The influence of vegetation and climate on clay earthwork slopes. Dr Joel Smethurst Faculty of Engineering and the Environment

Transcription

1 The influence of vegetation and climate on clay earthwork slopes Dr Joel Smethurst Faculty of Engineering and the Environment

2 Overview Background - climate, vegetation and slope failures Monitored site Southend, Essex Summary

3 Embankment construction end-tipped, dumped clay fill hidden defects shear surfaces high permeability lenses clod and matrix structure Image source:

4 Today embankments heavily vegetated

5 Ultimate slope failure London Clay embankment, failure occurred January 1994, closing the railway for two weeks Development of plastic shear forms deep seated mechanism Peak Post rupture Residual e Ellis and O Brien (2007)

6 Shallow slope failures West Coast Main Line, shallow failure, July 2012 Taken from

7 Use of field data to determine maximum wet winter pore water pressures Wet winter pore water pressures measured spring 2001 (~1:100 year wet winter) London Clay embankments, with a mix of vegetation covers From Briggs et al (2013) Data courtesy of London Underground/ Geo-Observations

8 Vegetation related seasonal shrink and swell m/c winter summer depth Photograph courtesy of Graham Birch, Network Rail

9 Hawkwell, Southend Poor track quality repeatedly causing speed restrictions Embankment covered in mature trees (some removed March 2007, all removed March 2010) Photo courtesy of Derek Butcher, Network Rail

10 Hawkwell - Instrumentation North side Inclinometer, extensometer, neutron probe access tube and Geo-Piezometers at crest and midslope Deep standpipe and Geo-piezometer at toe

11 Magnolia Road - Timeline Monthly rainfall (mm) Summer 2006 dry Summer 2007 and 2008 wet Summer 2009 and 2010 dry Vegetation removed from upper 2/3 of slope, instrumented section Vegetation removed from whole embankment, odd trees left at the toe

12 Magnolia Road - vertical displacements measured using magnet extensometers Apr Jun Aug Oct Dec Jan Apr Jun Aug Oct Dec-07 0 Displacement (mm) Trees removed from crest and mid-slope North side CREST - magnet 1.1m depth North side MIDSLOPE - magnet 0.62m Data courtesy of Network Rail/ Geo-observations

13 Magnolia Road pore water pressures and suctions measured using Geo-Piezometers Pore water pressure (kpa) Pore water pressure (kpa) Depth (m) Piezo BH06 Piezo BH05B Piezo BH04 Hydrostatic 0 Piezo BH Ash Embankment fill Underlying London Clay Pore water pressures, north side crest Measured range April 2006 November Standpipe BH23 Data courtesy of Network Rail/ Geo-Observations

14 Magnolia Road - climate Area east of London one of the driest and warmest parts of the UK Annual average rainfall Annual average potential evapotranspiration [calculated using Penman Monteith] ~640 mm ~700 mm

15 Influence of permeability on seasonal pore water pressure changes 5 x 10-6 m/s 5 x 10-7 m/s 5 x 10-8 m/s Finite element model From Loveridge et al (2010)

16 Track displacements at MR

17 Magnolia Road: soil water content measured with neutron probe Depth (m) Depth (m) Volumetric water content (m 3 /m 3 ) Volumetric moisture content (m 3 /m 3 ) Sep Mar Sep Dec-08 Ash Embankment fill Underlying London Clay Water content, north side crest April December Possible moisture deficit in underlying foundation 10

18 Magnolia Road pore water pressures and suctions measured using Geo-Piezometers Pore water pressure (kpa) Pore water pressure (kpa) Depth (m) Piezo BH06 Piezo BH05B Piezo BH04 Hydrostatic 0 Piezo BH Ash Embankment fill Underlying London Clay Pore water pressures, north side crest Measured range Sept 2009 March Standpipe BH23 Data courtesy of Network Rail/ Geo-Observations

19 Magnolia Road: Vertical displacements measured using magnet extensometers Trees removed from crest and midslope Swelling occurs as the embankment re-wets Magnet depths below ground level: 1.14 m Settlement (mm) Apr Jul Sep Dec Apr Jul Oct Dec-07 Shrink and swell movement observed over summer and autumn of Apr Jul Oct Dec Apr Jul Oct Jan Apr m 2.05 m 2.70 m 3.07 m 3.63 m 4.13 m 4.59 m 5.63 m 6.71 m 7.62 m m Data courtesy of Network Rail/ Geo-Observations

20 Magnolia Road Shrink/swell movements at Magnolia Road reduced by removal of mature vegetation, although swelling movements continue for 3 years as structure rewets. Removal of mature trees has allowed rewetting and loss of semi-persistent soil suction Near zero pore water pressures wont immediately threaten stability, but slope will be vulnerable to prolonged high intensity events

21 Wider effect of trees on track displacements Vertical seasonal track movement (mm) 1.25 D/H ratio (distance from the track / height of tree)

22 Compromise between shrink/swell movements and slope stability May not be easy answers trees some distance from the track have potential to cause damaging movements Could cut the vegetation off and then accept hard (and expensive) stabilisation methods (possibly reflects the current approach) May be possible to cut the high water demand trees off the upper slope (but leave at the toe), and replace with shrubby lower water demand vegetation further work required

23 Conclusions Embankments are partially saturated, and temporal changes in water content and pore water pressure can be quite complex Shrink/swell movements reduced by removal of mature vegetation although removal of mature trees has allowed rewetting and loss of soil suction Compromise between shrink/swell movements and slope stability required cut the HWD trees off the upper half of the slope?

24 Acknowledgements Network Rail Geo-Observations Arup Engineering and Physical Sciences Research Council Kevin Briggs Aingaa Sellaiya William Powrie Fleur Loveridge Derek Clarke

25 Publications Smethurst J., Clarke D. & Powrie W. (2006). Seasonal changes in pore water pressure in a grass covered cut slope in London Clay. Géotechnique, 56 (8), Smethurst J. & Powrie W. (2007). Monitoring and analysis of the bending behaviour of discrete piles used to stabilise a railway embankment. Géotechnique, 57 (8), Briggs, K. M. (2010). Charing embankment: climate change impacts on embankment hydrology. Ground Engineering, June 2010, Clarke D. & Smethurst J. (2010). Effects of climate change on cycles of wetting and drying in engineered clay slopes in England. Quarterly Journal of Engineering Geology and Hydrogeology, 43 (4), Loveridge, F. A., Spink, T. W., O Brien, A. S., Briggs, K. M. & Butcher, D. (2010). The impact of climate and climate change on UK infrastructure slopes. Quarterly Journal of Engineering Geology and Hydrogeology, 43, Smethurst, J. A., Clarke, D. & Powrie, W. (2012). Factors controlling the seasonal variation in soil water content and pore water pressures within a lightly vegetated clay slope. Géotechnique, 62, (5), Briggs, K.M., Smethurst, J., Powrie, W. and O'Brien, A. (2013) Wet winter pore pressures in railway embankments. Proceedings of the ICE - Geotechnical Engineering (Published online ahead of print). Briggs, K.M., Smethurst, J.A., Powrie, W., O'Brien, A.S. and Butcher, D. (2013) Managing tree removal from railway earthwork slopes. Ecological Engineering (In Press).