Geotechnical Background for the City of
The city of
Glasgow is located western of the central lowlands (Midland Valley) in
Scotland. The Midland Valley lies between the highlands and the Southern
Uplands and comprises of low-relied terrain (as is shown in Fig1).
years ago, during the Carboniferous period, the Midland Valley formed an area
of deposition with the bedrock in the region: predominantly sedimentary strata
with volcanic rocks that are both extrusive and intrusive. Largely, the
sedimentary rock in the area is made up of sandstone, limestone, siltstone,
mudstone with both coal and ironstone present.
1: The Midland Valley
In the past 2
million years, some of the softer sedimentary strata has experienced heavy
erosion – largely caused by glacial erosion. Extrusive volcanic rocks largely
make up areas of high ground in the Midland Valley with igneous sills and dykes
common within the basin. Due to deposits of till by glaciers, much of Glasgow’s
landscape has thus been formed by hills sculptured by till –heavily affecting
the location and development of urban property being built in the city.
Subsurface Geology of the City of Glasgow
Glasgow is predominantly
underlain by superficial deposits formed by glacier and river deposition along
the boundaries of the Clyde estuary under varying sea-levels. The highly
variable nature of these sediments gives rise to a range of potential hazards
which can lead to “unforeseen ground conditions” for construction and
development projects. Running sand, compressible ground and shrink-swell clay
hazards affect various parts of the City and are particularly associated with
regions of alluvial and glaciofluvial deposits that are found in the River
Clyde and its estuary.
thickness of sediment occurs in the north of the City, associated with a buried
channel beneath the Kelvin River valley. The channel is defined by an irregular
depression in the rockhead surface, with the sediment thickness exceeding 80 m.
Sediments 20 – 40 m thick also occur along the line of the River Clyde and
occupy a broad valley likely. Elsewhere local variations in sediment thickness
over 1 – 10 m scales result from the presence of drumlins along with the
irregular nature of the rockhead surface. Due to the significant change in
properties from poorly consolidated deposits to resistant bedrock, knowledge of
the level of rockhead will be important when considering constructing a flood
defence scheme at White Cart River.
superficial deposits sequence, glaciofluvial sand and gravel deposits along the
Clyde corridor and lying through the centre of the City form the main shallow
aquifer. The groundwater level is typically shallow, at approximately 3 m
depth, meaning that the shallow aquifers are vulnerable to contamination from
soils and surface water sources. Groundwater flooding is also a hazard in some
areas of the city. Heavy metal contamination of soils as a result of historic
industry, buried chromium waste and chromium contamination of the River Clyde
are major potential sources of contamination of shallow ground water aquifers.
Coal, limestone and
ironstone were formerly mined within the City. Glasgow lies within an area
known as the Lanarkshire Coal Field, and coal mining began in the area over 300
years ago, with intensive mining taking place between the mid 1800’s and early
1900’s. The final coal mine in the city closed in 1966. Shallow mine workings (within 30 m of
rockhead) underlie many parts of the city. In some mines, the pillars were
removed at the ending of the mines, resulting in the mine collapsing but in
many mines the pillars and stalls were left intact when the mine was abandoned.
The failure of pillars and the resulting mine collapse is a major cause of
subsidence in areas of abandoned shallow mine workings within the City. Glasgow
City Council has undertaken grouting to stabilise some near-surface mine
workings in order to halt or prevent subsidence and mine collapse in many areas
of the City.
The BGS borehole
database holds over 40,000 records of boreholes drilled within the City of
Glasgow. The borehole information is derived largely from site investigations
and wells drilled by private companies and deposited voluntarily within the BGS
archive. The BGS
initiated a programme of subsurface modelling for Glasgow in 2002 – 2003. Pilot
3D models of the superficial deposits and bedrock were developed using borehole
and geological map data held in the BGS archives, combined with additional
information from seismic data and mine records. Bedrock surfaces including
stratigraphic unit bases, faults and areas of worked coal seams have been
modelled from borehole records, mine plans and seismic models of some areas of
disused coal workings.
stratification consists of the made ground underlain by stiff to hard over-consolidated
clay layers. From approximately 10 m depth rock strata consisting of weak to
strong siltstones and sandstones as well as layers of coal were encountered.