Following substantial rainfall earlier this year, a second significant slope failure was identified on the GSM4 Old Dalby Test Track at the entrance to Grimston Tunnel, just 500 yards south of our existing slope repair works at the Old Dalby site. Whilst the shallow failure was only affecting a short section of track adjacent to the tunnel portal, the 60-metre slope length made for a challenging repair.
Having only just re-opened the GSM4 test track to rail traffic after the previous emergency slope repairs, the railway was once again at risk of imminent closure with the threat of the failed cutting slope encroaching closer towards the operational railway. So once again, the CML team were in emergency response mode, installing track mats across farmers fields to create access suitable for long reach excavators and heavy plant to operate from the crest of the cutting, with large lay down areas ready to receive the failed cutting slope material.
Whilst the access was being established, engineers were busy with ground investigation work to identify the cause and extent of the failure and agree the most practical design solution with the construction team. Initially, softer failed material was progressively removed from the upper cutting slope by 35 tonne long-reach excavator to form a 7m wide bench on the firmer under-lying mudstone found mid-slope. Plant could then operate from the mid-slope bench, enabling plant to access and install repairs to the remainder of the site, regrading slopes to a safe angle of repose. Over the course of 5 days, over 4,000 tonnes of material were removed.
The design included the installation a mid-slope cut-off drain with counterfort drains to the upper slopes to control surface water and natural springs, and a 6G stone blanket was benched into the upper slope at a 1:2 gradient. Following the installation of the drainage system, 6B aggregate, comprising 150mm-300mm clean granite, was installed to protect the mid slope bench and upper cutting slopes. A new cess drainage system and UTX complete the repair, allowing the new drainage systems to discharge effectively.
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