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Our patio’s fallen into a giant hole in the ground!
Basildon Recorder 15 April 2010
SinkholesWhy so many some years?
By Frank Nicholson, Huyton U3A
Tropical Storm Agatha caused a sinkhole to open in Guatemala City May 2010.
San Antonio, north of Guatemala city, on February 23, 2007.
Three people were killed in the collapse, as twelve homes
were swallowed up.
“When a sinkhole gobbled up a large rental condo at a resort
complex in Clermont, Florida, this week, I wasn't particularly
surprised. Hundreds of sinkholes form every year in Florida.
They really are not that unusual.” 14 August 2013. CNN report.
Jeff Bush fell into a sinkhole in his bed 3 April 2013 in Seffner,
Florida. It was not possible to even recover his body. This
house and two others were later demolished.
Sinkhole near Foolow, Derbyshire, December 2013
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So what are sinkholes? How do they form? Where and when
do they occur?
Not simple because there are different causes, variations of
hydrology, materials, and result.
Originally “sinkhole” was applied to solution hollows
(especially, but not only, in limestone) where water literally
sank into the ground.
Variations in terms used locally. In Britain – “shake hole” or
“swallow hole” in Northern England. In the South West
“swallet” is used.
Academic literature commonly uses “doline” from a
Slovenian word.
Different types of doline (from Farrant & Cooper, B.G.S.)
A suffosion doline is where there are unlithified sediments
over limestone rock and the sediment is simply washed into
fissures etc in the limestone. [Suffusion is flow of water over a
surface. Suffosion seems to be a word indicating suffusion
that causes erosion].
Underground water flow. Some sinkholes are caused not
by dissolution of limestone, but the erosion of weak
unconsolidated material by flowing water. Loose material can
be removed by a process called ‘soil piping’, creating large
voids within the sediment.
One spectacular example of this type of collapse occurred in
May 2010 in Guatemala City. Cavities developed in weak,
unconsolidated, volcanic deposits due to a tropical storm. May 2010 Tropical Storm Agatha caused a sinkhole to open in
Guatemala City. Cavities developed, which then collapsed, creating a shaft approximately 100 m deep and 20 m wide.
Sinkhole formed in a house 19-7-2011 north of Guatemala City.Guatemala City, built on volcanic deposits, is especially prone to sinkholes, often blamed on a leaky sewer system or on heavy rain.
Fire truck in “sinkhole” caused by a burst water main –
Los Angeles Sept 2009.
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A car at the bottom of a sinkhole caused by a broken water line in Toledo, Ohio on July 3, 2013. [Note driver still in the car!]
Toledo, Ohio. Car driver Pamela Knox (60) being rescued by
a firefighter. 3 July 2013.
So underground water flow collapses can be natural
underground water flows or manmade (water or sewer pipe
breaks).
Collapse of artificial cavities also occurs. Mine shafts, wells
or other mined cavities may all collapse. Many disused mine
shafts and wells are covered in various ways when
abandoned, and the coverings may fail in the long term e.g.
brick arches or even in the short term if timber was used.
There are a range of terms used but “well collapse”, “mine
shaft collapse” etc are clear.
Water flow collapses and mine collapses are usually all
termed “sinkholes” by the public and reporters.
A denehole is an
underground structure
consisting of a number of
small chalk caves entered
by a vertical shaft.
Denehole found at Rainham
Kent. Another found nearby
was found during excavation
for house construction. It
was infilled before the house
was constructed.
There were many theories for
the origin of deneholes.
Maybe druids used them! The
Danes (Vikings) were also
suspected. Others associated
them with concealment of
religious persons. Some
were wells, others flint mines,
but most were to extract
chalk for farmers marling
fields. Denehole at Capstone Country Park, Kent.
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A family have been told to leave their home after the ground
outside it collapsed into a 25ft-wide (7m) hole. The cause is a
denehole. Grays, Essex, Dec 2012. Rainham Mark Grammar School: surface expression of
denehole collapse that occurred on 12 February 2014.
Heavy rain in December 2013 produced this sinkhole near the
village of Foolow, in the Peak District, Derbyshire, England.
This was reportedly caused when the Mill Dam Lead Mine
below caved-in, producing this 160ft wide hole 130ft deep.
The M2 sinkhole opened up in the central reservation. 15ft
deep hole closed the motorway for nearly 2 days in early 2014.
Believed to be collapse into old mine workings (denehole?).
The collapse being triggered by heavy rainfall.
Sinkhole in Mancunian Way 14 August 2015. 40 feet deep
chasm opened when a 100 year old sewer collapsed after
heavy rain, causing the sinkhole to form.
The repair to the sewer was particularly
difficult. The sewer drainage water had to be
bypassed whilst the old sewer was repaired.
A new 130m of 6ft sewer was needed,
including shafts to install it at depth. 10,400
tons of sandstone had to be dug out as part of
the work. Total cost £6 million.
The Mancunian Way was reopened in later
June 2016. (Collapsed 14 August 2015).
