The effects of groundwater level and vegetation on creep of the Mam Tor landslip

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© Blackwell Publishing Ltd, The Geologists’ Association & The Geological Society of London, Geology Today, Vol. 26, No. 4, July–August 2010

FeatureThe effects of groundwater level and vegetation on creep of the Mam Tor landslip

Sam Green, Ernest Rutter & Robert HollowayRock Deformation

Laboratory, School of

Earth, Atmospheric and

Environmental Sciences,

University of Manchester,

Manchester M13 9PL, UK

The creeping Mam Tor landslide (North Derbyshire, UK) is sliding slowly downhill at up to 0.5 m per year. Annual measurements using conventional surveying techniques have shown that creep is exacerbated by periods of heavy rainfall, but to establish a detailed correlation between creep and groundwater levels requires spatially and temporally higher resolution monitoring. We have acquired three years of such data from wire creep meters, borehole piezometers and rainfall monitoring. These measurements showed that creep is strongly focused into the winter months and that it correlates well with groundwater levels. Summer grass and fern growth exerts a strong influence on rainfall infiltration, recycling much of the summer rainfall back to the atmosphere, and explaining the seasonal variability of creep motion.

The Mam Tor landslip near Castleton in Derbyshire, in the East Midlands of England, is in a perennial state of creep motion that is qualitatively known to be enhanced in periods of heavy and persistent rainfall. A rise in groundwater level increases the pore water pressure at the base of the slide, which has the effect of hydraulically lifting some of its weight, thereby reducing the frictional resistance to downslope slid-ing. Despite having formed about 3500 years ago, the landslip has the greatest surface slope (12°) of the several landslips on either side of the ridge separating Hope valley from the Vale of Edale (Fig. 1), and is the only one that still continues to be active. Situated in the Peak District National Park, the area around Mam Tor is scenically spectacular and is regularly visited by tourists in addition to school/university stu-dent groups interested in the geology and environ-ment. The landslipped area is traversed twice by the old Manchester–Sheffield highway as it makes its way downslope (Fig. 2) and the damage done to the road by continued creep movements since it was closed in 1979 is evident to all (Fig. 3). For this reason the results of the continuing study we have made of creep of the landslip since 1996 are of general interest.

The slipped mass is about 900 m long, 350 m wide and an average of 20 m thick. It slipped on a

spoon-shaped (listric) fault surface, now partly pre-served as the cliff face of Mam Tor, so that the upper part of the slip is strongly back-rotated (Fig. 4). Over most of its area, the slip involves mudstones of the Namurian Edale Mudstone Formation sliding over older rocks of the same unit and spreading downslope but also laterally to some degree to form an earthflow. The surface of the slip is very irregular, a characteris-tic feature of unstable ground (Fig. 2). The constant movement causes the formation of many small ponds in hollows from which there is no natural drainage outwards. We previously reported in Geology Today in 2003 the results of seven years of monitoring of a network of about 50 stations on the landslip by

Fig. 1. Google satellite photograph of the topographic ridge (highlighted with black dashed line) lying between the villages of Edale and Castleton, Derbyshire, UK. Several ancient (but now stable) landslips (highlighted) lie to the north-west and the active Mam Tor slip lies to the south-east. Arrows indicate directions of slip movement.

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means of annual electronic distance measurements (EDM). These measurements showed that the whole mass slides downhill (west to east) rather like a gla-cier, not as a single rigid mass but with the middle part moving faster (up to 50 cm/yr) than the edges (about 7 cm/yr), owing to frictional drag (Fig. 5). The movements are also unevenly distributed within the slip, so that a certain amount of resculpting of the landslip topography is continually taking place. The uphill portion is being stretched and thinned, as can be seen from the way that the upper arm of the high-way is being torn apart (Fig. 3). The lower part of the slide is being shortened in the direction of movement and thickened, whilst also spreading laterally to some degree. Figure 5 shows the results of cumulative EDM measurements from 1996, now extended to 2009, together with measurements reported by Walstra and colleagues in 2007, made on a succession of aerial photographs taken in different years.

