CASE STUDY 1
ALLHALLOWGATE, RIPON – PROPOSED RESIDENTIAL DEVELOPMENT
Client: Harrogate Borough Council
Contract Value: £120k
Ripon lies within an area of ground collapse hazards through the dissolution of gypsum located within the underlying strata. Gypsum dissolution has been shown to be greatest where fresh water can flow. Although at the surface this process is prevalent, the rate is somewhat reduced underground due to slower water-flow and sulphate rich water. However, as the bedrock is relatively shallow with gypsum rich beds underlying Karst – a landscape with abundant joints and fractures – dolostone and dolomitic limestone this dissolution process is present beneath Ripon. This combination provides, in suitable conditions, pathways for groundwater flow to reach the shallow gypsum deposits leading to dissolution and possibly catastrophic collapse of the overlying strata.
Harrogate Borough Council in consultation with the British Geological Society (BGS) and William Saunders required detailed geological assessment of the underlying solid geology to ascertain if Gypsum voids were present below the site which may cause issues with sinkholes in the future. The site investigation data would enable a sustainable foundation solution to be sought.
Fieldwork was agreed with the BGS and the client and comprised:
- 7no cable percussive boreholes with rotary Geobore-S follow on drilled to depths of 60.00mbgl.
- Boreholes drilled to determine the depth of potential unstable zones beneath the site and depth of competent rock strata.
- Coring took place once the base of the drift was reached. After the coring the boreholes were installed with solid 80mm diameter piping to facilitate geophysical testing. Grouting was undertaken around the pipes using a special sulphate resistant material.
- Verticality checks within each borehole were undertaken by European Geophysics Services Ltd.
- Crosshole Seismic tomography surveying comprising P-wave and S-wave analysis between each borehole at a target depth range of 25.00mbgl and 60.00mbgl.
The boreholes had to be drilled through the Edlington Formation which contains mudstone and the potentially unstable Gypsum deposits and into the underlying Cadeby Formation (limestone) at c.60m depth. The boreholes were then lined with soild HDPE pipework to allow geophysical testing.
The geophysical testing was provided to the BGS so they could form and underground 3D model of the site to show the Gypsum deposits and areas of voided ground. This was then used by Wm Saunders to design the final foundation solution.