OR/12/032 Engineering geology

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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).

General

The Lias Group consists of thick sequences of shallow marine deposits which have been subject to varying amounts of over-consolidation, cementation, and changes in clay mineralogy. This variability stems from the fact that the Lias Group was deposited in a series of geographically persistent subsiding basins. The Lias Group is dominated by mudstones and shaly mudstones, but with significant limestones and sandstones formed during shallowing of the sea and deltaic inputs, respectively. The dispositions and proportions of these lithologies vary between formations and across the country, and reflect changes between deep marine and deltaic environments throughout the Liassic period. However, the characteristically alternating sequences of mudstone and limestone usually result from cyclical patterns of deposition. The mudstones are variable in mineralogy, stress-history, and strength, but all weather at the near-surface to a clay-rich material, which is investigated by soil mechanics, rather than rock mechanics, methods. The result is that within the depth range of normal engineering operations, that is to about 20 m below ground level, the engineering behaviour of the Lias Group is frequently found to be close to the borderline between soil and rock. This same depth zone is applicable to the majority of entries in the geotechnical database forming the basis of this report.

Typical proportions of limestone lithologies and overall thickness are shown in Table 8.1. These figures are very approximate.

Table 8.1    Typical percentages of limestone lithologies and overall thicknesses.
Formation Typical % limestone Typical thickness (m)
Whitby Mudstone <30 40–120
Marlstone Rock >70 2–10
Dyrham <10 40–125
Bridport Sand <10 30–180
Charmouth Mudstone <20 100–290
Scunthorpe Mudstone <30 40–120
Blue Lias 20–50 40–150

Detailed lithological descriptions of the formations, their component members, and variation across the country, are contained in Geology.

Whilst increasing age and burial depth tend to be accompanied by a decrease in water content, weathering tends to result in an increase in water content. Weathering also produces progressive breakdown in cementation, reduction in strength in addition to an increase in water content, and a general homogenisation of the material, except very near to the ground surface where variability is re-established. Weathering also tends to produce an increase in plasticity. Unfortunately, adequate description of weathering state is patchy within the database and could not be used as a variable in a comprehensive geotechnical analysis. Further detail relating to weathering is contained in Weathering.

Foundations

Thaumasite attack

The high sulphate content of much of the Lias Group mudstones is responsible for high levels of thaumasite concrete attack reported within the last few years in the West Midlands. Typically, sulphate content varies with depth and weathering state. During construction of the M40 motorway in Oxfordshire, England, heave of the carriageway was caused when lime stabilisation of pyrite-bearing Charmouth Mudstone Formation (Lower Lias Clay) was attempted (Building Research Establishment, 1991[1]). One end product of pyrite oxidation, resulting for example from weathering, is the mineral thaumasite. Buried concrete structures within Lias Group mudrocks are subject to thaumasite attack (TSA), particularly where saturated Lias-derived fill is in contact (Longworth, 2002[2]), transforming the concrete into a weak paste. Clearly, this has serious consequences for the integrity of the concrete, and may result ultimately in failure. TSA was notable on bridge foundations where concrete contacted pyritic Lias Group clays and clay-fill on the M5 motorway in Gloucestershire (Floyd et al., 2002[3]).

Shrink/swell

The uppermost few metres of outcropping geological formations are subject to seasonal water content variations. These are often exacerbated by the presence of trees and shrubs, and soakaways and fractured water pipes. In the case of clays, a decrease in water content causes shrinkage (an overall volume decrease) and an increase in water content causes swelling (an overall volume increase). These conditions are neither permanent nor exactly reversible, and may take years to develop due to the extremely low permeability of clays. Neither are they intrinsic properties of the soil or rock, but rather a response to prevailing environmental conditions. It is significant that the relationship between shrink/swell and water content is also non-linear. The shrink/swell phenomenon becomes particularly significant where shallow, light foundations are concerned. This applies to houses and especially old properties where foundation depths were generally shallower than modern structures and foundation design did not make allowance for the effects of shrink/swell processes.

Clays containing the clay mineral smectite are particularly prone to swelling and shrinkage. The smectite content of the Lias Group is variable (See Mineralogy), and whilst the Lias Group overall has a ‘medium’ volume change potential rating, and the component Formations also have a ‘medium’ rating, certain data samples within them contain smectite-rich layers which have a ‘high’ rating. Such samples are found within the Whitby Mudstone, Charmouth Mudstone and Blue Lias Formations, and within the Dyrham and Bridport Sands Formations.

