University of Birmingham The mineralogy and fabric of 'Brickearths' in Kent, UK and their relationship to engineering behaviour Milodowski, AE; Northmore, KJ; Kemp, SJ; Entwisle, DC; Gunn, DA; Jackson, PD; Boardman, David; Zoumpakis, Aris; Rogers, Christopher; Dixon, N; Jefferson, Ian; Smalley, IJ; Clarke, M DOI: 10.1007/s10064-014-0694-5 License: Creative Commons: Attribution (CC BY) Document Version Publisher's PDF, also known as Version of record Citation for published version (Harvard): Milodowski, AE, Northmore, KJ, Kemp, SJ, Entwisle, DC, Gunn, DA, Jackson, PD, Boardman, D, Zoumpakis, A, Rogers, C, Dixon, N, Jefferson, I, Smalley, IJ & Clarke, M 2015, 'The mineralogy and fabric of 'Brickearths' in Kent, UK and their relationship to engineering behaviour', Bulletin of Engineering Geology and the Environment , vol. 74, no. 4, pp. 1187-1211. https://doi.org/10.1007/s10064-014-0694-5 Link to publication on Research at Birmingham portal Publisher Rights Statement: The Author(s) 2015. 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When citing, please reference the published version. Take down policy While the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has been uploaded in error or has been deemed to be commercially or otherwise sensitive. If you believe that this is the case for this document, please contact [email protected] providing details and we will remove access to the work immediately and investigate. Download date: 27. Sep. 2021 Bull Eng Geol Environ (2015) 74:1187–1211 DOI 10.1007/s10064-014-0694-5 ORIGINAL PAPER The mineralogy and fabric of ‘Brickearths’ in Kent, UK and their relationship to engineering behaviour Antoni E. Milodowski • Kevin J. Northmore • Simon J. Kemp • David C. Entwisle • David A. Gunn • Peter D. Jackson • David I. Boardman • Aris Zoumpakis • Christopher D. F. Rogers • Neil Dixon • Ian Jefferson • Ian J. Smalley • Miche`le Clarke Received: 8 October 2014 / Accepted: 9 October 2014 / Published online: 14 March 2015 Ó The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Mineralogical and petrographical investigation support for the calcareous and ‘non-calcareous’ brickearth of two loessic brickearth profiles from Ospringe and Peg- horizons being of different age or origins. The calcareous well Bay in north Kent, UK have differentiated two types brickearth is metastable in that it undergoes rapid collapse of brickearth fabric that can be correlated with different settlement when wetted under applied stresses. It is char- engineering behaviour. Both sequences comprise meta- acterised by an open-packed arrangement of clay-coated, stable (collapsing) calcareous brickearth, overlain by non- silt-sized quartz particles and pelletised aggregate grains collapsing ‘non-calcareous’ brickearth. This study has (peds) of compacted silt and clay, supported by an inter- demonstrated that the two types of brickearth are discretely ped matrix of loosely packed, silt/fine-grained sand, in different sedimentary units, with different primary which the grains are held in place by a skeletal framework sedimentary characteristics and an erosional junction be- of illuviated clay. The illuviated clay forms bridges and tween the two units. A palaeosol is developed on the cal- pillars separating and binding the dispersed component silt/ careous brickearth, and is associated with the formation of sand grains. There is little direct grain-to-grain contact and rhizolithic calcrete indicating an arid or semi-arid envi- the resultant fabric has a very high voids ratio. Any applied ronment. No evidence has been found for decalcification load is largely supported by these delicate clay bridge and being responsible for the fabric of the upper ‘non-cal- pillar microfabrics. Collapse of this brickearth fabric can careous’ brickearth. Optically-stimulated dates lend further be explained by a sequence of processes involving: (1) dispersion and disruption of the grain-bridging clay on saturation, leading to initial rapid collapse of the loose- A. E. Milodowski (&) Á K. J. Northmore Á packed inter-ped silt/sand; (2) rearrangement and closer S. J. Kemp Á D. C. Entwisle Á D. A. Gunn Á P. D. Jackson stacking of the compact aggregate silt/clay peds; (3) with British Geological Survey, Environmental Science