Case Study: Geoenvironmental Site Investigation at Rigmaden Bridge, Cumbria
Client: Cubby Construction
Services: Geoenvironmental site investigation, exploratory drilling and geotechnical assessment
Location: Rigmaden Bridge, Cumbria
Project summary & client purpose:
Earth Environmental & Geotechnical was commissioned by Cubby Construction to undertake a geoenvironmental site investigation at Rigmaden Bridge, Middleton, Westmorland and Furness. The investigation was required to assess ground conditions to support proposed strengthening works to the existing bridge structure, including potential reuse of the existing bridge abutments.
The aim of the investigation was to provide reliable geotechnical data to inform foundation design, confirm ground conditions and reduce uncertainty ahead of engineering design and construction planning.
Our services:
We delivered a programme of exploratory site investigation works across both sides of the bridge. The works included cable percussive boreholes, window sample boreholes and rotary coring to investigate the underlying strata and recover representative samples for analysis.
In-situ testing, including Standard Penetration Tests (SPTs) and super heavy dynamic probing, was undertaken alongside groundwater monitoring. Laboratory testing was also carried out on recovered samples to determine the engineering properties of soils and rock encountered.
The findings were compiled into a detailed geotechnical report presenting the factual records, interpretation of ground conditions and recommendations for foundation design to support the proposed bridge works.
The challenges:
The site presented several geological and practical challenges that required careful investigation. The bridge spans the River Lune and the surrounding landscape includes river terrace deposits with variable granular soils and cobbles, as well as underlying bedrock formations.
Ground conditions were found to vary between the eastern and western sides of the bridge, with made ground and reworked natural soils near the surface, underlain by sands, gravels and cobbles associated with river terrace deposits. The depth to bedrock also varied across the site, with siltstone encountered relatively shallow on the eastern side.
Understanding this variability was critical to ensure that the proposed strengthening works and foundation reuse would be technically viable while minimising the risk of unforeseen ground conditions during construction.
Results:
The site presented several geological and practical challenges that required careful investigation. The bridge spans the River Lune and the surrounding landscape includes river terrace deposits with variable granular soils and cobbles, as well as underlying bedrock formations.
The investigation confirmed that the underlying soils predominantly comprise medium dense to dense sands and gravels with cobbles, typical of river terrace deposits, with siltstone bedrock encountered beneath the eastern side of the bridge. Groundwater levels were identified during the investigation and considered within the geotechnical assessment.
Based on the investigation findings, it was concluded that the existing bridge abutments could potentially be reused, with an allowable bearing pressure of approximately 500 kN/m². Much of the anticipated settlement is expected to have already occurred due to the long-term loading from the existing structure.
These findings provided the client and design team with the technical confidence required to progress with the proposed bridge strengthening works, supported by robust ground investigation data and clear engineering recommendations.