Engineered fill and liners

Engineered fill is suitable material that has been placed to an appropriate specification under controlled conditions (Charles, 1993[4]). Key factors when assessing the Lias Group in terms of its use as engineered fill for construction purposes are strength, durability, excavatability, and compaction (Charles, 1993[4]). A further consideration is the possibility of sulphate attack on concrete, as outlined in Sulphate attack of concrete.

Durability depends on mineralogy, porosity, cementation, and structure. Whilst older mudstones tend to be more durable, changes on exhumation and exposure to air can result in rapid breakdown and loss of durability (Cripps & Taylor, 1981[5]).

The engineering properties of clay liners have been addressed by Murray (1998)[6]. In this case, the key parameter is the hydraulic conductivity of the material forming the barrier to advection or diffusion of contained liquids. Material selection and the method of placement both affect final hydraulic conductivity. Whilst low permeability is the aim, other properties such as shrink/swell and compaction behaviour must be considered. Thus a clay with ‘extremely high’ plasticity (e.g. some Gault, London Clay, and Fuller’s Earth Formation clays) may be deemed ‘unsuitable’ or ‘marginal’ based on factors other than its hydraulic conductivity. However, a plasticity index minimum is usually specified for a suitable clay, in addition to the requirement for a position above the Casagrande A-line (i.e. non-silt). Perhaps surprisingly, gravel contents of up to 30% are permissible from the hydraulic conductivity standpoint. A hydraulic conductivity of 1 x 10-9m/s is usually specified as the maximum acceptable following tests on the engineered liner material (Murray, 1998[6]). Other tests for construction control include Compaction (Light and Heavy), Moisture Condition Value (MCV), and undrained shear strength.

As mentioned earlier, the post-placement volume change characteristics of a fill need to be considered (Charles, 1993[4]). This is particularly true in the case of the Lias Group mudstones which contain varying amounts of pyrite, which oxidise on exposure and the products react with limestone (widely available within the Lias) to produce gypsum (section 5.3). Additionally, gypsum already exists in the form of selenite, particularly within discontinuities in the mudstone lithologies. Added to this the clay minerals within the mudstones themselves have the ability to change volume with water content change (Shrink/swell).

Slope stability

The Lias Group rocks are on record as having the highest incidence of landsliding in the UK (Jones & Lee, 1994[7]); the Upper Lias (Whitby Mudstone Formation) having as many as 42 landslides per 100 km2 of outcrop. Whilst this database is almost certainly incomplete, and the size of the landslides is not taken into account in this statistic, it is nevertheless an indication of the importance of slope instability when considering the engineering of natural slopes in the Lias Group.

The principal regions and types of inland landslides within the Lias Group are:

  • Avon — Somerset-Wiltshire (multiple rotational, cambering, debris slides)
  • Cotswolds (multiple rotational, cambering, debris slides, mudslides)
  • East Midlands plateau (cambering, rotational, mudslides, slab slides)
  • North York Moors (multiple rotational, cambering, toppling, debris flows).

Engineering within, or adjacent to, a landslide or landslide complex requires certain precautions and procedures to be carried out. These cover methods of site investigation, slope stability assessment, remediation (in the event of landslide activity), and monitoring. Landslide complexes within the Lias Group may be extensive, and though ancient in origin, may become partially re-activated by engineering and building activity or by changes in climate and water regime. Identification of landslide prone areas at an early stage of an investigation is made possible using stereo air photos and, more recently, by analysis of digital terrain models (DEMs). Remediation of potentially active landslides almost always involves enhanced drainage, often in combination with other engineering measures such as retaining structures, slope re-profiling, and re-vegetation.

Analysis of slope stability requires the input of geotechnical parameters, in particular strength and density, or alternatively, where appropriate, a rock mass rating value. These are used to calculate and compare the ‘driving’ and ‘resisting’ forces within a 2D or 3D model of the landslide. Key parts of the model are the positions of the slip plane and the water table (or piezometric surfaces). Such information is usually obtained from a ground investigation involving boreholes, trial pits, and in some cases instrumentation or monitoring to indicate water pressures, ground movement, etc. However, frequently such information is unavailable and the model remains purely conceptual.

A detailed account of the occurrence of landslides in the Lias Group is given in Landslides.

