Determining Confining Pressure for Multistage Triaxial Tests: A Geotechnical Insight

The determination of the correct confining pressure for multistage triaxial tests is a fundamental aspect of geotechnical engineering. This process is critical for accurately simulating the in-situ stress conditions of soil samples, which in turn ensures the validity and applicability of test outcomes in predicting soil behavior under field conditions. This article provides insights into the practical steps and essential considerations involved in this determination.

The quest to establish the right confining pressure starts with a fundamental understanding of the stresses exerted on soil at various depths. The theoretical formula to calculate stress is as below:

This formula provides a theoretical baseline for the initial confining pressure in the triaxial test, which simulates the lateral earth pressure at rest, similar to the stress present at a specific depth.

How can we get unit weight data of our soil sample? The Sand Cone Method, as per ASTM D1556, is a field testing procedure widely recognized is one of the method that can be used. By excavating a hole at the site and weighing the removed soil, the method employs a sand cone apparatus to fill the void and measure the sand's volume required to occupy it. The weight of the excavated soil divided by the volume of the hole provides the unit weight, a fundamental parameter for calculating the initial confining pressure.

As a geotechnical engineer, it's essential to transcend beyond theoretical formulas and envision the actual failure mechanisms within the soil. Understanding the stress conditions on a probable failure surface requires a deep dive into the material's behavior under various stress states and failure scenarios. It's here that the expertise of a geotechnical engineer shines, as they combine theoretical knowledge with practical, site-specific insights.

In this part, we delve into the significance of effective stress conditions on the probable failure surface. For a more nuanced approach, geotechnical professionals utilize limit equilibrium method software, such as Slide2 from Rocscience. This software calculates the stress acting at the base of the potential failure surface, offering a precise and contextual confining pressure for the triaxial test. This advanced analysis ensures that the confining pressures selected reflect the true stress conditions that influence soil stability. Figure below is the example of the result that we can get from Slide 2 software. You can use the "Base vertical stress (kPa)" as the reference for your triaxial confining pressure.

When it comes to the multistage triaxial test, a tiered strategy is often employed for confining pressures: 50% of the calculated stress value for stage 1, 100% for stage 2, and 150% for stage 3. This gradation not only challenges the soil sample across a spectrum of realistic stress scenarios but also establishes a thorough stress-strain relationship critical for understanding the soil's shear strength.

The preparation of the soil sample for the triaxial test is also a critical step. It involves reaching the target remolding density, which is influenced by the Standard Compaction Test. This test delineates the maximum dry density (MDD) and optimum moisture content (OMC), providing the parameters for sample compaction. Typically, samples are remolded to 95% of the MDD, a density that aligns closely with field compaction conditions and affects the soil's response during the test.

To summarize, the multistage triaxial test is a complex yet insightful process that requires a meticulous approach to sample preparation, a detailed application of testing procedures, and an analytical mindset for interpreting results. In the realm of geotechnical engineering, the accurate determination of confining pressure is not merely a technical task; it's an interpretive skill that reveals the intricate nature of soil behavior under stress.

As we conclude, the methodological determination of confining pressure underscores a broader commitment to excellence within geotechnical engineering. As our community continues to refine its practices, we are exceptionally positioned to address the multifaceted challenges of soil mechanics, thus ensuring the safety and stability of our infrastructure in the built environment.