OR/12/032 Weathering

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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).

Introduction

The weathering of rocks and soils alters the moisture content, density, spacing and type of discontinuities, material and mass strength, stiffness and, in many cases, the mineralogy. Some weathering products such as calcium sulphate (gypsum or selenite) are deleterious to man-made materials such as cement and concrete. The oxidation of iron pyrites, one of the chemical alteration stages that can produce calcium sulphate, may result in highly acidic conditions potentially attacking construction materials. It is important, also, to identify the depth to which the increased jointing and fissuring of weathered mudstones and clays increase the likelihood of slope or cut failure.

Different degrees of weathering have an important bearing on the engineering behaviour and precautions required for construction. This section describes the changes that occur during weathering and considers:

  • The length of time that weathering has taken place by considering weathering domains (Ballantyne and Harris, 1993[1]).
  • The influence of chemical and physical weathering.
  • The effect of different degrees of lithification relating to the maturation of the minerals within the sediment.

Much of the information available is confined to the mudstones, as these are the thickest and most widespread of the lithologies of the Lias Group. The weathering of this lithology is more likely to produce engineering problems and for this reason the description and classification of weathering for these materials are better understood.

Weathering domains

Time domains

In general terms, the deposits that have been exposed to subaerial weathering for longest will have better developed and (probably) thicker weathering profiles under similar weathering conditions. In the UK, time-related domains are generally described with reference to the extent of glaciations. Physical weathering will have progressed most rapidly during periods of periglacial climate that have dominated in the last 400 000 years. In comparison, it is likely that weathering in the relatively short interglacial periods has been slow, and there are no periods of time in the past 2 million years when the climate was sufficiently warm to give rise to ‘tropical or sub-tropical’ weathering as occurred before the Quaternary.

Using information on the extent and timing of the three main ice sheet advances in Great Britain, Ballantyne and Harris (1993)[1] differentiated five distinct periglacial regions each distinguished by the length of time over which the land surface has been exposed to interglacial and periglacial weathering (Figure 6.1 and Table 6.1). This is necessarily a highly simplified approach, neglecting variables such as altitude, local climate, erosion and the influence of periglacial and interglacial deposition.

Figure 6.1    Quaternary provinces of Britain (after Foster, et al., 1999[2]). Details of weathering regions 1–6 are described in Table 6.1.
Table 6.1    Weathering under periglacial and interglacial condition in the UK.
Glacial Provinces Beginning of Glaciation Weathering characteristics Region
Upland Glaciated and Periglaciated (Western Scotland) 10 000 BP (Loch Lomond Readvance) Weathering and dissolution features likely to have been removed. 1
Upland Glaciated and Periglaciated c.18 000 BP High water flow during deglaciation promoted dissolution 2
Lowland Glaciated and Peiglaciated (Northern and Western Britain) c.18 000 BP High water flow during deglaciation promoted dissolution 3
Lowland Glaciated and Peiglaciated (Central and Eastern England) c.400 000 BP (Not glaciated since Anglian Glaciation) Generally thin weathering profiles likely; Head deposits largely in valleys 4
Lowland Periglaciated at least 800 000 BP (Not glaciated during the Quaternary) Thick head deposits; often deep weathered bedrock profiles, including those due to Neogene (warmer climate) weathering 5
Upland Periglaciated at least 800 000 BP (Not glaciated during the Quaternary) Veneers of weathered deposits on variably weathered bedrock 6
Table 6.2    Weathering regions for each Lias Group Area (refer to Figure 1.1).
Lias Area Weathering region
1 2, 3 or 4
2 3 or 4
3 4
4 4 and 5
5 5
6 4

Table 6.2 shows the weathering domains in each of the Lias areas (Defined in Figure 1.1). It is likely that the deepest and most intensively weathered Lias Group deposits will be in area 5. This area has not been glaciated and has potentially undergone deep sub-tropical chemical weathering followed by permafrost and freeze/thaw processes during ice advance and retreat. However, in this area the Lias may be covered by, or mixed with, Head deposits typical of areas south of the (most extensive) Anglian Glaciation. Parts of Area 1 and 2 were glaciated during the most recent Devensian glaciation, and most of the earlier weathering products would have been removed. Much of the weathering of these materials would have occurred during the last 18 000 years. The depth of weathering will also be affected by erosion, largely controlled by local conditions.

Lithology-based domains

Weathering characteristics not only depend on the weathering conditions and time but also on lithology and the degree of induration. Rock and sediments compress and alter when buried and the amount of compression and alteration depends on the depth of burial, temperature and the length of time buried. In general the weathering depth will tend to be less in more indurated rocks. From the geological record, the estimated depths of burial for the Lias depositional basins are shown in Table 6.3. The Cleveland Basin and Wessex Basins have the greatest burial depths and the ‘highs’ at Market Weighton, Mendip, and the Bristol and Radstock shelf, along with the Bristol Channel Basin, have the shallowest depths of burial. The East Midlands Shelf and Worcester Basin are of intermediate depth. This suggests that the Lias of the Cleveland and Wessex Basins will be the most indurated and the ‘highs’ of Market Weighton, Mendip, Radstock and Bristol the least indurated.

Table 6.3    Estimated maximum depth of burial of the Lias for major depositional basins.
Cleveland Basin Market Weighton High East Midlands Shelf Worcester Basin Mendip High – Bristol
Radstock Shelf
Bristol Channel Basin Wessex Basin (Dorset)
Quaternary Ice (1,500) (1,000) (500) (100) (0) (200) (0)
Quaternary (50) (100) (100) (50) (0) (100) (0)
Tertiary (<50) (<50) (<50) (<50) (<50) (<50) (200)
U Cretaceous 700 500 400 300 200 200 300
L Cretaceous 400 0 100 50 50 50 50
U Jurassic 400 0 300 300 200 100 600
M Jurassic 300 0 50 200 100 100 250
L Jurassic (450) (100) (300) (550) (100) (350) (500)
Typical max burial depth 2,000 m 550 m 1,000 m 1,100 m 600 m 600 m 1,500 m

However, estimates of burial depth assessed from the mineralogy, most notably the presence of smectite and the ratio of illite to smectite in mixed layer clays (See Mineralogy), indicate somewhat different maximum burial depths (Table 6.4). These estimations suggest greater burial depths across the main depositional basins and the East Midlands Shelf, with the rocks of the Cleveland Basin and East Midlands Shelf having undergone deeper burial than those of the Worcester and Wessex Basins. On this basis it is likely that similar unweathered Lias rocks in the Cleveland Basin and East Midland Shelf areas will be stronger than the less indurated rocks of the Worcester and Wessex Basins.

