Self-Bored Pressuremeter Testing in the Arctic Offshore

Abstract

ABSTRACT Self-bored pressuremeter (SBP) tests are an important part of site investigations for gravity base structure (GBS) foundations. This applies in particular when such structures are expected to experience significant lateral loads, as most arctic structures are. The SBP data is used for both serviceability state analysis and for providing fundamental information on undisturbed soil behavior. SBP testing has been carried out offshore for Gulf Canada Resources Inc. (GCRI) during the past two years (1982-1983 arctic open water seasons). Testing has been carried out in a variety of soil conditions across the GCRI land interest on the Canadian continental shelf in the Beaufort Sea. The paper outlines the equipment used, operational successes and difficulties, and presents productivity data. Techniques and equipment for improving productivity are outlined. Experience with SBP operations has shown that the SBP is no more difficult to use offshore than the electric static cone penetrometer (CPT). One result in particular of the SBP testing stands out; the in-situ geostatic stress ratio (Ko) has been found to be not a unique function of past overburden hi story. This result is discussed in the geological context. It appears prudent to regard as an independent variable for engineering purposes. INTRODUCTION An estimated 12 percent of the worlds untapped hydrocarbon reserves occur with in the Canadian Offshore regions (Beaufort Sea, Labrador Coast, Scotian Shelf and Arctic Islands). Gulf Canada Resources Inc. (GCRI) has significant interest in all these areas and is a major operator in the Beaufort Sea. The Beaufort Sea is characterized by being frozen and ice covered for about 9 months each year. This ice cover presents the principal environmental constraint in offshore operations because of the large forces it may exert on stationary structures [1]. Artifical islands have proved successful in resisting large, horizontal ice forces in shallow waters. However, the average water depth of the GCRI land interest is about 35 m (115 ft). Artifical islands would be very expensive in such water depths so caisson technology has been developed as a technique for gaining the performance of islands without their cost. The GCRI system is called the Mobile Arctic Caisson (MAC). The MAC is a hybrid soil-steel structure and the principles of its design and operation have been presented elsewhere [2], [3]. A prerequisite to deployment of the MAC is a site investigation paying particularly attention to the foundation response under inclined and dynamic load vectors. GCRI has carried out site investigations at various sites within its land interest during 1981, 1982 and 1983. However, it was found during 1981 that conventional laboratory tests gave scattered and ambiguous results at some sites.