Network Rail – Multipoint Sampling and In-situ Testing in in Pits

Project Summary

Soil Engineering were appointed by Network Rail to carry out a structural and geotechnical investigation at London Bridge Station.

The works comprised an array of investigation techniques to ascertain the engineering properties of the ground and underlying strata. The structural investigations were to provide information on the depth, extent, strength and type of existing foundations of the station structure and five bridge structures associated with the station lines.

The Restrictions

In order to expose foundations Soil Engineering had to hand excavate pits and shafts up to 6.65m deep. The locations of the pits and shafts were very restricted, within the station, surrounding streets and occupied arches located below the station and station infrastructure.

The pits had to be of sufficient dimensions to allow access to the foundations to enable coring of be undertaken. Where man entry was required each pit had to be shored. At depths greater than 3m and where alluvium was present, the pits had to be treated as confined spaces and appropriate safe working methods adopted.

Soil Engineering Solution

Soil Engineering utilised their in-house teams of deep excavation specialists to hand dig the pits and provide appropriate shoring as the pits were extended to expose the foundations.

Cores of concrete and masonry were taken at designated locations from foundations and the side walls exposed in the trial pits. The cores were obtained using electrically powered core drills using 100mm diameter single tube core barrels. Cores were taken both vertically downwards (orientated 90 degrees) and horizontally (orientated 0 degrees).

At locations selected by the Engineer whole bricks were sampled from side walls exposed in the pits in order to determine compressive strengths and the composition of mortar used in the various structures.

In pits where excavation below and around particular structures was difficult or unsafe, and in order to obtain information below the base of these pits hand held window sampling equipment was used to extend the pits and gain the vital information. Where high qualityundisturbed samples of the Alluvium present below the deep foundations were required, access holes were cored through the foundation bases of sufficient diameter to accommodate 100mm diameter thin walled sampling equipment. Sample tubes were pushed into the underlying alluvium using a hydraulic ram and frame specifically designed by Soil Engineering for the London Bridge Station project. The samples retrieved were Class 1 undisturbed samples.

Additional information on the in situ strength and characteristics of the underlying strata was obtained by using a Geonor Inspection vane, again used in the vertical cored holes advanced below the base of the window sampler or thin walled sampler holes.

Value

Each pit was orientated and located so as to enable maximum sampling points and in situ testing opportunities within each location without the need for multiple pits at any one location.

In order to allow access to the observation pits over several shifts without impacting unduly upon the efficient operations of the station, cover plates were prefabricated to a standard size and the pits excavated such that the plates could be fitted over the excavations between shifts and lifted again when work recommenced.

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