CONSTRUCTION METHODS

Chennai - Gudur section is a part of the Eastern section of the Golden Quadrilateral. The laying of the III rd and IV th lines between Athipattu and Chennai Beach stations has become vital on account of rapid industrial development in North Chennai area and the creation of new ports. The proposed lines are passing through the marshy lands of Bay of Bengal for a length of 900m between Ennore and Athipattu stations-between kms. 15/900 -16/800. The base soil consists of highly decomposed organic clay as top layer resting on highly compressible clay layer of 12 m thick. The location is also subject to tides with sea water level varying from 3.0m to 4.5m. As a result, a permanent speed restriction of 80 kmph has been imposed on the existing Up and Down lines. The formation problems require perennial maintenance activities, higher inputs and permanent speed restrictions thereby resulting in reduction in the throughput.

Despite execution of several formation treatment techniques, the base soil required improvement in the form of consolidation of the clay layers in the subsoil before laying the formation. After studying the ground formation at Ennore - Athipattu, the Indian Institute of Technology, Chennai, suggested the ground improvement methodology with the provision of pre-fabricated vertical drains (PVDs) that are geo-textiles wrapped drains with plastic based central core. The PVDs are used for the acceleration of the rate of consolidation of the clay layer. Installation of the PVDs cause significant disturbance in the surrounding soil and the permeability of the clay layer is greatly reduced due to reorientation of the soil particles.

The advantages of the PVDs are many. Besides reduction in time required for primary settlement, they relieve excess pore pressures and increase the rate of consolidation. The strength of the base soil increases paving way for better use of construction machinery, better speed potential. Normal track maintenance and savings in construction cost are other advantages.

Adopting the technique suggested by IIT/Chennai, the base area of the embankment is divided into ponds and the creek bed is left to dry duly pumping out the sea water from the ponds. After removing the slush/organic material, sand is filled in layers and compacted with a vibratory roller. PVDs are then installed upto a depth of 12 m.