Remediation of a 125 year old Metropolitan London Underground chalk cutting at Chalfont

Remediation of a 125 year old Chalk cutting that forms part of the Metropolitan line, was carried out by Atkins and London Underground between 2012 and 2015. The site is located between Chalfont & Latimer and Chesham London Underground stations on the Metropolitan line. The 11m deep, 360m long cutting was originally excavated through the Lewes Nodular Chalk Formation of the Chiltern Downs in 1889. The chalk is overlain by a variable thickness of superficial deposits comprising Clay-with-flints Formation which included a number of dissolution features. Weathering and vegetation growth had seen the surface of the steep cutting slopes, exposed to vegetation, root ratcheting and frost shattering. This had led to localised chalk ravelling, and large cobbles of flint falling onto the single track line below, causing track serviceability issues as well as increased concerns over the operational safety of the railway. The enhancement measures designed by Atkins and implemented by London Underground as the Principal Contractor included: reprofiling of the cutting slopes; soil nailing of the over-steepened Clay-with-flints Formation and significant solution features; and placement of an erosion control mat over the slope surface secured with grouted restraint steel pins to reduce future ravelling of the Chalk slope. A site trial was conducted to assess the effectiveness, speed and installation feasibility of auger bored and driven restraining pins, and the results of the trials are reported. Advanced digital photographic/topographic laser survey equipment was utilised to enable the interface between the Chalk and overlying Clay-with-flints Formation deposits to be accurately plotted. This technology removed the need for geological face logging by abseiling, improving safety and enabled the final scheme requirements for soil nailing or re-grading to be targeted to specific areas.