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Geotechnical Study Area G12 Barton-on-Sea, Hampshire, UK

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GEOTECHNICAL STUDY AREA G12

BARTON-ON-SEA, HAMPSHIRE, UK

Plate G12 Damaged building on the cliff edge at Barton-on-sea, Hampshire 1. INTRODUCTION 1.1 Background

Barton-on-Sea, Hampshire, is situated on the south coast of England, UK and comprises approximately 1.75km of coastal cliffs and slopes (Figure G12.1). The developed coastal frontage was originally protected in the 1960’s. The cliff geology is comprised of a series of sands and clays which dip gently to the east enabling a series of geologically controlled slip planes at various stratigraphic horizons to develop. The presence of groundwater and inadequate drainage has contributed to the development of large cliff failures along these slip planes. This has resulted in some of the existing drainage schemes being lost due to ground movement. Coastal stabilization work has mainly involved regrading failed material into a series of benches and the more recent placement of rock armour along the entire length of the coast in the 1990’s. A number of short rock armour strongpoints extend into the sea at regular intervals along the site, promoting the formation of a discontinuous beach in front of the rock armour revetment to reduce the effect of ongoing marine erosion at the toe of the armour. Along the most landward bench of the cliffs there are many chalets, which in places are at the toe of the talus slope of the oversteepened cliff. Inland from the cliff edge a plateau serves as an area for recreation and car parking. Apart from some properties, including a café and a local shop, which lie precariously close to the cliff edge, a main road and the main residential area of Barton-on-Sea are some 40m to 60m from the cliff edge (Plate G12). Further west there are a few kilometres of unprotected cliffs at Naish Farm Estate, with recession rates in the order of 2 m/yr.

2. THE STUDY AREA


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2.1 Topography

The site is characterised by a series of terraced slopes (Plate G12a) and cliffs rising from a narrow beach to a plateau at approximately 31m to 33mOD. The upper cliff consists of a near vertical cliff face varying between 5m and 10m in height, with a talus slope typically inclined at 30° to 34° at its toe. Below this lies the undercliff, consisting off a series of steep scarps separated by benches between 5 and 10m in width, now used for access. The lowest bench is protected by rock armour along the entire length of the site. Five short rock armour promontories, referred to as ‘strongpoints’, extend into the sea at regular intervals along the site, promoting the formation of a discontinuous beach in front of the rock armour. The upper cliff is breached at three locations in the study area: Sea Road, Hoskins Gap and Fisherman’s Walk, where vehicular and pedestrian access is provided from the upper plateau to the undercliff. Whilst still reflecting the original topography, most of the undercliff has been regraded and modified over a number of years as part of continued attempts to stabilize the slopes.

2.2 Geology

The Barton Cliffs comprise a series of Eocene sediments overlain by Pleistocene deposits. The oldest exposed deposit is Barton Clay of the Middle Barton Beds. The Barton Clay is overlain by the Barton Sands and Chama Beds of the Upper Barton Beds. Approximately 4m to 8m of Pleistocene Plateau Gravels lie unconformable over the Eocene deposits. The Barton Clay is generally a stiff fissured overconsolidated clay with frequent vertical variations in lithology. The overlying Upper Barton Beds largely comprise silty fine sands over a sandy clayey silt unit known as the Chama Beds. The Plateau Gravel, which forms part of the near vertical cliff section, is generally a medium to very dense coarse sand gravel. See Figures G12.2 and G12.3. The geological structure of the area is simple with a gentle regional dip of ¾° to the east-northeast (Barton, 1973). The actual dip across the Study Area appears to vary slightly. No faults are indicated on the geological maps of the area (Geological Survey of Great Britain: Sheets 329 and 330) or the detailed geomorphological mapping. The Barton Clay can be divided into a number of zones with boundaries defined by lithological features in preference to palaentological zones (Barton (1973)). A zone of particular significance is the horizon locally known as the ‘Highcliffe Sands’. This zone contains regular layers of relatively permeable sand beds, generally 5 to 15mm thick, with grey clay, and a more prominent sand bed up to 0.6m thick at the top. The presence of this zone in the study area was confirmed by the ground investigations and its hydrogeological significance is discussed in section 2.3. Also of significance are the many hard bands of calcareous mudstone and nodule beds. These aid identification of the stratigraphy and appear also to control the position of slip surfaces within the landslides.