Year-on-year the movement vector at any given

station maintains the same transport direction. The amount of slip each year varies according to the amount of rainfall, but each year the ratio of move-ments between any pair of stations remains constant, except for station M10, which responds dispropor-tionately more strongly to rainfall than other stations. Thus the local slope immediately east of M10 (occu-pying approximately one-half a hectare) is becoming steeper year by year, and this may eventually trigger a rapid local collapse. Over the past decade there has been no acceleration of movements of M10, but the anomalous behaviour it demonstrates merits keeping a close watch on this sub-area.

Historical movements at Mam Tor

A key part of our ongoing study of Mam Tor land-slip creep has been annual EDM measurements since 1996, but with the aid of historic maps (e.g. the original Ordnance Survey map for the Hope Valley) and aerial photos to quantify the horizontal compo-nent of movement at Mam Tor since 1820, when the highway was constructed. A plot of horizontal movement against time displays a linear slip rate over a 190-year period (Fig. 6). The fastest moving survey station on the lower road (N10) is readily identifiable on all maps and images and was chosen as a point of comparison. The total movement over 190 years is 48 m, corresponding to an annual horizontal move-ment rate of 0.26 m/yr. This compares well with the average rate of 0.25 m/yr obtained from EDM measurements at N10 between April 1996 and April 2009. Thus there is no evidence of any anomalous change in behaviour in recent decades. Fluctuations of apparent rate on Fig. 6 can be attributed to normal year-on-year rainfall fluctuations.

At 0.25 m/yr, the total horizontal movement of N10 since the time of the initial collapse is 900 m, which is substantially greater than the distance from N10 to the footwall head scar on Mam Tor. Therefore the landslip has been sliding for the past 190 years at a rate substantially faster than the time-averaged rate

Fig. 2. View of the Mam Tor landslip from the north, with the disrupted highway that transects the slipped mass twice. The evening shadows accentuate the irregularity of the ground on the slip. The hill on the other side of the slip is the limestone spur around which the earthflow is deflected.

Fig. 3. Damage done to the western arm of the highway that crosses Mam Tor landslip respectively near (a) the southern edge and (b) the northern edge of the slip, and which led to the closure of the highway in 1979. The displacements across emergent faults show the multiplicity of repairs made by Derbyshire County Council.

Fig. 4. West-East cross-section through the Mam Tor landslip, showing the pre-collapse shape of the upper slope, the position of the basal slip surface, and the back-rotation of the Mam Tor beds (alternating sandstones and mudstones) arising from the curved shape of the upper part of the slip surface. ‘BH’ signifies boreholes made by Derbyshire County Council in the 1980s and which constrain the depth to the base of the slip.

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since the initial collapse of Mam Tor. This is taken to imply that at some time prior to the nineteenth cen-tury, climatic variability led to increased infiltration of rainfall at Mam Tor.

High resolution monitoring

Measurements made only once per year can at best indicate only a crude correlation between their mag-nitude and amount of rainfall. To understand better the relation between rainfall, groundwater level and creep motion a program of continuous high-resolu-tion creep measurements, rainfall measurements and groundwater pressure was required. Starting in 2004/5 we installed a number of continuously log-ging instruments to obtain such data. These com-prise:1. An automatic rain gauging station, also measuring

air temperature and pressure, sited immediately east of the toe of the landslip;

2. Two boreholes (LBH and UBH, upper and lower boreholes respectively), 50 m apart in the central part of the slip, each instrumented with two tilt meters (to measure internal readjustments of the landslip) and two piezometers (to measure groundwater pressure), at depths of about 5 m and 8 m respectively in each hole (Fig. 5); and,

3. Three invar wire creep meters, two of (NS4 to NS3, and NS3 to NS2) which are 20 m long, mounted end-to-end within the landslip (Fig. 5), at 45° to the southern boundary of the slip such that they undergo shortening. These measure differential movements within the slip. The third (NS2 to a

fixed point off the landslip to the south-west), 10 m long, traverses the southern margin of the slip. It is oriented 45° to the slip edge such that it undergoes extension. The wires are stretched by weights between pillars in buried pipes. Displacements are measured by potentiometric transducers of 0.3 mm resolution, and each transducer typically measures on the order of 10 cm of movement per year. A temperature sensor provides data from which corrections can be made for thermal expansion of the invar wires, and a dummy transducer is used to obtain corrections for data logger battery voltage fluctuations.

Data are collected at three-hourly intervals and can be accessed via GSM telephone connections to each data logger. Except for occasions when data loss has occurred owing to technical problems, we now have about three years of data, described below, that dem-onstrate in detail the relationships between creep, groundwater level and rainfall.