Table 6.4    Estimated maximum depth of burial for four depositional basins based on mineralogical data.
Cleveland Basin East Midlands Shelf Worcester Basin Wessex Basin (Dorset)
Typical max burial depth (m) 4,000 3,000 2,000 2,000

Overall Lias Group weathering domain

If the ‘time’ and ‘lithological’ domains are considered together, then the Lias of the Cleveland Basin could generally be expected to have the thinnest weathering profile (having potentially stronger rocks due to greater burial depths and undergoing glaciation during the Anglian, but effectively not the Devensian, period). The Lias of the Wessex Basin could be expected to have the thickest weathering profile (having been subjected to shallower burial depths and being unglaciated during the Quaternary). However, there will be other local factors that affect the degree and depth of weathering at specific sites.

Weathering processes

The weathering process involves the alteration and/or breakdown of rock and soil materials near the earth’s surface by physical disintegration and chemical decomposition. The type of weathering and the nature of the weathering products are greatly influenced by climate. Chemical decomposition is most active in hot, wet climates and least active in dry, cold climates. Intermediate rates occur in humid temperate climates. Even within Britain there are differences in weathering rates related to climate, chemical weathering being more active in the warmer south than in the mountains of Scotland.

Physical and chemical weathering processes often act together. For example, chemical weathering frequently occurs along joints that may be formed, or are opened up, during stress relief as a result of erosion of the surface rock or freeze-thaw mechanisms. Fractures may also open up because of chemical and mineralogical changes resulting in volume change within the soil or rock, for example as a result of gypsum formation.

Physical weathering

Physical weathering involves the breakdown of rock into fragments with little change (chemical alteration) in the minerals of the rock, and is by far the most important weathering process in very cold and dry, or very hot and dry, climates.

The first stage of disintegration is generally the development of jointing, due to stress relief where the rock is closer to the surface. The joints have considerably greater hydraulic conductivity than the rock material, which may result in chemical weathering along joint/discontinuity surfaces. In cool climates, further physical breakdown may take place because of volume change due to freeze-thaw action. Near surface, physical breakdown may also occur because of seasonal moisture content changes resulting in shrinking and swelling, most notably in clays and mudstones. A summary description of the types of physical weathering processes and their effects in clays and mudstones, such as those in the Lias Group, is shown in Figure 6.2, after Spink and Norbury (1993)[3].

These processes can be classified into a few main types:

  • Stress relief due to surface erosion
  • Periglacial conditions
  • Disruption due to subordinate chemical weathering
  • Moisture content change resulting in desiccation cracks.
Figure 6.2    The physical weathering, description and classification of grey clays and mudstones (After Spink and Norbury, 1993[3]).

These processes occur at different depths. Fissuring and shearing formed in periglacial conditions due to permafrost may occur at depths greater than 10 m; for instance borehole logs from some sites in Northamptonshire show fissuring to more than 17 m. Brecciation of the Whitby Mudstone Formation, attributed to periglacial freeze/thaw deformation and pore-pressure rise, has also been described at the Empingham Dam, Leicestershire, constructed in the early 1970’s to form Rutland Water reservoir (Horswill & Horton, 1976[4]). The dam was founded on, and formed largely from, Whitby Mudstone Formation material excavated locally. Brecciation is in the form of lithorelics of relatively stiff clay within a matrix of softer, highly disturbed clay. Individual lithorelics tend to retain the fabric and properties of the un-brecciated Lias, but the brecciated rock mass as a whole is highly heterogeneous with variable strength and deformation properties (Kovacevic et al., 2007[5]). The disturbance which caused the brecciation has been attributed to plane shear deformations extending considerable distances (>100 m) and at depths of 10 to 50 m, associated with cambering and valley bulging of the Gwash valley. The processes described above tend to reduce the value of the coefficient of earth pressure at rest (K0) from a value >1, related to geological over-consolidation, to a value close to unity (Kovacevic et al., 2007[5]).

Chemical weathering

Chemical weathering occurs when the rock-forming minerals are altered. The rate of chemical weathering depends primarily on water and temperature, with the highest rates occurring in hot humid climates. Chemical weathering processes include oxidation, hydrolysis, dissolution and re-deposition.

The overall effect of chemical weathering is determined by the accessibility of meteoric water, moisture content change and the types of minerals present. The effects of present-day chemical weathering of British Jurassic rocks seldom exceed the top 3 to 5 m.

Oxidation and acidification

In many deposits chemical weathering is, to a large extent, caused by oxidation. The most notable effects are colour changes in grey deposits, which on oxidation of iron sulphide (iron pyrites) become yellow, brown, orange and red iron oxide and hydroxides. The colour will also depend on the hydration (amount of water) in the mineral. The overall oxidation may be represented by equation 1:

4FeS2 + 15O2 + 14 H2O → 4Fe(OH)3 + 16H+ + 8SO42-          (1)

The oxygen is initially supplied by oxygenated water that flows along discontinuities or regions of preferential groundwater flow. This may act progressively, increasing the penetration of the oxidised region. Nearer the surface, there is an increased chance of the deposit drying and becoming unsaturated allowing for much greater ingress of oxygenated waters. This leads to more rapid and extensive oxidation. Here, the rock mass is likely to become oxidised and brown. This takes longer in low permeability rocks such as clays and mudstones than higher permeability sands and sandstones.

Chemical weathering by oxidation and acidification is greatly reduced where the ground in permanently saturated and there is little oxidising water flow. This greatly restricts or stops oxidation and, therefore, the colour changes and chemical alterations associated with weathering. This may occur beneath river deposits. However, such rocks may have fissures and joints caused by physical weathering.

Gypsum

A consequence of pyrite oxidation is the liberation of hydrogen and sulphate ions. The hydrogen ions lower the pH of the porewater/groundwater. The reaction between acid sulphate groundwater and calcium carbonate (calcite), which is commonly found in Lias Group deposits either as fine particles or as shells, produces hydrated calcium sulphate or gypsum (equation 2).

