Entwisle, David

Jefferson, Ian

Milodowski, Tony

Northmore, Kevin

English

NERC Open Research Archive

© NERC All rights reserved The engineering geology of Loessic Deposits in south east England David Entwisle+, Kevin Northmore+, Tony Milodowski+ and Ian Jefferson* +  British Geological Survey, Keyworth, Nottingham *  Civil Engineering Department, Birmingham University © NERC All rights reserved Synopsis • What are loessic deposits? • Where do they occur? • Why are they important? • Associated geohazards • Why they may collapse • Test site • Laboratory test • Microstructure © NERC All rights reserved What are Loessic Deposits? • Aeolian (wind blown) origin • Predominantly SILT   • Deposited in a dry climate (cold or hot) • Prismatic jointing • Open structure (large voids) – free draining © NERC All rights reserved Loessic deposit types Wind blow silt River loess Head loess Cryoturbation Soil forming process Loess Altered in situ Soil forming process Cryoturbation © NERC All rights reserved Loessic deposits - Distribution Cover ~10% of the Earth’ land surface •North America •South America •Eastern Europe and Western Russia •Central Asia  •China (>100 m thick)  For the distribution of European loessic deposits see http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VBC-4NF2NN0-1&_user=1001893&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000027978&_version=1&_urlVersion=0&_userid=1001893&md5=7d88bedcff30a0ed5294d45a1d6703a7.  © NERC All rights reserved Southern England Distribution - Geological Map Old names •Brickearth •Head silt •Head brickearth •River brickearth •Loess  Named Members •Crayford Silt •Dartford Silt •Enfield Silt •Ilford Silt •Langley Silt •Roding Silt  Not a Loessic deposit •Norwich Brickearth   This map is being updated! OS topography © Crown copyright. All rights reserved 10007897/2008 •Generally patchy distribution - redistributed •Thickest deposits in South East England generally < 5 m © NERC All rights reserved Southern England Distribution - Geological Map Mostly on high  permeability materials  •Terrace sands and gravels •Raised Beach deposits •Thanet Sand Formation •Chalk •Limestone OS topography © Crown copyright. All rights reserved 10007897/2008 © NERC All rights reserved Why are they important? • Cover ~10% of the Earth’s land • Extremely important agricultural soil • Brick manufacture  Engineering Hazard  • Hydro collapse • Landslides and unstable during Earthquakes (China) © NERC All rights reserved Hydro collapse – loading + saturation Loading + saturation     collapse of soil fabric      foundation distress Lower Loess 0.40.50.60.70.80.91.010 100 1000 10000Consolidation stress (kPa).Voids ratio (e)f f f f Painton, Devon F = Flooding stress © NERC All rights reserved Study Sites • Kent –  • Ospringe Brick Pit, (Faversham) – ‘Head Brickearth’  (NGR 599700, 161200) NERC/ESPRC Loessic Deposits test site • Pegwell Bay (Coastal Section)– ‘Head Brickearth’  (NGR 635374, 164402) Pegwell Bay Ospringe Brick Pit OS topography © Crown copyright. All rights reserved 10007897/2008 © NERC All rights reserved Example sites – Ospringe and Pegwell Bay Part of Section, Ospringe Bench Section, Pegwell Bay Two distinct Loessic deposits Upper Loess Lower Loess © NERC All rights reserved Example – Ospringe and Pegwell Bay Upper Loess Thanet Sand Formation Ospringe Pegwell Bay • Well drained light olive brown silt with clay • Open root channels + fissures   Vertical calcified root structures + nodular calcite (calcrete) Lower Loess Horizontal calcified root structures + cryoturbated fabric  Erosional surface © NERC All rights reserved Lower loess detail Vertical and horizontal calcified root structure Distorted laminae (Cryoturbation?)  below picture on the left © NERC All rights reserved Engineering Characteristics –Typical Consolidation Behaviour Lower Loess 0.40.50.60.70.80.91.010 100 1000 10000Consolidation stress (kPa).Voids ratio (e)f f f f 0.300.400.500.600.7010 100 1000 10000Consolidation stress (kPa).Voids ratio, ef f f Upper Loess No Collapse Collapse f = flooding stress © NERC All rights reserved Engineering Characteristics 0.00.51.01.52.02.53.00 10 20 30 40Water content (%)Depth (m)Water content Liquid Limit Upper Loess Lower Loess Key Plasticity Index Ospringe site © NERC All rights reserved Engineering Characteristics Particle Size distribution 01020304050607080901000.001 0.01 0.1 1 10 100PERCENTAGE PASSING (%)CLAYSAND GRAVELCOBBLESFine Medium Medium Coarse Fine MediummmSILTCoarseCoarse FineUpper Loess Lower Loess © NERC All rights reserved Microstructure upper loess e) Close-packed silt grain sometimes interlocked grains   Large voids representing root hair channels f) Silt grains completely coated with clay  © NERC All rights reserved Microstructure lower loess a) Loose packed framework of silt grains coated is clay film b) Clay forms meniscus ‘bridges’ (x) bonding dispersed silt grains c) Fine needle calcite (x) forming a meniscus film ‘bridges’ between silt grains d) Meshwork of fine needle calcite (x) forming meniscus ‘bridges’ forming scaffold for clay © NERC All rights reserved Lower loess - Structure  A - Open Fabric – loosely packed grains supported by meniscus clay bridges  Authigenic carbonates precipitate on the surface of the clay minerals – stronger bridges B – discrete domains of closely packed grains with clay grain coatings Grain-coating clay