Holdfast Training Services Trial Blog

Last week Beresford tricked me into attending what I thought would be a notoriously dull lecture at the Royal Society of Geologists. The talk was on a ground condition known as a Pingo, or using its more technical name, Drift Filled Hollows. It turned out to be mildly interesting because the ‘expert’ openly admitted that Geological understanding of this condition remains relatively basic at the moment and from engineer’s perspective, the implications for a large structure are potentially very severe.

To try keep this short, a Pingo is basically an unexpected disturbance and variation in the ground strata caused by (they think) perma frost occurring in the ground some millennia ago. This created a hole which is then essentially filled with differing material over time. In the London Basin this is an issue because it means, where you might expect to find a decent band of London Clay there could actually be a column of terraced gravel, or another material, penetrating to significantly greater depth.

The following image shows a cross section of the anomaly discovered during the Blackwall Tunnel project.

Some of these anomalies have been found to be more than 90m in depth and have relatively small surface areas. This is potentially a problem because if you insert a large load bearing column designed to use skin friction from the London clay, Lambeth group or Thanet sand, and it actually sits within a deep column of terraced gravel, the performance of the pile could reduce significantly. An Engineer in the audience stated that if you double the Pour Water Pressure the bearing capacity of the pile roughly halves. Clearly permeability in the terraced gravel is considerably greater than the clay.

This issue is even more of a problem because you could conceptually have a Pingo anomaly positioned between site bore holes. This means that you might be unaware of the issue on site until the point the pile is being installed. This is obviously too late in the day simply because by the point piles are being installed the design of the building and its foundations should have been finalised.

As I left the lecture it occurred to me this might make a decent thesis topic for a phase 2 student currently struggling for ideas. It looks like the Geologist lot are working hard to collect data on this issue. Combining this data with a detailed look at the risks a Pingo presents from a civil engineering perspective might prove interesting.

Laurie’s added site product from Phase 2