Site investigation

Routine site investigations carried out within the uppermost 5–10 m of the Lias Group are likely to utilise soil mechanics principles, that is, soils-type drilling, sampling, and laboratory testing, in order to characterise the materials in terms of engineering behaviour in accordance with British Standards BS5930 (1999)[8] and BS1377 (1990)[9]. This is certainly reflected in the prevalence of this type of information in the geotechnical database on which this report is based. Within this zone, the Lias Group materials are usually in a weathered state. This tends to alter the geotechnical properties, compared with the unweathered state, for example by increasing plasticity, reducing strength, and increasing fissuring. Care should therefore be taken in extrapolating data obtained for less weathered material to highly weathered material (See Weathering).

An important part of the preliminary stage of a site investigation is the walk-over survey. This enables the local geology to be checked and for potential geohazards to be identified for further desk study, survey, or ground investigation. Seasonal ground conditions can also be assessed. Self-boring pressuremeters and self-boring permeameters are becoming increasingly popular in site investigations for large engineering projects such as dams, cofferdams and tunnels. Instruments combining deformation/stress with pore-pressure monitoring are now available. In some cases these may be used to monitor ground conditions after construction, in addition to before and during construction. Other parameters increasingly becoming the focus for field testing and monitoring are suction and thermal properties.

An important parameter with respect to the Lias Group is that of permeability anisotropy.

As is the case with many sedimentary clay-rich formations, horizontal permeability may be expected to be greater than vertical by a factor of about 2.

Tunnelling

Information about tunnelling within the Lias Group is limited. Whilst there have been many tunnels excavated within the Lias Group, these have mainly been as part of the Victorian canal and railway networks, and little or no geotechnical information was obtained or retained. For example, the 2.85 km Blisworth Tunnel on the Grand Union Canal took 12 years to build (1793–1805) through Whitby Mudstone Formation and has been the subject of a re-alignment and much remedial work during its lifetime. Braunston Tunnel (1.85 km), north of Daventry, also on the Grand Union, was bored through Charmouth Mudstone Formation.

Modern projects involving tunnelling within the Lias Group include the proposed A417 re-alignment between Birdlip Hill and Crickley Hill, Gloucestershire (www.highways.gov.uk/roads) where a 2.8 km twin-bore tunnel has been proposed beneath a cambered and landslipped escarpment. The water-bearing properties of the Dyrham Formation and also the Bridport Sand Formation are important considerations for tunnelling. Artesian conditions may apply locally, particularly in landslipped terrain. In addition, the likelihood of mixed-ground working faces exposing clays, limestones, and sands, presents difficulties in the choice of tunnelling method. Such mixed ground conditions may be particularly variable and (in some cases) highly unpredictable in areas of cambering and deep-seated landslides.

The proportions of limestone to mudstone are an important consideration in the case of tunnelling in the Lias Group. This proportion varies from one formation to another and within formations. The Blue Lias Formation for example has a characteristic 50/50 to 60/40 ratio of mudstone to limestone for most of its thickness, a product of cyclic deposition processes. Each bed is typically 0.1 to 0.5 m in thickness, and is reasonably persistent laterally, except where faults have displaced them. Strong nodules of argillaceous limestone and ironstone are common within the Whitby Mudstone Formation. Strong cementstone nodules are found within the Dyrham Formation.

Weathering

The Lias Group mudrocks typically feature a high clay content, much of which consists of swelling clay minerals, and a laminated or shaly structure. These tend to produce high rates of breakdown on exposure, variation in water content, and stress relief fissures. In addition, chemical breakdown may occur very rapidly on exposure to air, and result in further mechanical breakdown. Where cementing agents strengthen the rock, breakdown may be gradual, but cycles of wetting and drying eventually produce failure. The development of tensile stresses due to desiccation and pore water suction also has a disruptive effect on mudrocks (Taylor, 1988[10]). In the East Midlands, examples of periglacial freeze/thaw action have been described that have resulted in deformation structures and brecciation (Kovacevic et al., 2007[11]). This has produced a weaker, more heterogeneous material compared with the un-brecciated source rock.

For the Lias rocks, a weathering classification scheme was developed, and the effects of weathering on strength assessed, by Chandler (1972)[12]. The current appropriate procedures for describing, and where possible classifying, weathering effects in variable mudrock sequences such as the Lias are described in British Standards BS5930 (1999)[8]. Further detail is contained in Weathering.

