International Journal of Geomechanics

This study applies the limit analysis to estimate the bearing capacity of a rigid foundation on a soil reinforced by a group of floating columns. The lower bounds of the bearing capacity are derived in terms of a dimensionless factor that depends on the characteristics of the soil and the inclusion, the area replacement ratio, the columns length, and a uniform surcharge surrounding the foundation. The established results are valid for a variety of reinforcing configurations in terms of arbitrary foundation shape and any randomly distributed column arrangements under the foundation. The computation of the bearing capacity is extended to the kinematic approach of the limit analysis for reinforcement of purely cohesive soils. For symmetrical column arrangement the upper bound of bearing capacity factor is calculated by the Prandtl’s mechanism failure. Previous results established for the reinforcement case by end-bearing columns remain valid when considering floating columns. Nevertheless, a limitation on the length of columns is required for some reinforcement cases. The reliability of the results of limit analysis is illustrated by comparison with two other published methods.

This paper proposes a three-dimensional numerical modeling of an embankment over a soft ground mass improved by vertical stiff piles, using a finite-difference continuum approach (FLAC3D). Arching occurs in the embankment granular material, leading to load transfer onto the piles and surface settlement reduction and homogenization. The embankment, the piles, and the soft ground are explicitly taken into account in the proposed numerical model. First, a unit cell from the pile grid is considered. Two sorts of soft clay deposits and two embankment materials are successively modeled. The soft soil behavior is simulated by the modified Cam Clay model and the embankment material behavior is successively simulated by an elastic perfectly plastic model with a Mohr–Coulomb failure criterion and then by an isotropic hardening elastoplastic model, the CJS2 model, in order to approach the real system behavior. The calculations are performed in drained conditions, simulating the long-term behavior. The impact of the soft soil deposit compressibility and of the embankment material characteristics are underlined. The academic case of a current embankment section including lateral slopes is then simulated. The embankment is $5\mathrm{m}$ high and $44\mathrm{m}$ wide and the lateral slopes have inclination of 26°. The piles are now subjected to horizontal movements. The three-dimensional aspect of the problem is highlighted.

This paper is concerned with the vertical vibration of a flexible foundation resting on a saturated layered soil half-space that consists of a dry soil layer and an underlying semi-infinite saturated soil. The problem is of the mixed boundary value type and, by means of the Hankel transform technique, the determination of dynamic compliance coefficient and contact stress distribution leads to a pair of dual integral equations. These equations are then reduced to a Fredholm integral equation of the second kind, which is solved by standard numerical procedure. Some limiting cases existing in the literature are revisited to validate the accuracy of the present solution. Extensive numerical results are presented to illustrate the influences of the thickness of the overlying dry soil, the foundation flexibility, and the shear modulus ratio between dry and underlying saturated soils on the dynamic responses of the flexible foundations.

In practice, natural soils exhibit strain-rate dependency and also soften as they are sheared and remolded. Traditional design approaches, however, have been developed based on idealized rate-independent perfectly plastic soil. This paper presents results for deep penetration of spudcan foundations in strain-softening, rate-dependent, soft clays quantifying the effects relative to results for ideal soil. The analysis was carried out using a large deformation finite-element (LDFE) approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength. Parametric analyses were undertaken varying the strain-rate parameter, the sensitivity and ductility of the soil, the normalized penetration rate, and the soil strength nonhomogeneity. Overall, penetration resistance for rate-dependent, strain-softening clays lay below that for ideal soil. Increased brittleness of the soil led to marked reduction in penetration resistance, only partly compensated by strain-rate dependency. Key results have been presented in the form of design charts, fitted by simple expressions to estimate the limiting cavity depth above the advancing spudcan and the limiting bearing capacity factor at depth.

Fatigue stresses associated with extreme storms, vessel movements, and vortex-induced vibrations are critical to the performance of steel catenary risers. The critical location for fatigue damage often occurs within the touchdown zone, where cyclic interaction of the riser with the seabed occurs. Developing a model for seabed stiffness requires characterization of a number of complex nonlinear processes including trench formation, nonlinear soil stiffness, soil suction, and breakaway of the riser from the seafloor. The analytical framework utilized in this research considers the riser-seafloor interaction problem in terms of a pipe resting on a bed of springs, the stiffness characteristics of which are described by nonlinear load-deflection $\left(P\text{-}y\right)$ curves. The $P\text{-}y$ model allows for first penetration and uplift, as well as repenetration and small range motions within the bounding loop defined by extreme loading. The backbone curve is constructed from knowledge of the soil strength, the rate of strength increase with depth, trench width, and two additional parameters, while three parameters are necessary for the cyclic response.