CaCO3 + 4H+ + SO42- → Ca SO4.2H2O + CO2          (2)

The simple replacement of calcite by gypsum causes an increase in volume by 103%. The expansion causes disruption to the rock fabric but also produces heave, which may damage buildings and other man-made structures (Hawkins and Pinches, 1987[6]). The net effect of the removal of calcium carbonate and formation of gypsum increases porosity and permeability, reducing the strength of the deposit and allowing greater movement of water or air thereby increasing the rate of weathering. Gypsum is commonly found in lenses and along fissures and joints generally where iron oxide staining is found, ahead of the main oxidation front.

The visible effects, processes and weathering Class are summarised in Figure 6.3, after Spink and Norbury (1993)[3].

Figure 6.3    Chemical weathering and classification for clays and mudstones. (After Spink and Norbury, 1993[3]).

Gypsum is relatively soluble and may be removed in solution, causing further increases in fabric disruption (including formation of voids), increased porosity and permeability. Gypsum is associated with sulphate attack on buried concrete, a hazard that is discussed more fully in Sulphate attack of concrete.

Gleying

Near-surface chemical weathering also takes place when brown or orange iron oxides and hydroxides are reduced to grey iron II oxide. This generally takes place in the presence of organic matter such as roots and initially produces pale grey streaks or mottles in oxidised, generally brown material that often follow root traces. The rotting of the roots and other organic materials requires oxygen and this may come from iron oxides, leading to reduction and the change from brown to pale grey. This process, known as ‘gleying’, increases with increasing biological activity towards the surface. Gleying may also occur where initially oxidised ground is waterlogged.

Calcium carbonate

Calcium carbonate may be deposited near the surface as fine localised powder or as hard nodules (often described by geologists as ‘race’). The formation of the calcium carbonate is likely to be partly due to the respiration of roots, which produce carbon dioxide that reacts with calcium in the pore water. In general, these deposits are localised and are sometimes seen associated with root systems.

Description & classification of weathered materials

Grey mudstones and siltstones dominate the Lias Group and most available information concerns these lithologies. Descriptions from ground investigation entered into the BGS National Geotechnical Properties Database occasionally contain weathering zones or grades based on early work on the weathering of the Lias Group from the East Midlands by Chandler (1972)[7] or BS5930 (1981)[8]. Chandler’s (1972)[7] weathering classification is shown in Table 6.5 and the classification according to BS5930:1981 in Table 6.6.

Table 6.5    Weathering of Whitby Mudstone Formation (Upper Lias Clay) at Rockingham and Gratton, Northamptonshire (after Chandler, 1972[7]).
Zone Description Fabric Discontinuities 1) % CaCO3
2) Gypsum
Landslip/
solifluction
Mottled light brown and light grey CLAY becoming grey with depth. Considerable oxidation near surface (frequent small haematite pellets), minimal oxidation at depth (i.e. >3 m). Extensive gleying at shallow depths. 0–2 m depth: Heterogeneous with small (<1 mm) rotated lithorelics

Greater depths: rotated lithorelics (up to 30 mm); otherwise as Zone II.
Fissure spacing:
Apparently intact (except for desiccation cracks) at shallow depths; 10–30 mm spacing at greater depths.

Shears: minor shearing in gleyed fissures is common.
1) 0.4%
2) None observed
IV Not observed – probably absent on slopes (mixed with landslip/solifluction material).
III Fissured CLAY with light grey (gleyed) fissure surfaces. Centre of lithorelics generally oxidised to pale brown. Lithorelics (up to 30 mm) have horizontal bedding; matrix occupies less than 50% of section, is often gleyed, and where limited in extent is usually sheared; larger areas of matrix show oriented bands. Fissures spacing:
10–30 mm

Shears: minor shearing in gleyed fissures is common.
1) 5–6%
2) Common
IIb Grey or blue grey CLAY with brown (oxidised) areas typically along fissures. Bedding horizontal; fabric as from depositional changes with brown staining usually in areas parallel to fissures. Fissures spacing:
20–100 mm

Shears: minor, 1–2 mm displacement, sometimes associated with oxidation.
1) 0–5%
2) Infrequent
IIa Blue grey weak MUDSTONE and very stiff CLAY. Bedding horizontal; fabric as from depositional changes; brown staining along fissures and joint surfaces only. Fissures spacing:
20–100 mm

Shears: none
1) 1–6%
2) rare
I Weak, blue-grey MUDSTONE. Bedding horizontal; fabric variations resulting from depositional changes. No oxidation. Fissures spacing:
>100 mm

Shears: none
1) 1–7%
2) None observed

In 1995 the Geological Society Engineering Group Working Party Report on the description and classification of weathered rocks for engineering purposes (Anon, 1995[9]) found that it was not feasible to use one all-encompassing scheme of weathering description and/or classification. The report proposed a number of ‘Approaches’ depending on different situations and scales. The most important recommendation was for a mandatory full description (‘Approach 1’) which might provide sufficient information for a classification be made for a particular purpose. Where appropriate, the formal classification must be unambiguous and used only where it is advantageous to do so.

Table 6.6    Scale of weathering classes of rock mass (after BS5930, 1981[8]).
Term Description Weathering Grade
Residual soil All rock material is converted to soil. The mass structure and material fabric are destroyed. There is a large volume change, but the soil has not been significantly transported. VI
Completely weathered All rock material is decomposed and/or disintegrated to soil. The original mass structure is still largely intact. V
Highly weathered More than half the rock material is decomposed or disintegrated. Fresh or discoloured rock is present either as a discontinuous framework or as corestones. IV
Moderately weathered Less than half of the rock material is decomposed or disintegrated to a soil. Fresh or discoloured rock is present either as a continuous framework or as corestones. III
Slightly weathered Discoloration indicates weathering of rock material and discontinuity surfaces. Weathering may discolour all the rock material. II
Fresh No visible sign of rock material weathering: perhaps slight discoloration on major discontinuities. I

The mandatory description is factual and should be carried out at material and mass scales as appropriate. It is often the only possible way of dealing with weathering where the full profile is not seen and aids interpretation of how the rock has reached its observed condition. Descriptions should use BS5930 (1999)[10] methods (adopted from Anon 1995[9]) and take particular note of colour including colour changes, discontinuities, strength and strength changes, and the nature and extent of weathering products.

