OR/12/032 Geotechnical properties
|Hobbs, P R N, Entwisle, D C, Northmore, K J, Sumbler, M G, Jones, L D, Kemp, S, Self, S, Barron, M, and Meakin, J L. 2012. Engineering Geology of British rocks and soils - Lias Group. British Geological Survey, Nottingham, UK. (OR/12/032).|
Data sub-division and presentation
The geotechnical properties of the Lias Group, are described in this section. Geotechnical data were primarily obtained from external commercial site investigation reports, and supplemented by a small number of tests carried out at the BGS laboratories. The acquired data was entered into the BGS National Geotechnical Properties Database, where it continues to be maintained and managed to QA standards. The data represent the results of tests carried out in accordance with contemporary British Standard procedures (e.g. British Standards BS5930: 1981, 1999; and BS1377: 1990).
It is stressed that the geotechnical values quoted in this report should be used as a general guide only and not as a substitute for adequate site investigation, or in detailed design calculations.
The data acquired relate mainly to the clay-rich lithologies of the Lias Group. This is due to the fact that these lithologies are the subject of the majority of ground investigations, as they cover the greatest area and have the potential for causing the greatest engineering problems (See Geohazards). Relatively few data exist for the limestone and sandstone lithologies and have been reported where appropriate. Data have been included only where they have been considered reliable and capable of lithostratigraphic attribution. Where less than 5 data points exist in the database for a particular formation/parameter combination, these data have not been included in the statistical analysis. Robust statistics have been used throughout. That is, they are portrayed in terms of ‘medians’ and ‘percentiles’, rather than ‘means’ and ‘standard deviations’ (Hallam, 1990). This is explained in greater detail in the text boxes below.
A basic statistical analysis of the data is presented in the form of tables. A variety of plots are also used to display the data. These include ‘extended box & whisker’, ‘scatter’, ‘line’, and ‘ternary’ plots and are used to show distribution and trends of various key geotechnical parameters.
|The extended box and whisker plot (Hallam, 1990) is a method of summarising a frequency distribution based on the robust median and quartiles. Unlike mean and standard deviation, it portrays all the data. It shows the percentiles as a central box icon (refer to Appendix B) the ends of which are the 25th and 75th percentiles (or quartiles), and the centre-line of which is the 50th percentile (or median), and the latter as values with the median shown in bold. Values lying above or below the box are shown as subsidiary boxes, representing percentiles above the 75th and below the 25th. The height of the box is proportional to the square root of the number of data. The median is the value which has 50 % of data above it and 50 % below, and in a normal distribution has the same value as the mean. Several extended box plots may be compared by placing them on the same scale.|
|Scatter plots show the direct relationship between two parameters, measured on the same sample, one scale on each axis. Each parameter pair is represented by a point (or locus) on the plot. The bubble plot is a form of scatter plot where the size of the point is proportional to the number of data having common values. For example, if there are 5 samples with identical loci, the point on the graph will be proportionately larger than one representing a single sample. This gives a clearer indication of data concentration than a simple scatter plot in which coincident points are simply overwritten.|
The extended box & whisker plots and summary statistical tables are presented in (Appendix B - Extended box and whisker plots (by Formation), Appendix C - Extended box and whisker plots (by Area)) and (Appendix D1 - Weathering classes). Parameter vs depth plots (depth profiles) in relation to weathering class and Formation are presented in Appendix D3 - Depth profiles (by weathering class and Formation). Other plots to show data trends are given in the text as appropriate.
Selected geotechnical parameters are plotted against depth or against one another (Figures 7.1 to 7.23), in order to determine variations caused by depth, and other related factors, and to characterise likely engineering behaviour at different depths. Weathering may be related in a general sense to depth below ground level, but this is not necessarily a simple relationship of decreasing weathering with increasing depth in the case of the Lias Group. Mechanical properties, such as water content, density, permeability, and strength may also relate to depth. A large proportion of the database was assembled from transport project site investigations (e.g. road and rail), supplied to the BGS by consulting and contracting engineers, and by government and local authorities. Many of these have been carried out on embankments or in cuttings, for example where existing routes are to be widened. This means that ground levels reported in the site investigation are not necessarily the original ground levels. Plots of parameters with depth should therefore be treated with some caution as they only give a general trend.
Data in the statistical tables and plots are subdivided according to a location code (Database Areas 1 to 6), nominally based on depositional basins and lithostratigraphy (Formations and Members). In the case of some Areas and Formations few data are available, and may not be shown. The Areas do not have any significance outside the database, and are not represented by uniformly distributed data. The samples from any general location identified should not necessarily be considered as representative of that area as a whole. The Area codes (Figure 1.1) represent the following general locations:
|2||East Midlands Shelf (North)|
|3||East Midlands Shelf (South)|
|4||Worcester Basin & adjoining shelf areas|
The great majority of data is taken from site investigation reports supplied to BGS by commercial consulting and contracting engineering companies. These data have been vetted to some extent, using the criteria of reliability, clarity, test methodology, traceability, and stratigraphical provenance. A significant proportion of site investigation reports have been supplied in the Association of Geotechnical Specialists (AGS) digital format, which is reflected in the structure and nomenclature of the National Geotechnical Properties Database.
Geotechnical data reported in this section of the report are derived in the main from routine laboratory testing using either British Standards, e.g. British Standards: BS1377, (1990); BS5930 (1999); Head, (1992, 1998), or recommended British or American procedures, e.g. International Society for Rock Mechanics, ISRM (1981). In general, research data are not included unless stated otherwise. Geotechnical tests on soils and rocks may be broadly sub-divided into ‘index’ and ‘mechanical’ property tests. The term ‘index’ implies a simple, rapid test, the equipment and procedure for which are recognised worldwide (e.g. liquid limit) and which can be repeated in any competent geotechnical laboratory; or a test which measures a fundamental physical property of the material (e.g. particle density). A mechanical property test may measure the behaviour of the material under certain imposed conditions (e.g. a triaxial strength test), and be more complex and time consuming. If conditions are changed the result of the test will be different. Mechanical property tests tend to require carefully prepared ‘undisturbed’ specimens. Index tests, often carried out on ‘disturbed’ or ‘bulk’ samples, tend to be used to generally characterise a deposit and to plan further testing, whereas mechanical property tests may be used for design calculations. For mechanical properties where little or no data are available (e.g. shrink/swell, permeability, durability), index tests are often used as a guide if relationships have been established elsewhere.
Density and water content
The density of a soil is the amount of material within a given volume. It may be expressed in units of (mass) density, ρ (g/cc = Mg/m3) or unit weight, γ (kN/m3); the ratio of the two being the acceleration due to gravity (g ≈ 9.807 m/s2). Density determines the self-weight and hence the load applied by the soil to underlying strata. The density of a soil in its natural state, or any other non-specific state, is called the bulk density, ρ, and is defined as follows:
The voids within this soil may contain air or water, or both in various proportions. This fact leads to the definition of the quantities: dry density, ρd (density after oven drying at 105°C), and saturated density, ρsat (density with all voids filled with water) which may be considered as a specific value of bulk density. It should be noted that the density of a soil is usually measured by preparing a test specimen of known volume (in a ring or cylinder) and weighing it. This means that the specimen is relatively small and intact, and does not necessarily represent the bulk of the soil mass, which will have a lower density due to the presence of discontinuities.
