The GIS approach to evaporite-karst geohazards in Great Britain

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Environ Geol
DOI 10.1007/s00254-007-0724-8
ORIGINAL ARTICLE
The GIS approach to evaporite-karst geohazards
in Great Britain
A. H. Cooper
Received: 5 June 2006 / Accepted: 6 March 2007
Ó British Geological Survey 2007
Abstract Evaporite karst in Great Britain has formed in
Permian and Triassic gypsum, and in Triassic salt. Active
dissolution of these deposits can occur on a human rather
than a geological timescale causing subsidence and building damage. The British Geological Survey has taken two
approaches towards understanding and advising on hazards
caused by dissolution of these soluble rocks. At a detailed
level, a national database and GIS of karstic features is
being populated. Information gathered includes dolines,
springs, stream sinks, caves and building damage. At a
national level, the soluble rocks in Great Britain have been
identified and digital-map polygon information relating to
them was extracted from the British 1:50,000-scale digital
geological map. These areas have been assessed, and in
places their margins extended to include some overlying
rocks where subsidence features are known to penetrate
upwards through the overlying sequence. The national
areas have then been assessed using detailed local information to assign a susceptibility rating from A (extremely
low) to E (high), depending on the nature and regularity of
the subsidence events that occur. This national zonation of
the soluble rocks can be used for planning, construction
and in the insurance businesses. This has proved useful for
assessing the potential stability of linear routes, such as
roads and pipelines or for other important structures such
as bridges and buildings. The information can also be used
to delineate zones of karstic groundwater flow.
Introduction
Engineering problems, such as subsidence and irregular
rockhead developed over soluble (karstic) rocks, pose difficulties for planning and development and can be very
expensive for the construction and insurance industries. In
the extreme they can cause properties to collapse and put
lives at risk. The carbonate rocks (mainly limestone and
chalk) are well known for their karstic development;
however, karst in gypsum and salt is less well known.
These rock types dissolve faster and are much more soluble, allowing karst to develop very quickly in them. To
understand the problems associated with soluble rocks in
Great Britain, the British Geological Survey (BGS) is
constructing a database of karst features. This has been
utilized in conjunction with digital geological map and
scientific information to generate a karst hazard susceptibility map of Great Britain. The map and karst database are
important for understanding the severity of the problem
and constitute useful tools for hazard avoidance that have
relevance to planning, engineering, development and the
insurance industry. Developers, planners and local government can only operate effectively if they have advance
warning about the hazards that may be present and have
access to relevant geological information. The British
Geological Survey is the main national supplier of this
geological and geohazard data.
Keywords Evaporite Karst Subsidence GIS Hazard assessment
Evaporite karst in Great Britain
A. H. Cooper (&)
British Geological Survey,
Keyworth, Nottingham NG12 5GG, UK
e-mail: [email protected]
Because evaporite rocks are highly soluble, areas underlain
by them in Great Britain tend to form low ground, which is
often extensively covered with superficial deposits. The
evaporites are not often seen at outcrop, but can be mapped
123
Environ Geol
from borehole data and may be inferred from the sinkholes
or dolines that develop across the outcrops and on the
overlying strata. The main evaporite deposits at and near
outcrop in the Great Britain include Permian gypsum,
Triassic gypsum and Triassic salt sequences (Fig. 1). They
all dissolve to varying degrees depending on the local
geological and hydrogeological situation. Gypsum also
occurs in some Jurassic rocks in southern Britain, where
Fig. 1 Distribution of the main evaporite karst sequences at outcrop
in England
Fig. 2 Cross-section through the typical Permian gypsum sequence
at Ripon, North Yorkshire. The Dolomite at the base of the sequence
is the Cadeby Formation, which is overlain by gypsum and mudstone
of the Edlington Formation, dolomite and dolomitic limestone of the
Brotherton Formation, gypsum and mudstone of the Roxby Forma-
123
some evidence of dissolution and tectonic brecciation exists in the form of brecciated strata known as the Broken
Beds (West 1964), but no evidence of modern dissolution
or subsidence has been noted.