The classification requires knowledge of the unweathered material as well as the various stages of weathering. In addition to the mandatory description (Approach 1), four additional ‘Approaches’ are proposed for the classification of varying material types and weathering states (see Appendix D1 - Weathering classes). These comprise:

Approach 1: Factual description of weathering (mandatory).
Approach 2: Classification for uniform materials that tend to weather gradationally (may not be applicable to the Lias Group).
Approach 3: Classification for heterogeneous masses that tend to weather gradationally and develop profiles which comprise a mixture of relatively strong and weak material in the mass.
Approach 4: Classification incorporating material and mass features (for describing rocks that weather in a gradational manner, but where the material and mass characteristics cannot be readily or usefully separated, including ‘stiff’ to ‘hard’ clays and ‘weak’ mudstones and siltstones, such as the Charmouth Mudstone and Whitby Mudstone Formations).
Approach 5: For rocks whose weathering state does not follow the above patterns, such as karst (which can only be described by reference to other characteristics such as landforms) and the particular effects of arid climates (not applicable to the Lias Group).

Note that BS EN ISO 14689-1 (BSI, 2003) does not currently support Approaches 2 and 3.

The effect of weathering on the lias group

Previous studies of the Whitby Mudstone Formation in the East Midlands by Chandler (1972)[7] and of the Charmouth Mudstone Formation in Gloucestershire by Coultard and Bell (1993)[11], both found an increase in moisture content with increased weathering and, in the latter case, a general increase in liquid limit with increased weathering.

Most of the information on the Lias Group in the BGS National Geotechnical Properties Database is for those Formations that have the greatest surface exposure, that is, the Blue Lias, Charmouth Mudstone, Scunthorpe Mudstone and Whitby Mudstone Formations. The assessment of the changes due to weathering of moisture content, plasticity, strength, sulphate and pH required a weathering classification method based on Anon (1995)[9] and BS5930 (1999)[10]. The weathering ‘class’ adopted for classification in the present study was based on colour as this was generally well described. This is a simplification of the current code but provides a useful guide to the weathering condition of the argillaceous rocks. Change from grey to brown is commonly used to distinguish between unweathered and weathered horizons. The ‘classes’ used are listed below:

‘Disturbed’ Predominantly light grey, soliflucted or landslipped material (where there is sufficient data, landslip, reworked and soliflucted materials are shown separately in depth profile plots).
Class D Brown with light grey streaks.
Class C Brown.
Class B Grey with brown on fissure surfaces or mottled brown and grey.
Class A Grey or dark grey (unweathered).

Depth of weathering

Differing burial depths and weathering domains indicate that the likely depth of weathering for Lias Group formations should also differ. Table 6.7 shows the general maximum depths of the weathering ‘classes’ for data taken from the geotechnical properties database. Some Class A material may be found near the surface where deposits are permanently saturated (thus inhibiting oxidation) or because surface material has been removed by erosion, mass movement or as part of earlier engineering activities and there has not been sufficient time for colour changes to develop.

The data (Table 6.7) indicate that there is a general increase in the depth of weathering, for all classes, in the more southerly Lias depositional areas (see Figure 1.1). This is the case for each formation. The deepest weathering profiles are found in the Dyrham and the Charmouth Mudstone Formations and shallowest in the Blue Lias and Scunthorpe Mudstone Formations. This indicates that the general maximum depth of colour changes tends to follow the weathering domains and degree of lithification due to depth of burial as proposed above.

Table 6.7    General maximum depths below
ground level of weathering 'classes' (except Class A).
Formation Area

Depth of ‘weathering class’ (mbgl)

Class B Class C Class D Disturbed
Blue Lias 3 4 3 6
Blue Lias 4 12 6 5 9
Charmouth M. 3 11 6 4 7
Charmouth M. 4 14 9 8 8
Charmouth M. 5 17 10 4 8
Dyrham 3 15 6 3 3
Dyrham 5 20 13 10 10
Scunthorpe M. 2 11 6 5
Whitby M. 3 16 6 6 5

Weathering of formations

Blue Lias Formation

The unweathered Blue Lias is generally a weak, thinly laminated to thinly bedded, grey to dark grey mudstone with bands of strong pale grey argillaceous limestone. The weathering of the mudstones is similar in character to that of the Whitby Mudstone and Charmouth Mudstone Formation as described by Spink and Norbury (1993)[3].

The limestone may remain strong and pale grey even when the mudstone is highly weathered resulting in an increased contrast in strength between the two rock types. Near surface, the limestone may be moderately weak to moderately strong, highly jointed pale grey and orange, or may be broken down into angular gravel or cobbles particularly where the limestone is the more important component.

The depth of weathering of the Blue Lias varies due to local conditions but completely weathered rocks are generally found within the top 5 m. At depths greater than 10 m below ground level most rocks are generally unweathered.

Summaries of the effects of weathering on selected Blue Lias geotechnical index and effective strength parameters are given in Table 6.8 and Table 6.9.

Table 6.8    The effect of weathering on the Blue Lias Formation.
Parameter Box and whisker plots (Appendix D2) Profile Plots (Appendix D3)
Moisture Content i)  Wide range of values in each weathering class.
ii)  Moisture content of class A generally lower than other classes.
i)  General trend of decreasing moisture content with depth in top 10–12 m.
ii)  Highest moisture contents generally found in weathered material in top 5 m.
Liquid limit i)  Wide range of values in each weathering class.
ii)  Liquid limit generally increases with increasing weathering, most notable between class A and B.
i)  Most of the higher values are in the weathered material in the top 5 m.
Plasticity Index i)  Wide range of values in each weathering class.
ii)  Plasticity Index generally increases with increasing weathering class, apart from ‘reworked’.
i)  Most of the higher values are in the weathered material in the top 5 m.
Liquidity index i)  Wide range of values in each weathering class.
ii)  Most of the lower values are class A, which tends to be slightly lower than the other classes.
i)  A slight trend of higher liquidity index in the more weathered upper 5 m.
Bulk Density i)  Wide range of values in each weathering class.
ii)  Most of the higher values are class A.
i)  Most of the lower values are in the upper 5 m, which tend to be weathered.
Cohesion i)  Wide range of values in each weathering class.
ii)  Class A generally has higher values than class B.
iii)  Little data for the more highly weathered materials.
i)  A trend of increasing strength with depth.
ii)  Most of the lower values are in the upper 5 m, which tend to be weathered.
Total sulphate and pH i)  Little difference in total sulphate between class A and class B.
ii)  pH of class A slightly higher than Class B.
i)  General decrease in total sulphate with depth in upper 10 m. Highest values generally in top 5 m.
ii)  No observable trends of pH with depth.
Table 6.9    Blue Lias Group median effective
stress parameters for each weathering class.
Weathering class Number of tests

Median effective stress parameters

c´ (kPa) ф′ °
A 4 85 35
B 5 11 27.5
C 5 26 26
D 1 5 28
E/reworked 0
  • Moisture content, liquid limit and plasticity index tend to be lower in unweathered class A materials than other weathering classes.
  • Cohesion is generally greater in class A materials.
  • Sulphate content is not controlled by weathering class.
  • pH tends to be slightly higher in class A rocks. There is little data for the Class C, D and ‘reworked’materials.
  • Changes in behaviour due to weathering tend to occur mainly in the top 5 m.
  • The limited data set of effective test data indicates that Class A has higher effective cohesion, c’, and friction angle, ф′ ˚, than the other weathering classes; however, there are too few data to make firm conclusions.

