Suction Caissons In Sand Research Articles

Simplified approximation for seepage effect on penetration resistance of suction caissons in sand

ABSTRACTDuring the installation process of suction caisson in sandy seabed, the applied suction not only produces a downward driving force, but also decreases the penetration resistance to the caisson. In this paper, numerical studies were performed to investigate the characteristics of seepage flow around the caisson as well as its effects on the wall friction and the caisson tip resistance. A simplified approximation method was proposed to predict the reduction of penetration resistance. In particular, for the case of sandy seabed interbedded by a layer of low-permeability soil, the seepage flow will be suppressed with the caisson tip getting closer to the low-permeability layer. The seepage flow around the caisson was first studied, and then a new approximation for the seepage-induced resistance reduction, considering the effect of low-permeability layer, was proposed for the engineering practice.

Numerical modelling of transient cyclic vertical loading of suction caissons in sand

This paper presents numerical investigations of the monotonic and cyclic behaviours of suction caissons upon vertical transient loading. Both drained and partially drained conditions are investigated. Monotonic compression and traction simulations are carried out to qualitatively compare results with the literature and validate the model. They highlight the different modes of reaction of the caisson to both compression and traction loading. A sensitivity analysis points out the strong influence of some parameters on the resistance of the caisson but also on the failure mechanism. The transient behaviour of the caisson upon different kinds of cyclic load signals is analysed. Results reproduce the settlement and pore water pressure accumulations observed during experiments. The influence of the key design parameters on the settlement accumulation is also assessed. Finally a cyclic diagram is proposed to describe the evolution of the final settlement upon different magnitudes of loading.

Centrifuge model tests on installation of suction caissons in sand

Centrifuge model tests on installation of suction caissons in sand

Numerical Analysis of Inclined Uplift Capacity of Suction Caisson in Sand

Numerical Analysis of Inclined Uplift Capacity of Suction Caisson in Sand

Model testing of upright and tapered suction caissons in sand

Behaviour of upright suction caissons with regard to their applications, installation, load-bearing, etc., has already been investigated by a number of researchers. Some researchers have also addressed the behaviour of tapered piles under different loading conditions. However, there has not been any previous experimental experience regarding the performance of tapered suction caissons. This paper reports the results of a primarily experimental investigation intended as a first step to understanding the performance of tapered suction caissons. The 1g experiments have been carried out on small-scale caisson models having upright and sloping walls. The caissons have been installed in a very fine saturated silty sand, and the effects of geometrical parameters, wall slope and the soil density on the overall penetration force required for installation and the pull-out capacity of the cassion have been studied. Influence of pull-out rate on the ultimate capacities has also been investigated.

Extraction of suction caissons in sand

This technical note examines the suction caisson extraction mechanism under steady seepage conditions using a series of centrifuge tests supported by finite-element predictions of these tests. An approximate methodology for prediction of the extraction resistance is subsequently proposed, which is shown to provide a simple means for estimation of suction caisson extraction resistance in un-aged free-draining materials.

Experimental studies on the anti-uplift behavior of the suction caissons in sand

A series of model tests was conducted in sand to explore the anti-uplift behavior of suction caissons, considering the effects of aspect ratios, load inclination angles and loading positions. This paper emphasizes on analyzing the deformation characteristic and the mechanism of the suction caissons under various loading conditions. The movement modes of the suction caisson are different when the load inclination angle increases from 0° to 90° corresponding to various mooring positions. The pull-out bearing capacity decreases with load inclination angles increasing. When the load inclination angle changes from 0° to 60°, the bearing capacity reduces more significantly than that between inclination angle of 60° and 90°. While the load inclination angle is relatively small, the pull-out capacity of the suction caisson decreases after reaching the peak as the loading position moves downwards. Moreover, the optimum loading position locates between 2/3 and 3/4 of the caisson length. The optimum loading position is at the bottom of the caisson when the load inclination angle exceeds 60°. However, the influence of the loading position on the pull-out capacity of the caisson can be ignored while the load inclination angle equals to 90°. The pull-out bearing capacity increases as the aspect ratio increases but the aspect ratio has no effect on the deformation characteristic of the suction caisson.

Discussion of “CPT-Based Method for the Installation of Suction Caissons in Sand” by Marc Senders and Mark F. Randolph

Discussion of “CPT-Based Method for the Installation of Suction Caissons in Sand” by Marc Senders and Mark F. Randolph

CPT-Based Method for the Installation of Suction Caissons in Sand

A CPT-based method is presented for estimating the self-weight penetration and suction required during the installation of suction caissons in medium dense to dense sand. Existing methods, although fundamental in nature, suffer from the difficulty in estimating key input parameters such as an appropriate bearing factor for the tip resistance and frictional coefficients for skirt friction. Instead, the proposed method is based directly on the cone resistance profile, starting with the Det norske Veritas approach for calculating the installation resistance of skirted foundations. The approach is modified to include a simple linear reduction in end bearing on the skirt tips and internal friction with the level of suction, with those components of resistance assumed to reduce to zero once the suction reaches the critical level corresponding to hydraulic failure of the soil plug. The method is coupled with a seepage model so that, for a given pumping rate, the penetration and corresponding resistance can both be expressed as a function of time. Results of calculations with this complete model are shown to compare well with data from centrifuge model tests and with published data from suction caisson installations.