The water content, w of a soil or rock is defined as follows:
The water content is usually expressed as a percentage. From the above it will be seen that water content can exceed 100%. Note: in some scientific and technological fields (other than geotechnical) water content may be defined as the weight of water divided by the total weight (i.e. solids and water) rather than the weight of solids; in which case water content cannot exceed 100%. The water content (or moisture content) of a soil in-situ is normally referred to as its ‘natural water content’. If this is not the case the condition of the soil should be specified (e.g. remoulded, compacted etc.). The water content determined in the laboratory may be slightly lower than the true natural water content as some water may be lost during transport and storage. The natural water content itself may fluctuate seasonally, so that it is rarely a fixed quantity, except at considerable depth.
The database contains 3644 values of bulk densities. The density and natural water contents of the Lias Group are affected principally by lithology, but also by cementation, and other structural features. Considerable lateral and vertical variations on a scale of metres and centimetres are seen. These changes also affect strength. Bulk density (r or BD) for undisturbed samples is recorded in units of Mg/m3. Values of bulk density range from 1.14 to 2.72 Mg/m3. The median values for the Formations are similar, lying in the range 1.99 to 2.05 Mg/m3. Analysis of all the data shows a relatively small but perceptible overall decrease in bulk density southward. Some higher values within the Blue Lias Formation may be due to the presence of ironstones.
The dry density, ρd (or dry unit weight, gd) is the density of the oven-dried soil, i.e. with no ‘free’ water contained in the voids. Dry densities are fewer in number than bulk density; there being only 1505 data for which the overall median dry density is 1.66 Mg/m3. The Scunthorpe and Whitby Mudstone Formations have median dry densities greater than 1.70 Mg/m3, whereas the Bridport Sand, Blue Lias, Dyrham, and Charmouth Mudstone Formations all have median dry densities less than 1.65 Mg/m3. As with bulk density, the data show a relatively small but perceptible overall decrease in dry density southward. Particle density (specific gravity) medians range from 2.62 to 2.72 Mg/m3; the overall median being 2.66 Mg/m3 (number of data results, n=263).
There are 12 464 natural water content results in the database, for which the overall median value is 21%. The Redcar and Scunthorpe Mudstones are the only formations with median water contents below 20% (18 and 17%, respectively). The remainder lie between 21 and 23%, with the exception of ‘undifferentiated’ Lower Lias (Blue Lias and Charmouth Mudstone) which has a median water content of 31%. The reason for this is unclear. There is a general trend of increasing water content from north to south, although the overall difference, based on analysis of all data, is relatively small. Variation in water content is largely a consequence of the differences in lithology between the clay rich basin areas and the more granular platform deposits. Cripps and Taylor (1987) point out that due to variation in the lithologies the ranges are wide and reduced overburden and weathering action typically accounts for a 60 to 100% increase in moisture content in depth profiles.
Plots of water content with depth (Figures 7.1) and water content vs. bulk density (Figure 7.2) show considerable scatter with no clear overall trend. However, it is possible to see that the upper bound on water content values decreases with depth, reaching a constant value of around 12% below a depth of about 50 m. The greatest variation is to be found near-surface where weathering and seasonal effects operate. There are no clear distinctions in the relationships between water content and bulk density for formations or areas, so these have not been shown.
Particle size data for soils are usually carried out by combining the results of sieving and sedimentation analyses (British Standards: BS1377: Part 2: 1990; Head, 1992). The coarse fraction (>0.060 mm) is determined by dry or wet sieving, and the fine fraction (<0.060 mm) by sedimentation using the hydrometer or pipette methods (automated indirect methods may also be used such as the Sedigraphä or the Coulter Counterä). The results are usually shown as triangular (or ternary) plots with clay/silt/sand+gravel fractions or clay+silt/sand/gravel as the three axes, and also as grading curves, which have ‘percentage passing’ (0 to 100%) on the Y-axis and particle size as a log scale (0.002 to 60 mm) on the X-axis (see Table 7.1 for size ranges and definitions).
|clay size||<0.002 mm|
|silt size||0.002 to 0.06 mm|
|fine silt size||0.002 to 0.006 mm|
|medium silt size||0.006 to 0.02 mm|
|coarse silt size||0.02 to 0.06 mm|
|sand size||0.06 to 2.0 mm||fine sand size||0.06 to 0.20 mm|
|medium sand size||0.2 to 0.6 mm|
|coarse sand size||0.6 to 2.0 mm|
|gravel size||2 to 60 mm||fine gravel size||2.0 to 6.0 mm|
|medium gravel size||6 to 20 mm|
|coarse gravel size||20 to 60 mm|
A total of 671 particle-size tests were selected for the database, of which 383 have full particle-size distributions shown as grading curves. Ternary (triangular) plots of selected test results (Figures 7.3 and 7.4) show the distributions for the main formations. Three of the main mudstone formations (Charmouth, Scunthorpe, and Whitby) show similar trends of high ‘silt’ and ‘clay’ particle concentrations, the silt content predominating slightly, and low ‘sand+gravel’ contents (particularly so for the Whitby Mudstone Formation). The Dyrham Formation has a distribution similar in pattern to those of the main mudstone formations, but with more silt and less clay. The Bridport Sand Formation shows a reasonable scatter between the ‘silt’ and ‘sand+gravel’ apices, with a clay content consistently less than 30%.
The Blue Lias, Scunthorpe, Whitby, and Charmouth Formations have similar median clay contents of 42, 41, 40, and 37%, respectively. Median silt contents for the same formations are 35, 48, 52, and 48%, respectively. The Bridport Sand and Dyrham Formations have median clay and silt contents of 15 & 51% and 22 & 64%, respectively. The Bridport Sand Formation, whilst having the highest sand content (28%), does have much higher silt content (51%) and therefore the lowest clay content. Selected median gradings are shown in Table 7.2.
|Formation||n||Clay %||Silt %||Sand %||Gravel %|
|Bridport Sand F||53||15||51||28||0|
|Blue Lias F||9||42||35||10||2|
|Charmouth Mudstone F||159||37||48||5||0|
|Scunthorpe Mudstone F||263||41||48||19||0|
|Whitby Mudstone F||46||40||52||3||0|
The grading medians for the different depositional areas (defined in Figure 1.1) are presented in Table 7.3 and show an overall decrease in clay content from 42% (Area 2) to 23% (Area 5). Areas 3 and 5 show the highest silt contents, 58% and 59%, respectively. Lowest silt contents occur in Areas 2 and 4, 48% and 45%, respectively. It should be noted, however, that Areas 1 and 6 are not represented in the database and the results are affected by the relative proportions of data from each formation within each area. For example, a major proportion of Area 5 consists of Bridport Sand Formation and Dyrham Formation data.
|Area||n||Clay %||Silt %||Sand %||Gravel %|
Plasticity is a property of clay soils which is largely dependent on clay mineralogy and particle size. If a clay contains enough water to form a slurry it behaves as a viscous fluid (liquid state). If the clay begins to dry out it reaches a point where it is capable of withstanding a shear stress (plastic state). On further water loss the clay becomes stronger and brittle (semi-solid state). The water content above which the clay changes from a plastic to liquid state is defined by the liquid limit, wL (or LL); the water content below which the clay changes from a plastic to semi-solid state is defined by the plastic limit, wp (or PL). The range of water content over which the clay soil behaviour is plastic (and often ‘sticky’) is usually expressed in terms of the plasticity index, Ip (or PI), defined as follows:
The liquid and plastic limits (Atterberg, or consistency limits) are universally recognised empirical values on the water content scale, as is shrinkage limit (Section 5.2.11). In addition, the liquidity index, IL is defined as follows:
The results of liquid and plastic limit tests, for individual formations, are shown in Figure 7.5 as Casagrande, or A-line, plasticity plots (liquid limit, wL vs. plasticity index, IP).