Permian gypsum karst
In northeast England, karst developed in Permian gypsum
occurs in a belt of about 3 km wide and 100 km long
stretching from just north of Doncaster in the south to
Hartlepool in the north. The Permian sequence (Fig. 2)
comprises two thick units of gypsum underlain by dolomite
aquifers. The gypsum is heavily karstified especially in
places where the major rivers and buried valleys have cut
through the Permian sequence producing major pathways
for the escape of groundwater from the bedrock into the
fluvial system. By comparison with known phreatic gypsum cave systems (such as those in the Ukraine, Klimchouk et al. 1997) and from the pattern of subsidence, it is
inferred that there are phreatic cave systems in the gypsum
caused by the allogenic recharge from the adjacent ground,
particularly the dip slopes of the dolomite aquifers and the
overlying sandstone aquifer, into the major valleys. The
rapid solubility rate of the gypsum means that the karst is
evolving on a human time scale and active subsidence
occurs in many places, especially around the town of Ripon
(Cooper 1986, 1989, 1998; Cooper and Calow 1998). The
active nature of the dissolution and the ongoing subsidence
features here cause difficult ground conditions for planning
and development (Thomson et al. 1996; Paukštys et al.
1997; Cooper 1998) and for road and bridge construction
(Cooper and Saunders 2002; Jones and Cooper 2005). In
this area water abstraction can aggravate the problem and
lead to enhanced dissolution and collapse (Cooper 1988).
Gypsum karst is also present in the Permian rocks of the
tion. The Permian sequence is capped by the arenaceous Sherwood
Sandstone Group of Triassic age. The sequence is cut into by the
buried valley of the River Ure and perforated by breccia pipes caused
by collapse following gypsum dissolution
Environ Geol
Fig. 3 Cross-section through Triassic gypsiferous strata of the
Cropwell Bishop Formation (Mercia Mudstone Group), south of
Derby. The gypsum caps the hill and is partially dissolved both to the
south and north of where it has been mined; the quantity of
dissolution has limited the extracted area. To the north of this there is
a zone of greater dissolution, approximately at the present water table
and then down-dip from this the dissolution decreases and the amount
of gypsum increases again
Vale of Eden (Ryder and Cooper 1993), but here it is less
extensive as the gypsum is sandwiched within a mudstone
sequence, which restricts the passage of water through the
gypsum.
difficult ground conditions for road construction, south of
Derby (Cooper and Saunders 2002).
Triassic salt karst
Triassic gypsum karst
Gypsum karst is present in the Triassic strata, but the effects are much less severe than those in the Permian rocks.
The difference is mainly caused by the thickness of Triassic gypsum (typically less than 5 m) and the fact it is
interbedded mainly with weakly permeable mudstone sequences (Fig. 3). In places subsidence does occur with
sinkholes largely triggered by the infiltration of surface
water carrying down fine material into subsurface cavities.
Leakage of water from installations, such as power generation stations, has been reputed to have aggravated dissolution and caused subsidence (Seedhouse and Sanders
1993). The presence of gypsum karst has also produced
Salt near surface in Great Britain occurs mainly in the
Triassic strata of central and north-western England. The
towns on the Triassic salt strata commonly have ‘‘wich’’ or
‘‘wych’’ in their names, a term derived from the old
English word for a salt spring. These names indicate that
the towns are sited on former salt springs, which emanated
from the actively dissolving salt karst (Cooper 2002).
Starting with the exploitation of natural brine, these saline
spring sites later became the focus for shallow mining and
near-surface brine extraction (Fig. 4). The method used
was to sink wells or drill boreholes to intersect the near
surface ‘‘brine runs’’, a technique that was called ‘‘wild’’
brine extraction and which exacerbated the salt karstification (Arup Geotechnics 1990; Calvert 1915; Collins 1971).
Fig. 4 Cross-section though
Triassic salt deposits in
Cheshire. At wet rockhead there
is a zone of intense dissolution
and collapse where the salt is
overlain by brecciated and
collapsed strata
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Environ Geol
The exploitation of ‘‘wild’’ brine has resulted in near-linear belts of subsidence trending towards the abstraction
point and partly controlled by the geological structure.
Most extraction of natural brine has ceased, and modern
exploitation is mainly in dry mines or by deep controlled
brine extraction leaving brine-filled cavities. Since the
cessation of natural brine pumping, the saline ground water
levels have returned to their pre-pumping state. Brine
springs are being re-established and natural karstification
and subsidence might occur although heavily influenced by
the man-made brine runs.
