Residential

Ever had that sinking feeling

Residential | 16/02/16   Chris Woodhouse

Our Director and Chartered Surveyor, Chris Woodhouse, shares his thoughts on sink holes.

Readers will probably remember the St Albans sink hole hitting the news in October 2015.

By “good fortune” I was asked to survey a property in St. Albans in mid-October so I took the opportunity of going to have a look.  I should add that the property I was looking at was over a mile from the site. 

Apparently what started off as a small hole in the pavement necessitating the area being cordoned off with yellow tape for safety purposes suddenly turned into a massive sink hole of some twenty metres diameter and ten metres depth literally overnight on the 1st of October 2015 – quite a scary thought!

For obvious reasons the area was fenced off so a really close look wasn’t possible but I could see enough.  It was almost perfectly round and the sides were roughly vertical with the mettled surface sitting some ten meters or so below where it should have been – quite a sight.

The cause of this phenomenon was as always water, thought to have been eroding compressed infill from an old clay excavation of the London Brick Company.  It is thought this excavation was in excess of 100 years of age. 

Sink holes are apparently quite common in minor form in Yorkshire and areas where limestone and chalk are prevalent generally known as ‘Karst’ areas.  Unrepaired water mains over a long period and unknown underground streams can be common causes of sinkholes occurring.  For some reason Norwich seems to be a favourite.  Who can forget the photograph of a double decker bus which had wedged itself at 45 degrees in a Norwich sink hole. 

Fortunately sink holes tend to occur in roads rather than directly beneath where houses are built which would suggest to me that vibration and service pipes are major contributory factors.  Very fortunately my understanding is that nobody has been killed in the UK as a result of a sink hole although I believe that this is not the case in the USA. 

A word of comfort – so far sink holes do not appear to have occurred in areas of boulder clay where there have been no major excavations so that tends to make this area a low risk one boulder clay being prevalent in the area of Essex.  However not far away from here there are areas of chalk where there is a greater risk. 

As always a detailed inspection of any property purchase by a chartered surveyor is prudent.  The photographs beneath may be of interest. 

Chris Woodhouse FRICS