Engineering Properties of Marine Sediments-Gulf of Paria, Trinidad, W. I.

ABSTRACT The engineering properties of high plasticity marine sediments in the Gulf of Paria were investigated for the design of a five mile long submarine pipeline and marine structure at the request of Texaco Trinidad, West Indies. The strength-depth relationships determined by using hand vane shear tester and laboratory tests performed on undisturbed samples indicated linear variation to a depth of approximately 80 feet. Thereafter, the rate of strength increase reduced resulting in almost constant shear strength at the maximum depth explored -- 212 feet below sea bottom in 90 feet of water. Design charts determining the anticipated settlement of the submarine pipelines and foundation pile capacities were developed. INTRODUCTION The construction of engineering works on the sea bed requires knowledge of the physical properties of the bottom sediments to varying depths, depending upon the type of facility contemplated. Certain types of facilities are involved only with the shallow deposits, other types of facilities with both shallow and deep deposits. This paper is involved with both shallow and deep strata - the shallow for a submarine pipeline and the deep for the foundations for a platform and an anchorage for a Single Point Mooring (SPM). The accurate determination of the physical properties of each sediment stratum is essential to the development of a sound engineering design. The accuracy with which these determinations can be made is intimately related to the ability to either obtain undisturbed samp1e which can be tested, or to perform in-situ tests. The problem is further compounded when operations have to be performed in exposed sea locations. Several approaches to in-situ testing have been attempted with only limited degrees of success, and only in relatively shallow penetrations of the bottom. This approach, therefore, still remains in the research and development stage and is not suitable for consideration in a production type of project. The use of undisturbed sampling currently affords the best opportunity for a successful program at moderate cost. This project required operations in 90 feet of water and utilized the cased boring techniques with almost continuous thin wall tube sampling. Shear testing was performed on site by means of a small vane shear tester and compared with the results of laboratory unconfined compression and triaxial shear tests. A design chart was developed to determine the estimated settlements of the submarine pipeline as a function of pipe diameter and specific gravity. Another chart was developed to establish the pile capacities in tension and compression.