Slope Stability Investigation and Analysis - Tonbridge Castle, Kent
Background
The site represents the eastern limb of the outer keep at Tonbridge Castle, which is topped to the south by the remains of the outer castle wall. A tarmac footpath runs along the crest over the north but steps down alongside the outer wall as it approaches the river on the south. The 6-9m high embankment slopes stand at about 40º, but slip scars were visible just below the footpath in February 2006, with voids beneath the footpath where it steps down over the south. The slip scars stretch about 30m along the crest of the embankment. Downslope there are several lengths of toe bulging, with slumping and sloughing of materials, which result in localised steeper slopes of over 50º.
Ground Investigation
Detailed site investigation was undertaken by Southern Testing, which comprises several 3-6m deep windowless sampler boreholes, Installation of slip indicators and a series of DPL dynamic penetration tests along the crest of the embankment. The soils encountered comprise made ground of sands and clays, and natural soils were encountered at depth. No groundwater was encountered during the investigation. Field monitoring results indicate that the slope is experiencing quite shallow landslip. No significant movement has been recorded at lower depths.
Slope Stability Analysis
Comprehensive slope stability analysis was undertaken, comprising:An analytical model including soil parameters and groundwater condition was established based on field test data and laboratory test results for slope stability analysis, which is verified by back analysis of the failed slope.- Various remedial measures, such as soil nailing or regrading the existing slopes together with soil reinforcement such as gabion wall under the footpath, are recommended to achieve both the local and overall long term slope stability,- Close attention was paid to the historical site to avoid any damage to the existing foundations of the castle walls which may be caused by remedial treatments.Southern Testing undertook the full ground investigation, performed the slope stability analysis and recommended effective and practical remedial measures.
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