St. Albans Sinkhole - Update

According to the BBC's web site today, the void which opened up at Fontmell Close at the beginning of the month has now been backfilled, but that geophysical surveys (gravity) below the surrounding roads and footpaths suggest that there is at least one other void in the same area.  A copy of the image showing the survey reuslts (corutesy Hertfordshire County Council via the BBC News Web Site) is shown at the bottom of this post.

It does also appear that the site is a former clay pit on which housing was later built.  It is suspected, as was the case in Reading (e.g. Field Road), that there has been mining of the Chalk below the clay pit and it is these mine workings that have collapsed leading to the formation of the void.

The next step is expected to be intrusive investigations to search for voids and back up the geophysical surveys. 

Backfilling the existing void apparently required 48 lorry loads of foamed concrete.  Sadly the residents closest to the now backfilled void have still not been able to return home due to ongoing safety concerns.

St. Albans Sink Hole

Overnight last night (30th September - 1st October 2015) a large sink hole has formed below part of Fontmell Close including a front garden in St. Albans.

(Photograph courtesy BBC News Web Site and hertfordshire Fire and Rescue Service)

Residents reported that the hole had substantially opened up to form a void 20 m wide and up to 10 m deep.  As a precaution 10 people have been evacuated and water, electricity and gas supplies have been switched off to 58 homes in Fontmell Close and Bridal Close.

There are no reports of injuries nor cars being swallowed up (as happened with a sink hole in Walter's Ash, near High Wycombe some while ago (February 2014).

Prior to last night apparently there had been a "small hole" which had been scheduled for backfilling.  It now looks like a more substantial engineering solution will be required.

The press have not stated a cause, but it is likely to be the result of a solution feature migrating up to the surface, given that St. Albans is underlain by the Chalk

The British Geological Survey (BGS) have not provided any information on their web site but may well do before too long now that the sink hole has been reported on the BBC web site.

Seminar 18 - Glacial Geology Part 2

Hot off the press, the recording for Seminar 18 which was delivered at lunchtime today.

Seminar 18 is the second of two on Glacial Geology during which I look at Glacial Deposits, some of the features they can form and some of the engineering issues which can occur when working in glacial deposits.

The first Seminar on Glacial Geology was Number 16.

Seminar 3 - Sedimentary Rocks

Finally after having to do a complete re-record Seminar 3 has been nailed and is now ready for publication.  Due to the number of slides and the resulting file size without unacceptable loss of quality I have had to split the presentation into two parts.  Part 2 naturally should be watched after Part 1.

As a reminder Seminar 3 covers the first of the three major rock groups, the Sedimentary Rocks.  An important group of rocks for those in the UK and in the UK construction business rocks from this group will generally be those which structures are either founded on or lie within.  This rule certainly applies to most of England and large parts of Scotland, Wales and Northern Ireland as well, plus our friends in the Republic of Ireland.

Part 1:

Part 1 covers what sedimentary rocks are, how geologists classify them, the processes of erosion, transport and deposition of sediment, sedimentary structures and the lithification and diagenesis.

Within Part 1 I am not convinced that my verbal description of how cross bedding forms is that clear so I have reproduced the decription from Wikipedia, which gives as good a description as I have come accross. 

  Cross-bedding is formed by the downstream migration of bedforms such as ripples or dunes in a flowing fluid (or under the action of wind in the air). The fluid flow causes sand grains to saltate (migrate or carried within the fluid close to the sediment surface) up the upstream ("stoss") side of the bedform and collect at the peak until the angle of repose is reached. At this point, the crest of granular material has grown too large and will be overcome by the force of the depositing fluid, falling down the downstream ("lee") side of the dune. Repeated avalanches will eventually form the sedimentary structure known as cross-bedding, with the structure dipping in the direction of the paleocurrent (reproduced under Wikimedia Creative Commons Share-Alike License).

I hope that this description together with the images in the set of slides for part 1 (Slide 27) will make how cross beddings form understandable.

and Part 2:

Part 2 looks at a number of common sedimentary rocks. 

The posting of Seminar 3 means I am now up to date with Seminars being delivered since I started this blog, although it is slightly out of sequence in terms of posting date.  The next Seminar will be No. 18, which will be the second of two on Glacial Geology (the first one was No. 16).  This will be a new one not delivered previously.  In terms of the re-runs, No. 7 is the next in sequence which will be the first of several on aspects of Palaeontology or the study of fossils.  As usual when the recording files are available they will be posted for the reader to view. 

Seminar 6 - Metamorphc Rocks

Still a problem with the recording file for Seminar 3, but there is now a recording for Seminar 6 which is on Metamorphic Rocks, the third and last of the three major rock groups.  The Seminar was delivered to a live audience and via Lync on 15th September but technology problems required a re-record!  I suspect I will be forced to do the same for Seminar 3 in due course.


Within Seminar 6 I talk about metamorphic processes and describe some common metamorphic rocks and textures.  I also look at metamorphic grades and terranes. 

Sharp eyed viewers will also notice that the template has changed, this is for corporate reasons in case I decide to post the recording on a company version of You Tube which is shortly due to be launched. 

Seminar 18, the second part of the talk on Glacial Geology is programmed for next week and follows on from Seminar 16.