The karst database and GIS
It has been recognized for some time that the availability of
baseline data is essential for the assessment of geological
hazards. Guidance for the development of unstable land is
written into British Government planning policy in the
‘‘Planning Policy Guidance note 14: Development on
unstable land’’ (Department of the Environment 1990), and
the supplementary ‘‘Annex 2’’ (Department of Transport,
Local Government and the Regions 2002). To underpin this
policy, rudimentary baseline data were collected in an
initial database of natural cavities commissioned by the
Department of the Environment and produced by Applied
Geology Limited (1993). This study showed the national
distribution of karst and other natural cavities, but did not
include all the details that were available and some of the
spatial recording was not very accurate. Consequently, in
2000, the British Geological Survey embarked on con-
Table 1 Datafields gathered
for dolines or sinkholes
Sinkholes: record item
structing a more comprehensive Geographic Information
System (GIS) and database of karst information (Cooper
et al. 2001). Over the past 6 years this system has been
populated and karst features for most of the evaporite areas
have been added. In addition, karst features for about onequarter of the limestone area and one-half of the chalk karst
area in the country have also been included in the database,
the population of which is ongoing.
Information gathered during fieldwork is either recorded
digitally on portable tablet computers or on proforma field
data sheets that have the same data fields as the GIS and its
underlying database. Data are gathered either in the field or
from existing datasets such as scanned and georegistered
copies of the geologists field maps, historical and modern
georegistered Ordnance Survey maps, papers and historical
documents. The information is added directly into the GIS
and five categories of data are collected: dolines or sinkholes, springs, stream sinks, caves and building damage.
The data are entered into the GIS using the British
Geological Survey desktop data capture methodology, the
‘‘Geological Spatial Database’’ (GSD) system, developed
by Keith Adlam. Initially this system used ArcView3
(Cooper et al. 2001), but has now been migrated to run on
ArcGIS9. The data are stored in ArcGIS format on central
servers, but the point information and database tables are
also copied to centralized Oracle databases to allow compatibility with the main BGS datasets. In common with all
BGS databases, the information added to the system has
common header data including National Grid co-ordinates,
date entered, user ID and reliability (this is not shown in
Tables 1, 2, 3, 4, 5).
Parameters
Sinkhole Name
Free text
Size
Size x, Size y, Size z, meters.
Type
Compound, collapse, suffusion, solution, no data, buried
Shape
Round, oval, irregular, modified, compound, no data
Surface profile
Pipe, cone, inverted cone, saucer, complex, levelled (filled), no data
Infill deposits
British Geological Survey rock and stratigraphical codes with thicknesses
Subsidence type
Gradual, episodic, instantaneous, no data
Evidence of quarrying
Primary data source
Yes, no, no data
Field mapping, air-photo, site-investigation, database, maps and surveys,
literature, Lidar remote sensing, DoE database, no data
Reliability
Good, probable, poor, no data
Property damage
Yes, no, no data
Oldest recorded subsidence
dd/mm/yyyy
Intermediate subsidence events dd/mm/yyyy
Most recent subsidence
123
dd/mm/yyyy
Other data
Free text
References
Free text
Environ Geol
Table 2 Datafields gathered
for springs
Springs; record item
Parameters
Spring name
Free text
Elevation
Meters
Situation
Open surface, borehole, concealed, submerged, submarine,
underground inlet, no data
Yes, no
Proven dye trace
Flow
Table 3 Datafields gathered
for stream sinks
Ephemeral, fluctuating, constant, flood overflow, ebbing and flowing,
no data
Water type
Normal/fresh, saline, sulfate, tufaceous, other mineral, no data
Size
Trickle, small stream, medium stream, large stream, small river,
medium river, large river, no data
Primary data source
Field mapping, air-photo, site-investigation, database, maps and surveys,
literature, Lidar remote sensing, DoE database, no data
Artesian
Yes, no, no data
Thermal
Yes, no, no data
Karstic
Yes, no, no data
Uses
None, public, agricultural, industrial, other, no data
Character
Single discrete, multiple discrete, diffuse, no data
Reliability
Good, probable, poor, no data
Estimated discharge
Liters per second (l s–1)
Other data
Free text
References
Free text
Stream sinks; record item
Parameters
Sink name
Free text
Elevation
Meters
Proven dye traces
Yes, no, no data
Morphology
Discrete compound, discrete single sink, diffuse sink, losing stream,
ponded sink, cave entrance, concealed sink, no data
Flow
Perennial, intermittent, ephemeral (flood), Estavelle, no data
Size
Trickle, small stream, medium stream, large stream, small river,
medium river, large river, no data
Primary data source
Field mapping, air-photo, site-investigation, database, maps and surveys,
literature, Lidar remote sensing, DoE database, no data
Reliability
Good, probable, poor, no data
Estimated discharge
Liters per second (l s–1)
Other data
References
Free text
Free text
For dolines and sinkholes, the data can be gathered
either as a point for a small collapse, or depending on the
scale, as a polygon for more extensive areas. Once a point
or polygon is captured, the GSD presents a drop down list
of information to be populated. The details gathered are
listed in Tables 1 (Dolines or sinkholes), 2 (springs), 3
(stream sinks), 4 (caves) and 5 (building damage). The size
of springs and stream sinks are recorded, but it is generally
subjective and weather dependent on the time of year and
recent rainfall. Furthermore, for the majority of historical
information gathered from published maps and geologist
field maps, no precise description of spring or stream sink
flow is given. Information gathered for caves is also collected as either point data for cave entrances, or if it is
known, as linear data for the approximate centre lines of
the caves themselves. The functionality is there in the
software to include full cave plans, but commonly these
have copyright restrictions and cannot be included. Many
of the doline and sinkhole affected areas also suffer from
building damage and damage to infrastructure.
