OR/12/032 Weathering - Revision history

Ajhil: /* Description & classification of weathered materials */

2021-08-06T09:37:42Z

Description & classification of weathered materials

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	==Description & classification of weathered materials==		==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)<ref name="Chandler 1972">CHANDLER, R J. 1972. Lias clay: weathering processes and their effect on shear strength. ''Geotechnique'', 22, 403–431.</ref> or BS5930 (1981)<ref name="BS5930 1981">~~British Standards~~: BS5930. 1981; 1999. Code of practice for site investigations. ''British Standards Institution'', BS5930.</ref>. Chandler’s (1972)<ref name="Chandler 1972"></ref> weathering classification is shown in Table 6.5 and the classification according to BS5930:1981 in Table 6.6.		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)<ref name="Chandler 1972">CHANDLER, R J. 1972. Lias clay: weathering processes and their effect on shear strength. ''Geotechnique'', 22, 403–431.</ref> or BS5930 (1981)<ref name="BS5930 1981">BRITISH STANDARDS: BS5930. 1981; 1999. Code of practice for site investigations. ''British Standards Institution'', BS5930.</ref>. Chandler’s (1972)<ref name="Chandler 1972"></ref> weathering classification is shown in Table 6.5 and the classification according to BS5930:1981 in Table 6.6.

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Ajhil: /* Description & classification of weathered materials */

2021-08-06T09:37:14Z

Description & classification of weathered materials

← Older revision		Revision as of 10:37, 6 August 2021
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	==Description & classification of weathered materials==		==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)<ref name="Chandler 1972">~~Chandler~~, R J. 1972. Lias clay: weathering processes and their effect on shear strength. ''Geotechnique'', 22, 403–431.</ref> or BS5930 (1981)<ref name="BS5930 1981">British Standards: BS5930. 1981; 1999. Code of practice for site investigations. ''British Standards Institution'', BS5930.</ref>. Chandler’s (1972)<ref name="Chandler 1972"></ref> weathering classification is shown in Table 6.5 and the classification according to BS5930:1981 in Table 6.6.		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)<ref name="Chandler 1972">CHANDLER, R J. 1972. Lias clay: weathering processes and their effect on shear strength. ''Geotechnique'', 22, 403–431.</ref> or BS5930 (1981)<ref name="BS5930 1981">British Standards: BS5930. 1981; 1999. Code of practice for site investigations. ''British Standards Institution'', BS5930.</ref>. Chandler’s (1972)<ref name="Chandler 1972"></ref> weathering classification is shown in Table 6.5 and the classification according to BS5930:1981 in Table 6.6.

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Ajhil: /* Gypsum */

2021-08-06T09:36:44Z

Gypsum

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	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<ref name="Hawkins 1987">HAWKINS, A B, and PINCHES, G M. 1987. Cause and significance of heave at Llandough Hospital, Cardiff-a case history of ground floor heave due to gypsum growth. ''Quarterly Journal of Engineering Geology'' 20, pp.41–57.</ref>). 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 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<ref name="Hawkins 1987">HAWKINS, A B, and PINCHES, G M. 1987. Cause and significance of heave at Llandough Hospital, Cardiff-a case history of ground floor heave due to gypsum growth. ''Quarterly Journal of Engineering Geology'' 20, pp.41–57.</ref>). 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)<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.		The visible effects, processes and weathering Class are summarised in Figure 6.3, after Spink and Norbury (1993)<ref name="Spink 1993">SPINK, T W, and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.

	[[Image:OR12032fig6.3.jpg\|thumb\|center\|500px\| '''Figure 6.3'''    Chemical weathering and classification for clays and mudstones. (After Spink and Norbury, 1993<ref name="Spink 1993">SPINK, T W, and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]		[[Image:OR12032fig6.3.jpg\|thumb\|center\|500px\| '''Figure 6.3'''    Chemical weathering and classification for clays and mudstones. (After Spink and Norbury, 1993<ref name="Spink 1993">SPINK, T W, and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]

Ajhil: /* Blue Lias Formation */

2021-08-06T09:35:56Z

Blue Lias Formation

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	==Weathering of formations==		==Weathering of formations==
	===Blue Lias Formation===		===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)<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.		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)<ref name="Spink 1993">SPINK, T W, and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.

	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 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.

Ajhil: /* Gypsum */

2021-08-06T09:35:35Z

Gypsum

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	The visible effects, processes and weathering Class are summarised in Figure 6.3, after Spink and Norbury (1993)<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.		The visible effects, processes and weathering Class are summarised in Figure 6.3, after Spink and Norbury (1993)<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.