Fig. 5. Map of the emergent boundary of the main shear surface of the Mam Tor landslip showing total displacements as arrows with circle and station number attached (note separate scale) over the period 1996 to 2008, measured during annual electronic distance measurement (EDM) surveys. These data are consistent with those measured by Walstra, Dixon and Chandler in 2007, from the analysis of aerial photographs and scaled to the same time period and shown as arrows without an attached circle. The present positions of the two arms of the old Manchester to Sheffield highway are shown, and also the position of the lower arm of the highway as it was on the 1880 Ordnance Survey map. The highway has moved downhill by up to 40 m in 130 years, a rate consistent with movement rates measured using EDM over 12 years. The positions of the creep meters and instrumented boreholes are also shown. Displacement vectors at the creep meter stations are also scaled to 12 years of movement.

Fig. 6. Graph showing the distance moved by station N10 on the lower arm of the road since 1820. The trend line shows that movement since 1820 has occurred at the same rate except for short-term fluctuations attributable to natural rainfall variation and there has apparently not been any substantive variation from this trend in recent years.

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Results

Creep meters and rainfall Figure 7 shows creep meter data for the period May 2006 to September 2009. The total change in the lengths of the wires was 63 cm during that time. This corresponds well to the measurements of changes of wire lengths obtained from annual electronic distance measurements (Fig. 7). Resolving the length changes along the wires into corresponding shear displacement parallel to the slip vector shows that the total shear displacement along the array (between stable ground and NS2) is 89.1 cm. Each of the creep meters shows that normally movements occur in the winter months but not in the summer. Exceptionally, a movement occurred in the summer of 2007. This corresponded to a period of unusually heavy and persistent rain during June and July (Fig. 8), which led to major flooding in central England. A period of heavy rain preceded the onset of winter creep in late 2007, but the much steadier accumulation of rainfall through both summer and winter of 2006 and into 2007 showed no particular correlation with the onset of creep at the end of 2006.

The records from all three creep meters are closely in-phase. Thus any creep rate discontinuities within the slip (for example across internal fault surfaces) would require observations over a larger length-scale than the size of the creep array permits.

Groundwater pressure Figure 7 also shows groundwater pressure (expressed as head of water in metres) records for the lower piezometric transducer in the lower borehole. Records from the piezometers in the two boreholes were found to vary in sympathy, and could be brought into coincidence using a single scaling factor. In this way some gaps in data coverage could be filled. At first sight the upper and lower piezometer traces in each hole seemed to vary in-phase, with a pressure difference corresponding to their vertical separation. Rainfall events are marked by a rapid rise in water pressure, within a few hours, followed by a slower decay over several days as lateral seepage out of the slope occurred. By examining the difference between the two traces, however, it was found that the higher transducer responds to a rainfall event first, so that the apparent pressure difference falls at the onset of a rainfall event, followed by a slower recovery to the normal pressure difference. This behaviour corresponds to a ‘wave’ of groundwater pressure passing downwards from the surface as rainfall begins. It implies that groundwater pressure fluctuations following rainfall events vary throughout the thickness of the slope, such that the short term fluctuations become progressively more damped out at deeper levels within the slope. This is why we used the lower transducer of the two

records to seek correlations between groundwater level and creep events. Nevertheless, at 8 m depth the groundwater pressure fluctuations with individual rainfall events are still very marked. These short-term fluctuations are superimposed on the longer-period variations in water pressure that can be related to the creep movements.

Fig. 7. Time-series data obtained from the three creep meters (above) and the groundwater pressure (metres) obtained from the lower piezometer in borehole LBH (lower graph). The piezometer lies 8.0 m below the present ground surface. The horizontal line at 4.5 m is arbitrarily drawn as a reference line to help contrast the periods of relatively high and low groundwater pressure. The short term fluctuations correspond to individual rainfall events. The vertical grey bars correspond to principal creep periods that occur when groundwater levels are high. Creep displacements are shown parallel to the wires, and the plotted points with error bars are changes in wire lengths measured by EDM at the ends of each wire.