The GIS allows building damage to be recorded and has
the functionality to database up to three separate inspec-
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Table 4 Datafields gathered
for natural cavities
Natural cavities, record item
Cavity name
Free text
Length
Meters
Vertical range
Meters
Elevation
Meters
Type
Open cave natural, infilled cave natural, gull cave, lava tube, boulder,
peat cave, sea cave, stoping cavity, palaeokarst, hydrothermal,
borehole cavity, no data
Rock units penetrated
(bedrock and superficial)
British Geological Survey rock and stratigraphical codes
Primary data source
Field mapping, air-photo, site-investigation, database, maps and surveys,
literature, Lidar remote sensing, DoE database, no data
Streamway
Yes, no, no data
Other entrance
Yes, no, no data
Evidence of mining
Yes, no, no data
Reliability
Good, probable, poor, no data
Other data
Free text
References
Free text
Table 5 Datafields gathered for property damage
Property damage,
record item
Parameters
Address
Free text
Postcode
Postcode format
Elevation
Meters
Damage survey 1
Date (dd/mm/yyyy), notes, damage rating (1–7)
Damage survey 2
Date (dd/mm/yyyy), notes, damage rating (1–7)
Damage survey 3
Date (dd/mm/yyyy), notes, damage rating (1–7)
Suspected cause
Reliability
Natural subsidence, mining subsidence, landslip,
compressible fill, building defect
Good, probable, poor, no data
Other data
Free text
References
Free text
tions allowing multi-temporal analysis of the data. The
proforma and GIS allow information on suspected cause
and reliability of the data source to be included. The
methodology and dataset are also applicable to mining and
landslip subsidence, and the recording scheme has been
designed to cope with information from those sources. The
building damage classification has seven classes. The first
five classes are based on the National Coal Board (NCB
1975) Subsidence Engineers Handbook classification. This
has been extended to include partial collapse (Category 6)
and total collapse (Category 7). In addition to damage to
buildings, the scheme has information relevant to the
recording of damage to roads, pavements and land
(Table 6). The recording of building damage using the
original five categories of the NCB scheme has been
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Parameters
successfully applied to Ripon in Great Britain (Griffin
1986; McNerney 2000) and to Calatayud in Spain
(Gutiérrez and Cooper 2002).
To understand the karst of Great Britain and to make a
dataset that can be used for the assessment of karst geohazards, the British Geological Survey has utilized this
detailed karst information to constrain the GeoSure dissolution dataset.
The GeoSure dissolution dataset
Over the past decade, the British Geological Survey has
invested considerable resources in the production of digital
geological map data for the UK. Digital geological maps
are available for most of the country (except for a small
part of Wales) at a scale of 1:50,000 with the entire country
covered at the 1:250,000 and 1:625,000 scales; in addition,
for a significant part of the country 1:10,000 scale digital
coverage is available. All these datasets include the bedrock and the 625,000, 50,000 and 10,000 scale datasets also
include data for the superficial deposits. 1:50,000 and
1:10,000 scale digital data are also available for artificial
deposits and mass movement (mainly landslip) deposits.