A total of 5930 combined (i.e. for all formations) liquid and plastic limit results are held in BGS National Geotechnical Properties Database. The overall medians for liquid and plastic limit are 52% and 24%, respectively, giving a median plasticity index of 28%. Values for liquid limit range from 20 to 134%. Medians for the individual formations range from 41% to 61% (liquid limit) and 22% to 24% (plastic limit).
As shown in Table 7.4 the range of plastic limit medians is very small (two percentage points). The variation in plasticity index, IP, is thus due principally to variations in liquid limit.
|Formation||n||wL %||wP %||IP %||Plasticity Class|
|Bridport Sand Fm||55||41||23||18||Intermediate|
|Blue Lias Fm||380||51||23||28||High|
|Charmouth Mudstone Fm||3182||52||24||28||High|
|Redcar Mudstone Fm||36||51||23||28||High|
|Scunthorpe Mudstone Fm||439||46||22||24||Intermediate|
|Whitby Mudstone Fm||1013||53||24||29||High|
The ‘best-fit’ linear formulae derived from the formations’ Casagrande plasticity plots are shown in Table 7.5.
|Blue Lias F||380||IP = 0.72wL–8.51||0.82|
|Charmouth Mudstone F||3182||IP = 0.76wL–11.06||0.84|
|Dyrham F||586||IP = 0.84wL–6.21||0.85|
|Redcar Mudstone F||36||IP = 0.87wL–16.24||0.86|
|Scunthorpe Mudstone F||439||IP = 0.70wL–7.76||0.87|
|Whitby Mudstone F||1013||IP = 0.77wL–12.07||0.77|
For comparison, the equation of the Casagrande A-line is:
Thus, all the formations have a best-fit line with a steeper gradient than the A-line, with the exception of the Scunthorpe Mudstone Formation.
The range of plasticity values within the Lias Group, and within its Formations, is high (Figure 7.5). Data are distributed from the ‘low’ to the ‘extremely high’ categories, although the great majority are contained within the ‘intermediate’ and ‘high’ categories. The Bridport Sand and Scunthorpe Mudstone Formations, and to a lesser extent the Redcar Mudstone and Dyrham Formations, tend to have lower plasticities than the remaining formations. However, the differences are not great. Statistically, the Scunthorpe Mudstone Formation has the lowest plasticity of the ‘mudstone’ formations. Wide variations in numbers of samples per formation make direct comparison difficult in some cases. Of the well-represented areas (Areas 2 to 5) (Figure 1.1), Area 2 stands out as having a trend of lower plasticity than the remainder, though the differences are small. Areas 1 and 6 have very few data points. The ‘best-fit’ lines for the Casagrande plots are generally similar. The gradients of the best-fit lines (Table 7.6) increase from Area 2 to Area 5, the former being almost parallel with the A-line (0.72).
|2||IP = 0.74wL–9.87||0.85|
|3||IP = 0.75wL–11.35||0.84|
|4||IP = 0.755wL–10.42||0.81|
|5||IP = 0.81wL–13.33||0.90|
Liquidity index, IL is a ratio (defined above) which allows the locus of the in-situ condition of a soil to be placed in its consistency range (that is in relation to the Atterberg limits. A value of zero indicates that the natural water content equals the plastic limit, while a value of +1.00 indicates that the natural water content equals the liquid limit. Values of liquidity index may be used as a guide to desiccation or, where equilibrium water content is established, the remoulded strength of a soil. There are 5523 liquidity indices in the Lias Group database, for which the overall median is -0.06. The median liquidity index values for the various formations range from zero to -0.22, the lowest being for the Scunthorpe and Redcar Mudstone Formations (Table 7.7). This reflects the overall north to south trend (from Areas 2 to 5) of increasing plasticity, decreasing density, decreasing lithification and decreasing over-consolidation (Table 7.8) (see Mineralogy).
|Formation||LI (median)||Ac (median)|
|Bridport Sand F||0.00||0.88|
|Blue Lias F||0.00||-|
|Charmouth Mudstone F||-0.06||0.73|
|Redcar Mudstone F||-0.19||-|
|Scunthorpe Mudstone F||-0.22||0.55|
|Whitby Mudstone F||-0.09||0.70|
|Area||LI (median)||Ac (median)|
Activity is a parameter intended to describe the contribution of clay minerals, within the clay fraction of the soil, to shrink/swell and other engineering behaviour. Activity, Ac, was defined (Skempton, 1953) as follows:
A soil with a relatively low clay-size fraction, but where that fraction consists mainly of the very ‘active’ clay mineral sodium smectite will, in theory at least, produce a high Activity whereas a soil with a high clay-size fraction consisting mainly of illite will not. The Lias Group formations and areas are not uniformly represented in terms of amount of Activity data. The Activity data summarised in Tables 7.7 and 7.8, and shown in Figure 7.6, are widely scattered. Median values range from 0.55 to 1.00. The highest values are likely to be unreliable as a result of incorrect particle-size preparation. For example, a few values over 5.00 were obtained for the Dyrham and Scunthorpe Mudstone Formations. The Dyrham and Bridport Sand Formations give high Activity values, possibly due to their smectite content. It is probably the case that these sand- and silt-rich formations are not comparable directly with the clay-rich formations in terms of Activity, and it is doubtful whether Activity should be applied to clays with low clay-size content (see Mineralogy), in common with parts of the Charmouth Mudstone Formation in the south. The presence of the clay mineral smectite would tend to increase activity significantly. The data contained in other BGS databases (Hobbs et al., 1998) suggest that silt-sized particles (i.e. >0.002 mm) can result in measurable liquid and plastic limits, and hence plasticity indices (for such materials it is theoretically possible for an Activity value of infinity to be obtained).
Using the Activity classification shown in Table 7.9 the Lias Group data are scattered across all categories (Figure 7.6). However, the median values are confined to the categories ‘inactive’ and ‘normal’. From a total of about 500 test results less than 5% fall in the ‘very active’ class and almost 70% within the ‘inactive’ class. It is interesting to compare the results for the Dyrham and Scunthorpe Mudstone Formations. The Dyrham Formation has double the Activity of the Scunthorpe Mudstone Formation, apparently solely as a result of the former’s clay content being half that of the latter (plasticity indices are identical). There appears to be an overall increase in Activity from north to south (Areas 2 to 5), possibly due to the increased presence of more active clay minerals, i.e. smectite, in the Lias of southern England, though comparison of individual areas is not useful due to non-uniformity of data. It has been noted (refer to Chapter 3; Kemp & McKervey, 2001; Kemp et al., 2005) that significant proportions of the clay mineral smectite, normally associated with high plasticity and high activity, are also accompanied in Southern England and the West Midlands by significant proportions of carbonate. This carbonate (in the form of calcite and dolomite, and probably in part authigenic and fine-grained) tends to ‘dilute’ the effect of the higher smectite in the southern England and West Midlands deposits. This may also explain the relatively low values for Activity and the poor distinction in terms of plasticity and activity between the Lias of southern and Northern England.
Sulphate, pH, and other chemical tests
A small group of relatively simple chemical tests for soils is usually included in geotechnical testing. These are: total sulphate content, aqueous extract sulphate content, pH, carbonate content, and organic content (BS1377:1990; Head, 1992). In addition, there is a test for the sulphate content of ground water used in modern chemistry laboratories.
