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Structural Geotechnical Environmental Other services

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►CS11 New Line, Bacup

Development: Foundation appraisal for 53 residential properties on a reclaimed brownfield site
Year: 2007-2008
Client: Harron Homes

The Brief

Coopers were commissioned by Harron Homes to carry out an appraisal of a site for a proposed residential development in Bacup. The available site investigation data had been prepared by two previous consultants and the remediation contractor employed by the land owner.

 

There were such substantial variations in the findings of the trial pit logs and borehole logs provided that it was not possible for us to complete a realistic appraisal of the anticipated foundations without confirming the true situation with the strata present on site.

Ground Investigation

When we attended site, the remediation contractor and his appointed groundworks contractor had demolished the former mill building and regraded the site with significant depths (3m+) of fill in certain areas. The remediation contractor was apparently only commissioned to undertake remediation on site, however was also partially guaranteeing the geotechnical properties of the site. The geotechnical aspects supervised by the remediation contractor involved both the grading and compaction of the fill strata. We excavated a series of trial pits across the site as well as trial trenches.

During the course of the additional ground investigation undertaken by Coopers, intact foundations and ground floor slabs were found to still be in situ, and significantly oversized cobbles and boulders were within the regraded fill. The presence of the obstructions was initially refuted by the remediation contractor as well as the groundworks contractor, and had to be proved by additional phases of trial pits with the contractors, our client and the vendor present. This additional work was supported by the photographic records and logs completed by Coopers. During the second phase of ground investigation we invited piling contractors to attend site to assess the suitability of the filled ground for pile installation. Five piling contractors attended site and all bar one stated their techniques could not work on this site due to the obstructions present.


Additional Observations Made by Coopers

Additional issues associated with the site comprised a culvert which had been partially installed by the groundwork`s contractor, the slope of the site compared to adjacent land and the lack of a condition survey for adjacent properties with visible cracks along walls. Japanese Knotweed had been encountered and buried on site by the remediation contractor; however extensive swathes of this invasive plant were still present along the site boundary within private gardens. Coopers recommended that these plants should be treated or the site would inevitably become affected by this invasive species.

The culvert was historically present underlying two gardens and had been re-culverted within the development site at significant expense to tie into the existing culvert. We recommended to the client that this was unlikely to be approved by the Environment Agency due to potential blocking of culvert entries. We conjectured that it would require re-culverting in its entirety beneath the proposed development site. The site was subsequently rejected for planning approval due to the position of the culvert and the Environment Agency recommend it should be re-aligned to underlie the new development site.


Conclusion of Coopers Investigation

The findings of our investigation determined that the actions of the groundwork`s contractor and remediation contractor had altered the anticipated land value of the site, effectively halving the value due to foundation issues. The original foundation recommendations presented by the remediation contractor comprised 28 No. plots requiring shallow strip foundations and 26 No. plots required vibro-replacement stone columns. The revised appraisal by Coopers conjectured that 37 No. plots would require steel tube piling, with 12 No. plots requiring a combination of shallow pier and beam with piling. 4 No. plots were lost due to culvert issues.

Recommendations were made to the remediation contractor to both re-excavate the fill to remove all oversized materials in a methodical manner (supervised by an independent consultant), complete large scale loading tests over areas of compacted fill, and slope stability assessments for the boundary with the existing properties which also required condition surveys.

Due to the issues which had been recognised by Coopers and indicated to the client, this site was not developed as the potential liability to the developer was too great. Although our client was prepared to pursue this site at the right price, the vendor was not prepared to accept the true value to the site.

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