The coverage of digital data is listed on the Internet on the
BGS Internet site http://www.bgs.ac.uk/ under the theme of
‘‘GeoIndex’’ which is a web-based GIS index of all the
major BGS datasets.
In the digital geological map dataset, every polygon of
digital geological data is attributed with a two-part seed
(LEX-ROCK) that gives its lithostratigraphy and its
lithology. All the lithostratigraphical (LEX) codes are
Environ Geol
Table 6 Classification of building damage for karst and other subsidence recording
Damage category
Description of typical building damage
Description of associated damage to roads and land
0
Hairline cracking, widths to 0.1 mm. Not visible from
outside
Not visible
1
Fine cracks, generally restricted to internal wall
finishes; cracks rarely visible in external brickwork.
Typical crack widths up to 1 mm. Generally not
visible from outside
Not visible
2
Cracks not necessarily visible externally, some
external repointing may be required. Doors and
windows may stick slightly, typical crack widths
up to 5 mm. Difficult to record from outside
Generally not visible
3
Cracks which can be patched by a builder.
Repointing of external brickwork and possibly a
small quantity of brickwork to be replaced. Doors and
windows sticking, slight tilts to walls, service pipes
may fracture. Typical crack widths are 5–15 mm or
several of say 3 mm. Visible from the outside
Slight depression in open ground or
highway, noticeable to vehicle users,
but may not be obvious to casual
observers. Repairs generally
superficial, but may involve limited
local pavement reconstruction
4
Extensive damage that requires breaking-out and
replacing including sections of walls and especially
over doors and windows. Windows and door frames
distorted, floors sloping noticeably. Walls leaning or
bulging noticeably; some loss of bearing of beams,
some distortion of structure. Service pipes disrupted.
Typical crack widths 15–25 mm, but depends on
number of cracks. Noticeable from outside
Significant depressions, often
accompanied by cracking in open
ground or highway. Obvious to the
casual observer. Small open hole may
form. Repairs to the highway generally
require excavation and reconstruction
of the road pavement
5
Structural damage which requires a major repair job,
involving partial or complete rebuilding. Beams
loose, bearing walls lean badly and require shoring.
Windows broken with distortion. Danger of
instability. Typical crack widths are greater than
25 mm, but it depends on the number of cracks.
Very obvious from outside
Rotation or slewing of the ground or
significant depression, often accompanied
by cracking. In open ground or highway;
open crater formed with large void.
General disruption of services in highways.
Significant repair required
6
Partial collapse
Collapse of ground or highway,
significant open void, services severed
or severely disrupted
7
Total collapse
Large open void or landslip scar
listed on the Internet at http://www.bgs.ac.uk/lexicon/
lexicon_intro.html where they can be actively searched by
name or code; many of the entries include extended
information describing the units and their type localities.
The lithological codes (ROCK) are also explained and
listed on the Internet at http://www.bgs.ac.uk/bgsrcs/
home.html and can be searched by name or code. The
1:50,000 scale digital geological map dataset is now being
developed in its third edition.
The availability of digital map data linked to GIS software has opened new doors for the interrogation and utilisation of geological data in the UK. The British Geological
Survey has produced new digital products for geological
hazards, which it markets under the name of GeoSure
(http://www.bgs.ac.uk/products/geosure/home.html). Several derived datasets have been produced using a variety of
algorithms to provide geohazard data for soluble rocks
(dissolution); landslides (slope instability); compressible
ground; collapsible rocks; shrink–swell deposits and running sand. The methodology that underlies the construction
of the dissolution dataset is described here.
Identification of the evaporite and overlying
collapse-affected formations
The first step in generating the soluble rock geohazard
layer in the GeoSure dataset was to identify all the rocks in
Great Britain, which contain a significant quantity of
evaporites and which are susceptible to dissolution and
sinkhole development. Basically all these formations included a substantial amount of gypsum and salt at or near
outcrop. These were obtained from the digital 1:50,000
scale bedrock data, supplemented in a few places by
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1:250,000 scale data. A search of all the lithological codes
for evaporite rocks attached to the digital geological map
polygons generated the first listing. Secondly a similar
search was done for any formations and groups that were
known to include evaporite rocks, but were a lesser constituent and thus not shown by the main lithological code.
These selections were then displayed in the GIS (Fig. 5a)
and compared with the known distribution of karstic features from the Department of the Environment Natural
Cavities Database (Applied Geology Limited 1993) and the
BGS karst database (Fig. 5b).