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Soluble rocks and their
distribution in Great Britain.
Soluble rocks cover quite a
lot of Britain, especially
England.
However, natural dolines
especially on the stronger
limestones cause relatively
few problems as sinkholes,
because they relatively
rarely collapse.
Slowly developing sinkholes
give warning and are often
avoided or remedied e.g. by
concrete grouting.
Dropout sinkhole near Cheddar (Carboniferous Limestone)
formed when loessic soil collapsed into an underlying cave
(GB Cave) after heavy rain in 1968. The hole was partly
filled with old cars, but continues to slump today.
Typical suffosion sinkhole developed in thick clays over
limestone, formed by clay gradually washing into a small
cave below. Brimble Pit Swallet, near Cheddar, Somerset.
A few British rocks are more soluble.
Particularly gypsum – especially around Ripon in North
Yorkshire.
Gypsum (calcium sulphide) is much more soluble than
limestone, so if subjected to an underground water flow then
cavities develop much more quickly.
Another, even more soluble rock is salt, but this is so soluble
that most of the salt very near the surface was dissolved
away long ago. Cheshire is where there is lots of salt in
Britain (also North Yorkshire and Carrickfergus, Northern
Ireland). Subsidence/sinkholes in Cheshire were especially
associated with poor salt extraction practices (brining).
Sinkhole over gypsum, Sutton Howgrave, North Yorkshire, c.
2001
Frequency of sinkhole collapse
Ripon periodically gets sinkholes; in the 1980s and 1990s,
one was appearing every two to three years. The last
recorded hole was on the flood plain of the River Ure, north of
Ripon, there have not been any reported in the last seven
years so the occurrence of one in winter 2013/2014 was not
unexpected.
The triggering mechanisms for these sinkholes to collapse
can be one, or a combination of mechanisms, the most
common being:
Enlargement of the caves due to subsurface solution,
followed by infiltration of water from the surface washing
down fine materials from the covering deposits.
Fluctuations in the groundwater levels rising to wet the
covering materials, and then falling, leaving the cover
materials saturated and without the hydraulic support offered
by the water.
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Collapse of four garages into subsidence hollow caused by
the dissolution of gypsum at Ripon, North Yorkshire, 1997.
Partial house collapseOn the evening of Monday 17 February 2014, Mr and Mrs Cunningham returned home to their house in Magdalen's Close, Ripon, to find they could not open their back door, they heard noises, looked up and saw that the back of the house was collapsing.
They called the emergency services who broke down the front door and rescued their dog before shutting off utilities to make the house safe from gas, electricity and water problems. Residents of neighbouring houses were also evacuated.
By the following morning, the rear third of the house was severely damaged and a crack widening from ground to roof was wrenching the house apart.
This crack in the house continued to widen to about 40 cm and the press gathered in anticipation of its total collapse.
Damage caused to 26 Magdelen’s Close, Ripon by sinkhole
that developed on Monday 17 February 2014. Another view of the damage to 26 Magdelen’s Close, 17 Feb.
August 2018 Sainsburys in Ripon closed for safety reasons
– possible sinkhole.
The sinkhole opened in an alley close to Sainsbury's
supermarket in Ripon on Tuesday 21st August morning.
Dropout sinkholes formed on Chalk due to a burst water main
at Fontwell, Sussex 1985.
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21 February 2014. Grave shaped “sinkholes” in a
graveyard in Gravesend. This is compaction of soil over
burials brought on by very wet soil.
British Geological Survey map of sinkholes that occurred in
Southern England in February 2014.
Florida has porous Mesozoic limestone. Often overlain by
sandy soils deposited as beach deposits.
Most modern sinkholes are in West-Central Florida, though
some sink holes are found over much of the state, even
where there is very little superficial material over the
limestone. Sinkholes are more of a problem where there are
clay layers in the sands overlying the limestone, making the
superficial deposits more cohesive, and more susceptible to
dropout dolines.
In a few areas of Florida over 200 feet of sediments cover the
underlying limestone. These sediments are cohesive
because of the clay and layers of limestone they contain.
Although there are not many sinkholes in these areas, the
ones that occur are deep and wide.
This aerial view shows a large sinkhole that claimed several
sports cars, a house, and the deep end of the city swimming
pool, in Winter Park, Florida, on May 11, 1981
Sinkhole destroying road near Deltona, Florida during a
hurricane in December 2004. It grew to 225ft wide and 50ft
deep.
San Diego California: After an underground pipe ruptured on
Feb. 23, 1998, a hole opened up severing 2 major roads.
These roads were closed for 5 months. (Hair raising escapes).
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Seattle 11-11-1957. Huge hole due to failure of 6ft diameter
sewer pipe. No homes damaged or any injuries, but the hole
took 2 years to fill and repair!
Workers look into a sinkhole caused by a broken water main in Chicago, Illinois, April 18, 2013. This sinkhole swallowed 3 cars and one person was injured.
