Behaviour Of A Gravity Foundation Under Working And Failure Conditions
Authors: Burland John B. , Penman Arthur C.M. , Gallagher Kenneth A. 
Published: December 2007

The paper describes the behaviour of the foundation of the NMI Experimental Tower that as placed in Christchurch Bay in 1975 to measure wave forces. The formation was of the gravity type and was 10.5 m in diameter resting directly on the seabed which consisted of compact sand overlying stiff clay. The foundation was instrumented with total earth pressure cells and piezometers were placed through the base into the sand. During a series of severe storms the structure and the foundation underwent significant tilt and progressive settlement. The observations have progressive settlement. The observations have pinpointed an important mechanism of failure which, pinpointed an important mechanism of failure which, though recognised qualitatively, has not yet been quantified. The mechanism is due to movement of the foundation under waveforces causing erosion by pumping at the interface between the gravity base and the seabed. It is clear that further study of this important phenomenon is required. phenomenon is required

Introduction

The foundation described in this paper is that of the Christchurch Bay experimental waveforce measurement tower. The project, which was conceived as a part of the Department of Energy's offshore structures part of the Department of Energy's offshore structures fluid loading advisory group (OSFLAG) research programme, was to place a simple structure at sea to programme, was to place a simple structure at sea to measure the forces and pressures exerted by moves and currents on vertical cylinders. The fluid loading measurement experiments were designed by the Division of Maritime Science at the National Physical Laboratory (DMS, NPL) which subsequently became a part of the National Maritime Institute (NMI). The development of the project has been reported in papers by Wheatley (1976) and Bishop (1977) in which the detailed objectives of the project are given. In summary the intention was to measure the environmental conditions, the water particle motions and the wave and current induced pressures and forces simultaneously.

The Department of Energy's advisory group on seabed stability (AGSBS)recommended that the project should be extended to include the monitoring of the foundation so that the performance of a gravity type foundation under accurately measured loads could be observed. The Building Research Establishment (BEE) Geotechnics Division designed the foundation monitoring experiment. As an ancilliary experiment the BEE Structural Design Division were to study the dynamic characteristics of the structure and its response to wind and wave excitation.

The tower was built with its experimental foundation during the latter part of 1975 at the Blackwall Dock, London, and first placed in Christchurch Bay in December 1975. During 1976 the tower experienced a wide range of sea conditions including several severe storms. In this paper the design and instrumentation of the foundation are reviewed and the behaviour of the foundation and the tower during 1975 and 1976 are described. Some of the measurements made during this period are briefly presented and from these and the period are briefly presented and from these and the observed behaviour of the tower, the most likely mechanism of failure is established. The performance of this foundation is compared with those of a number of other coastal and offshore gravity type structures on which practical expedients have prevented similar types of failure.

TOWER SITE AND INSTRUMENTATION

The Christchurch Bay tower has been described in detail by Wheatley (1976)and Bishop (1977). It consisted of a structural column 2.4 m in diameter and 13m high surrounded by a series of outer cylinders (sleeves), 2.8 m in diameter and instrumented to measure wave loads and pressures. This was mounted on a concrete base 1 m high and 10-5 m in diameter and carried a main platform with the instrumentation cabin and several subsiduary platforms. A second, smaller instrumented cylinder (wave staff), 0.48 m in diameter, was mounted with its axis parallel to the tower and 6.85 m from it, to the south east. The completed tower is shown in Fig 1.

The selection of the site and the design of the foundation have been described in detail by Penman and Gallagher (1976). Christchurch Bay was chosen to suit the hydrodynamics experiments and for its operational convenience.

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