Identification of marginal areas
From the superimposition of the map polygon information
with the karst database information (Fig. 5b) and by
incorporating previous local knowledge of groundwatercontrolling features, such as buried valleys (Fig. 5c), it was
possible to pinpoint areas of interstratal karst. It was also
possible to identify several formations that are not karstic
themselves, but which are affected by karstic subsidence
emanating from the underlying evaporite sequences. For
example in the Ripon area, the Permian sequence (Table 7)
from bottom to top comprises dolomite of the Cadeby
Formation, gypsum and mudstone of the Edlington Formation, dolomite of the Brotherton Formation and gypsum
and mudstone of the Roxby Formation. The sequence dips
gently eastwards (Fig. 2) and is capped by the Triassic
Sherwood Sandstone Group. The Edlington and Roxby
formations include significant thicknesses of gypsum (up to
40 and 10 m thick, respectively) which is locally heavily
dissolved and karstified, but the Brotherton Formation and
the lower part of the Sherwood Sandstone Group are also
both affected by severe subsidence due to the dissolution of
the underlying gypsum. The whole of the Brotherton Formation can be affected by subsidence emanating from
gypsum dissolution, but only the western part (from a few
hundred meters to a kilometer or so) of the Sherwood
Sandstone is affected. Although both the Cadeby and
Brotherton Formations are dolomite, they are only slightly
affected by karstification of this rock.
To utilize this knowledge and to generate the GeoSure
dissolution dataset for the Permian rocks in north-east
England, it was necessary to combine the polygons for the
Edlington Formation, the Brotherton Formation, the Roxby
Formation and part of the Sherwood Sandstone Group that
was affected. This generated a merged polygon for all the
rock that was susceptible to subsidence (Fig. 5d), but it
gave no indication of the severity of the collapses that have
occurred or may occur in that area.
Zonation of the karst-collapse prone areas
Fig. 5 These figures show the way the national dissolution dataset is
built from digital map data and the karst database information
combined with local geological knowledge to construct the national
zonation detailing the dissolution susceptibility for gypsum and salt
123
Using the detailed BGS karst database and the National
Cavities Database (Applied Geology Limited 1993) the
severity of the dissolution hazards was assessed and related
to the local bedrock and superficial geology. This allowed
the subsidence prone areas with good information to be
geologically characterized and zoned (Fig. 5d). This
assessment was then used to generate the rankings
(Tables 7, 8), which relate to the degree to which future
problems may locally occur. The extension of this ranking
into areas where the database of subsidence events is patchy (due to variability in the information) is slightly
Environ Geol
Table 7 Geological sequence
and karstic features that affect
formations in the Ripon area
Unit
Lithology and karstic features
Quaternary
Numerous glacial and
post-glacial deposits
Glacial till with sand and gravel, glaciolacustrine clays and silts,
river terrace deposits and alluvial areas. The deposits range up to
40 m thick and are affected by karstic collapse emanating from the
underlying Permian sequence
Triassic
Sherwood Sandstone
Group
Sandstone up to 300 m thick with the most western part locally
affected by subsidence features emanating from the underlying
Permian sequence. The Sherwood Sandstone Group is the major
aquifer in the area
Permian
Roxby Formation
Calcareous mudstone with up to 10 m of heavily dissolved massive
gypsum and abundant collapse features
Brotherton Formation
Dolomite and dolomitic limestone with sparse dissolution features,
but abundant collapse features
Edlington Formation
Calcareous mudstone with up to 40 m of heavily dissolved massive
gypsum and abundant collapse features
Cadeby Formation
Dolomite with sparse dissolution features and very few caves.
The rock is a significant aquifer resting unconformably on
the underlying Carboniferous strata
Carboniferous
Numerous formations
Table 8 Parameters used to
define the hazard ranking for
gypsum dissolution prone areas
Ranking
Sandstones and mudstones
Details
A—extremely low Areas where gypsum is present, but the thickness of deposits is known to be thin,
where the adjacent rocks are not aquifers and there is no recorded subsidence.
