Bincombe Shaft
Currently under construction, the Weymouth Relief Road passes over a Victorian railway tunnel access shaft which extends vertically down to an active railway tunnel lying some 40m below the current ground level. Investigation was required to determine its location and backfill characteristics and hence to provide parameters for designing a shaft cap.
The investigation comprised rotary percussive sampling carried out using a tracked Klemm drilling rig and supervised by Geotechnics’ Engineer, Ben Tucker. The work was carried out over several night shifts under Network Rail possession. An overhead 20kV electricity cable had to be isolated as it ran directly over where the shaft was predicted to be.
Following Skanska’s excavations to locate the shaft visually, the drilling rig was set up over the centre of the shaft. Due to the uncertain nature of the chalk backfill material the rig was positioned on a steel platform, usually used for mining investigation projects, that extended across the shaft preventing the rig becoming unstable should the shaft collapse beneath. This had to be manoeuvred into place by a tracked 360o excavator. Railway sleepers and boards were laid across the steel platform for the drillers to work on.
Once drilling had commenced the borehole made good progress and we were aiming to finish within two or three shifts. However the weather had a difference of opinion. The temperature dropped, the snow fell relentlessly and the country came to a stand still. As most major roads were blocked by thick snow, Network Rail required the use of the line overnight to keep at least some parts of the country running. This meant permission to drill was not given for several nights until Network Rail could grant us rail possession again.
After two nights of heavy snow we were granted re-possession to complete the borehole to a depth of 22.50m. A control borehole was sunk adjacent to the shaft to provide information on the natural chalk material nearby. On completion a plain standpipe was inserted into the shaft borehole to allow a Geophysical Logging survey (Gamma logging) to be carried out to determine the relative density of the backfilled chalk and the location of any possible voids or remaining structure of the old access shaft.