Installation of Suction Caissons in Sand with Silt Layers

Suction caissons are increasingly being used for offshore anchors because of their ease of installation. However, for soil profiles that have sand overlain by layers of silt, there are reasons to believe that it may not be possible to install suction caissons. This may happen because the low permeability of the silt will create a hydraulic blockage, and thus diminish the upward hydraulic gradient required to reduce the penetration resistance in the sand to enable caisson penetration. The tendency of the silt to be sucked up, and its effect on the underlying sand, are not clearly understood. Furthermore, the blockage by the silt layer may be less than if a clay layer is present. This is because scouring of the silt may occur, allowing an upward seepage gradient to develop, and hence causing a reduction in penetration resistance. A series of suction caisson installation tests have been conducted in a geotechnical centrifuge to provide data on the penetration resistance and mechanisms for soil profiles where silt layers are present, either on top, or interbedded between layers of sand. In these tests, the thickness of the silt layer and its depth below the sand surface have also been varied. It has been found that if silt overlies the sand, a larger suction force is required for installation than when installation occurs in homogeneous sand, but that this force is still much smaller than the soil resistance when the caisson is pushed into the ground. When a silt layer is present, the suction pressures are observed to follow a similar trend that is independent of the position and thickness of the silt layer. Scouring of silt was evident in the tests, and significant upward movement of the soil plug inside the caisson was observed in all soil profiles when silt layers were present. The plug height was seen to reduce after the suction was turned off, suggesting that the resulting heave was unstable.

Field trials of suction caissons in sand for offshore wind turbine foundations

A programme of testing on suction caisson foundations in an artificially prepared sand test bed near Luce Bay, in Scotland, is described. The tests are relevant to the design of either monopod or quadruped foundations for offshore wind turbines. Records are presented for suction installation of the caissons, cyclic moment loading under both quasi-static and dynamic conditions to simulate the behaviour of a monopod foundation, and cyclic vertical loading and pullout of caissons to simulate one footing in a quadruped foundation. Variations of stiffness with loading level of the foundation are observed, with high initial stiffness followed by hysteretic behaviour at moderate loads and degradation of response at high loads. Some implications for the design of wind turbine foundations are briefly discussed.

Design procedures for installation of suction caissons in sand

Suction-installed caisson foundations are being used or considered for a wide variety of offshore applications ranging from anchors for floating facilities to shallow foundations for offshore wind turbines. In the design of the caissons the installation procedure must be considered as well as the in-place performance. The scope of this paper is to consider the calculations appropriate for the installation of caissons in sands. Calculation methods are presented for determining the resistance to penetration of open-ended cylindrical caisson foundations both with and without the application of suction inside the caisson. Comparisons are made with case records. A companion paper addresses the calculation procedure for installation in clays as well as in other soils.

Design procedures for installation of suction caissons in sand

Suction-installed caisson foundations are being used or considered for a wide variety of offshore applications ranging from anchors for floating facilities to shallow foundations for offshore wind turbines. In the design of the caissons the installation procedure must be considered as well as the in-place performance. The scope of this paper is to consider the calculations appropriate for the installation of caissons in sands. Calculation methods are presented for determining the resistance to penetration of open-ended cylindrical caisson foundations both with and without the application of suction inside the caisson. Comparisons are made with case records. A companion paper addresses the calculation procedure for installation in clays as well as in other soils.

Performance of suction caissons in sand and clay

The use of suction caissons (suction piles) in marine environments has been increasing in the last decade. A suction caisson is a steel pipe with an open bottom and a closed top that is inserted into the ground by pumping water out of it. Pumping creates a differential pressure across the caisson's top that pushes it into place, thus eliminating the need for pile driving. There are a number of uncertainties in the design of suction caissons. First, the state of stress and soil conditions adjacent to a suction caisson differs from those around typical driven piles or drilled shafts. Second, the axial load capacity of suction caissons depends on the rate of loading, hydraulic conductivity, drainage length, as well as the shearing strength properties of the foundation material. Finally, during pullout, volume change characteristics of the surrounding soils may change the theoretical suction pressures. A review of the existing knowledge relating to the design and construction of suction caissons is presented in this paper along with the results of a laboratory study on model caissons in sand and clay. Test results indicate that the use of suction pressure for installation of caissons is a viable alternative to conventional methods. Suction was also shown to resist some axial tensile loads.Key words: suction, pile, bucket, foundation, anchor, capacity.