Fig. 8. The relationship between creep (lower graph) and rainfall (upper graph) at Mam Tor. Cumulative rainfall and monthly rate of rainfall are shown. There is a strong correlation of creep with the marked rainfall periods in summer 2007 and winter 2007-08. The uppermost graph (MORECS) shows the notional loss of infiltrating rainfall due to evapotranspiration by plants, and this amount has been subtracted from the total rainfall rate graph beneath for the corresponding period (shaded interval). Evapotranspiration significantly reduces the amount of penetrating rainfall in the summer months, and this helps explain the polarization of creep into the winter months. However, as the summer 2007 shows, this effect can be swamped by extreme rainfall.

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Relating groundwater pressure to creep movements In Fig. 7, groundwater pressure data are laid alongside creep meter data with the same timescale. The vertical grey bars indicate the time limits of significant creep events. It can be seen that these correlate with periods of high groundwater pressure. In most cases the onset of accelerated creep seems to lag a little behind the onset of elevated water pressure. The periods of quiescence correlate strongly with low groundwater pressures. The overall seasonal variation in groundwater pressure is small, only about ± 0.7 m. The highest groundwater level attained lies about 2.6 m below the present land surface at the site of borehole LBH.

Figure 9 shows how log creep rate (as mm/day, re-solved parallel to the overall downhill transport direc-tion) responds to variations in log groundwater level. The groundwater level record has been smoothed over monthly intervals to remove the short-term fluctuations associated with individual rainfall events. The log/log plot can be fitted well to a straight line. The creep rate is very sensitive to small changes in groundwater level.

The best-fit line is extrapolated to a groundwater level corresponding to total saturation of the slope, leading to a predicted slip rate of 0.62 m/day. It is extremely unlikely that such a degree of saturation could be attained. It would require an extraordinary period of heavy and persistent rainfall, to overcome groundwater loss through lateral seepage, which would in turn occur faster at higher degrees of slope saturation.

Factors affecting groundwater level Our results show that creep is initiated on the Mam Tor landslip when groundwater level rises sufficiently, and that the rate of creep is related to the groundwater level averaged over (arbitrarily) one month intervals to smooth out the effects of short term fluctuations due to individual rainfall events. Figure 8 shows how rainfall is related to creep events. Rain data are presented as cumulative rainfall and as rate of rainfall (in units of mm/month). Rainfall is obviously a prime influence on groundwater level. As the creep event of midsummer 2007 shows, a period of persistent and heavy rainfall can raise groundwater level sufficiently to precipitate creep. However, this episode excepting, rainfall alone is not sufficient to explain the general polarization of creep preferentially into the winter periods, for average summer rainfall rate is often as high as winter rainfall. A further factor to consider is the role of vegetation, which grows vigorously through the summer months then dies back during the winter months. The traditional view of the effect of vegetation, especially trees and bushes, is that the root structure ramifies the slipped material and resists deformation. It is also recognized that vigorous

summer growth of trees and bushes draws moisture out of the soil and lowers the groundwater level, but it is quite difficult to quantify the latter effect for heterogeneous distributions of variously sized trees.

The Mam Tor slip is almost devoid of trees and bushes, but it has a homogenous cover of grass and ferns whose shallow root structures probably do not contribute a significant stabilizing effect. On the other hand they may restrict infiltration of summer rain through the evapotranspiration effect, whereby water is rapidly returned to the atmosphere. Grass and fern cover also retains water droplets on the leaves, which are returned to the atmosphere by evaporation. The UK Meteorological Office Rainfall and Evaporation Calculation System (MORECS) provides data in terms of equivalent weekly rainfall returned to the atmos-phere by evapotranspiration for specific vegetation types in a given area for a particular time window. Figure 9 shows how this effect modifies the effective rainfall available for infiltration at Mam Tor for the period up to May 2008. The modified rainfall figure also includes that unknown proportion that will be lost to runoff and lateral subterranean seepage, so it is not possible to compute groundwater levels from the rainfall data. However, the data show that eva-potranspiration can reduce summer rainfall available for infiltration substantially, so that the modified rain-fall data now show a stronger correlation with the creep events. The data also show how the exception-ally heavy rainfall of summer 2007 overwhelmed the capacity for plant activity to recycle summer rain to the atmosphere, leading to the summer creep event. The marked impact of the evapotranspiration effect in the stabilization of Mam Tor creep is probably due to the homogeneous nature of the grass cover and lack of individual large trees, which would be expected to exert large local effects. These results suggest that ho-

Fig. 9. Plot of log measured creep rate (mm/day) measured between station NS4 and stable ground, resolved parallel to overall slip direction, versus log Ground Water Level (GWL). GWL is averaged over one month periods to eliminate the effect of short period fluctuations due to individual rainfall events. The linear trend implies a power-law relation between creep rate and GWL, with creep rate extremely sensitive to small changes (less than 1 m) in GWL. The linear fit on this plot is extended to indicate the predicted creep rate (62 cm/day) that might arise if complete saturation of the landslip could occur.

