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The hole problem

Historic chalk mining in Kent meant finding a foundation solution for a new housing development was far from straightforward. Andy O’Dea reports.

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Chalk can be a challenging material to work with - solution features in the upper surface are often hidden from view by the overlying clay, but some of the hazards are man made.

Historic mining of chalk and the passing years mean that these mines - known as dene holes - can also be obscured by the geomorphology, presenting a challenge for developers.

Springhead Park, a development of 650 new low-rise homes by Countryside Properties, overlooking the High Speed 1 rail line and Ebbsfleet International Station in Kent, faced just this challenge recently.

Both historic chalk mining and natural dissolution features are known in this area and early ground investigations confirmed the presence of a number of anomalies across the northern part of the site.

Investigations showed that the depth to the top of the Upper Chalk below the sandy head deposits and Thanet Sands increased significantly towards the south and west of the site.

No weak ground that could be associated with chalk dissolution features was found, but an open void that had the potential to be a denehole was located in the phase one development area. As a result, the possibility of other deneholes could not be ruled out.

Ground penetrating radar, electroresistivity and electromagnetic surveys were carried out around the known denehole. It was hoped that, if correlation between the geophysics data and the known denehole could be established, this would provide a quick and cost-effective means of surveying the entire site for other deneholes.

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Unfortunately, the geophysical trial proved inconclusive and the foundation design for the first phase of the development adopted lightly reinforced strip foundations and suspended ground floor slabs in excess of 1,000 probe holes up to 12m in depth carried out to check for significant voids.

During the construction, three deneholes - including the one highlight by the ground investigation - were located in the phase one area. All of these deneholes were excavated, surveyed and backfilled from the surface.

When it came to developing the second phase, the developer appointed a new consulting engineer - Walsh Associates - and CGL was in turn appointed to provide specialist geotechnical and geoenvironmental advice.

CGL carried out a detailed review of all previous site information to see if an alternative approach to the existing foundation solution could be found.

It was found that there were significant differences in the ground conditions between development phase areas that could allow for a revised stability risk assessment and simplified approach to the foundations that would avoid the need for further structure-specific probing.

Confidence in this approach was derived from the greater depth to the top of the chalk, suggesting that hand excavation to reach the chalk was unlikely.

Calculations of predicted ground movement beneath a collapsing denehole confirmed that, if a denehole was present in an area where the overburden thickness was greater than 7m - and it was 7.5 to 15m in phese two - a collapse over time would result with no ground movement at the ground surface due to bulking.

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Despite the proven lower risk of the presence of deneholes in the phase two area, their presence could not be completely discounted and a foundation design to account for this was needed.

CGL proposed the use of a new modular expanded polystyrene (EPS) raft foundation using the Insulslab SFRC (steel fibre reinforced concrete) system. The system uses series of interlocking EPS pods to form a substantially rigid “waffle”-shaped slab over which a SFRC is poured to form the finished foundation.

In addition to meeting the structural demands of the site, the solution was also cheaper than the reinforced strip approach, and offered a higher thermal performance, adding to credit scores under the Code for Sustainable Homes

Although there was a lower ground instability risk in phase two and the foundation solution further reduced the risk, it was recognised that a construction watching brief and specific training were required.

These included toolbox talks with ground workers to convey the potential for chalk mines, discussion and warnings of potential deneholes in all site inductions and visual inspection of formations by a geotechnical engineer.

Construction work on the second phase of residential development at Springhead Park is now well under way as the foundation design not only reduced costs, but has also contributed to significant programme savings on the project.

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