As noted in some detail in Sulphate, groundwater and pore-water containing sulphate can attack concrete and other materials containing cement. A reaction takes place between the sulphate and aluminium compounds in the cement, causing crystallisation of complex compounds. This process causes expansion and internal damage. A former classification for sulphate in soils is given by the Building Research Establishment, BRE, (Building Research Establishment, 1981; 1991) as shown in Table 7.10. This would have informed many of the test regimes carried out prior to 1996 and hence a significant proportion of the Lias Group sulphate database. The more recent BRE classification (Building Research Establishment, 1996) is slightly more complex and requires assessment of total sulphate followed, if above the threshold for class A, the aqueous sulphate test to decide on the appropriate cement type.
Excessive acidity or alkalinity of the ground can also have detrimental effects on concrete below ground level and even moderate acidity can corrode metals. Some soil stabilisation agents may be unsuited to alkaline conditions; pH also affects the solubility of some ions.
Results from five ‘chemical’ laboratory test parameters are contained in the database: total (soil) sulphate, aqueous extract (soil) sulphate, sulphate in ground water, pH, and organic content.
Total sulphate (T.Sul.) is the acid-soluble sulphate content, whilst aqueous 2:1 water/soil extract sulphate (A.Sul.) is the water-soluble sulphate content. Both are obtained from liquid extracts but give the content of the soil itself rather than of the ground water, and are expressed as a percentage by weight (T.Sul.) and as grams per litre (A.Sul.). Unfortunately, it is often the case that sulphate content data are quoted as below detection level. This makes statistical assessment of the raw data difficult if these data are included. If they are not included, then the dataset is unrepresentative and may be biased. However, the use of classes for cement type does not have this problem as the ‘below detection level’ data will all be class 1.
A summary of the sulphate results for the main Lias Group Formations is shown in Table 7.11. The highest total sulphate content median was obtained for the Scunthorpe Mudstone Formation and the lowest for the Blue Lias and Dyrham Formations. Fewer data are available for the aqueous extract sulphate content and the sulphate content of groundwater. A plot of Total vs. Aqueous sulphate is shown in Figure 7.7. Using the 1996 BRE scheme (Building Research Establishment, 1996), sixty-five percent of the Total sulphate data lie below 0.24%, placing them in BRE’s Class 1. Of the remainder, the majority are Class 2. Correlation of sulphate content parameters with depth is generally poor. However, no results for Total sulphate in excess of 1.0% (BRE Sulphate Class 4) are recorded below 10 m.
|Formation||T.Sul. (%) median||A.Sul. (g/l) median||W.Sul. (%) median|
|Bridport Sand F||-|
|Blue Lias F||0.06||0.34||0.17|
|Charmouth Mudstone F||0.19||1.06||0.20|
|Redcar Mudstone F||0.22*||-||-|
|Scunthorpe Mudstone F||0.38||1.23||0.10|
|Whitby Mudstone F||0.18||0.71*||0.14|
* less than 10 data points
As noted above, excessive acidity or alkalinity of the groundwater in soils can have a highly detrimental effect on buried metals and reinforced concrete. Acidity and alkalinity is quantified in terms of the pH value. This is a measure of the ‘active’ acidity rather than the acid content and is a logarithmic scale. The test is usually carried out in tandem with the sulphate content test (British Standards: BS1377:1990; Head, 1992). Results of pH tests in the database show that all medians for Formations lie between 7.3 and 7.9, i.e. slightly alkaline. The Scunthorpe Mudstone and the Blue Lias Formations have the highest pH medians, while the Dyrham and Whitby Mudstone have the lowest. Of the Areas, 3 and 4 have the highest and 5 the lowest pH medians, though the variability is slight.
There are two BS tests dealing with organic matter in the soil. The first is the 'Walkley and Black' method, which determines the percentage by dry mass of organic matter within a soil by dichromate oxidation and titration (British Standards: BS1377: Part 3:1990, Test 3). The second determines the mass loss on ignition, LOI, as a percentage of the dry mass of the soil (British Standards: BS1377: Part 3:1990, Test 4). The LOI is related to organic content, but not for all soil types. Organic content data are poorly represented in the database. A total of 28 samples, most from Area 2, gave a range of 0.01 to 3.5% with an overall median of 2.0%. The highest values for organic content were obtained for the Charmouth Mudstone Formation.
The strength of a soil or rock is a measure of its capability to withstand a stress (or stresses) in a particular direction or configuration. Strength is not a fundamental property of a soil or rock, but is dependent on the soil/rock condition and the type of stresses applied to it. The strength of soils is particularly sensitive to the drainage conditions and duration of the test. If drainage is allowed the test is capable of measuring effective strength parameters. If the conditions are undrained the test is assumed to measure total strength parameters, unless pore-water pressures are measured, in which case the effective stress parameters may be calculated. Strength may be analysed as a compressive, shear, or tensile strength parameter. Strength is usually determined on intact laboratory specimens but may be determined by tests on the soil/rock mass in the field (either at surface outcrops, in trial pits, or in boreholes) or, in the case of soils, on remoulded specimens.
There are a variety of tests that measure strength. In both laboratory and field the methods differ from soils to rocks although the principles are the same. The most common tests for rock, usually acquired during normal site investigations, are the uniaxial (unconfined) compression test (UCS), and the point load index test (PTL), whilst for soils it is the triaxial test of which there are several versions, and which, for a cohesive soil, produce the parameters of cohesion, c, and internal friction angle, Φ. Total shear strength is usually defined by the Mohr-Coulomb failure criteria, the equation of which is as follows:
For a fully saturated, intact specimen, prevented from draining at all stages of the test, the value of the internal friction angle, Φ, is zero. The undrained shear strength, su, thus equals the undrained cohesion, cu. This failure criterion is known as the Tresca criterion. However, if triaxial test specimens are consolidated at each stress level by allowing drainage, as in the consolidated-undrained (CU) or consolidated-drained (CD) tests, effective shear strength may be measured if pore-water pressure (u) is measured and subtracted from the total stresses. This is reported in terms of the ‘effective’ cohesive and frictional strength parameters c' and Φ'. The effective shear strength, s', is then calculated from the Mohr-Coulomb equation as follows:
It is difficult to give typical or average values of strength for the Lias Group, or individual Formations and Members within it, because of the variability of lithology, fabric, structure, and cementation and the post-depositional processes of weathering and consolidation it has undergone. This results in variable depth profiles for intact strength on a scale of metres or centimetres, whether these are determined in-situ or in the laboratory. It should be borne in mind that the great majority of data within the Lias Group database are from ‘routine’ engineering investigations where samples are taken at relatively shallow depths, either from trial pits or from shallow drilling; over 75% of the triaxial strength data are derived from sample depths less than 10 m. At depths greater than 20 m the absence of weathering and other stress-relief factors means that the strength of the rock mass tends to be much greater than is represented here in the database. In some cases one or even two orders of magnitude difference may be anticipated, depending on the precise nature of the strength test (Haydon & Hobbs, 1977).