	[[Image:OR12032fig6.3.jpg\|thumb\|center\|500px\| '''Figure 6.3'''    Chemical weathering and classification for clays and mudstones. (After Spink and Norbury, 1993<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]		[[Image:OR12032fig6.3.jpg\|thumb\|center\|500px\| '''Figure 6.3'''    Chemical weathering and classification for clays and mudstones. (After Spink and Norbury, 1993<ref name="Spink 1993">SPINK, T W, and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]

	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 [[OR/12/032 Geohazards #Sulphate attack of concrete \|Sulphate attack of concrete]].		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 [[OR/12/032 Geohazards #Sulphate attack of concrete \|Sulphate attack of concrete]].

Ajhil: /* Physical weathering */

2021-08-06T09:35:20Z

Physical weathering

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	* Moisture content change resulting in desiccation cracks.		* Moisture content change resulting in desiccation cracks.

	[[Image:OR12032fig6.2.jpg\|thumb\|center\|550px\| '''Figure 6.2'''    The physical weathering, description and classification of grey clays and mudstones (After Spink and Norbury (1993)<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]		[[Image:OR12032fig6.2.jpg\|thumb\|center\|550px\| '''Figure 6.2'''    The physical weathering, description and classification of grey clays and mudstones (After Spink and Norbury (1993)<ref name="Spink 1993">SPINK, T W, and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]

	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<ref name="Horswill 1976">HORSWILL, P, and HORTON, A. 1976. Cambering and valley bulging in the Gwash Valley at Empingham, Rutland. ''Phil. Trans. Roy. Soc''., London. Vol. A283, pp.427–462.</ref>). 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<ref name="Kovacevic 2007">KOVACEVIC, N, HIGGINS, K G, POTTS, D M, and VAUGHAN, P R. 2007. Undrained behaviour of brecciated Upper Lias Clay at Empingham Dam. ''Geotechnique'', 57, No. 2, pp.181–195.</ref>). 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 (K<sub>0</sub>) from a value >1, related to geological over-consolidation, to a value close to unity (Kovacevic et al., 2007<ref name="Kovacevic 2007"></ref>).		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<ref name="Horswill 1976">HORSWILL, P, and HORTON, A. 1976. Cambering and valley bulging in the Gwash Valley at Empingham, Rutland. ''Phil. Trans. Roy. Soc''., London. Vol. A283, pp.427–462.</ref>). 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<ref name="Kovacevic 2007">KOVACEVIC, N, HIGGINS, K G, POTTS, D M, and VAUGHAN, P R. 2007. Undrained behaviour of brecciated Upper Lias Clay at Empingham Dam. ''Geotechnique'', 57, No. 2, pp.181–195.</ref>). 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 (K<sub>0</sub>) from a value >1, related to geological over-consolidation, to a value close to unity (Kovacevic et al., 2007<ref name="Kovacevic 2007"></ref>).

Ajhil: /* Physical weathering */

2021-08-06T09:34:57Z

Physical weathering

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	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.		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)<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.		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)<ref name="Spink 1993">SPINK, T W, and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.

	These processes can be classified into a few main types:		These processes can be classified into a few main types:

Ajhil: /* The effect of weathering on the lias group */

2021-08-06T09:34:33Z

The effect of weathering on the lias group

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	Previous studies of the Whitby Mudstone Formation in the East Midlands by Chandler (1972)<ref name="Chandler 1972"></ref> and of the Charmouth Mudstone Formation in Gloucestershire by Coultard and Bell (1993)<ref name="Coultard 1993">COULTARD, J M, and BELL, F G. 1993. The influence of weathering on the engineering behaviour of Lower Lias clay. In: "The engineering geology of weak rocks". Editors, J C Cripps, J M Coulthard, M G Culshaw, A Forster, S R Hencher and C F Moon. ''Proceedings of the 26th annual conference of the Engineering Group of the Geological Society'', Leeds, 9–13 September 1990. </ref>, both found an increase in moisture content with increased weathering and, in the latter case, a general increase in liquid limit with increased weathering.		Previous studies of the Whitby Mudstone Formation in the East Midlands by Chandler (1972)<ref name="Chandler 1972"></ref> and of the Charmouth Mudstone Formation in Gloucestershire by Coultard and Bell (1993)<ref name="Coultard 1993">COULTARD, J M, and BELL, F G. 1993. The influence of weathering on the engineering behaviour of Lower Lias clay. In: "The engineering geology of weak rocks". Editors, J C Cripps, J M Coulthard, M G Culshaw, A Forster, S R Hencher and C F Moon. ''Proceedings of the 26th annual conference of the Engineering Group of the Geological Society'', Leeds, 9–13 September 1990. </ref>, 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)<ref name="Anon 1995">~~Anon~~ 1995. The description and classification of weathered rocks for engineering purposes. ''Geological Society, Engineering Group Working Party Report''. </ref> and BS5930 (1999)<ref name="BS5930 1999"></ref>. 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:		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)<ref name="Anon 1995">ANON 1995. The description and classification of weathered rocks for engineering purposes. ''Geological Society, Engineering Group Working Party Report''. </ref> and BS5930 (1999)<ref name="BS5930 1999"></ref>. 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:

