Terra Firma (Wales) Ltd was retained by a Local Authority during 2010 and 2013 as independent specialist Geotechnical Consultants to assess the stability of a landslip adjacent to a new dual bypass highway: a Category 3 Geotechnical project.
Geologically the site is underlain by variable deposits of Made Ground, underlain by Glacial Head Deposits which is further underlain by the Productive Coal Formation (Middle Coal Measures). Land slippage has been a recurrent problem in the local area of the site for many years, with first anecdotal evidence dating back to 1938. Major slippage events occurred during 1954, 1983 and 1989. No further landslip movement was observed after 1990.
A ground investigation and laboratory soil and rock testing provided robust information to perform a slope stability assessment which confirmed that the landslip was only marginally stable with factors of safety of just above unity. The slope stability assessment concluded that the slope was very sensitive to groundwater and the soil was behaving under residual shear stress conditions. Slope stability modelling indicated a number of intersecting marginally stable shear surfaces which extend beneath the landslip itself and beyond, beneath adjacent haul roads and down gradient slopes with the potential for retrogressive failure. The modelled slip surfaces were largely circular and deep seated within the cohesive Head Deposits and close to rockhead.