Undrained (total) triaxial strength data are reported in site investigations either with the assumption that the friction angle, Φ, is zero, or that it has a positive value, despite this being contrary to the principles of the test (Head, 1992; 1998). Undrained strength data containing a positive friction angle have been omitted from the database. A total of 2,965 results were obtained for cu, and 204 for c' and Φ'. A summary of triaxial test median values for these strength parameters is given in Table 7.12.
|Formation||cu (kPa)#||c' (kPa)||Φ' (degrees)|
|Blue Lias F||139||34||26|
|Charmouth Mudstone F||125||12||28|
|Scunthorpe Mudstone F||127||26*||23*|
|Whitby Mudstone F||123||15||26|
* less than 10 results # friction angle, Φu = 0
The range of undrained cohesion values, cu, for the Lias Group was from 7 to 1650 kPa, for drained cohesion, c', from 0 to 400 kPa, and for drained friction angle, Φ', from 0 to 47 degrees. The Scunthorpe Mudstone, Whitby Mudstone, and Charmouth Mudstone Formations show a remarkable similarity in terms of undrained cohesion; their medians being separated by only 4 kPa. The Blue Lias and Dyrham Formations medians for cu are higher and lower, respectively. Undrained cohesions are lowest for Area 5 and highest for Area 4.
The profile with depth of cu is highly scattered (Figure 7.8) although an overall trend of increasing strength with depth is seen.
Effective cohesion, c′, for the Blue Lias Formation was higher than the other formations tested, possibly as a result of calcareous cementation. Effective angles of friction did not vary greatly between formations, medians lying between 23 and 33 degrees.
Residual shear strength is the minimum strength of a soil achieved after continuous shearing along a pre-determined shear plane, usually within a remoulded sample, in a laboratory shear box test . The results are expressed in terms of the residual angle of internal friction, Φ'r, and residual cohesion, c'r, obtained from a plot of effective normal stress vs. shear stress. A summary table of both peak (cp', Φp) and residual shear-box strength results for those formations tested is shown in Table 7.9. The table also shows the ratio of peak to residual friction angle (Φp'/Φr) for those formations with greater than 5 results; the average ratio based on data from 38 Lias Group samples being 1.75.
|Formation||c'p (kPa)||Φ'p (degrees)||c'r (kPa)||Φ'r (degrees)||Φ'p/Φ'r|
|Bridport Sand F||11*||38*||8*||36*||1.04|
|Charmouth Mudstone F||15||24.5||9||14||1.89|
|Scunthorpe Mudstone F||-||-||13*||13*||-|
|Whitby Mudstone F||-||-||2||8||-|
* less than 10 results
Residual shear strength (angle of internal friction) has been plotted against plasticity index in Figure 7.9. This shows a good inverse correlation between residual shear strength and plasticity index as demonstrated for various formations by Cripps & Taylor (1981), Lupini et al. (1981), and Voight (1973). The plot suggests an approximate division of residual behaviour at a plasticity index, Ip, between 20 and 35%. Burland et al. (2001) stated that for ‘low’ plasticity clays (Ip <25%) undrained shear strength, cu was mainly a function of water content, and the ratio Φp'/Φr equalled 1.0. However, for more plastic clays (Ip >25%) cu was mainly a function of maximum previous overburden stress, pc’ and initial stress, pi', and ratio fp¢/ fr¢ was greater than 1.0. This change in behaviour was attributed to the occurrence of turbulent shear in low plasticity clays and sliding shear in high plasticity clays, at large strains (Burland et al., 2001). There also appears to be a similar pattern with the Lias Group samples (though with the Ip division closer to 20%), where the ratio Φp'/Φr' increases from 1.0 for samples with Ip <20% (e.g. Bridport Sand Formation) up to 3.0 for the samples with Ip >20% (e.g. most of the Charmouth Mudstone Formation).
A relationship suggested by Wesley (2003), related to residual shear strength of a sample and its plotted position on the Casagrande plasticity chart, was also investigated. Specifically, this correlated the residual strength with the distance of the sample point from the A-line on the plasticity chart (+ve above the A-line and –ve below it). Improved correlations of the type described above were reported for a wide variety of soil types with liquid limit in excess of 50%. However, in the case of the Lias Group test data a poor relationship was obtained. This was probably due to the fact that the Lias Group data are relatively uniform in terms of their position on the Casagrande chart.
Because of the strength variability within the Lias, both ‘soil’ and ‘rock’ testing methods may be undertaken to determine strength parameters. An important difference between 'rock' and 'soil' tests is the method of specimen preparation. Rock specimens for Uniaxial Compressive Strength (UCS) testing require machining of undisturbed samples. Soil specimens may be undisturbed, remoulded, or compacted. Reference should be made to Head (1998). Data from ‘rock’ strength tests are few, and consist mainly of uniaxial compression test (UCS) results. The results are summarised in Table 7.14. These tests were carried out mainly on calcareous mudstones, mudstones, argillaceous limestones, and shales.
|Formation||n||UCS (MPa)||IS(50)||IS(50) Mst.||IS(50) Lst.|
The results of UCS tests are plotted (on a log scale) against depth for Blue Lias and Charmouth Mudstone Formations in Figure 7.10. The plot shows a breakdown by broad lithological type. It can be seen that carbonate content has an influence on uniaxial compressive strength and that samples without limestone do not exceed 5 MPa (with the exception of two samples from the Blue Lias Formation).
Results from the Point load index test (IS(50)) are also summarised in Table 7.14 with an additional breakdown according to lithology. These tests were carried out on mudstones and limestones on a variety of sample types, but all with diameter corrections to 50 mm. There is an order of magnitude covering the four formations represented; the Blue Lias and Whitby Mudstone formations providing the highest and the Charmouth and Scunthorpe Mudstone Formations the lowest. The lithology breakdown shows a 28% greater point load median for the Blue Lias limestone compared with mudstone, but the opposite, though lesser, trend for the Whitby and Scunthorpe Mudstone Formations.
Consolidation is the process whereby pore water is expelled from a soil as the result of applied, static, external stresses, resulting in structural densification of the soil. For most purposes, the external stress is considered to be unidirectional, and usually vertical. Swelling strain data may also be obtained from the oedometer test. The oedometer is a simple laboratory apparatus, which applies a vertical load to a small disc-shaped soil specimen, laterally confined in a ring. The consolidation test is normally carried out on undisturbed specimens by doubling the load at 24-hour intervals, and measuring the resulting consolidation deformation (BS1377: 1990; Head, 1998).
The rate at which the consolidation process takes place is characterised by the coefficient of consolidation, cv (m2/yr), and the amount of consolidation by the coefficient of volume compressibility, mv (m2/MN). Consolidation data derived from the oedometer test on undisturbed specimens are used in the calculation of likely foundation settlement, and may also provide information on the stress history, geological history, state of disturbance, permeability, and elastic moduli of clay soils.