Charmouth Mudstone Formation

The weathering profile of typical Charmouth Mudstone Formation is described in Coultard and Bell (1993)[11] and can be classified using Anon. (1995)[9] weathering ‘Approach’ 4 (see Description & classification of weathered materials).

Geotechnical property plots showing variations of water content, plasticity index and total cohesion with depth for the Charmouth Mudstone Formation, with the data distinguished according to weathering state (‘class’), are given in Figures 6.4, 6.5 and 6.6.

Figure 6.4    Plot of Water content vs. Depth for Charmouth Mudstone Formation classified by weathering class.
Figure 6.5    Plot of Plasticity index vs. Depth for Charmouth Mudstone Formation classified by weathering class. Cohesion (kPa).
Figure 6.6     Plot of Total Cohesion (triaxial) vs. Depth for Charmouth Mudstone Formation classified by weathering class.

A brief description of the effect of weathering on selected parameters is given in Table 6.10 and effective strength parameters in Table 6.11.

Table 6.10    The effect of weathering on the Charmouth Mudstone Formation.
Parameter Box and whisker plots (Appendix D2) Profile Plots (Appendix D3)
Moisture Content i)  Wide range of values in each weathering class.
ii)  Moisture content generally increases with weathering class.
i)  Most weathered material generally occurs in the top 10 m, with highest water contents within top 5 m.
ii)  General trend of increasing moisture content between 15 m depth to ground surface.
Liquid limit i)  A wide scatter of data for all weathering classes
ii)  A slight trend of a increasing liquid limit with increasing weathering class from A to D.
i)  The highest liquid limit values are generally in the top 10 m, with most weathered material associated with highest moisture contents in the topmost 5 m.
Plasticity Index i)  Wide range of values in each weathering class.
ii)  A slight trend of increasing plasticity index with weathering classes from A to C.
iii)  Classes D and E are similar to C.
i)  Greatest range of plasticity indices occur in the top 5–10 m within the most weathered materials.
ii)  Within the top 5 m, soliflucted materials tend to record the majority of higher PI values.
Liquidity index i)  Wide range of values in each weathering class.
ii)  Trend of higher values with increasing weathering.
i)  Highest values generally in the upper 10 m, in most weathered materials.
ii)  In the top 7 m some class A samples record very high liquidity indices.
Bulk Density i)  Wide range of values in all weathering classes.
ii)  Bulk density tends to decrease slightly with increasing weathering.
i)  Slight trend of bulk density increase with increasing depth.
ii)  Most of the lower values are in the upper 10 m, in most weathered materials.
Cohesion i)  Wide range of values, particularly in weathering class A.
ii)  Nearly all the higher values (>200 kPa) are class A samples.
iii)  ‘Reworked’ samples tend to be weaker than samples from other classes.
i)  A general trend of increasing strength with depth.
ii)  Majority of the lower values occur in the upper 5–10 m generally, but not entirely, in most weathered materials.
iii)  In the top 10 m the strength of weathered and unweathered (class A) samples are often similar.
Sulphate and pH Total Sulphate
i)  Wide range of values in each weathering class.
ii)  Values tend to generally increase with weathering class.
iii)  Weathering class A and reworked samples tend to have the lowest values.

Water soluble sulphate
i)  Weathering classes A, B and C have generally similar values.

pH
i)  Similar values for the different weathering classes.
ii)  Most samples tend to fall within the range pH 7.5 to 8.5.
iii) Lowest values tend to be class A.
Total Sulphate
i)  Higher values generally occur in the topmost 5 m.
ii)  Higher values, in the top 5 m, are generally associated with weathering classes B, C and D.
iii)  Between 5–10 m a few class A (unweathered) samples record high total sulphate contents.

Water soluble sulphate
i)  There appears to be no clear trend with depth although the highest values occur in the top 5 m.

pH
i)  There is a very slight trend of increasing pH with increasing depth and less weathered samples.
Table 6.11    Charmouth Mudstone F. median effective
stress parameters for each weathering class.
Weathering class Number of tests

Median effective stress parameters

c´ (kPa) ф′ °
A 48 25 24.5
B 27 10 30
C 8 18.5 24.5
D 1 0 39
E/reworked 8 10.5 22
  • Moisture content, liquid limit, plasticity index and liquidity index tend to increase with increasing weathering; the highest values being found in the top 5–10 m.
  • Bulk density and cohesion both appear to be controlled more by depth than by weathering, but there is a trend of lower values with increasing weathering near surface (within the topmost 5 m).
  • Weathering appears to control total sulphate. Most class A values have values below that required for aqueous extraction sulphate testing, whereas about half of class B samples and most class C samples would require further testing. The reworked samples generally had low total sulphate content, presumably because the sulphate had already been removed by groundwater. Aqueous soluble sulphate does not appear to be controlled by weathering; however there are few data for the more highly weathered materials. pH values do not appear to be controlled by depth or degree of weathering.
  • The variation in sulphate and pH may be partly explained by oxidation of samples during storage.
  • There is a trend of decreasing effective cohesion, c´, with increased weathering; however, there is no clear trend for ф′ ° apart from that shown by ‘reworked’ material.

Dyrham Formation

Typical descriptions of Dyrham Formation deposits for each weathering class are given below.

Class A
Generally weak to moderately weak, very thinly bedded to thinly laminated, grey or dark grey, micaceous MUDSTONE or SILTSTONE.