Centre, increasing stress further consolidation may result from Keyworth, Nottingham NG12 5GG, UK e-mail: [email protected] deformation and break up of the peds as they collapse into the inter-ped regions. Smectite is a significant component D. I. Boardman Á A. Zoumpakis Á C. D. F. Rogers Á I. Jefferson of the clay assemblage and will swell on wetting, further School of Civil Engineering, The University of Birmingham, encouraging disruption and breaking of the clay bonds. In Edgbaston, Birmingham B15 2TT, UK contrast, the ‘non-calcareous’ brickearth already possesses N. Dixon a close-packed and interlocking arrangement of silt/sand School of Civil and Building Engineering, Loughborough grains with only limited scope for further consolidation University, Loughborough, Leicestershire LE11 3TU, UK under load. Minor authigenic calcite and dolomite may also I. J. Smalley form meniscus cements between silt grains. These have Department of Geography, Leicester University, either acted as ‘‘scaffolds’’ on which illuviated clay has Leicester LE1 7RH, UK subsequently been deposited or have encrusted earlier- formed grain-bridging clay. In either case, the carbonate M. Clarke School of Geography, University of Nottingham, University cements may help to reinforce the clay bridge fabrics. Park, Nottingham NG7 2RD, UK However, these carbonate features are a relatively minor 123 1188 A. E. Milodowski et al. feature and not an essential component of the collapsible northeasterly winds. These re-worked silt materials are brickearth fabric. Cryoturbation and micromorphological from outwash sandurs and glacigenic deposits around the features indicate that the calcareous brickearth fabric has North Sea Basin lobe of the Late Devensian ice-sheet (cf. probably been developed through periglacial freeze–thaw discussions in Ballantyne and Harris 1993 and Antoine et al. processes. Freezing could have produced the compact 2003 and references therein). Subsequent re-working of silt/clay aggregates and an open porous soil framework these aeolian sediments through solifluction, colluvial and containing significant inter-ped void space. Silt and clay alluvial processes has produced a range of deposits were remobilised and translocated deeper into the soil variously described as ‘brickearth,’ ‘head brickearth,’ profile by water percolating through the active layer of the ‘river brickearth’ or ‘loam’ (Dines et al. 1954; Ballantyne sediment profile during thawing cycles, to form the loosed- and Harris 1993; Bell et al. 2003; Jefferson et al. 2003). packed inter-ped silt matrix and grain-bridging meniscus Some loess was also deposited during the Early Devensian clay fabrics. In contrast, the upper ‘non-calcareous’ and pre-Ipswichian periods (Parks and Rendell 1992). The brickearth may represent a head or solifluction deposit. deposits may once have been extensive, but as a result of Mass movement during solifluction will have destroyed erosion and re-working they are now preserved only as any delicate grain-bridging clay microfabrics that may isolated remnants; for example, in Jersey or in dolines in the have been present in this material. chalk of the Chiltern Hills (Parks and Rendell 1988, 1992; Antoine et al. 2003; Bates et al. 2003). Keywords Loess Á Brickearth Á Mineralogy Á Fabric Á Loess and loessic soils characteristically possess highly Collapse behaviour Á Palaeoenvironment porous metastable grain fabrics with relatively low densi- ties, and commonly have void space in their undisturbed state sufficient to hold their liquid limit water content at Introduction saturation (Bell and Culshaw 2001; Jefferson et al. 2001; Bell et al. 2003). Normally these materials are well drained Loess and loessic soils are aeolian deposits typically con- and, while in their natural, low-water content phase, pos- sisting of highly porous soil fabrics dominated by silt-sized sess high apparent strength. However, they have the po- quartz particles. In the UK, Quaternary loessic deposits tential to undergo rapid collapse and settlement when occur as discontinuous ‘spreads’ across southern and east- saturated under applied loads, a process referred to as hy- ern England, and parts of Wales and the English Midlands droconsolidation or hydrocollapse (Derbyshire et