Mainly the Triassic Mercia Mudstone Group where fibrous gypsum has been
recorded
B—very low
Areas where gypsum is present in substantial thicknesses, but where the adjacent
rocks are not aquifers and where there is no recorded subsidence. Mainly the
Triassic Mercia Mudstone Group where thick gypsum is present
C—low
Areas where gypsum is present in substantial thicknesses, where the adjacent rocks
may or may not be aquifers, but where there is no recorded subsidence. Mainly the
Triassic Mercia Mudstone Group where thick gypsum is present and some
karstification has occurred. Similarly, the majority of the Permian gypsum in the
Vale of Eden and some of the Permian gypsum of eastern England are also
included
D—moderate
Areas where gypsum is present in substantial thicknesses, where the adjacent rocks
are aquifers and where there is some recorded subsidence. Mainly the Permian
gypsum of eastern England, including areas peripheral to Ripon, Darlington,
Tadcaster, Church Fenton etc
E—high
Areas where gypsum is present in substantial thicknesses, where the adjacent rocks
are aquifers, where buried valleys cut through the sequence and where there are
numerous records of ongoing subsidence. Mainly the Permian gypsum of eastern
England including south of Darlington, Ripon, and near Brotherton
subjective, but it does allow national geohazard coverage
based on the geological parameters to be generated. The
fivefold subdivision is used and this is an internal British
Geological Survey standard for assessing geological
hazards; similar ratings of severity have been applied
to landslips, compressible ground, collapsible ground,
running sand and shrink–swell clays. For gypsum, five
subdivisions were compiled with Ripon in North Yorkshire
taken as the worst-case scenario and areas where soluble
rocks exist, but where little or no known subsidence has
occurred taken as the least severe case; for the gypsum
sequences the zonation is shown in Table 8. The geological
parameters for the salt sequences are different (Table 9),
but generate the same categories with subsidence
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Table 9 Parameters used to
define the hazard ranking for
salt dissolution prone areas
Ranking
Details
A—extremely low
Areas where salt is present, but the thickness of deposits is known to be thin and
covered with impervious material
B—very low
Areas where salt is present in substantial thicknesses, but where the deposits
are covered with impervious material
C—low
Areas where salt is present in substantial thicknesses and present at rockhead
(wet rockhead)
D—moderate
Areas where salt is present in substantial thicknesses, present at rockhead
(wet rockhead) and where salt springs are present in the area
E—high
Areas where salt is present in substantial thicknesses, present at rockhead
(wet rockhead) and where wild brining or nearby mining has occurred,
salt springs are present and there is some recorded subsidence in the vicinity;
mainly the Triassic salt of Cheshire and Worcestershire
geohazards rankings comparable to those used for the
gypsum sequences.
Although the datasets have been subdivided into five
categories (Tables 8, 9), the extremely low (A) and very
low categories (B) are not generally significant for most
uses. Consequently, for commercial and public use, only
the three higher ratings of low, moderate and high (C, D
and E) are used. http://www.bgs.ac.uk/products/geosure/
pdf/soluble.pdf. These are the subdivisions that are also
used on the interactive web GIS which explains these
hazards and which can be accessed through http://
www.bgs.ac.uk/britainbeneath/guide.html.
property will collapse, but it acts as a warning that the area
is susceptible to dissolution and may be prone to subsidence. The recommendation for house buyers in such areas
is that a full structural survey is undertaken and that the
surrounding properties and infrastructure are also examined
for damage. If some evidence of subsidence is found in the
immediate or surrounding area, further investigation is
recommended.
Urban and national planning and construction
The national dissolution dataset is available commercially
and has found uses in the insurance industry. Insurance
companies have used it to define problematical areas where
they wish to limit their exposure to risk or charge a slightly
increased premium to reflect the increased claims that
would occur in such areas. The availability of the GeoSure
datasets enables the insurance industry to correlate their
claims history with the likely geological causes.
The Local and National Government have a responsibility
to protect the public from foreseeable hazards. Development on unstable ground is covered by the Planning Policy
Guidance PPG 14 and its Annex 2 (Department of the
Environment 1990; Department of Transport, Local Government and the Regions 2002). Local Government
through their Local Development Plans have a responsibility to consider unstable ground in their local areas. In
some places, such as Ripon, they have had specific local
advice (Thomson et al. 1996; Paukštys et al. 1997), which
is now included in the local planning policy, but for most
of the country this has not been done. The national dissolution dataset and the detailed karst database provide the
baseline information from which the Local Government
can obtain an assessment of the local stability of their area.
