Horizontal Capacity of Offshore Mat Foundations under Cyclic Waves

Offshore foundations · Soil–structure interface · Cyclic loading

Research question

How do the foundation–seabed interface and the natural state of clay control horizontal bearing-capacity evolution under cyclic wave loading?

The modeled mechanism is direct: cyclic shear generates excess pore-water pressure at the interface, reducing vertical effective stress and, in turn, the horizontal resistance available to the mat foundation. The study therefore evaluates capacity through the coupled evolution of pore pressure, effective stress, and soil state rather than through load amplitude alone. (Zhu et al., 2022)

Constitutive and numerical model

The soil model extends Modified Cam-Clay to represent clay structure, over-consolidation, and stress-induced anisotropy. It was checked against consolidated-drained and consolidated-undrained triaxial tests on Shanghai clays, including the strain-softening behavior associated with structure. The constitutive law was then implemented through a user material in a coupled pore-fluid/stress finite-element model. (Zhu et al., 2022)

The numerical study used a rigid $2\,\mathrm{m}\times2\,\mathrm{m}$ mat embedded 1 m into a single saturated Shanghai-clay stratum. It varied:

• initial structure $R_0^{\ast}=1,0.5,0.25,0.125$, where smaller values represent stronger structure;
• over-consolidation ratio $\mathrm{OCR}=1,2,4,8$;
• cyclic frequencies from $0.05$ to $0.4\ \mathrm{s}^{-1}$; and
• cyclic shear-stress amplitudes equal to 0.3, 0.4, 0.5, and 0.6 of the reference undrained capacity $H_u$.

Fully coupled finite-element domain and refined mesh around the rigid mat foundation; the soil domain extends $10B$ in length, width, and depth. Fig. 6 from (Zhu et al., 2022).

What controls capacity evolution

The analyses separate a temporary weakening capacity, reached as excess pore pressure accumulates, from a later permanent weakening capacity, reached as accumulation and dissipation approach a dynamic balance. For normally consolidated, unstructured clay at $0.4\ \mathrm{s}^{-1}$, the normalized capacity fell to about 0.5 near $T=0.001$ and later recovered to about 0.6. Stronger structure and larger OCR generally reduced the weakening, while higher frequency and amplitude generally increased it. At the lowest modeled frequency, $0.05\ \mathrm{s}^{-1}$, the two strongest structured-clay cases showed a slight capacity increase, which the paper attributes to a consolidation-like effect. (Zhu et al., 2022)

The evolution of horizontal capacity closely followed the vertical effective stress at the foundation–clay interface. The paper concludes that the interface controls the weakening effect on horizontal bearing capacity. (Zhu et al., 2022)

Scientific contribution

• Natural soil state is retained in the analysis. Structure and over-consolidation are represented explicitly rather than treating all seabed clay as normally consolidated and remolded. (Zhu et al., 2022)
• Hydraulic and mechanical responses remain coupled. Capacity loss is traced through pore-pressure accumulation, effective-stress reduction, consolidation, and interface resistance. (Zhu et al., 2022)
• Transient and equilibrium effects are distinguished. The framework separates the short-term minimum capacity from the longer-term capacity reached during continued cyclic loading. (Zhu et al., 2022)
• Loading and soil state are evaluated together. Frequency and amplitude are interpreted jointly with clay structure and OCR, revealing conditions with substantial loss and conditions with little loss. (Zhu et al., 2022)

Scope and limitations