OR/17/006 Engineering geology (geotechnical data)

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Monaghan, A A, Dochartaigh, B O, Fordyce, F, Loveless, S, Entwisle, D, Quinn, M, Smith, K, Ellen, R, Arkley, S, Kearsey, T, Campbell, S D G, Fellgett, M, and Mosca, I. 2017. UKGEOS - Glasgow geothermal Energy Research Field Site (GGERFS): initial summary of the geological platform. British Geological Survey Open Report, OR/17/006.

This section provides an assessment of the geotechnical data for the different lithostratigraphical units that are present within a rectangular area enclosing the Clyde Gateway area (SW corner 258000, 660850, NE corner 265000, 665000) within the BGS National Geotechnical Properties Database (NGPD). Where there is little data available for a unit, a wider area was used. This focused primarily within the central Glasgow area (NS56SE, NS56NE, NS66SW and NS66NW, 255000, 660000 to 265000, 670000) as this is where most of the data are located. A summary of the engineering geological descriptions of the superficial deposits and bedrock unit lithologies are in Tables 20 and 21 respectively.

The superficial deposits and bedrock geological units considered here are:
Superficial deposits:

Clyde Valley Formation (Law Sand and Gravel Member); Gourock Formation;
Killearn Sand and Gravel Member; Paisley Clay Member;
Bridgeton Sand Member;
Broomhouse Sand and Gravel Formation (including clay facies);
Ross Sand Member;
Bellshill Clay Member; Wilderness Till Formation

Bedrock:

Scottish Upper Coal Measures Formation;
Scottish Middle Coal Measures Formation;
Scottish Lower Coal Measures Formation;
Passage Formation;
Upper Limestone Formation;
Limestone Coal Formation;
Lower Limestone Formation;
Western Midland Valley Westphalian to Early Permian Sills

The graphs presented in this section (below) and Appendix 2 provide the basic summary geotechnical data for the majority of the units that will be encountered by any potential boreholes in the Clyde Gateway area. There was little data for the Law Sand and Gravel Member, Lower Limestone Formation and no data for the Lawmuir or West Lothian Oil-Shale formations. Analysis of the anthropogenic deposits was not included as they are highly variable and site specific assessment is preferable. This could be undertaken once specific sites are delineated for the Clyde Gateway baseline and monitoring boreholes.

