The vertical capacity of grillage foundations

Grillage foundations may provide an economical alternative to offshore ‘mudmat' foundations for seabed infrastructure, owing to their improved hydrodynamic characteristics, which are important during installation. Grillage foundations consist of a mesh of vertical grilles that penetrate the seabed during loading. Offshore loadings on these types of foundation are likely to consist of vertical (mostly dead weight) loading and horizontal ‘in-service' loads. However, to date there is no accepted method of design, as foundation capacity may differ significantly from that of conventional solid shallow foundations. This paper presents an analytical method designed to calculate the variation of vertical bearing capacity with grille penetration in sand. The results show that grillages are able to achieve the same capacity as solid foundations of the same breadth, but this requires significant penetration of the grillage. Consequently, design choices are likely to depend on the amount of settlement the structure can tolerate. Simplified analytical equations have been presented to allow calculation of the load–settlement response, and to calculate how much settlement is required to mobilise the flat-plate capacity of a solid mudmat of the same overall breadth. The methodology has been validated by comparing results with those from model tests. </jats:p

Strategies for quantifying the installation reliability of skirted subsea foundations

An increasing number of well-related and pipeline-related subsea structures (e.g. PLETs, ILTs, buckle initiators, etc.) are being placed on the seabed. Many of these structures are founded on unskirted and skirted mudmat foundations as a costeffective, low risk solution. Much attention has been placed in the literature on the geotechnical capacity of subsea mudmat foundations, but less attention on how these types of foundations are installed. In some soil conditions this can be a critical aspect of design: the requirement to add skirts to ensure sufficient foundation capacity comes with the penalty of increasing the necessary foundation weight to ensure that these skirts can be reliably installed. The addition of weight to a foundation increases its installation and fabrication cost and so requires careful treatment in design. As an example of this design optimization process, attention is given to dealing with uncertainty associated with variable soil conditions. It is shown how probabilistic geotechnical analysis can give a clearer perspective of cost and risk and how carefully targeted site investigation can also be used in this context.</p

Interaction of caisson foundations with a seismically rupturing normal fault:Centrifuge testing versus numerical simulation

Dramatic failures have occurred in recent earthquakes due to the interplay of surface structures with outcropping fault ruptures, highlighting the need to account for fault induced loading in seismic design. Current research into the mechanisms of fault rupture–foundation–structure interaction has revealed a potentially favourable role of caissons in comparison to other foundation types. This paper explores the mechanisms of normal fault rupture interaction with caisson foundations, with an integrated approach using both experiments and analysis. A series of centrifuge model tests were first conducted to study the response of a square (in plan) caisson foundation of dimensions 5 m x 5 m x 10 m, founded on a 15 m thick layer of dry dense sand. Nonlinear 3-D numerical simulation of the problem was then developed and adequately validated against centrifuge test results. Depending on its position relative to the fault, the caisson is found to interact with the fault rupture, sometimes modifying spectacularly the free field rupture path. Acting as a kinematic constraint, the caisson “forces” the rupture to divert on either one, or both, of its sides. The numerical study was extended to gain further insight into the effect of the exact position of the caisson relative to the fault outcrop. Different mechanisms taking place for different caisson positions are identified, and their effect on the response of the system is discusse