The consolidation data selected for inclusion in the database are confined to oedometer tests where loading increments are doubled, as recommended in British Standards: BS1377 (1990). The stress range is 25 to 3,200 kPa, but with only a small proportion of tests reaching 3,200 kPa, this range is inadequate to characterise the Lias Group materials fully, but adequate for most engineering purposes. There are a total of 284 consolidation data points in the database. Statistical analyses of the coefficients of volume compressibility, mv, and coefficient of consolidation, cv, at specific stresses are shown in Appendix B. A summary of the data for specific applied stresses is shown in Table 7.15. The overall median values for cv range from 1.24 to 14.79 m2/yr. A plot of medians by Formation is shown in Figure 7.11. This reveals almost uninterrupted decreases in cv with stress increase for all Formations except the Dyrham Formation. The Dyrham Formation curve stands apart from the others with significantly higher overall values of cv, and a pattern of decreasing cv from 50 to 200 kPa but increasing cv from 200 to 3,200 kPa. Above 100 kPa, the Blue Lias, Charmouth Mudstone, Scunthorpe Mudstone, and Whitby Mudstone Formations are very closely grouped. The results place the Lias Group in the ‘medium’ cv class, typical of medium plasticity soils, with the exception of the Dyrham which is placed in the ‘high’ category, typical of low plasticity, more permeable soils (e.g. silts) (Lambe & Whitman, 1979).
|Formation||n||cv (m2/yr) @100kPa||cv (m2/yr) @400kPa|
The overall median values for mv range from 0.013 to 0.160 m2/MN. A summary of data for specific applied stresses is shown in Table 7.16. A plot of medians by Formation is shown in Figure 7.12. This reveals uninterrupted decreases in mv with increasing stress (above 200 kPa) for all Formations. As with the cv data, the mudstone formations are grouped closely together, while the Dyrham Formation (silt) lies below. The mudstone data tend to merge between 200 and 400 kPa. These results place the Lias Group coefficient of volume compressibility results in the 'very low' to ‘medium’ category.
|Formation||n||mv (m2/MN) @100kPa||mv (m2/MN) @400kPa|
(note: Figures 7.11 and 7.12 do not represent individual test curves, but are simply a visual representation of the statistics at each stress increment). Initial void ratio medians for Formations, from the oedometer consolidation test, range from 0.52 to 0.63.
A total of 11 oedometer consolidation tests were carried out at BGS on samples of Lias Group clays undertaken as part of an assessment of shrink/swell geohazards (Nelder & Jones, 2004). The oedometer results, along with matching plasticity results, are summarised in Table 7.17. Plots of voids ratio vs. stress and coefficient of volume compressibility vs. stress are given in Figures 7.13 and 7.14, respectively. These data show a wide variety of behaviour, apparently unrelated to formation, with some samples having very high voids ratios and coefficients of volume compressibility at low applied stresses compared with the medians shown in Figure 7.12. This may be due to fissuring and softening caused by stress-relief, despite the use of carefully hand-prepared ‘undisturbed’ tube samples. It may be considered, however, that the use of such preparation methods has captured the true behaviour of weathered Lias Group mudstones. The oedometer results do not appear to relate to plasticity in any straightforward manner. A plot of compression index (the change in voids ratio for one log cycle of pressure change), Cc, vs. liquid limit, wL, for some Lias Group formations is shown in Figure 7.15 (BGS data). Also shown are Skempton’s Cc vs. wL relationships for undisturbed and remoulded soils, where Cc = 0.009 (wL-10) is representative of undisturbed and Cc’ = 0.007 (wL-10) of remoulded soils. The data do not conform to the Skempton relationship, though this has been found elsewhere with indurated mudrocks (Hobbs et al, 1998). There are too few data with which to establish such a relationship, should it exist, for the Lias Group.
|Sample Site||Formation||Diam (mm)||Voids ratio
|mv range (m2/MN)||cv range (m2/year)||wL (%)||wP (%)||cc||ce|
|Blockley Site 1||CHAM||50||0.731||0.005–0.501||1.83–33.87||54.7||32||0.21||0.07|
|Blockley Site 2||CHAM||75||0.595||0.012–2.614||0.98–160.52||56||34||0.16||0.07|
|Conesby Site 2||WHM||75||0.299||0.006–0.121||1.13–58.92||61||38||0.06||0.03|
BLI = Blue Lias, CHAM = Charmouth Mudstone, WHM = Whitby Mudstone
The state of consolidation (i.e. normally, over-, or under-consolidated) of a soil in its current natural condition, and hence its maximum previous overburden stress, can be estimated from oedometer consolidation tests, where a yield point can be identified from the voids ratio vs. stress curve.
An over-consolidated clay is one in which the maximum previous overburden exceeds the present overburden, resulting in a denser, stronger, and less deformable soil.
Clays are often classified or discussed in terms of their degree of consolidation in their natural state, i.e. the natural, geological stress history. The over-consolidation of a clay is an important engineering descriptor, particularly where the degree of over-consolidation is high. Over-consolidation affects undrained shear strength, lateral stress, pore-water response, and allowable bearing pressure and settlement (Borowczyk & Szymanski, 1995). It is not always possible to obtain stress-history information from standard oedometer tests, due to the fact that near-surface disturbance tends to remove the effects of over-consolidation. Disturbance may be caused by natural factors such as weathering and glaciation, whilst man-made factors might include drainage, loading, and excavation.
Deformability (the terms compressibility and stiffness may also be used) is a measure of the strain undergone by a soil or rock subjected to a particular stress amount and direction. This strain may be unidirectional or volumetric. Deformability may be measured in both laboratory (intact) and field (rock or soil mass). Usually, test data are interpreted from stress-strain plots, with several parametric variants of deformability available. The elastic properties of a material are defined by the fundamental properties: bulk modulus, K, and shear modulus, G. Bulk modulus represents the change in all-round stress per unit change in volume, whereas shear modulus represents the change in shear stress per unit change in shear strain. The simplest form of deformability measurement is that of Young’s modulus, E, which is derived from a uniaxial compression test and is defined as follows:
The relationship between strain in the direction of stress and strain at right angles to it is defined by the Poisson’s ratio, v, as follows:
Shear modulus, G, is defined as:
Shear modulus may be measured in a variety of ways from the stress vs. strain plots. The most commonly quoted are the initial shear modulus, Gi, (determined from the initial slope of the stress vs strain curve) and the unload/reload modulus, Gur.
Only one deformability result is contained in the database. This is for a highly weathered mottled greenish-grey/brown, highly fissured, mudstone (Scunthorpe Mudstone F.) from 8 m depth at Cotgrave, Nottinghamshire, the Young’s Modulus and Poisson’s Ratio for which were 2.55 MPa and 0.25. This sample had compressional and shear velocities of 0.0012 m/s and 0.0007 m/s, respectively. Dynamic Poisson’s ratios, nDYN, ranging from 0.30 to 0.34 and dynamic Young’s moduli, EDYN, are shown in Table 7.18 for geophysical laboratory tests on fine-grained argillaceous limestones from the Dorset coast (Area 5). This also shows a positive correlation between Young’s Modulus (dynamic) and both dry unit weight, gd, and compression wave velocity, V.
NOTE: the dynamic versions of these deformation properties are determined by interpretation of velocities of ultrasonic compressional and shear-waves passed through the rock.
|Formation||γd (Mg/m3)||E DYN (GPa)||vDYN||V (km/s)|
(Black Ven Marl M.)
Permeability, or hydraulic conductivity, in the geotechnical context, is a measure of the ability of soil or rock to allow the passage of fluid subject to a pressure gradient. The permeability measured on intact specimens in the laboratory (kLAB) is usually distinct from that measured in the field (kFIELD), as a result of the huge scale difference, and hence the involvement in the field tests of discontinuities and gross lithological variations. The database contains 56 field permeability determinations, representing the major litho-stratigraphic units. These are of the ‘falling head’, ‘rising head’, ‘variable head’, and ‘packer’ type tests applied to boreholes. Packer tests enable a particular horizon to be isolated for testing whereas the other test types allow water transmission throughout its unlined length of the borehole.
The field permeability results range over seven orders of magnitude, from 1.8 x 10-11 to 1.7 x 10-4m/s. The median value is 1.7 x 10-7m/s. A plot of permeability vs. depth is shown for the formations in Figure 7.16. The Whitby Mudstone Formation appears to cluster at lower permeabilities than the other formations. It is notable that for the data examined, the Dyrham and Bridport Formations do not register higher permeabilities than the more mudstone-dominant formations.
