Bounding Residual Shear Strength in Tailings: FVST Large-Strain Strength vs. CPT Sleeve Friction, or, FVST: Don’t Throw Out the Baby with the Bath Water

Tailings, often a mix of fine silts and clays, are hydraulically deposited, leading to loosely packed, often contractive materials. Their behavior under shear loading can be complex, frequently exhibiting a propensity for strain-softening and potential liquefaction. Therefore, determining a conservative yet realistic residual undrained shear strength (s_u​(r)) is paramount for robust tailings storage facility (TSF) design and risk assessment.  Yes, there are robust CPT (and SPT)-based correlations for this parameter, but they are just that, correlations.  Direct in situ measurements are available.

The stability of TSFs hinges critically on an accurate understanding of the tailings’ shear strength, particularly s_u​(r). This parameter represents the minimum strength a saturated soil can maintain after experiencing large deformations, often triggered by events like seismic activity or static/flow liquefaction. However, direct measurement of s_u​(r) in fine-grained tailings can be challenging, especially when partial drainage occurs during in-situ testing. Let’s explore how combining the Field Vane Shear Test (FVST) and the Cone Penetration Test (CPT) with its sleeve friction measurement can provide a valuable upper and lower bound estimate for s_u​(r), even in the presence of partial drainage.

The Field Vane Shear Test (FVST): A Direct Measurement with Caveats

The FVST is a widely used in-situ test that directly measures the shear strength of cohesive soils. A four-bladed vane is inserted into the soil and rotated at a constant rate until failure. The torque required to cause shearing is measured, which can then be converted to undrained shear strength. The FVST is particularly attractive for tailings as it can directly capture both peak and post-peak (remolded or residual) shear strengths.

The “remolded” strength obtained from a FVST, achieved by rapidly rotating the vane after initial failure, is often considered an approximation of the undrained residual strength. This is because the rapid rotation induces large strains, effectively breaking down the soil structure to its lowest strength state.

However, a significant challenge with FVST in tailings, particularly those with higher permeability (e.g., silty tailings), is the potential for partial drainage to occur during the test. If the vane rotation rate is too slow relative to the soil’s permeability, pore water can dissipate, leading to an overestimation of the “undrained” strength. This “drainage effect” means that the measured residual strength might be higher than the true undrained residual strength, making it an upper bound estimate if proper undrained conditions are not ensured. Modified FVST procedures, including high-speed vane rotation, are being developed and implemented to mitigate these drainage effects and achieve more truly undrained conditions.

CPT Sleeve Friction (fs​): An Indirect but Often Conservative Indicator

The CPT is a rapid and continuous in-situ test that provides a profile of soil resistance. While the cone tip resistance (q_c​) is commonly used to infer shear strength, the sleeve friction (f_s​) also offers valuable insights into the soil’s residual strength.

The sleeve friction measures the frictional resistance along a cylindrical sleeve located above the cone tip as it penetrates the soil. In fine-grained soils and tailings, especially those prone to liquefaction, the f_s​ has been observed to strongly correlate with the remolded or residual undrained shear strength. The theoretical basis is that the soil along the sleeve undergoes significant shearing and remolding as the cone penetrates, bringing it close to its large-strain, or residual, state.

Crucially, the CPT penetration rate (typically 2 cm/s) is generally fast enough to induce largely undrained conditions in fine-grained soils. While some consolidation can occur along the sleeve away from the cone tip, the overall process often leads to a more consistently “undrained” representation of large-strain strength compared to a standard FVST in partially draining materials. Therefore, the CPT sleeve friction can often provide a more conservative, lower bound estimate of the residual undrained shear strength, particularly in situations where FVST might be influenced by partial drainage. Some studies even suggest that in certain tailings, f_s​ can be directly assumed to represent s_u​(r).

Establishing Bounds: A Complementary Approach

By utilizing both the FVST and CPT sleeve friction, engineers can establish a practical range for the residual undrained shear strength of tailings, even when partial drainage is a concern:

• Upper Bound (from FVST): When a standard FVST is performed, especially in silty tailings where some drainage might occur, the measured remolded strength can serve as an upper bound. While efforts should always be made to ensure undrained conditions (e.g., by adjusting rotation rates), acknowledging the potential for drainage provides a more conservative interpretation.
• Lower Bound (from CPT f_s​): The CPT sleeve friction, due to its faster penetration rate and the extensive shearing it induces, often provides a more reliably undrained, and therefore typically lower, estimate of the large-strain strength. This can be considered a practical lower bound for s_u​(r), particularly in contractive tailings.

This dual-tool approach offers significant advantages in the characterization of complex tailings deposits. It acknowledges the inherent difficulties in perfectly replicating undrained conditions in the field and provides a robust framework for design. By understanding the potential range of residual undrained shear strengths, engineers can make more informed and conservative decisions regarding the stability of TSFs, ultimately enhancing safety and minimizing environmental risks. Ongoing research continues to refine the interpretation of these in-situ tests, emphasizing the importance of site-specific correlations and careful consideration of drainage effects in tailings engineering.