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Fig. 10. Long term variations in rainfall at Mam Tor (since 1931, data from Derbyshire County Council: since 2004 data from our rain gauging station). The long term trend is very linear (1.30 m/yr), but deviations from the best-fit straight line are plotted here. The 1990s were relatively dry, but since year 2000 cumulative rainfall has increased substantially, to a level not seen in north-central England since the 1880s.

mogenous plant cover can make a larger contribution than formerly realized to slope stabilization.

Long term trends in rainfall at Mam Tor are il-lustrated in Fig. 10. This shows deviations from the best-fit to a plot of cumulative rainfall vs. time since year 1931. The average rainfall during all of that time was 1.297 m/year. During the 1990s rainfall was below average but since year 2000 the rainfall deviation has been increasing, such that the average annual rainfall during the past decade was 1.640 m. It is presently unknown how much further the rain-fall will deviate in the direction of increasing wetness, but it should be noted that north-central England has previously been as wet as now, for example in the 1880s. It is not the overall annual rainfall amount that determines whether Mam Tor landslide moves, but how rainfall is distributed in terms of magnitude of events and their frequency, and the time of the year. There is no simple way to relate rainfall to creep events.

Conclusions

By means of high-resolution creep metering, coupled with groundwater pressure measurements and rain-fall monitoring, we have demonstrated the episodic nature of the creep movements that affect the Mam Tor landslide. We have found that movements are en-hanced by quite small rises in groundwater level and are normally focused over the winter months. The latter tendency is enhanced by the summer growth activity of plants (grasses and ferns) that remove in-filtrating water that would otherwise contribute to raising the pore water pressure at the base of the landslip and precipitating creep motions. Sufficiently heavy and prolonged rainfall can, even in summer, overwhelm the stabilizing effect of evapotranspiration by plants.

Acknowledgements

This work was funded by UK NERC grant NER/B/S/2003/00194 and carried out with the permission of the National Trust and English Heritage. We are grateful to the staff of the High Peak National Trust Office, particularly Chris Millner and Stephen Trotter, and to Chris Arkwright, Tony Barrett, Steve Boult and Steve May of Manchester University Earth Sci-ences for support during fieldwork.

Suggestions for further reading

Arkwright, J.C., Rutter, E.H. & Holloway, R.F. 2003. The Mam Tor Landslip; still moving after all these years. Geology Today, v.19, pp.59–64.

Greenwood, J.R., Norris, J.E. & Wint, J. 2004. As-sessing the contribution of vegetation to slope sta-bility. Proc. Institute of Civil Engineers, Geotechnical Engineering v.157, GE4, pp.199–207.

Ridley, A., McGinty, B. & Vaughan, P. 2004. Role of pore water pressures in embankment stability. Proceedings of the Institute of Civil Engineers, Geotech-nical Engineering v.157, GE4, pp.193–198.

Skempton, A.W., Leadbeater, A.D. & Chandler, R.J. 1989. The Mam Tor landslide, North Derbyshire. Philosophical Transactions of the Royal Society of Lon-don, v.A329, pp.503–547.

Rutter, E.H., Arkwright, J.C. & Holloway, R.F. 2003. Strains and displacements in the Mam Tor land-slip, Derbyshire, England. Journal of the Geological Society, v.160, pp. 735–744.

Walstra, J., Dixon, N. & Chandler, J.H. 2007. Histori-cal aerial photographs for landslide assessment: two case histories. Quarterly Journal of Engineering Geology and Hydrogeology, v.40, pp.315–332.

Waltham, A.C. & Dixon, N. 2000. Movement of the Mam Tor landslide, Derbyshire, U.K. Quarterly Journal of Engineering Geology and Hydrogeology, v.33, pp.105–124.

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The effects of groundwater level and vegetation on creep of the Mam Tor landslip