ABSTRACT Because of its irregular shape, it is inaccurate to calculate the bearing capacity of the A-shaped mat foundation by a conventional method, which may have potential safety hazards. In this paper, the vertical bearing capacity of the mat is obtained by using the equivalent calculation method (ECM) and the finite element method respectively, and compared with the centrifuge test results. A correction method based on the ECM is proposed. Influences of foundation dimensional parameters on the H, M and H-M combined bearing capacity are studied. Expressions of H-M failure envelopes related to the dimensional parameters are established. A new form of V-H-M envelope expression for mats is suggested according to the effect of vertical load on H-M capacity. The modified ECM and the V-H-M envelope proposed in the paper can be used to quickly evaluate the vertical bearing capacity of the A-shaped mat and its stability under coupling loads.

The bearing capacity of A-shaped mat foundations on cohesive soil

It is very well observed that offshore sites mostly contain soft/loose soils may be of sand or clay or may be multiple layer of soil mixture under saturated condition. This particular soil undergoes large deformation on the installation of any offshore structure like jack up. It is also observed that jack up may tilt or sink under its own weight without any lateral load or hydrodynamic pressure or any natural hazard like liquefaction or hurricane. It may undergo punching shear failure due to the presence of soft soil. It may suffer failure due to jack up leg and foundation element connection failure. It gets affected by rotational stiffness, foundation fixity parameters. To overcome this situation, several techniques had been proposed by various researchers. Spudcan, anchors, mud mat, bucket foundation, suction caisson foundation, etc., are used to overcome such difficulties. These techniques of mitigation subsequently affect bearing capacity failure zones of soil. Sinking and tilting effects are due to overlapping of bearing capacity failure zones. Hence, by advancing changing in shape and spacing of jack up legs, sinking and tilting effects can be considerably reduced. In particular research paper, an attempt has been made to investigate the effects of various mitigation techniques by previous researchers. All field and laboratory studies along with soft computing with FEM-based software had been discussed. Considerable researchers had shown that provision of mud mat in jack up bottom increases stability of jack up structure. To visualize the various effects, FEM-based Plaxis 3D program had also been used.

S. R. Gandhi

Papers

Reviews on Mitigation Techniques of Offshore Jack Up Hazard by FEM Analysis