Class B
Weak to moderately weak locally strong widely fissured very thinly bedded grey micaceous clayey SILTSTONE. Occasional red-brown staining on discontinuity surfaces. Fissures are sub-vertical. Slightly weathered.
Firm to stiff horizontally laminated grey mottled orange-brown micaceous SILT with a little clay and fine sand. Contains occasional thin (<4 mm) laminae of very stiff dark grey silty clay.

Class C
Stiff fissured grey brown and light brown shaly CLAY with iron staining on larger fissures. Occasional ironstone fragments.
Firm to stiff, extremely closely to closely fissured, thinly bedded multi-coloured grey-brown, brown-grey, orange-brown, red-brown, very micaceous clayey SILT with a trace of fine sand. Fissures are sub-vertical.

Firm to stiff, extremely closely to closely fissured, very thinly irregularly bedded, multi-coloured yellow-brown, light grey orange-brown, and red-brown, micaceous sandy SILT with a trace of clay. Locally calcareous. Fissures are sub-vertical.

Class D
Stiff, extremely closely fissured orange-brown and light grey mottled calcareous silty CLAY with a trace of sand. Gleyed. Occasional small red-brown weathered siltstone nodules (<5 mm). Fissures randomly orientated.
Firm very closely fissured thinly interbedded (100 mm) dark brown mottled brown-grey clayey SILT and silty CLAY. Weakly gleyed. Fissures are sub-vertical.

Class E and reworked
Firm to stiff brown-orange mottled clayey SILT with a trace of sand.

The effects of weathering on the Dyrham Formation are shown in Table 6.12 (index properties) and in Table 6.13 (effective strength parameters).

Table 6.12    The effect of weathering on the Dyrham Formation.
Parameter Box and whisker plots (Appendix D2) Profile Plots (Appendix D3)
Moisture Content i)  Wide range of values in each class.
ii)  Moisture content generally increases with weathering class.
i)  Majority of the highest moisture content values are in the top 5–10 m.
ii)  Below 10 m weathered material tends to have higher moisture contents than class A material.
Liquid limit i)  Class A, B and C have similar ranges of liquid limits.
ii)  Class D and ‘reworked’ materials tend to have higher liquid limits.
i)  Virtually all the highest liquid limit values (above 60%) are in the top 5 m and are generally class D or ‘reworked’ material.
Plasticity Index i)  Wide range of values in each class.
ii)  Similar to liquid limit, class A, B and C all having generally similar ranges of plasticity index.
iii)  Class D and ‘reworked’ materials tend to have higher plasticity indices.
i)  Most high plasticity index values (>30%) are in the top 7 m and are predominantly, but not exclusively, in class D or ‘reworked’ material.
Liquidity index i)  Wider range of values for weathering classes A to C.
ii)  Trend of higher values with increasing weathering most notably for class D.
iii)  Classes A, B and C contain majority of the lowest values (-0.25).
i)  Values below 10 m are less variable (-0.4 to 0.3) than those in overlying more weathered material (mainly -1 to 0.6).
Bulk Density i)  General trend of bulk density decreasing with increasing weathering, with classes D and ‘reworked’ giving lowest values. i)  Bulk density tends to increase with increasing depth (and decreasing weathering class).
ii)  All the low values (<1.95 Mg/m3) occur in the top 10 m.
Cohesion i)  Wide range of values in each class.
ii)  There is a general trend of decreasing strength with increasing weathering class.
iii)  The median values of classes A and B are markedly higher than those of more weathered materials.
i)  A general trend of increasing strength with depth.
ii)  The majority of the lower values are in the upper 10 m, in weathered material.
Sulphate and pH Insufficient data. Insufficient data.
Table 6.13    Dyrham Formation, median effective
stress parameters for each weathering class.
Weathering class Number of tests

Median effective stress parameters

c´ (kPa) ф′ °
A 1 114 21.5
B 12 20.5 32.5
C 12 17.5 33
D 3 24 27
E/reworked 2 5.5 33
  • Moisture content tends to increase and bulk density decrease with increased weathering.
  • Liquid limit, plasticity index and liquidity index tend to increase in the most weathered classes, that is class D and ‘reworked’.
  • Cohesion tends to decrease with weathering class; most of the low values (<100 kPa) are in the top 10 m.
  • The limited set of effective strength data indicates a general reduction in cohesion with increased weathering but little change in ф′ °; however, there are too few data to make firm conclusions.

Scunthorpe Mudstone Formation

Typical descriptions of Scunthorpe Mudstone Formation for different weathering classes are given below:

Class A
Very weak, locally thinly to thickly laminated, dark grey, calcareous silty MUDSTONE.

Very stiff, fissured, dark bluish grey, slightly sandy, very silty CLAY with occasional fossils, limestone bands and silt partings.

Weak, fissured, thinly laminated, dark grey silty MUDSTONE.

Class B
Very weak, closely to medium fissured, laminated, grey and grey brown, silty MUDSTONE. Firm to stiff, fissured, grey with a little brown mottling, CLAY.

Very stiff, fissured, dark bluish grey with a little brown mottling, slightly sandy CLAY with occasional fossils.

Stiff, fissured, yellowish brown and dark grey mottled slightly sandy CLAY with occasional calcareous nodules and shell fragments.

Class C
Stiff, thinly laminated, green grey mottled grey and orange brown, calcareous CLAY.

Firm, very closely fissured, grey brown CLAY with trace of shells and fine to coarse gravel. Class D Firm becoming stiff, fissured, light brown and light grey mottled, slightly sandy CLAY with occasional calcareous nodules and rootlets.

‘Reworked’
Firm, becoming stiff, yellowish grey and grey mottled sandy becoming slightly sandy very silty CLAY with some weak calcareous nodules and decayed roots.

The effects of weathering on the Scunthorpe Mudstone Formation are in Table 6.14 (index properties) and in Table 6.15 (median effective stress parameters).

