Thiam-Soon Tan

PROPERTIES OF SINGAPORE MARINE CLAYS IMPROVED BY CEMENT MIXING

Stabilization of soft ground by the deep cement mixing (DCM) method has become an increasingly popular method to improve stability in an excavation in soft clay and to limit movement in adjacent sub-structures. The desired increase in strength and stiffness to fulfil the intended functions can be achieved provided that the right mix proportion is adopted. To proceed with this kind of soil improvement, prediction of the strength and stiffness of the improved soil is necessary. Due to a short history of the DCM method in Singapore, there is limited data on the improved properties of local clays. This study is conducted to bridge that gap and also extends its usefulness to clays elsewhere. In the paper, the influences of three main constituents of the mixture, namely clay, water, and cement on the strength development of Singapore marine clays improved by cement mixing are investigated. Based on the experimental results, it is shown that a convenient normalization can produce a consistent pattern for evaluation of improved strength of clays from different parts of Singapore. This normalization is also shown to work for one Japanese clay. Correlations between strength and stiffness of the improved clay are also obtained. Lastly, it is shown that for a cement mixed clay there is a continual increase in strength and stiffness with time. This will help to reduce ground movement, and it will also increase the bending moment in the retaining wall. Both aspects must be considered in a design.

Effect of soil microstructure on the compressibility of natural Singapore marine clay

The effect of soil microstructure on the compressibility of natural Singapore marine clay is studied in this paper. It was found that natural Singapore marine clay can sustain higher void ratio than reconstituted Singapore marine clay at the same effective stress in one-dimensional compression. This difference is most noticeable between yield stress and two times the yield stress. This void ratio difference is commonly attributed to soil microstructure, and the decrease in this difference at higher stresses is attributed to progressive destructuration. As a consequence of progressive destructuration, the virgin compression curve is nonlinear and the compression index along the virgin compression range is variable, which is only noticeable during the constant rate of strain consolidation test on high-quality samples. In the absence of high-quality samples and constant rate of strain consolidation tests, an empirical method is proposed to obtain a first-order estimation of a nonlinear virgin compression cur...

Shear Strength of Very Soft Clay-Sand Mixtures

To treat a deposit of clay slurry in situ, sand is often added with the aim of trapping the sand in the upper portion of the slurry. The presence of the trapped sand may directly improve the shear strength of the slurry or contribute indirectly to the improvement by accelerating the consolidation. For this treatment to be effective, an important consideration is the improvement in the shear strength of the slurry as sand is added. A laboratory study of this is reported here. To rapidly determine the shear strength of the mixture, the fall cone method and a penetration test that was developed to measure very low shear strength were used. For the clay slurries studied, the added sand grains seemed to “float” in the clay without increasing the shear strength if they were sufficiently far apart. Treating the clay slurry as a fluid filling the “voids” in the sand, the void ratio of the sand for which a marked increase in shear strength is observed is about 5. The liquid limit of the mixture is also found to follow approximately a linear mixture law unless the amount of sand added is enough to ensure that the void ratio of the sand is less than 5, which is in agreement with the earlier result.

In Situ Evaluation of Radioisotope Cone Penetrometers in Clays

A comprehensive site investigation was performed at a 12-year-old man-made island in Singapore, which was reclaimed using dredged clay lumps. The objective of the site investigation was to evalute the performance of the radioisotope (RI) cone penetrometers to measure in situ wet density and water content, which are the two important parameters in the characterization of soils formed from clay lumps. The site investigation program included RI cone penetration tests, high quality soil sampling, and laboratory testing. The in situ water content and wet density profiles measured with RI cone penetrometers were compared with data obtained independently from the laboratory tests on undisturbed samples. The comparison shows very good agreement, and 92% of the RI cone measurements were found to be within ±5% of the laboratory measurements.

Responsibilities and Activities of Singaporean Production Managers

A recent survey conducted on a sample of 101 Singaporean production managers, studying their responsibilities and activities in a number of decision areas, is examined. The findings are summarised and the differences between production managers in local and foreign firms are discussed. Results are compared with two similar studies conducted earlier in the UK and India.