Karst Subsidence, South Wales
Introduction:
Phased investigation enabled delineation of solution features in a karst setting where roadway subsidence presented a significant risk and public access was halted. The region is known to be susceptible to limestone solution with extensive evidence of karst landscapes; the only turlough in mainland Britain, extensive caves and sinkholes.
Our investigation provided the basis of a remediation options appraisal and informed technical earthworks advice for remediation of the roadway. Our investigations and assessments provided a successful remediation solution in an area of limited access and challenging logistics.
Details:
We were engaged by the Local Authority to assess potential solution features which had resulted in surface deformation, subsidence and temporary closure of a publicly accessible roadway. A phased assessment was adopted to allow for an assessment of the ground hazards associated with the karst subsidence landscape.
Information provided by the client indicated the presence of a natural sink located in the vicinity of the study area draining surface waters. Subsidence features had historically formed with localised ad-hoc repair works to the roadway undertaken over time.
Our initial site walkover identified a number of possible sinkhole features along the roadway, spanning approximately 50m. We identified the underlying bedrock to comprise Carboniferous Limestone, which is susceptible to chemical weathering and the formation of karst subsidence features.
Karst terrain is created from the dissolution of soluble rocks, principally limestone and dolomite. Karst areas are characterised by distinctive landforms like springs, caves, sinkholes and a unique hydrogeology that results in highly productive aquifers.
Records held for a number of karst features (e.g., sinkholes and caves) in the locality were identified in a local memoir “Caves of Carmarthen” (Oldham, 1975). Further evidence included the presence nearby of the only turlough recorded in mainland UK (Pant y Llyn).
A turlough is a lake with no surface water inflow/outflows and is fed by underlying groundwater via solution features, geological joints and faults. Typically the lake is seasonal with high groundwater levels filling the lake in autumn/winter and subsequently drying out during the summer months.
We implemented an exploratory investigation comprising surface mapping, geophysics and trial pitting to assess the underlying ground conditions, whilst allowing for challenging site access and risks posed by potential sudden ground collapse.
Supplementary intrusive investigation comprising rotary boreholes to confirm underlying bedrock strata and inform detailed remediation design. Due to the potential for ground instability, a drilling platform was utilised to mitigate sudden ground collapse.
Our investigation idenitified solution feature infill up to a thickness of 5.4m, over Oxwich Head Limestone bedrock. The findings of the investigation confirmed that the area of the access track is likely representative of a Complex Karst kIV setting (Waltham and Fookes, 2003).
Using the findings of the phased investigations, we developed a Remediation Options Appraisal to identify a cost-efficient and feasible engineering solution for the unique site. Based on the findings, and consideration of site limitations (e.g., access and land sensitivity), a flexible earthworks solution was recommended, comprising reinstatement of the roadway with engineered fill and reinforcement geosynthetics.
A Planned Maintenance Approach was also recommended to include regular site inspections. The remediation approach was selected considering existing surface conditions, cost-efficiency, site restrictions and land sensitivity.