Progress of Consolidation of a Marine Clay in Borne

Abstract

In conjunction with the design and installation of production platforms on the Continental Shelf off the coast of Borneo, soil borings were made through a Quaternary deltaic clay deposit that is up to 150 ft thick. Field and laboratory tests on samples recovered from borings determined classification properties, undrained shear strength and soil compressibility. Undrained shear strengths determined from samples were corrected for sample disturbance. These strengths were used to predict footing penetrations for a jack-up rig. A comparison of predicted and observed penetrations shows a close correlation, indicating that undrained shear strengths corrected for sample disturbance can be used in support of practical design decisions. Plasticity test results for these clays and Gulf of Mexico deltaic clays are similar and encouraged using Gulf of Mexico correlations to estimate the degree of consolidation of this deposit. The degree of consolidation was estimated from both liquidityindex and undrained shear strength data. These estimates compare favorably with results of consolidation tests indicating that this deposit is under consolidated and that correlations developed for Gulf of Mexico clays can be used to estimate strength and consolidation properties of these Borneo deltaic clays in Southeast Asia. INTRODUCTION Soil borings at potential platform locations on a major delta on the Continental Shelf off the coast of Borneo were drilled and sampled using equipment and techniques developed in the Gulf of Mexico(2). Typical stratigraphy of the clay deposit, which is up to 150 ft thick, and engineering soil properties, including undrained shear strengths, Atterberg Limits and water contents, are shown in Fig. 1. Plasticity index and liquid limit data determined for samples of this clay are reasonably consistent, plot above the "A" line on the plasticity chart, indicating inorganic clays of high plasticity, and plot around a linear regression line described by the equation: (Mathematical equation available in full paper) The slope and intercept of this line are similar to corresponding values for a regression line developed for Gulf of Mexico clays(6), indicating general similarity between the deltaic clays from these two regions. Soil plasticity characteristics, including liquid limits, plastic limits and liquidity indices, provide a useful basis for correlating other data, as they are not influenced by sample disturbance. Published correlations of the relationship between liquidity index and remolded undrained shear strength(9) are presented in Fig. 2 as an average line for Gulf of Mexico deltaic clays and limiting lines for published data. Also plotted on this graph are data for the Borneo clays. The remolded undrained shear strengths of the Borneo clays generally fall within the limits of published data and on either side of the average line for Gulf of Mexico clays. Sensitivities of the Borneo deltaic clays computed as the ratio of undisturbed to remolded shear strength from unconfined compression testsare between 1.0 and 2.5 to 20-ft penetration below the seafloor and between 1.5 and 3.5 below that penetration. The average sensitivity is 2.3, which is similar to the average sensitivity of Gulf of Mexico clays.

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“…2). For instance 25-30kPa at 45m depth was measured in the Quaternary deltaic clay off Borneo (Gemeinhardt and Wong 1978). Micic et al (2003) and Bellato et al (2015) also state that typical values of such sediments ranging between 5 and 40kPa may cause some problems when offshore platforms have to be founded in these soft marine deposits.…”