Investigation on Historical Clay Pit

We recently undertook a ground investigation on an interesting site in the city of Oxford. At first glance, it appeared that the site would involve a straight forward investigation for the redevelopment from a commercial land use to a residential land use. However, when undertaking the Phase 1 Desk study, it was revealed that this would not be the case. The site is recorded to be situated on the Oxford Clay Formation and the West Walton Formation, and historical maps showed the site was historically located on a clay pit during the late 1800s. These clay pits were historically used to extract clay which was predominantly used for brick making at the time.


Due to the presence of the historic clay pit on the site, and the likelihood of this being backfilled, it was considered that deep Made Ground could be encountered on the site. There were several geoenvironmental risks and geotechnical risks expected from this deep Made Ground.


Focusing on the geotechnical risk, it was considered that a piled foundation solution may be necessary at the site, and therefore as part of the Phase 2 Ground Investigation we undertook a deep (30m) cable percussion borehole. The purpose of the deep borehole was to understand the ground conditions, to perform in-situ testing such as SPTs, and to take samples for geotechnical testing. A standpipe was also installed into the borehole, for monitoring of ground gas and groundwater.


Geotechnical testing on the samples included Atterberg tests to understand the volume change potential of the soils, triaxial tests on U100 samples to provide the undrained shear strength of the soils, and soluble sulphate tests to give an indication of the aggressivity of the ground in relation to buried concrete; the soluble sulphate tests were particularly important due to the natural soils of the Oxford Clay Formation and West Walton Formation typically having significant concentrations of sulphates.


The results of the geotechnical testing allowed us to provide the Client with guidance on design parameters for the soils, calculate preliminary working loads for piled foundations, and give initial recommendations on an appropriate piling method for the site, all of which could then be passed on to their piling contractor to formulate a suitable piling technique and type for the site.



This site highlights the importance of a thorough and well-researched Phase 1 Desk Study before the commencement of a Phase 2 Ground Investigation. Should you require a Phase 1 Desk Study at your site, or boreholes to be undertaken at your site to understand deeper ground conditions, contact us on this email link or 01280 816409.

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