Table 20    Engineering geological classification of the Clyde Gateway area superficial deposits.
Engineering geological classification Engineering geological description Lithostratigraphical units
Organic deposits Highly compressible organic deposit Peat
Mixed fine and coarse-grained deposits Highly variable, very soft to very stiff, very loose to very dense, clay, silt, sand, gravel, cobble and or boulders of natural and man-made materials Anthropogenic deposits
Loose to medium dense SAND and GRAVEL or soft to firm (stiff) CLAY or SILT Gourock Formation Head
Firm to very stiff gravelly sandy CLAY with occasional cobbles and boulders or very dense SAND and GRAVEL or firm to stiff CLAY or SILT Wilderness Till Formation
Baillieston Till Formation
Fine deposits (Very soft) soft to firm laminated sometimes sandy SILT or CLAY with occasional gravel Lacustrine deposits
Strathkelvin Silt and Clay Member
Linwood Clay Member
Paisley Clay Member
Firm to stiff (very stiff) sometimes laminated CLAY/SILT Bellshill Clay Member
Broomhill Clay Formation
Coarse deposits Loose SAND, gravelly SAND, sandy GRAVEL or GRAVEL Law Sand and Gravel Member
Loose to dense (silty) SAND or SAND and GRAVEL Killearn Sand and Gravel Member
Ross Sand Member
Bridgeton Sand Member
Medium dense to very dense SAND/GRAVEL occasional boulders Broomhouse Sand and Gravel Formation
Broomhouse Sand and Gravel Formation (sand facies)
Cadder Sand and Gravel Formation
Sand and Gravel (Undifferentiated)
Table 21    Engineering geological descriptions of the Clyde Gateway area bedrock units and their lithologies.
Geological formation Rock type Engineering description
Western Midland Valley Westphalian to Permian Sills (WMVAS) Olivine dolerite and olivine microgabbro Medium strong to extremely strong, greenish grey or greyish black DOLERITE/BASALT sometimes close to widely spaced horizontal discontinuities
Upper Coal Measures Scotland Formation (UCMS) Sandstone Weak to medium strong, sometimes very strong, laminated to medium bedded, pinkish grey, reddish grey, greyish pink, sometimes purple mottled, fine to medium SANDSTONE
Siltstone Very weak to medium strong, laminated, purplish brown, red, greyish green, reddish grey SILTSTONE
Mudstone Very weak to weak (sometimes medium strong very occasionally strong (seatearth), laminated MUDSTONE
Middle Coal Measures Scotland Formation (MCMS) Sandstone Weak to very strong sometimes very weak or extremely strong, laminated to thickly bedded, white to greyish white fine to coarse SANDSTONE
Siltstone Very weak to medium strong, sometimes strong, laminated, grey to black SILTSTONE
Mudstone Extremely weak to medium strong, thinly laminate to medium bedded, grey to black MUDSTONE
Ironstone Strong to extremely strong, sometimes medium strong, grey or grey brown to pale yellowish brown IRONSTONE
Coal Extremely weak to medium strong, black COAL
Lower Coal Measures Scotland Formation (LCMS) Sandstone Very weak to very strong, white to grey, laminated to medium bedded, greyish white to grey, fine to medium SANDSTONE
Siltstone Weak to medium strong (sometime extremely weak to very weak or strong), laminated, grey to black SILTSTONE
Mudstone Extremely weak to medium strong, occasionally strong, thinly sometimes medium bedded, grey to dark grey MUDSTONE
Coal Extremely weak to very weak, black COAL
Passage Formation (PGP) Sandstone Medium strong to strong, sometimes very weak, white to grey fine to medium SANDSTONE
Siltstone Weak to medium strong, sometimes strong, light grey to grey SILTSTONE
Mudstone Extremely weak to medium strong, thinly laminated to thinly bedded, grey to dark grey, greyish brown, MUDSTONE
Coal Extremely weak to weak, black COAL
Limestone Medium strong to strong, grey or dark grey, sometimes greenish grey, sometimes with closely spaced vertical and subhorizontal sand filled joints, LIMESTONE
Upper Limestone Formation (ULGS) Sandstone Weak to very strong, greyish white to grey, sometime brownish grey, fine to medium SANDSTONE
Siltstone Weak to medium strong, thinly to thickly laminated to sometimes thickly bedded, grey SILTSTONE
Mudstone Very weak to medium strong occasionally strong, laminated to thickly bedded, sometimes jointed grey to dark grey MUDSTONE. Joints sometimes medium space and rough
Coal Extremely weak to very weak, sometimes fractured, black COAL
Limestone Medium strong to very strong, sometimes jointed, grey to dark grey LIMESTONE
Limestone Coal Formation (LSC) Sandstone Weak to very strong, (sometimes very weak or extremely strong), light grey to greyish white, fine to coarse SANDSTONE. Sometimes with calcite veins
Siltstone Very weak to strong occasionally very strong, laminated, thinly to thickly bedded grey SILTSTONE
Mudstone Extremely weak to medium strong, sometimes jointed, generally laminated, grey, dark grey or black, sometimes greyish brown MUDSTONE. Joints are smooth to rough
Coal Extremely weak to medium strong, or very strong when burnt
Seatearth Extremely weak to strong, grey, sometimes brecciated, SEATEARTH
Limestone Medium strong top very strong, grey, sometimes argillaceous, LIMESTONE
Ironstone Strong to extremely strong, sometimes medium strong, thickly laminated, pale brown, brown or black, sometimes colour banded IRONSTONE sometimes with mudstone band
Lower Limestone Formation Sandstone Medium strong to strong, thin to medium bedded, sometimes with thin siltstone or mudstone beds, pale to very pale grey or grey brown to yellow brown (weathered) SANDSTONE
Siltstone Medium strong to strong, sometimes with thin bedding, dark grey SILTSTONE
Mudstone Extremely weak to medium strong occasionally strong, sometimes widely jointed, sometimes laminated, grey to dark grey or black. MUDSTONE. Joints are generally undulating and smooth
Coal Extremely weak to very weak, fractured, black COAL
Limestone Strong to very strong, grey, generally argillaceous LIMESTONE
Ironstone Very strong, sometimes with slightly undulating to rough planar joints, light grey to grey fine IRONSTONE

