USE OF JUTE GEOTEXTILE IN LAYERED CLAY-SAND RECLAMATION SCHEME

Abstract

The concept of the use of jute geotextile in the layered clay-sand reclamation is developed to supplement and extend the well established layered clay-sand scheme of land reclamation to slurry deposits with large water content. The main feature of the layered clay-sand scheme is the formation of a horizontal sand layer on top of sedimented clay slurry in order to reduce drainage path and accelerate consolidation of the clay layers during surcharge application. In the layered clay-sand scheme it is found that the slurry of water content 275 % or greater is too weak to arrest the sand particles and if the sand particles are greater than 600µm, most of them penetrate to the bottom even when the slurry water content is as small as 225 % . Furthermore, the recommended small doses of sand spreading operation to ensure the build up of sand layer will tend to increase operation time which may be reduced with the use of geotextiles. In cases of scarce and expensive sand supply, it is desirable to minimize the loss of sand through penetration in the very soft clay slurry during the process of forming the horizontal sand layers. It is found that the use of inexpensive woven jute geotextile as an interlayer on the surface of the soft hydraulically placed clay slurry can aid in minimizing the sand loss through penetration in the clay slurry. The behavior of layered clay - jute geotextile - sand could provide an economical modification to the existing layered clay-sand scheme of land reclamation. This study provides some insights into the nature of sand penetration in the clay slurry through a jute interlayer. The most significant factor controlling the sand retained on jute is the ratio of particle size to the AOS (Apparent Opening Size) of the jute geotextile. It is apparent that for particle sizes greater than 1/10th of the AOS, nearly all of the sand particles can be retained by the jute. For particle sizes finer than 1/10th of the jute AOS, the slurry strength plays the most important role of preventing large sand penetration. With the use of a 500g/m2 jute interlayer, even fine sand seams can be formed above a slurry of 300% water content. Four index properties namely specific gravity, thickness, apparent opening size (AOS) and tensile strength, which are important to relate natural geotextile to layered clay-sand scheme of land reclamation, have been determined in the case of jute geotextile. In order to subject all fabrics to the same simple procedure for pore-size measurement, a modified method has been developed using a dry sieve analysis which aims at establishing a characterization of a fabric with respect to size and uniformity. The AOS for jute geotextile 600g/m2 and 500g/m2 is found to be 2. 7mm and 5.5mm respectively. The maximum strength of jute geotextile determined by wide width strip tensile strength is found to be 10kN/m to 15kN/m. In the layered clay-jute geotextile-sand scheme, the interface resistance developed between the jute and clay slurry plays a major role in supporting a substantial amount of sand for a drainage seam on the surface of soft clay slurry, which will eventually pave the way for successful completion of the reclamation economically. A new technique has been adopted to obtain the interface behavior between jute geotextile and clay slurry. The test results indicate very low jute/slurry interface adhesion of 3 to 7 Pa for slurry water content from 600% to 300%, with rapid increase in interface adhesion values of 7 to 35 Pa for 300% to 200% water content. With the aid of jute geotextile laid on the slurry surface, sand layer can easily be formed by careful spreading of a thin layer of sand through water onto the jute geotextile lying on the slurry surface. Model bearing capacity tests on a rectangular loaded area of a 500g/m2 jute geotextile suggest that bearing capacity factor, Ne of jute on clay slurry is about 5 to 6.5 times the jute/slurry friction for slurry water contents of 150% to 300%. The thickness of sand formed prior to failure can be reasonably estimated from the measured jute/slurry interface friction and the bearing capacity factors of clay slurry from 250% to 500% water contents reinforced with a jute geotextile layer in cylindrical columns of different diameters. Thus, the use of a low cost jute geotextile allows expedient means of forming a sand seam on top of clay slurry with minimal sand losses.