Dealing with sinkholes (from BGS Web Site)
The hazards associated with sinkholes can be
mitigated by appropriate planning, good site
investigation (with geophysics and boreholes),
appropriate design and proper maintenance of
infrastructure such as drains and services.
Care is required when installing any structures that
could affect the local groundwater flow or
groundwater levels including soakaways and open
loop ground source heat pumps; in some places on
soluble rocks these may be impractical.
[continued]
Actions needed when a sinkhole appears:
• Ensure that areas and a safety perimeter are cordoned off
and keep people away.
• Notify the landowner and/ or emergency services as
appropriate.
• If services pipes, e.g. gas or water are left suspended
ensure that infrastructure managers are contacted, e.g.
Safety and emergencies (National Grid) and Emergencies
(United Utilities).
• Ensure that any triggering processes, e.g. leaking drains
are managed to minimise the potential for ongoing
subsidence.
• Contact your local council/building control to notify them;
find out if there is a history of such features in the area, and
request a list of consulting engineers that are able to give
appropriate advice on the correct procedures for stabilising
them.
The 2m-wide (7ft)
hole appeared at
about 1300 on 25
May 2019 in
Sheringham, Norfolk.
Anglian Water said a
sewer pipe was
"significantly
damaged" and it was
working "as fast as
possible" to fix the
pipe and repair the
hole.
Twelve properties in Ripon were evacuated after a 20ft (6m)
deep hole appeared in 2016
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Sinkholes formed 11 June 2019 caused closure of the
M25 near Sevenoaks for a period. Following an accident
two sinkholes were found in the central reservation.
Subsidence in Northwich. This has been caused by the mining
of salt in Cheshire. Photo probably about 100+ years ago.
In 1891, the rear of Castle Chambers in Castle Street ,Northwich, slipped spectacularly backwards – but thanks to its timber-framed construction, the building, including the glass windows, remained intact. Subsidence damage due to salt mining in Victorian times.
Flash near
Winsford,
Cheshire.
The result of salt
mining.
Salt extraction began in Roman times. Saline water from
natural springs was evaporated.
In 1779 a deeper bed of pure rock salt was found beneath
Northwich. This was mined at about 90m depth. Large pillars
were left supporting the roof, so there was no subsidence.
Natural drainage water entered the workings, and began to
dissolve the remaining salt. However the water soon became
saturated with salt, so there was only limited solution.
There has always been some limited subsidence due to salt
dissolving and timber frames have been used for houses in
areas where this was liable to occur.
When timber frames are used the houses can be jacked back
up or even moved bodily to a new location. Very attractive
Tudor appearance houses result.
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Distinctive timber-framed buildings in Northwich
The big problems began however, when there was more
competition, brining became common, and companies began
pumping in more water into the old mine workings and
pumping out the brine to produce salt. This so called
“bastard brining” (into old mines) lead to much subsidence.
One early 1900s resident, describing the effect of subsidence
on the buildings, explained: ‘They don’t go down suddenly;
it’s a slow process, and when we think they have gone far
enough we lift ‘em up again with hydraulic jacks.
I have heard of folks going to bed with saws so that if the
door would not open in the morning, they could saw
themselves out.’
The full extent of the problem in Northwich was only
uncovered in 1988, with plans for expansion of the town
centre northwards. A few test boreholes discovered that the
salt pillars of old mines beneath the town were unstable. All
major development in Northwich was immediately halted.
In the late 1990s studies of ground movements indicated a
major danger of further subsidence/collapse.
Although coal mines have government assistance, salt mines
were not covered. John Prescot, then Deputy Prime Minister,
was convinced of the need and the Land Stabilisation Fund
was extended to cover mining other than coal mining.
Four mines needed to be grouted to stabilise them – volume
780,000m3 (= over 1 million tons of cement grout).
The grout would need to be pumped 2.5 kilometres from a
central mixing plant. The mix chosen was 96% pfa
(pulverised fuel ash), 4% cement and a small amount of salt
to keep the grout fluid when pumped but allow it to set solid
when in place.
This work was carried out between January 2005 and
November 2007 at a cost of £32 million, but Middlewich is
now much safer.
The only currently working salt mine at Winsford is free of
any seepage and is used for secure document storage!
Brining is a major extraction method for chemical sodium
chloride, but only to limited volumes at 150-300m depth. The
chambers are left filled with saturated brine. The voids can be
used to store waste or chemicals for use e.g. ethylene, or if
suitable shapes and smaller sizes, natural gas.
Concrete is poured into a 15 feet wide sinkhole in Walters Ash,
High Wycomb, Bucks. 17 Feb 2014.Sinkhole 17 February 2014 in Hemel Hempstead, Herts.
Reported as 20 ft deep and 35ft wide – 17 houses evacuated.
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A construction vehicle lies where it was swallowed by a
sinkhole on Saint-Catherine Street in downtown Montreal,
August 5, 2013
Thank you for your
patient attention.
If anyone wants a pdf copy (pictures & notes) of this presentation, please
give me their email address OR send an email to:
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