meniscus around grains – bridges across pore throats – supports open loose packing – fabric A © NERC All rights reserved Lower loess - Microstructure Formation of Calcium Carbonate  •Calcium ions from breakdown of minerals (plagioclase, chalk and microfossils) •Carbonate ion  from root respiration, soil atmosphere, organic decomposition, dissolution of carbonate minerals •Calcium Carbonate precipitates out when the pore water is supersaturated from evaporation  © NERC All rights reserved Review - Differences between Loess types  Upper Loess Non collapsing – fairly consistent structure  Mineralogy + particle shape proportion of glacial origin Very little calcareous material (fossils and recent precipitation in root tracts) Saturation ratio generally > 75%   More clayey > 18% wL >30  Origin – Solifluction deposit (Head) - OSL Dating 18-19000 yrs BP  Lower Loess Collapsing – variable, some open structure Mineralogy + particle shape – mostly local origin (Thanet Sand Formation) Groundwater calcrete and rhizocrete Saturation ratio generally < 75%  Less clayey <18% wL < 30 Origin – An original loess altered by freezing/thaw in Tundra conditions OSL Dating 23-24000 yrs BP Erosion © NERC All rights reserved Conclusions • Loess and loessic deposits are silt-rich deposits of aeolian origin • Cover c10% of the earths surface • Post depositional processes have a marked influence on their engineering behaviour  • In Kent (and Essex) two distinct loessic deposit are present - laid down during separate depositional events. • Upper loessic deposit does not collapse: probably a solifluction (Head) deposit • Lower loessic deposit is meta-stable is probably a primary loess modified in situ by freeze/thaw giving rise to open structure. Structure maintained by bridging inter-particle bonds. • The upper is not a leached horizon. © NERC All rights reserved Acknowledgements David Gunn, Peter Jackson, Simon J Kemp, British Geological Survey. David Boardman, Chris Rogers, University of Birmingham. Aris Zoumpakis, formerly University of Birmingham. Ian Smalley, University of Leicester. ex University of Birmingham.  Michèle L Clarke, University of Nottingham.  © NERC All rights reserved References Project related papers Jackson, P D, Northmore, K J, Entwisle, D C, Gunn, D A, Milodowski, A E, Boardman, D I, Zourmpakis, A, Rogers, C D F, Jefferson, I, Dixon, N . 2006. Electrical resistivity monitoring of a collapsing meta-stable soil: In: Quarterly Journal of Engineering Geology & Hydrogeology Vol. 39 pt/no 2 (2006) p. 151-172. Northmore, K.J.; Jefferson, I.; Jackson, P.D.; Entwisle, D.C.; Milodowski, A.E.; Raines, M.R.; Gunn, D.A.; Boardman, D.I.; Zourmpakis, A.; Nelder, L.M.; Rogers, C.D.F.; Dixon, N.; Smalley, I.J.; 2008. On-site characterisation of loessic deposits in Kent, UK.  Proceedings of the Institution of Civil Engineers Geotechnical Engineering Vol. 161 pt/no 1, 3-17.  Rodriguez MA, KJ Northmore, DC Entwisle, PD Jackson & L Nelder, 2006. Geotechnical characterisation of loessic brickearth deposits at Faversham, Kent, UK. . 5th European Congress on Regional Geoscientific Cartography and Earth Information Systems, Barcelona, Catalonia, Spain, 13-16 June 2006  A. Zourmpakis, D.I. Boardman, C.D.F. Rogers, R.S. Karri, D.A. Gunn, P.D. Jackson, K.J. Northmore, D.C. Entwisle, A.E. Milodowski, M.G. Raines, L.M. Nelder, N. Dixon, I. Jefferson, I.J. Smalley, A. 2006.  Case Study of a Loess Collapse Field Trial in Kent, South East-England.  Quarterly Journal of Engineering Geology and Hydrogeology, 39, 131-150. Gunn DA, Nelder LM, Jackson PD, Northmore KJ, Entwisle DC, Milodowski AE, Raines MR, Boardman DI, Zoumpakis A, Rogers CDF, Karri RS, Dixon N & Jefferson I. 2006.  Shear Wave Velocity Monitoring of Collapsible Loessic Brickearth Soil. Quarterly Journal of Engineering Geology and Hydrogeology, 39, 173-188. Jackson PD, Northmore KJ, Entwisle D C, Gunn DA, Nelder LM, Milodowski AE, Raines MR, Boardman DI, Zoumpakis A, Rogers CDF, Karri RS, Dixon N & Jefferson I. 2006.  Resistivity Monitoring of a Collapsing Metastable Soil. Quarterly Journal of Engineering Geology and Hydrogeology, 39, 151-172. Clarke, M L, Milodowski, A E, Bouch, J F, Leng, M J, Northmore, K J. New OSL dating of UK loess : indications of two phases of Late Glacial dust accretion in SE England and climate implications: In: Journal of Quaternary Science Vol. 22 pt/no 4 (2007) p. 361-371  Clarke, M L, Milodowski, A E, Bouch, J F, Leng, M J, Northmore, K J. 2008 Replay: evidence for episodic dust accretion in SE England:  Journal of Quaternary Science Vol. 23 pt/no 4 , p. 361-371. Catt, J A. 1985. Soil particle size distribution and mineralogy as indicators of pedogenic history: examples from the loessial soils of England and Wales. In: Richards, K S, Arnett, R R, Ellis (eds), Geomorphology and Soils. Allen and Unwin, London, pp 202-216.  Web sitses .http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VBC-4NF2NN0-1&_user=1001893&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000027978&_version=1&_urlVersion=0&_userid=1001893&md5=7d88bedcff30a0ed5294d45a1d6703a7.   http://loessletter.co.uk/ 

The engineering geology of Loessic Deposits in south east England

http://nora.nerc.ac.uk/500279/1/TVRG_presentation_0309_compressed_alt1.pdf

The engineering geology of Loessic Deposits in south east England

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