Table 6.14    The effects of weathering on the Scunthorpe Mudstone Formation.
Parameter Box and whisker plots (Appendix D2) Profile Plots (Appendix D3)
Moisture Content i)  Wide range of values in each class.
ii)  There is a marked increase in moisture content between unweathered materials (class A) and the other classes.
i)  Majority of highest moisture content values occur in the top 10 m, with weathered material predominating in the top 5 m.
Liquid limit i)  There is a marked increase in liquid limit between unweathered materials (class A) and the other classes.
ii)  75% of class A samples have liquid limit values <50%, whereas the other classes have liquid limits generally >50%.
i)  Most of the higher liquid limit values (above 60%) are in the top 10–15 m with weathered material dominating the top 5 m.
ii)  Most samples below 15 m have liquid limits <50%.
Plasticity Index i)  Wide range of values in each class.
ii)  As for the liquid limit, class A samples generally have lower values than other weathering classes.
i)  Wide range of plasticity indices in the top 10–15 m (10 ->50%), dominated by weathered samples.
ii)  Below 10 m the great majority of samples are unweathered with plasticity indices of less than 30%.
Liquidity index i)  Class A samples tend to have generally lower values than other weathering classes.
ii)  Class A materials comprise the majority of samples with liquidity indices <-0.25.
i)  General trend of decreasing liquidity index with increasing depth.
ii)  Nearly all samples below 5 m are unweathered (class A) with liquidity indices predominantly <0.0.
iii)  About 25% of values above 5 m are >0.0, mainly comprising class B, C, D and ‘reworked’ samples.
Bulk Density i)  Class A samples tend to have greater bulk density values than weathered samples. i)  General trend of increasing bulk density with increasing depth.
ii)  Virtually all values of bulk density below 5 m are > 2.0 Mg/m3.
iii)  Wide range of bulk density values in top 3–5 m in predominantly weathered material, with c. 50% of values less than 2.0 Mg/m3.
Cohesion i)  Over 50% of class A samples have cohesion values >200 kPa.
ii)  Over 75% of the weathered samples have cohesion values less than 200 kPa.
i)  Great majority of cohesion values less than 100 kPa occur within the top 5 m.
ii)  Majority of the lower values are in the upper 10 m, in predominantly weathered material.
Sulphate and pH Total Sulphate
i)  Based on a limited dataset, Class B samples may tend to have higher values than class A samples.

pH
i)  Class A samples tended to have generally higher pH values than weathering classes B and C.
Total Sulphate
i)  All values greater than 1% are found in the top 6 m.

pH
Insufficient data.
Table 6.15    Scunthorpe Mudstone Formation median effective
stress parameters for each weathering class.
Weathering class Number of tests

Median effective stress parameters

c´ (kPa) ф′ °
A 3 8 23
B 3 23 27.5
C 2 18 16
D 0
E/reworked 0
  • Class A (unweathered) samples tend to have a narrower range and generally lower moisture contents, liquid limit, plasticity index and liquidity index values than weathered classes.
  • Most of the higher values of moisture contents, liquid limit, plasticity index and liquidity index are within 5–10 m of the ground surface.
  • There is a marked reduction in undrained strength in weathered materials compared to unweathered classes A samples, with a clear trend of increasing cohesion with depth and decreased weathering.
  • There appears to be no weathering class trend of total sulphate or pH values.
  • There are insufficient effective stress results to draw meaningful relationships with weathering class.

Whitby Mudstone Formation

Typical descriptions of Whitby Mudstone Formation for different weathering classes are given below:

Class A
Stiff, extremely closely fissured, thinly laminated, dark grey, slightly calcareous CLAY with weak mudstone regions.

Very stiff, very closely fissured, thickly laminated, dark grey CLAY.

Very stiff, very closely to closely vertically fissured, thinly (<2 mm) laminated to very thinly bedded dark grey calcareous micaceous silty CLAY with abundant shell fragments. Rare selenite.

Stiff, extremely closely fissured, thinly laminated, dark grey silty CLAY. Firm to stiff fissured dark grey silty shaly CLAY.

Class B
Very stiff, very closely fissured, dark grey CLAY, with occasional silt lenses and rare calcareous siltstone nodules (<80 mm). Fissures <60 mm. Slightly fossiliferous. Faintly oxidized.

Stiff to very stiff, dark grey, very closely fissured, silty CLAY. Occasional shell fragments. Rare calcareous siltstone nodule (<40 mm). A trace of oxidation along fissure surfaces. Minor shears.

Firm, multicoloured grey, orange and yellow CLAY.

Stiff extremely closely fissured thinly to thickly laminated bluish dark grey occasionally mottled yellow brown silty CLAY with a trace of fine gravel sized pockets of gypsum and rare partings of light grey silt.

Class C
Stiff to very stiff, very closely fissured mottled olive and grey, silty CLAY. Fissure lumps <60 mm. Abundant Selenite.

Stiff, fissured light brown micaceous silty CLAY. Occasionally brown on fissure surfaces with occasional selenite crystals becoming locally abundant on fissures. Lithorelicts 40%.

Class D
Soft to firm, closely fissured, light grey mottled orange, CLAY, highly gleyed and oxidized. Stiff, extremely closely fissured, thinly laminated, light grey mottled orange brown, CLAY.

Soft, extremely closely fissured, light grey mottled brown CLAY with occasional rootlets. Fissures are columnar. Minor shear surface <2 mm thick, showed undulating striated surface of soft grey clay.

Firm, extremely closely vertically fissured light grey mottled orange CLAY. Approximately 70% pale grey (gleyed) matrix encloses small (<10 mm) orange-brown relic fissure blocks. Abundant rootlets.

‘Reworked’
Stiff, extremely closely fissured, mottled orange-brown and grey CLAY. Occasional ironstone nodules (<10 mm). Extensively gleyed and oxidized.

Firm to stiff, mottled dark grey and orange-brown, silty CLAY with occasional rootlets and occasional calcareous mudstone concretions (<3 mm). Abundant shear surfaces. Highly oxidized.

Soft to firm, light grey and orange-brown, silty CLAY with occasional rootlets and rare ironstone fragments towards top. Gleyed and highly oxidized. Minor shear surfaces at 0.70 m. Major shear surface at 0.90 m. Occasional lenses of orange-brown silty sand.

The effects of weathering on the Scunthorpe Mudstone Formation are in Table 6.16 (index properties) and in Table 6.17 (effective strength parameters).