A plot of field permeability with depth for broad lithological types is shown in Figure 7.17. This shows that whilst all samples below 10-8m/s are clay or mudstone, the higher permeabilities include all lithology types. This may be due to weathering and water movement through fissures. The general ranges shown in Table 7.19 may be used for comparison.
|Very fine or silty sands||10-5–10-8|
|Fissured & weathered clays||10-4–10-8|
Permeability is an important parameter in the selection of materials for engineered (reworked and compacted) structures such as clay liners (see section 8.3), where an upper limit of 1 x 10-9m/s is common (Murray, 1998).
The anisotropy of permeability has not been examined here, due either to the fact that data in the database refer to a ‘global’ value, or that the orientation of the test has not been reported. However, it should be noted that horizontal permeabilities may be expected to exceed vertical permeabilities by a factor of about 2, though locally in the presence of limestone and sandstone layers this will tend to increase.
Compaction is the process whereby soil, usually reworked, is densified, usually in layers, in order to produce an engineering fill of known properties. This is achieved by applying dynamic forces, using special plant, such as rollers, vibratory rollers, rammers, or by special ground improvement processes. The densification is achieved by the solid soil particles packing closer together and producing a stronger soil mass. The water content of the placed fill and the amount of energy input are critical to the density that can be produced. The process is not the same as Consolidation.
Compaction properties may be measured in the field and in the laboratory. Field methods include the nuclear probe and the cone penetrometer. The laboratory method is standardised and usually employs the 1 litre compaction mould where layers of a disturbed soil sample are compacted into the mould and sub-samples taken for water content determination (British Standards: BS1377, 1990). There are two forms of the laboratory test, the ‘light’ (2.5 kg rammer) and the ‘heavy’ (4.5 kg rammer) Proctor Test. The results are shown in the form of a dry density vs. water content relationship, from which the maximum dry density (MDD) and corresponding optimum water content (OMC) are calculated.
A total of 238 compaction test results are contained in the dataset. For the Lias Group overall, the test data give median values for maximum dry density (MDD) of 1.76 Mg/m3 and optimum water content (OMC) of 16%. Formation medians are in the ranges 1.70 to 1.78 Mg/m3 and 15.4 to 18%, respectively. A summary table showing formation medians is given in Table 7.20. A plot of MDD vs. OMC is given in Figure 7.18.
|Formation||MDD (Mg/m3)||OMC (%)|
|Bridport Sand F||1.70||18.0|
|Blue Lias F||1.75||18.0|
|Charmouth Mudstone F||1.78||16.0|
|Scunthorpe Mudstone F||1.77||15.6|
|Whitby Mudstone F||1.71||15.4|
The permeability (hydraulic conductivity) for a soil increases dramatically below the OMC (Murray, 1998). This is particularly true of field compacted permeability where ‘clod’ size influences the development of fissures at water contents below OMC.
The Moisture Condition Value (MCV) is a test capable of being used in the laboratory or field, and is becoming an increasingly popular means of selection, classification, and specification of fill material (British Standards: BS1377, 1990; Highways Agency, et al., 1991; Caprez & Honold, 1995). The test aims to determine the minimum compactive effort required to produce near-full compaction of a single 1.5 kg sample passing a 20 mm mesh sieve. The test differs from the traditional Proctor compaction test in that the compaction energy is applied across the entire sample surface, and compaction energy can be assessed as an independent variable. The degree of compaction involved in the MCV test probably lies between those of the standard and heavy compaction tests (Murray, 1998).
A total of 684 MCV data values for the Lias Group are contained in the database, ranging from 0 to 19.8%, with an overall median of 11.7%. The medians for each formation tested are shown in Table 7.21. The formation medians range from 9.7 to 13.2%. MCV values less than 7 tend to indicate very poor trafficability.
|Bridport Sand F||42||9.7|
|Blue Lias F||23||13.2|
|Charmouth Mudstone F||212||12.6|
|Scunthorpe Mudstone F||5||12.0|
|Whitby Mudstone F||155||12.5|
n = number of samples
Swelling & shrinkage
Swelling and shrinkage are two mechanical properties of a soil which, though driven by related physico-chemical mechanisms, are usually treated separately in the laboratory. Swelling sensu-strictu is mainly a function of the clay minerals present in the soil or rock. The engineering phenomenon of heave may be caused by factors other than swelling of clay; for example, by stress relief. The geological processes affecting swelling and shrinkage were described by Gostelow (1995). Assessment of swelling and shrinkage usually does not involve direct measurement, but rather indirect estimation of volume change potential from index tests on reworked samples. Direct shrink/swell tests are not well catered for in British Standards.
The wide variety of test methods applicable to shrink/swell was described in Hobbs & Jones (1995). Laboratory tests may be carried out on undisturbed or disturbed samples. Undisturbed samples are as near to their in-situ condition as possible, whereas disturbed samples may be reworked, reconstituted, or compacted depending on the engineering application. Swelling tests usually either measure the strain due to swelling, resulting from access of a partially saturated sample to water, or the pressure produced when the sample is restrained from swelling. Swelling strain samples may be disc-shaped oedometer-type samples for 1-D testing of soils and slaking rocks, or cubes for 3-D testing of non-slaking rocks. The 1-D samples are laterally restrained. Swelling pressure samples are usually oedometer discs and may be mounted in a normal oedometer or a special swelling pressure apparatus. There are two shrinkage tests specified by British Standards: BS1377:1990. These are the Shrinkage Limit Test, carried out on undisturbed or disturbed samples, and the Linear Shrinkage Test, carried out on reworked soil ‘paste’ (prepared as for Atterberg limits). It should be noted that the shrinkage limit is a specific water content below which little or no volumetric shrinkage occurs, whereas the linear shrinkage is a percentage reduction in length (strain) on oven drying.
The clay-dominant formations of the Lias Group are generally considered to be of ‘low’ to ‘medium’ shrink/swell potential depending on lithology and mineralogy, while the dominantly sandy formations are ‘low’. This is borne out by the database (see below).
The Building Research Establishment (BRE) Digest 240 (1993) gives a scale of susceptibility to volume change (i.e. swelling or shrinkage), or volume change ‘potential’, for over-consolidated clays in terms of a modified plasticity index, Ip' (Table 7.22).
where: Ip’ (modified plasticity index):
|Ip||Volume change potential|
The purpose of the modified plasticity index is to take account of the proportion of fines in relation to the total sample and to reduce the measured plasticity index in proportion. Many Atterberg limit data in the database do not include <0.425 mm results. A total of 3,671 (out of 5,930) contain <0.425 mm results. This may be because the sample did not require sieving, or that a small number of coarse particles were removed by hand, without sieving. The modified plasticity index and volume change potential data are shown in Table 7.23:
|Formation||Median Ip‘ (%)||Volume change potential|
Overall, the percentage of data falling in the ‘high’ category was 2.5%, these being distributed across the clay-dominant formations. Only four samples gave a ‘very high’ classification. These were from the Charmouth Mudstone and Redcar Mudstone Formations. The Building Research Establishment (BRE) Digest 240 (1993) classification does not indicate the actual volumetric shrinkage to be expected for each of the volume change potential categories. Net volume changes depend on the initial saturation condition of the test sample. In the case of the shrinkage limit test this is usually natural moisture content, whereas in the case of the linear shrinkage test it is close to the liquid limit.