2.3 Groundwater

Prior to the construction of stabilization works in the 1960s, the surface drainage at Barton would have been similar to that of the natural slopes below the Naish Farm Estate, just west of the site, where several seepage points can be observed in the scarp slopes of failures and significant ponding occurs on the benches within the landslide complex. Occasionally small streams issue from the more prolific seepage points or from the margins of mudslides.


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Seepage points occur in the Study Area, but to a lesser extent, since the engineering drainage scheme was installed in the 1960’s. This consisted of a sheet pile cut-off wall installed along the upper part of the undercliff, with a deep gravel filled drainage trench on its landward site, with a carrier pipe at or above the Barton Sand/Barton Clay interface and outfalls to sea at regular intervals. Seepage horizons exist within the debris slope at the toe of the upper cliff, at the lithological boundary between the Barton Sands and the underlying Middle Barton Beds and possibly at the hard band separating the F1 and F2 zones of the Barton Clay. Localised drainage measures have been constructed at various times, mainly consisting of gravel filled trench drains, to deal with persistent seepages. Major instability has historically locally disturbed the main cut-off drain, sometimes requiring complete replacement of short sections. The main cut-off drain and outfalls have recently undergone a major programme of refurbishment during the late 1990’s, mainly by slip lining with MDPE pipes. Based on the piezometric data and observation of seepages, a hydrogeological model has been determined based on five main hydrogeological units: a) Plateau Gravel b) Upper Barton Beds c) Middle Barton Beds (zones F2 to B) d) the Highcliffe Sands (zone A3) e) Landslide debris, colluvium and fill. The Plateau Gravels and the Upper Barton Beds together form a partly confined aquifer. Infiltration from precipitation and other sources is prevented from draining downward due to the presence of the low permeability clay of the Middle Barton Beds. The main component of flow is horizontally towards the cliff edge. Here, however, flow is partly restrained by the presence of lower permeability colluvium or slip debris, which leads to an accumulation of pore pressures and locally high water tables. In addition the sheet pile cut-off wall described above intercepts groundwater at these higher levels. Within the Middle Barton Beds, the Highcliffe Sands (zone A3) behave as a confined aquifer of sufficiently different hydraulic properties to be distinguished from the surrounding clay and may be regarded as a separate hydrogeological unit. Piezometric heads in zone A3 are typically a few metres above sea-level, indicating flow towards the sea, and are much lower than hydrostatic below the phreatic surface in the Plateau Gravel/Upper Barton Beds, or the colluvium, indicating the presence of a perched water table in these units and underdrainage of the Barton Clay (zones B to F2) into zone A3.

3. THE IMPACT OF INSTABILITY 3.1 The Problems

The condition of the site and mechanisms of slope degradation has been determined by geomorphological mapping and ground investigations. The two main mechanisms of instability contributing to slope degradation are described below: i) A compound failure system involving non-rotational failure seated along near horizontal

shear surfaces at the lithological boundary between Upper and the Middle Barton Beds. This type of failure involved the lateral displacement of a block to form an elongate ridge parallel to the cliff and the creation of a low-lying depression or ‘graben’ immediately


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upslope of the failed block. At the rear of the slide, displacement was accompanied by some rotation, often associated to the slumping of a narrow wedge of the upper cliff.

ii) Secondly, shallow translational mudslide failures principally developed within the Barton

Clay, but also within the colluvial material. These are relatively slow moving masses of clay rich debris sliding on translational shear surfaces, often at lithological boundaries.

Mechanism i) corresponds to sliding on the shear at the interface between the G/F2 zones as defined by Barton (1973), this being the uppermost shear surface in the Barton Formation. Mechanism ii) corresponds to sliding on the F1/F2 shear surface and, at the western end of the site, on the C/D interface (see Figure 2).

3.2 Stabilization Work

Apart from early sporadic attempts to stabilize the beach by groyne construction, the first serious attempt at stabilising the slope consisted of works to control groundwater in the upper part of the undercliff by means of a deep cut-off drain supported on the downslope side by a sheet pile wall. To protect the toe of the slope from further sea erosion a comprehensive system of groynes and wooden piled breastwork was also constructed. These stabilization works, constructed in the 1960’s, were generally successful in reducing both the rate of toe erosion and that of cliff top retreat. However, major failure of short sections of coastline occurred through the years, sometimes requiring complete replacement of the breastwork and/or the cut-off drain. In particular, significant failures occurred in the section between Hoskins Gap and Fisherman’s Walk in 1974 and again in 1987/88, and below the Cliff House Hotel. Localised failures, especially of the lower scarp slopes, have been widespread, requiring extensive treatment by ground replacement and shallow drainage. See Plate G12b. In 1991 the groynes and breastwork were replaced by new rock armour revetment along the entire length of the site, the cliff toe being moved seaward by up to three metres and associated regrading of the lower slopes. A programme of staged refurbishment of the cliff drainage system was also implemented.