Bedrock permeability[edit]

One of the parameters of great interest for Clyde Gateway area is permeability, or, as measured in ground investigations, hydraulic conductivity. There are 31 values for the Clackmannan Group (Passage to Lower Limestone formations) and Scottish Coal Measures Group within the Clyde Gateway area, and a total of 34 values. All the values are shown in Figure 53 for the different lithologies and geological formations.

Figure 53    In situ permeability values vs depth for the Clyde Gateway area.

Porosity[edit]

Other parameters that might be used to indicate permeability include porosity, pore size, pore connectivity and the characteristics of mechanical discontinuities, for example joints. The laboratory porosity test measures connected pores (effective porosity). The particle size of the rock is indicative of pore size, in decreasing pore size sandstone > siltstone > mudstone. However, there is little bedrock porosity data available but porosity can be related to dry density via the particle density.

e = (rs/rd) – 1                    1
n = e/(1+e)                        2

Where e is voids ratio, and the relationship between porosity, rs is the density of the particles, rd is the dry density and n is porosity. The dry density vs depth for each of the bedrock units are in Figure 54 to 58.

Figure 54    Scottish Upper Coal Measures Formation, the dry density at depth below ground level.
Figure 55    Scottish Middle Coal Measures Formation, the dry density at depth below ground level.
Figure 56    Scottish Lower Coal Measures Formation, the dry density at depth below ground level.
Figure 57    Passage Formation, the dry density at depth below ground level.
Figure 58    Upper Limestone Formation, the dry density at depth below ground level.

The dry density vs porosity graph for all bedrock lithologies is in Figure 59.

Figure 59    Porosity vs dry density for all bedrock units.

There are insufficient data to calculate the relationship for siltstone or for limestone but the sandstones are assessed as in Figure 60. The regression line has an equation of:

ρd = 0.0318n + 2.7446                    3
r2 of 0.9717 for 26 values

This equates to:

n = -30.542ρd +84.288                    4

Equation 3 indicates a particle density (that is at zero porosity) of about 2.7446 Mg/m3, which is higher than expected and as m = 0.0318 and the value of r2 is not 1, this suggests that there is some variation in the value of particle density. However, equation 4 is used to calculate the value of porosity for sandstones only as shown in Figure 61.

Figure 60    The porosity vs dry density for the sandstones with a linear regression line.
Figure 61    Dry density vs calculated porosity for sandstone.

Figure 62 shows the porosity (mostly calculated) with depth for each bedrock unit.

Figure 62   &nbspDepth vs porosity (mostly calculated) vs depth.

Discontinuities[edit]

Mechanical discontinuities identified in borehole core, which might increase rock mass permeability, are indicated in ground engineering in the description and by the fracture discontinuity state as fracture index, FI, (discontinuities per metre) or fracture spacing, If (mm). In the dataset for this area, fracture index is used more commonly. Rock quality designation (RQD), which is used in engineering rock mass assessment, can also be used. However, these data are all affected by the interpretation of the discontinuities by the logger as only natural discontinuities as in the ground should be included. Commonly, ‘man-made’ breaks, particularly along bedding planes, are included, generally, in error, and, although these can removed from the descriptions, breaks along bedding planes cannot be removed from the fracture state data. Fracture index values of 50 (20 mm or gravel sized) are given for material that is not intact, given as 99 in the database. The fracture index is plotted against depth for each unit in Figure 63 to Figure 67.

Figure 63   &nbspScottish Upper Coal Measures Formation, depth vs fracture index.
Figure 64   &nbspScottish Middle Coal Measures Formation, depth vs fracture index.
Figure 65   &nbspScottish Lower Coal Measures Formation, depth vs fracture index.
Figure 66   &nbspPassage Formation, depth vs fracture index.
Figure 67   &nbspUpper Limestone Formation, depth vs fracture index.

Further graphs of geotechnical data are given in Appendix 2.