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Ajhil: /* Gypsum */

2021-08-06T09:33:00Z

Gypsum

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	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<ref name="Hawkins 1987">HAWKINS, A B, and PINCHES, G M. 1987. Cause and significance of heave at Llandough Hospital, Cardiff-a case history of ground floor heave due to gypsum growth. ''Quarterly Journal of Engineering Geology'' 20, pp.41–57.</ref>). 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 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<ref name="Hawkins 1987">HAWKINS, A B, and PINCHES, G M. 1987. Cause and significance of heave at Llandough Hospital, Cardiff-a case history of ground floor heave due to gypsum growth. ''Quarterly Journal of Engineering Geology'' 20, pp.41–57.</ref>). 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)<ref name="Spink 1993">~~Spink~~, T W, and ~~Norbury~~, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.		The visible effects, processes and weathering Class are summarised in Figure 6.3, after Spink and Norbury (1993)<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>.

	[[Image:OR12032fig6.3.jpg\|thumb\|center\|500px\| '''Figure 6.3'''    Chemical weathering and classification for clays and mudstones. (After Spink and Norbury, 1993<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]		[[Image:OR12032fig6.3.jpg\|thumb\|center\|500px\| '''Figure 6.3'''    Chemical weathering and classification for clays and mudstones. (After Spink and Norbury, 1993<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]

Ajhil: /* Physical weathering */

2021-08-06T09:32:40Z

Physical weathering

← Older revision		Revision as of 10:32, 6 August 2021
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	* Moisture content change resulting in desiccation cracks.		* Moisture content change resulting in desiccation cracks.

	[[Image:OR12032fig6.2.jpg\|thumb\|center\|550px\| '''Figure 6.2'''    The physical weathering, description and classification of grey clays and mudstones (After Spink and Norbury, 1993<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]		[[Image:OR12032fig6.2.jpg\|thumb\|center\|550px\| '''Figure 6.2'''    The physical weathering, description and classification of grey clays and mudstones (After Spink and Norbury (1993)<ref name="Spink 1993">SPINK, T W and NORBURY, D R. 1993. The engineering geological description of weak rocks and overconsolidated soils. In: The engineering geology of weak rock, Engineering Geology Special Publication, 8, ''Balkema'', Rotterdam.</ref>). ]]

	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<ref name="Horswill 1976">HORSWILL, P, and HORTON, A. 1976. Cambering and valley bulging in the Gwash Valley at Empingham, Rutland. ''Phil. Trans. Roy. Soc''., London. Vol. A283, pp.427–462.</ref>). 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<ref name="Kovacevic 2007">KOVACEVIC, N, HIGGINS, K G, POTTS, D M, and VAUGHAN, P R. 2007. Undrained behaviour of brecciated Upper Lias Clay at Empingham Dam. ''Geotechnique'', 57, No. 2, pp.181–195.</ref>). 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 (K<sub>0</sub>) from a value >1, related to geological over-consolidation, to a value close to unity (Kovacevic et al., 2007<ref name="Kovacevic 2007"></ref>).		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<ref name="Horswill 1976">HORSWILL, P, and HORTON, A. 1976. Cambering and valley bulging in the Gwash Valley at Empingham, Rutland. ''Phil. Trans. Roy. Soc''., London. Vol. A283, pp.427–462.</ref>). 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<ref name="Kovacevic 2007">KOVACEVIC, N, HIGGINS, K G, POTTS, D M, and VAUGHAN, P R. 2007. Undrained behaviour of brecciated Upper Lias Clay at Empingham Dam. ''Geotechnique'', 57, No. 2, pp.181–195.</ref>). 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 (K<sub>0</sub>) from a value >1, related to geological over-consolidation, to a value close to unity (Kovacevic et al., 2007<ref name="Kovacevic 2007"></ref>).