Soil nails support the excavation for the north abutment of the new Godington Road Bridge in Ashford town centre. The bridge carries local traffic and a major service diversion over the existing Railtrack Southeast coast line and forms enabling works for the Channel Tunnel Rail Link.
The bridge diversion, now completed, lay on the critical path for contract 430, which in turn is on the critical path for completion of the whole rail link project.
Soil nailing, carried out from an existing cutting beside the working lines, was completed in four weeks, one week ahead of the original schedule. The time gain was achieved, says Howard Roscoe of contractor Kvaerner, by close co-operation at site level between its site team and design and project manager consortium Rail Link Engineering.
RLE preferred the soil nail concept to a bored pile wall option because it avoided the potential complications of operating heavy plant near the crest of a steep railway cutting, and also reduced disruption to local residents.
Kvaerner Cementation Foundations carried out the detailed design of the temporary slope. Nail length was restricted by the position of houses immediately behind the slope and in order to ensure adequate safety, internal stability and the vertical and horizontal spacing of the soil nails was analysed using the computer code Goldnail.
This limit equilibrium slip circle program takes account of the limiting nail tensions at the point where they intersect a potential slip circle. A global factor of safety approach was used, as artificially high slice forces and nail tensions can result if soil weights are factored using a partial factor approach, explains Roscoe. The limiting bond stress was estimated using the approach given in HA68/94 as there was no scope for precontract pull out tests.
To limit ground movements to acceptable values, Kvaerner installed six rows of nails on a square grid pattern of 1m spacing.
Three additional boreholes were sunk at the start of the contract to define the variable ground conditions in greater detail. The slope is 60m long and 6.2m high, excavated through Head deposits, predominately a firm sandy clay, overlying the Hythe Beds, generally a silty sand with layers and lenses of sandy clay.
Groundwater level was about 2m above the base of the slope.
Kvaerner Cementation Foundations used a self drilling grout flush system to install 380, 6m long nails, supplied by Ischbeck Titan.
Immediately on completion of each section, the contractor applied a 100mm sprayed concrete facing.
Wellpoints controlled ground- water in the lower part of the slope, and careful scheduling of work prevented this impeding overall progress. Dewatering significantly improved the excavation working surface, with development of significant suctions preventing water flow through the face.
Throughout the work, precise levelling of studs in the pavement between the slope and the terrace of houses, monitored ground movements. Kvaerner took readings after each level of excavation, and throughout the work recorded movements of between 0.5mm and 1mm. Pull out tests on six anchors checked anchor capacity, with ultimate capacity proving to be between three to six times design load.