Linear Shrinkage results for the Lias Group were obtained from samples collected as part of an assessment of the shrink/swell hazard (Hobbs et al., 2007). Sample details are given in Appendix D4 - Summary shrink-swell data by Formation. A plot of linear shrinkage versus plasticity index is given in Figure 7.19. The dashed line shows a relationship (Ls = 0.47Ip) suggested in British Standards: BS1377:1990. This compares with a best-fit line (solid line) for all the Lias samples tested of (Ls = 0.22Ip + 5). The plot shown in Figure 7.19 suggests that at low plasticity indices, the linear shrinkages are less scattered.
Standard Penetration Test (SPT) results
The Standard Penetrometer Test (SPT) is a long-established method of in-situ geotechnical testing. This dynamic method employs a falling weight to drive a split-sampler and cutting shoe (or solid 60° cone in the case of coarse soils or soft rock) 300 mm into the ground from a position 150 mm below the base of a borehole; the initial 150 mm being the 'seating’ drive. The use of the test is described in British Standard 5930 (1999) and the methodology in British Standard 1377: Part 9: Clause 3.3 (1990). There has been much discussion concerning the test method, test apparatus, and test interpretation (Stroud & Butler, 1975; Stroud, 1989). International variations in practice have been a feature of its use.
It was recommended (Clayton, 1995; British Standard 5930, 1990; International Society for Rock Mechanics, 1988) that test results be reported in the form of six 75 mm penetration increments; the first two representing the 'seating’ drive and the final four the 'test’ drive, the sum of the latter providing the SPT 'N’ value. This is often not the case in site investigation reports, though it does form part of the Association of Geotechnical Specialists (AGS) format.
The Standard Penetration Test (SPT) may be regarded as crude, but it is inexpensive and effective. In most cases site investigation reports included a record of the incremental blows and penetrations. These have been entered into the geotechnical database for analysis. The summaries presented for the SPT are derived from over 2,000 tests. The data from these tests were processed in the following stages:
|Seating blows and penetration. The current test standard (British Standard BS1377, 1990) has specified that the seating drive is complete after an initial penetration of 150 mm or 25 blows, whichever is first achieved. This recognises that, in harder soils and weak rocks, the test equipment is adequately seated with a penetration of <150 mm. Thus it is no longer permissible to report tests as ‘seating blows only’. Few of the site investigations used to acquire data for this study had complied with this requirement. Some drillers had apparently carried out the test correctly, and yet the formal report records these tests as incomplete. Where the blow count to 150 mm was greater than 25, the data within this interval were examined. The seating penetration was taken at the increment for which the total blow count was proportionately closest to 25. In the cases where this occurred at the first increment, all the subsequent data were shifted to the right and the next four increments taken as the main test drive. Tests in which data have only been recorded for a single increment were discarded, as they left no scope to distinguish 'seating' from 'test drive'’.|
|Variability of the incremental blows. The purpose in recording the SPT test in 75 mm increments is not clear, either in the British Standard or in the otherwise comprehensive report by Clayton (1995). In this study, the incremental data were analysed on the premise that in each test an attempt is made to derive a measure of resistance for a single and, at least locally, consistent material. Thus, when the test interval crosses between two materials of differing resistance, the result will reflect the properties of neither, nor the overall properties of the two materials when considered together. Data from the final increment of each test were examined. Where the penetration for this increment had been recorded as zero, the test was discarded. It was taken that the test interval had reached a second material of very much higher resistance, for which the available data indicated an infinite N value. For the remaining tests, the penetration rate (blows/mm) for the final increment was compared to the rate for the first increment of the main test drive. Where this ratio was higher than about 4:1, the tests were again discarded.|
For a small proportion of tests neither digital nor incremental data were available. In these cases, tests giving a full N value were accepted, together with those in which a partial main test drive could be distinguished from a seating drive. For the remainder, extrapolated N values were calculated from the main test drive penetration (where less than 300 mm) and the corresponding blow count. Much of the discarded data represents high but unquantifiable N values.
A total of 3,812 SPT N values are contained in the database. The majority of these represent the Charmouth Mudstone, Whitby Mudstone, and Dyrham Formations. The overall median value is 42, with a range from 2 to 277. The summary statistics are shown in Table 7.24. Plots of SPT vs. depth for the principal formations are shown in Figures 7.21 to 7.23. These show trends of increasing N value with depth to 25 m (there are few data below this depth). The scatter of data is considerable and linear correlations between N and depth are poor. However, some trends can be discerned, for example the rate of increase in N with depth is less for the Whitby Mudstone Formation than for the other mudstone formations. In fact, the Whitby Mudstone Formation has about half the rate of increase of the Charmouth Mudstone Formation, and about two-thirds that of the Dyrham Formation. The results analysed by Area show that the rate of increase of N with depth appears greater for Areas 2 and 4 compared with the remainder.
|Formation||N (blows/300 mm)|
|median||range||25th percentile||75th percentile|
For information, Stroud (1989) and Clayton (1995) give log-log correlations between N value (derived from SPT, pressuremeter, and pile tests) and unconfined compressive strength data for various clay and mudrock formations, obtained from a variety of sources. From this Clayton (1995) quoted the following relationship for 'clays’:
Assessment of this correlation for the Lias’clays’ SPT data acquired for this report has not yet been undertaken.
Analysis of the Lias Group geotechnical data, described in Geotechnical data, has revealed some trends in the properties and engineering behaviour of the principal formations. Data have been analysed by Formation and Member (where available), combined with correlation according to defined geographical Areas.
In terms of particle size, the Bridport Sand and Dyrham Formations are distinct from the other mudstone-dominant formations, having lower clay contents and higher silt and sand contents. In terms of Areas, the clay fraction data for Area 5 are notably lower (and notably skewed) compared with the other Areas represented. Area 2 has the highest clay content, with Areas 3 and 5 having displaying the highest silt contents. Compared to the formation clay contents, the sand and gravel data in all areas are also highly skewed (see Appendix C - Extended box and whisker plots (by Area)).
Variations in geotechnical properties analysed by Area usually result in north-south trends. For example, from north to south there are trends of decreasing density, increasing water content, increasing plasticity, and decreasing sulphate content. Whilst these variations are consistent, they are, overall, relatively minor in the case of density and water content.
Plasticity classifications range from ‘low’ to ‘extremely high’, although the great majority of data lie within the ‘intermediate’ and ‘high’ classes. As is the case with many clay-rich rocks and soils, the best-fit lines for the Casagrande plasticity chart are steeper than the A-Line, with the exception of the Scunthorpe Mudstone Formation, and their equations are reported. Plastic limits fall within a very narrow range. This means that plasticity index variations are largely caused by variations in liquid limit. The Scunthorpe Mudstone Formation has the lowest plasticity of the mudstones, while the remainder are very similar. Whilst Area 5 shares the lowest liquid limit median with Area 2, it has a skewed upper range of liquid limit compared with the other Areas. This is not repeated in the case of plastic limit. There is little variation in liquidity index across Areas.
The calculation of Skempton’s Activity (Ac), designed to highlight the relative influence of the clay mineral content on plasticity, revealed that the formations with the lowest clay-size fraction returned the highest activity. This may in part be due to the presence of smectite, but is probably also due to the inapplicability of the concept to clay-poor materials.
The majority of sulphate data fall within BRE classes 1 and 2. Plots of total sulphate content against depth show that there is a reduction below a depth of about 10 m. This may be associated with the transition from the weathered to the unweathered state with increasing depth. The highest median total sulphate content was recorded for the Scunthorpe Mudstone Formation and for Area 3, although the Area differences are, overall, relatively small.
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