Summary

A summary engineering geological assessment is shown in Table 8.2. This describes in general terms some key engineering geological factors affecting engineering behaviour. The bearing capacity column was derived from guidelines given in BS 8004, Foundations (British Standard: BS8004, 1986[13]). The diggability column was formulated using SPT, UCS and Point Load data, in addition to lithological description and case histories and the application of methods described in Pettifer & Fookes (1994)[14] and Reeves et al., Chapter 11 (2006)[15].

Table 8.2    Summary engineering geological assessment of the Lias Group.
Formation Main lithologies Bearing capacity# Plasticity Shrink/swell potential Slope stability (natural) Diggability* Trafficability Concrete attack potential
Bridport Sand Sand, sandstone, siltstone Moderate Intermed. Low Moderate-Poor Medium Good Low
Blue Lias Mudstone, limestone (+sandstone) Moderate High Medium Moderate Medium to Hard, hard ripping Good Low
Charmouth Mudstone Mudstone, limestone Good High Medium Poor Medium to Hard, easy ripping Moderate-Poor High
Dyrham Mudstone, ironstone, sandstone, siltstone Moderate Intermed. Medium Moderate Medium Moderate-Good Medium
Redcar Mudstone Mudstone, siltstone, limestone, sandstone Moderate High Medium Moderate Medium to Hard, hard ripping Moderate-Good High?
Scunthorpe Mudstone Mudstone, limestone Very good Intermed. Medium Good Medium to Hard, hard ripping Good Medium
Whitby Mudstone Mudstone, siltstone, limestone Moderate High Medium Moderate Medium to Hard, easy ripping Moderate High

* refer to Pettifer & Fookes (1994)[14], Reeves et al., Chapter 11 (2006)[15]; # refer to British Standards BS8004 (1986)[13].

References

  1. Building Research Establishment. 1991. Sulphate and acid resistance of concrete in the ground. Building Research Establishment, BRE Digest 363.
  2. Longworth, T I. 2002. Contribution of construction activity to aggressive ground conditions causing the thaumasite form of sulphate attack to concrete in pyritic ground. Proc. 1st Int. Conf. on Thaumasite in Cementitious Materials. BRE, Garston, UK. June 2002.
  3. Floyd, M, Czerewko, M A, Cripps, J C, and Spears, D A. 2002. Pyrite oxidation in Lower Lias Clay at Concrete highway structures affected by thaumasite, Gloucestershire, UK. Proc. 1st Int. Conf. on Thaumasite in Cementitious Materials. BRE, Garston, UK. June 2002.
  4. 4.0 4.1 4.2 Charles 1993. Building on fill: geotechnical aspects. Building Research Establishment Report BR230.
  5. Cripps, J C, and Taylor, R K. 1981. The engineering properties of mudrocks. Quart. Journ. of Eng. Geol., London, Vol. 14, pp.325–346.
  6. 6.0 6.1 Murray, E J. 1998). Discussion on: ’Observations on soil permeability, moulding moisture content and dry density relationships’ by Wright, S P, Walden, P J, Sangha, C M, and Langdon, N J. Quaternary Journal of Engineering Geology, 31, Part 1, pp.73–74.
  7. Jones, D C K, and Lee, E M. 1994. Landsliding in Great Britain. London HMSO, 361p.
  8. 8.0 8.1 British Standards: BS 5930. 1981; 1999. Code of practice for site investigations. British Standards Institution, BS 5930.
  9. British Standards: BS 1377. 1990. Methods of test for soils for civil engineering purposes. British Standards Institution, BS 1377.
  10. Taylor, R K. 1988. Coal Measures mudrocks: composition, classification and weathering processes. Quarterly Journal of Engineering Geology, 21, 85–99.
  11. 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.
  12. Chandler, R J. 1972. Lias clay: weathering processes and their effect on shear strength. Geotechnique, 22, 403–431
  13. 13.0 13.1 British Standards: BS8004. 1986. Code of practice for foundations. British Standards Institution, BS8004.
  14. 14.0 14.1 Pettifer, G S, and Fookes, P G. 1994. Memoirs of William Smith LLD, author of the Map of the Strata of England and Wales by his nephew & pupil John Phillips FRS, FGS. First published 1844. Bath Royal Literary and Scientific Institute.
  15. 15.0 15.1 Reeves, G M, Sims, I, and Cripps, J C (editors). 2006. Clay Materials Used in Construction. Geological Society, London, Engineering Geology Special Publication, 21.