3.3 Current Instability

The main area of current instability of the undercliff is between the Cliff House Hotel to the west and Barton Court. Several active areas of localised instability can be found in the lower scarp slopes along this stretch of coastline, and their extent is increasing with time. Monitoring of four inclinometers installed in 1991 shows continued seaward movement of the upper part of the undercliff in this area. The area of active movement on the undercliff below the Cliff House Hotel extends 90m from top to bottom and 115m across, with up to 2m settlement of the upper track and complete disruption of the cut-off piled wall and drain (Plate G12c). Ground movement extended inland to the upper cliff talus slope and affected the whole undercliff at this location. Further cracking became apparent in October 1993 indicating an extension of landslide activity to the west and east following lateral unloading due to the large movement of the main slip. It is now evident that the whole undercliff from the Sea Road access track to the untreated section towards Naish Farm is affected by active instability. Below Cliff House Hotel, the basal shear of mechanisms i) and iii) passes below the rock armour placed in 1991 to outcrop in the sea bed. The rock armour has been displaced seaward accordingly as a single body and to date without excessive distortion. High strains exists where the rock armour crosses the eastern edge of the main slip and severe disturbance is apparent in this area. East of the main slip, active instability of the lower slopes between the middle and lower bench can be observed, probably a precursor to an eastern extension of the main slip.

3.4 The Role of Groundwater


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The role of groundwater is causing instability of the cliff and upper bench has long been appreciated. Installation of sheet piled cut off wall and drain formed an essential element of the stabilization scheme devised in the 1960’s. Based on observations of piezometric levels and seepage points, it is considered that while these drainage works have been successful in limiting groundwater flow to the lower slopes of the undercliff, they have resulted in only localised drawdown of the phreatic surface upslope. Based on monitoring, geomorphological mapping and direct observation on site, it is evident that the whole of the upper area of the undercliff west of Barton Court is currently moving, to varying degrees, often in association with cracking and slumping of the cliff top. The stability of the upper area of the undercliff, and therefore its rate of movement, are closely related to groundwater levels. Groundwater has an equally if not more important, yet subtle, influence on the instability of the lower slopes, especially in relation to deep seated slips. Historically, coastal retreat occurred at a relatively fast rate of approximately 1m per year. The associated unloading resulted in a zone of depressed pore pressures in the thick clay sequence of the Middle Barton Beds, which temporarily prevented deep-seated instability of the oversteep undercliff. Other mechanisms of slope degradation intervened before deep-seated failure of the whole cliff or of a major part of the undercliff could occur in response to progressive rise in groundwater pressures. However, since coastal retreat has been slowed down significantly by the stabilization measures implemented in the 1960’s, pore pressures are slowing recovering, making deep-seated large-scale failures, such as that currently observed below the Cliff House Hotel, increasingly likely. Analyses demonstrate that similar deep-seated slips could occur at lease as far east as Hoskins Gap. Pore pressures in the thick clay sequence of the Middle Barton Beds are still depressed below their long term equilibrium levels. It is essential to take this into account when evaluating the stability of the site or any further stabilization measures that may be required.

3.5 The Role of Marine Processes

Removal of slip debris or erosion of undisturbed material by the sea prevents the formation of a slope which is stable in the long term. While prevention of toe erosion is a necessary condition for stability, it is not, alone, sufficient to stabilize slopes in a timescale of engineering significance. The process of natural degradation to a stable condition may take several thousands of years and involve significant changes to the slope profile, as demonstrated in several ‘abandoned’ cliffs elsewhere on the south coast.

3.6 Public Perception

The public recognise that there is ground movement in the form of cliff failure at Barton-on-Sea. This is highlighted during a period in which failure is evident, however memories even with local residents are somewhat short. The local press has an interest in the cliff failure at Barton and features are frequently presented on coastal instability.

