A detailed geotechnical investigation was undertaken at the Kinsale Head Gas Field in the Celtic Sea. Five-inch-diameter cores were recovered to a terminal penetration of 400 ft below the seafloor from two foundation borings drilled in a water depth of about 300 ft. Chalk extended continuously below 10-ft-thick surficial deposits of sand and gravel. Laboratory tests were conducted on the chalk samples to determine chemical and physical properties, and laboratory model techniques were developed to evaluate adhesion along a chalk-grout interface. The engineering properties of the chalk as determined from the field and laboratory study were used to develop suitable design parameters for two fixed offshore platforms recently installed in the Kinsale Head Gas Field. Graphical correlations of water content, unit dry weight, unconfined compressive strength and modulus of elasticity of the chalk are presented along with the chalk strength profiles with penetration. Chalk-grout adhesion results are discussed. The Celtic Sea chalk appears to be stronger and more dense than the English Upper Chalk and shows similarities to the chalk of Northern Ireland. Tests indicated that samples of fractured chalk softened and disintegrated rapidly when exposed to saltwater, unlike competent chalk which was unaffected by similar exposure.
The constitution and properties of chalk from many regions of the British Isles are extensively documented. There is, however, an appreciable lack of information concerning chalk from the Celtic Sea area. In the fall of 1973, a detailed study was made to determine foundation conditions at two locations in the Kinsale Gas Field in the Celtic Sea (see Figure 1). The study was aimed at providing sufficient information for the design and installation of two template-type structures, requiring that two deep foundation borings (350 to 400-ft penetration) be made in the chalk. This paper presents the results of a comprehensive series of laboratory tests made to determine the geotechnical properties of chalk samples recovered from the borings.
Two 8-pile platforms were recently installed successfully at the study locations. The foundation system consisted of large-diameter pipe piles driven to about 50-ft penetration with smaller-diameter pipe piles grouted into predrilled holes in the chalk below 50-ft penetration. Information concerning foundation design parameters and installation details of these piles will hopefully be published in the near future.
The Kinsale Head Gas Field is located approximately 35 miles southeast of Cork, Ireland, as shown in Figure 1. In the area of the gas field, water depths range from 283 to 310 ft. The measured water depth at the boring locations was 295 ft, subject to tidal variations of about 13.5 ft. The seafloor is generally level, except for isolated areas of small sand waves. Shallow seismic information indicated that there were areas where unconsolidated sediments were present at the seafloor and areas where chalk outcropped at the seafloor. At the boring locations, these sediments were approximately 10 ft thick, consisting mainly of coarse sand and gravel. Beneath the surficial deposits, chalk exists continuously to below the terminal penetration of the borings.