Table 6.16    Effects of weathering on the Whitby Mudstone Formation.
Parameter Box and whisker plots (Appendix D2) Profile Plots (Appendix D3)
Moisture Content i)  Wide range of values in each weathering class.
ii)  Moisture content generally increases with increasing weathering.
i)  There is a clear trend of decreasing moisture content with depth in the upper 5–10 m. This is particularly marked in the upper 5 m where highest moisture contents occur for all weathering classes.
ii)  Below 10 mbgl there are a few high moisture content values (>20%) for class A samples, but no general trend with depth is observed.
Liquid limit i)  Wide range of values most notably for classes A, B and ‘Reworked’.
ii)  There is a general increase in liquid limit with increased weathering.
i)  Liquid limit values are very variable in the upper 10–15 m.
ii)  No clear trend with depth is observed other than less variability in liquid limit values (Class A) below c. 15 m depth.
Plasticity Index i)  Wide range of values in each class particularly for class A and ‘Reworked’.
ii)  There is a general increase in plasticity with weathering class apart from class B, which tends to have lower plasticity values than the other classes.
i)  Plasticity indices are very variable in the upper 10–15 m.
ii)  No clear trend with depth is observed other than less variability in plasticity indices (Class A) below c.15 m depth.
Liquidity index i)  There is a general trend of increasing liquidity with increasing weathering.
ii)  Class A is very variable, with higher values similar to more weathered material.
i)  A large majority of values below 5 m are <0.0 mostly represented in class A and B samples.
ii)  Higher values >0.0 increase from 10 m depth to the surface.
iii)  Highest values markedly occur in the top 5 m, predominantly in weathered materials.
Bulk Density i)  General trend of decreasing bulk density with increasing weathering class.
ii)  Great majority (c.90%) of class A samples have bulk densities greater than 2.0 Mg/m3.
i)  Marked trend of increasing bulk density with depth in the upper 15 m.
ii)  Below 10 m majority of bulk density values are above 2.0 Mg/m3 and tend to be class A (unweathered) samples.
iii)  Approximately half the bulk density values above 5 m are less than 2.0 Mg/m3.
Cohesion i)  General trend of decreasing cohesion with increasing weathering.
ii)  Over 75% of class A samples have cohesion values >100 kPa.
i)  General increase of cohesion values with increasing depth.
ii)  Generally wide spread of values below 15 m depth in unweathered Class A samples.
Sulphate and pH Total Sulphate
i)  Based on limited data, Class B samples tend to show slightly higher values than class A samples.

pH
i)  Class A samples tend to show higher pH values than class B.
ii)  Classes A and B have a few very low values.
Total Sulphate
i)  Limited data indicate that higher values are generally found between 2 and 8 mbgl.

pH
i)  pH becomes more variable near surface.
ii)  Lower values (<6.5) are observed in the top 10 m, with most acidic values (<6) in the upper 4 m.
Table 6.17    Whitby Mudstone Formation median effective
stress parameters for each weathering class.
Weathering class Number of tests

Median effective stress parameters

c´ (kPa) ф′ °
A 5 20 30
B 3 25 26
C 0
D 3 5 21
E/reworked 0
  • Depth of sample appears to be a major control on moisture content in the top 5–7 m.
  • Class A samples are generally denser than weathered samples and the lower bound of densities appears to be depth controlled.
  • The liquid limit and plasticity index of near surface ‘disturbed’ samples are very variable probably due the variation of mixing of coarse and fine material; no clear trend with depth other than less variability (dominantly Class A samples) below c.15 m depth.
  • Class A samples are generally stronger than more weathered material. Cohesion generally increases with depth.
  • Total sulphate values tend to be higher between 2 and 8 mbgl.
  • The limited data indicates that there is a reduction in effective cohesion, c’ and friction angle, ф΄ with increased weathering.

Summary

The effects of weathering on the argillaceous deposits of the Lias Group are generally well understood. The depth and degree of weathering are generally controlled by the duration of exposure, i.e. the time since the last ice age, and the degree of lithification which is controlled by temperature and the depth of burial. These factors indicate that the thickest weathered deposits occur in the south of England, an area that has not been glaciated and with the shallowest burial depth. The thinnest weathering profiles occur in the Cleveland Basin, which was partly glaciated in the last ice age and has the greatest depth of burial. In general, the data collected for this project indicates that this is the case. However, local conditions will also have an effect at specific sites.

The two main weathering processes, physical and chemical weathering, are dominant under different climatic conditions. Physical weathering is most important in cold dry and hot dry climates, whereas chemical weathering is most active in hot and wet conditions. Jointing and fissuring due to physical weathering would have been most active to the south of the glaciations in Southern England during the most extensive Anglian glaciation, and in much of the Midlands and Southern England again during the most recent Devensian glaciation. Here permafrost including ice lensing at depth and freeze-thaw may result in the formation of joints and fissures at significant depths, potentially to 10’s of metres.

The greatest depth of chemical weathering occurs to the south of the Anglian glaciation particularly where there are remnant materials that were subjected to subtropical weathering. Chemical weathering occurs generally where there is ingress of oxygen, which reacts with iron pyrites changing the colour from grey to red, brown and orange. Gypsum forms in grey mudstone and siltstone where iron sulphide (iron pyrites) is oxidised and one of the products of this reaction, sulphuric acid, reacts with calcium carbonate. The formation of gypsum disrupts the mudstone/clay fabric. However, gypsum is soluble and may be removed near surface where there is water flow, further disrupting the fabric and increasing porosity.

The weathering changes for the Blue Lias, Charmouth Mudstone, Dyrham, Scunthorpe and Whitby Formations are described. The effect of weathering and depth on moisture content, plasticity, liquidity index, bulk density, strength, sulphate and pH were investigated using box and whisker plots and depth profiles annotated for weathering class (‘class’ with a lower case ‘c’ is used to distinguish it from the weathering classification scheme ‘approach D’ in BS5930 (British Standards, 1999[10]), which requires a full description). The current classification system of weathering is given in Appendix D1 - Weathering classes.

The weathering class can be used as a proxy for chemical weathering, and changes with depth as an indicator of physical weathering where the trends are different from the weathering class. In general, there is an increase in moisture content, liquid limit and plasticity index and a decrease in density and strength with increasing weathering although there are variations between the different formations. Aqueous sulphate content does not appear to be controlled by weathering class, but the total sulphate content increases with weathering class for Charmouth Mudstone Formation. However, this may be due to oxidation of samples during storage. There is no observable trend of pH with weathering class which may be due to buffering of sulphuric acid and calcium carbonate, or alteration during storage.

Increases in moisture content, liquid limit, plasticity index and reductions in cohesion are generally greatest between Class A and Class B material for Blue Lias and Scunthorpe Mudstone Formations. The same parameters for the Charmouth Mudstone Formation change more gradually with weathering class.

In general, the highest values of moisture content, liquid limit, plasticity index and lowest values of cohesion occur within 10 m of the ground surface, apart from Blue Lias Formation where this occur within 5 m of the surface.

References

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