TUNNEL BORING machines and diaphragm walling rigs would not have been able to reach the southern approach for the Thames Tunnel had it not been for Tensar geosynthetics.
Around 40,000m 2of Tensar SSLA30-G geocomposite was rolled out by two man crews to create haul roads and initial working platforms across the Swanscombe marshes. The weak, wet marshy ground, between 9m and 13.5m deep, has a typical undrained shear strength of 15kPa.
Tensar International and main contractor Hochtief/Murphy jv worked together to develop the solution, which comprises the geosynthetic overlain with crushed concrete fill.
The SSLA30-G geocomposite comprises a biaxial geogrid with a non-woven filter geotextile bonded onto the underside, so it acts as a separator between the marsh and the fill. The geogrid interlocks with the fill to create a stiff reinforced layer with minimal lateral movement and high shear strength. A layer of Tensar SS20 biaxial geogrid covered by a sub-base material was placed on top.
Keller Ground Engineering built two, 17m by 400m, load transfer working platforms for the diaphragm wall plant installing the 700m long approach cut.
These comprise 1m of granular fill reinforced with three layers of Tensar biaxial geogrids supported by vibro-concrete columns.