iranian Journal of Science and Technology, Transactions of Civil Engineering, 45(3), 1737-1758.

Ng, C. W. W., Sadeghi, H., Jafarzadeh, F., Sadeghi, M., Zhou, C., & Baghbanrezvan, S. (2020). Effect of microstructure on shear strength and dilatancy of unsaturated loess at high suctions. Canadian Geotechnical Journal, 57(2), 221-235.

Pande, P. B., Khandeshwar, S. R., & Bajad, S. P. (2021). Review of Experimental Techniques for Evaluating Unsaturated Shear Strength of Soil. In Advances in Civil Engineering and Infrastructural Development (pp. 579-590). Springer, Singapore.

Sadeghi, H., Kiani, M., Sadeghi, M., & Jafarzadeh, F. (2019). Geotechnical characterization and collapsibility of a natural dispersive loess. Engineering geology, 250, 89-100.

Shariatmadari, N., Zeinali, S. M., Mirzaeifar, H., & Keramati, M. (2018). Evaluating the effect of using shredded waste tire in the stone columns as an improvement technique. Construction and Building Materials, 176, 700-709.

Wang, K., Jin, Z., Shen, K., Du, W., Zhang, Q., & Gao, S. (2021, August). MCC and OCR-to-CPTU method for strength assessment in Hiep Thanh offshore wind power project. In 2021 International Conference on Wireless Communications and Smart Grid (ICWCSG) (pp. 485-491). IEEE.

Yin, K., Liu, J., Vasilescu, A. R., Di Filippo, E., & Othmani, K. (2021). A Procedure to Prepare Sand–Clay Mixture Samples for Soil–Structure Interface Direct Shear Tests. Applied Sciences, 11(12), 5337.

Yin, Z. Y., Hicher, P. Y., & Jin, Y. F. (2020). Introduction of Laboratory Tests for Soils. In Practice of Constitutive Modelling for Saturated Soils (pp. 61-81). Springer, Singapore.

Zheng, H., Zheng, J., Xie, X., & Yang, K. (2021). Rigidity indices of sands from their image-based intrinsic properties. Acta Geotechnica, 1-9.

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