Uncertainty in cyclic triaxial tests on undrained dense marine sand

Offshore foundations are frequently subjected to cyclic loads that cause particle rearrangement, pore pressure buildup, and strength degradation in surrounding sand. The cyclic triaxial (CTX) test is commonly used to evaluate cyclic resistance. The reliability of results is critical for design. This study conducted undrained, stress-controlled CTX tests on dense marine sand under three cyclic stress ratios (CSR), with four to five repetitions at each level, to quantify the uncertainty in excess pore water pressure (EPWP) evolution, stiffness degradation, damping ratio, and number of cycles to failure (N<sub>f</sub>). A literature database was also compiled and analysed using N<sub>f</sub> residuals as a measure of scatter. Results show that failure envelopes remain stable across stress levels, while phase transformation lines evolve toward steady values. Axial strain increases rapidly once the EPWP ratio exceeds ~65%, marking the onset of failure. Variability is greatest during EPWP buildup (40-65%), and N<sub>f</sub> scatter is larger at low CSR. A cyclic resistance ratio (CRR) corresponding to 50 cycles provides a reliable reference for design. Literature datasets confirm limited uncertainty, with 66% of N<sub>f</sub> residuals falling within ±0.1 on a logarithmic scale. These findings improve understanding of CTX test variability and its implications for offshore foundation design.