Geotechnical Running sand, compressible ground and shrink-swell clay hazards

Geotechnical
Report

 

 

Geotechnical Background for the City of
Glasgow

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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).

 

350 million
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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure
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.

The greatest
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 the proposed flood defence
scheme at White Cart River thus, further borehole information must be required.

 

Groundwater:

Within the 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. Contaminated soil due to heavy metal as a result of historic
industry, buried chromium waste and chromium contamination of the River Clyde
are all major possible sources of contamination for shallow ground water
aquifers.

 

The Mining industry and Deep-Earth Environment:

Coal, limestone and ironstone
were previously mined within the City. Glasgow is situated within an area known
as the Lanarkshire Coal Field, where the coal mining industry beginning over
300 years ago with further and more intensive mining between the mid 19th
century and early 20th century. The last coal mine in the city to
close was in 1966. Shallow mine
workings (within 30 m of rockhead) underlie many parts of the city.

In certain
mines, the pillars were removed at the closing of the mine, causing mines to
collapse 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.

All of the
City of Glasgow is regarded as a ‘development low risk area’ according to the ‘Glasgow
City Development Risk Map’ (Gov, 2017) with certain areas regarded as ‘development
high risk areas’. Two sites of concern relating to the proposed flood defence
scheme in this report are the areas of Linn Park and Pollok House. As can be
seen in FIG 2, probable
shallow coal mine workings and mine entries with potential zone of influences
are present in the Linn Park vicinity. A large area of Pollok House is also
regarded as a ‘development high risk area’ with coal outcrops, probable shallow
coal mine workings and mine entries with potential zone of influence all
present (as shown in Fig 3).

 

Key: Development high risk area: dangerous
mine areas at White Cart (box shaded), Coal
Outcrops (purple), Probable Shallow Coal Mine Workings (pink), Probable Shallow Coal Mine Workings (blue), Mine Entries with Potential Zone of
Influence (red)

As Linn Park and
Pollok House are both sites regarded as ‘Development High Risk Area’ and areas of
interest for Natural Flood Management (NFM) mitigation in the proposed scheme, a
coal mining assessment to the City of Glasgow Planning Authority will be
submitted to help support the planning application.

 

Subsurface
Information:

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.

Soil
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.