House purchase
Linear route assessments—roads, pipelines, railways
For the house buyer, the recent Government initiative to
speed house sales transactions called for a ‘‘Homebuyers
information pack’’ which was to include information derived from this dataset; however, the scheme has been
cancelled. Third-party information providers and the British Geological Survey utilize the information and supply it
to the public as part of their environmental information
searches. The presence of a moderate or high dissolution
rating (class E or D) does not mean that any particular
Linear structures such as railways, roads and large airfields
are very susceptible to subsidence damage; even small
amounts of settlement can be disastrous for fast moving rail
traffic. Oil and gas pipelines are susceptible to subsidence
movements, which can cause them to be run at lower and
less economical pressures (Hucka et al. 1986). The GeoSure dataset and the karst dataset allow the rapid assessment of new routes and the likely stability and risk to
existing structures to be determined (Gibson et al. 2006).
Uses of the datasets
Insurance
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Environ Geol
Water abstraction and ingress
The national karst dissolution dataset helps to define areas in
gypsum karst where there is strong hydrogeological connectivity from the surface to the subsurface gypsum karst.
This connectivity largely takes place down breccia pipes,
collapsed areas and the bottoms of dolines. The connectivity
through the sequence is important for aquifer modeling and
aquifer protection. The karstic nature of the sequence and
the active dissolution of gypsum explain why the Sherwood
Sandstone, which is usually a very good aquifer, can contain
significant quantities of sulfate-rich water at its western
limit where it directly overlies mudstones that in turn overlie
gypsum. Similarly, the dolomites of the Cadeby Formation
may contain sulfate-rich water derived from the overlying
gypsum in the Edlington Formation. The mudstones in the
sequence do not act as an effective aquitard because they are
perforated by breccia pipes caused by gypsum dissolution,
and this fact must be considered when modeling the hydrogeology of the area. Areas of salt karst are not affected in
the same way because the presence of brines makes them
unattractive as aquifers. Water ingress also affects salt karst
less as the salt at wet rock head may be protected in places
by a layer of dense brine.
Waste disposal sites
Sinkholes in some places look like disused quarries and
have in the past been used as waste disposal sites; in the
east of Ripon, five holes were filled with domestic rubbish
during the 1960s or early 1970s. Because there is such a
good hydrogeological connectivity through the sinkholes
and into the underlying breccia pipes to the aquifer, sinkhole areas should be avoided for waste disposal. Any
leachate from these types of landfill can find its way very
rapidly to the springs that drain the karstic system. Where
landfill activities are unavoidable, consideration should be
given to ascertain the stability of the ground and to the
provision of hydrological barriers and membranes. The
karst database and the national dissolution dataset provide
some background information for studies looking into the
provision of waste disposal areas.
Site-specific enquiries and automated enquiries
Both the site-specific information contained in the karst
database and the national GeoSure dissolution dataset can
be tailored to allow automated reporting for geological
enquiries and studies. The British Geological Survey
GeoReports http://www.shop.bgs.ac.uk/georeports/ utilize
the GeoSure dissolution dataset to help provide background
information for the BGS enquiry system. Many parts of the
reports are automated, but except for the basic Ground
Stability Reports, the final interpretation and reporting is
currently done manually. It is possible to subdivide the
national dissolution dataset even further based on local
geology and subsidence history. It is planned to generate
paragraphs of locally specific text that will be attached to
each of these database subdivisions and automatically recalled to populate part of the local GeoReport. Further
details could also be added from the detailed karst database
with information such as the distance from a sinkhole and
the subsidence history of the sinkhole included. The generation of this type of automated reporting is the start of
building an expert system for geological reporting.
Conclusions
The combination of digital map information and detailed
karst database information has enabled the construction of
a national dataset detailing the susceptibility of evaporite
rocks (gypsum and salt) to dissolution problems. This
dataset allied with the detailed karst dataset is a powerful
tool for planning and hazard avoidance with the potential
for automated geological reporting of the problems.
Acknowledgments This work has benefited from discussions and
help from numerous colleagues, especially Professor Martin Culshaw,
Dr Francisco Gutiérrez, Keith Adlam, Al Forster, Dr Andy Gibson, Dr
Andy Farrant, Matt Harrison, Dave Bridge, Jenny Walsby, Rhonda
Newsham and Kathrine Linley. The karst database has been largely
populated by Dr Andy Farrant, Sarah Doran, Nathan Williams,
Amanda Richardson and the author. Published with permission of the
Executive Director, British Geological Survey (NERC).
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