The soil strength of soft clay is influenced by strain rate effect. Models considering strain rate effect always ignore the impact of loading rate on pore pressure and have poor applicability to 3D engineering problems. Based on the classic inelastic core boundary surface model, a logarithmic rate function representing the strain rate effect of soft soil was introduced to the hardening law. A new parameter H was added to adjust the plastic modulus while another new parameter mu is introduced to account for the strain rate effect. A rate-effect boundary surface constitutive model suitable for saturated clay was subsequently proposed. Combined with the implicit integral numerical algorithm and stress-permeability coupling analysis, the innovative model was implemented in the finite element software and validated by comparing with the results of triaxial tests. By analysing the rate-effect of 11 types of soft soil, a formula to calculate the rate parameter was derived. The developed model was used to calculate the undrained vertical bearing capacity and sliding resistance of a movable subsea mudmat. The mudmat frictional coefficient from soil undrained to partial drained and finally undrained state was obtained and compared with those from the Modified Cam-Clay model. Identical results were obtained without considering the rate effect. When considering the strain rate effect on the improvement of soil strength, the friction resistance coefficient initially decreases and then increases with the decrease of the sliding speed, eventually stabilising after reaching the limit value. The rate-effect on the friction resistance coefficient is most prominent under undrained conditions with high sliding speeds. The soil strain rate effect is suggested to be considered in the design of the subsea mudmat avoid underestimating the friction resistance.

Determining the deformation trend of silt subsoil under long-term aircraft loading by conventional numerical methods based on finite elements is challenging and poses several limitations. In this study, a boundary surface model for remolded saturated silt considering the influence of the soil dry density was developed, and an explicit integral algorithm with error control was used to incorporate the model into a user-defined material subroutine that the finite element software (ABAQUS 6.14) could call. In this way, the consolidated undrained dynamic triaxial test of a soil unit was established for simulation and model validation, which corroborated that the model could describe the dynamic properties of the saturated silt. Then, a numerical model of the runway with layered compaction and different compaction degrees was also developed to numerically analyze the deformation of the subsoil under cyclic aircraft loading. The results showed that the subsoil deformation increased continuously with the increase of cycle number. However, the deformation rate decreased gradually, and the silt subsoil deformation remained stable after 50 loading cycles. After the same number of loading cycles, the cumulative plastic deformation of the subsoil model with the overall compaction degree of 94% was smaller than that of the model with layered compaction. It was also shown that different aircraft speeds have minimal effect on the cumulative plastic deformation of the subsoil. Nevertheless, the ultimate cumulative plastic deformation is larger, as the loading duration is longer at low aircraft speeds. It indicates that strictly controlling of the compaction degree within a certain range of load influence is imperative in practical engineering, as it reduces the associated costs.