4. THE ROLE OF KEY AGENCIES

The Local Authority responsible for the Barton-on-Sea area is New Forest District Council (NFDC). The coastal defence strategy for the area is outlined in the Shoreline Management Plan, in which the preferred policy option is ‘Hold the Line’. The Council has a budget for beach and cliff monitoring partly funded by MAFF. All instrumentation monitoring is undertaken by the Council. The data is continuously reviewed by NFDC and analysed by the Council’s consultants on an annual basis. If problems occur


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throughout the year the data is analysed more frequently, as and when required. The Council is aware of the risk and the likely maximum extent of ground failure, which they believe to be sufficiently far from property at present not to warrant concern.

5. INVESTIGATION AND MONITORING 5.1 Introduction

As part of the recent programme of landslide investigation, a series of geomorphological and ground behaviour maps of the site were prepared in 1991, 1992 and 1998. This enabled the ongoing development of the landslides within the undercliff to be determined. The mapping indicated areas of active or marginal movement of the whole undercliff from Naish Farm to the west extending eastwards beyond Hoskins Gap. Various phases of ground investigation were carried out to supplement historical borehole information and instrumentation. This included three boreholes to depths of between 20.0m and 35.5m and 35 static piezocone penetration tests to depths of up to 23.3m and were used to correlate the strategraphy and landslide systems within the undercliff. A programme of slope monitoring was initiated, to provide information on the depths and rates of sliding and to give and early warning of worsening conditions. This has included the monitoring of existing and supplementary instrumentation, some of which has now been lost by ongoing landslide activity, and comprised: · 5 borehole inclinometers to depths of between 17.5m and 25.5m · Slip indicator tubing monitored using brass mandrels to 20m depth in boreholes · Monitoring of over 200 No. Surface Ground Markers:-

- on 7 selected section lines - points either side of developing cracks on the slope - linear measurements between fixed points - lines of fixed points along the length of the revetment

· Visual Inspections – weekly inspections on site - Groundwater Monitoring of 6 vibrating wire piezometers, 25 push-in standpipe

piezometers to depths of between 5.0m and 15.5m and 4 standpipe piezometers in boreholes to depths of between 20.0m and 34.5m.

· Manhole Flow Surveys – manual inspection of drainage catch-pits and manholes · Rainfall Records from local weather station The results of these investigations, coupled with the monitoring programme, have enabled the mechanisms to slope failure to be more clearly identified and stability analyses undertaken.

5.2 Correlation of Rainfall with Ground Movements

Results from monitoring of instrumentation described above show clear relationships of ground movement with the onset of the wet winter period (Figure G12.4). A comparison of monthly rainfall with ground movement has been made for those monitoring points that show significant seasonal trends in movement (Fort et al 2000). Monthly rainfall records and antecedent rainfall for between 1994 and 1999 are presented in Figure 4 which also shows the start and end of significant ground movement for selected locations derived from borehole inclinometer data and topographic surveys. Ground movement for the winters of 1994/95 to 1998/99 generally start to accelerate during October/November. The rapid response between the onset of winter rainfall and ground movement is probably a reflection of the relatively high soil permeability of the ground. Accelerating ground movements in October/November generally appear to commence when monthly rainfall for the current and preceding months (antecedent 2-month average) is in


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excess of about 80mm/month. The onset of ground movement in October/November 1996, however, appears to have initiated at a lower average rainfall threshold of about 55mm/month. Once high rates of movement have commenced during the winter period, monthly rainfall has to reduce to relatively low levels for rates of ground movement to reduce back to summer levels with average monthly rainfall typically being about 40mm to 50mm.

5.3 Monitoring Strategy

Monitoring at Barton-on-Sea is comprehensive and has intensified in the areas where ground movement is putting the public and property at risk. A considerable amount of field data is being collected from the slopes at Barton. As with any monitoring programme, its effectiveness in serving as a ‘predictive’ method for slope instability relies on the rapid processing of the data into a clear/informative format and the continuous and ongoing review and interpretation of such data.

6. CURRENT STATUS AND APPROACH AND LESSONS LEARNT

Analysis of the data has shown there to be a reasonable correlation between rainfall levels and slope movement. Based on this it should be possible to predict the onset of accelerating ground movement. The data suggests that the threshold average monthly rainfall level for this is of the order of 55 to 80mm/month and that extra vigilance/attention should be made in the monitoring/inspection of the site for slope instability. The monitoring shows that once the accelerating rate of ground movement commences, it continues until the start of the drier months, typically when average monthly rainfall falls below 50 to 40mm/month. What cannot be predicted with any certainty, however, is the point where ground movement will continue to accelerate and result in a major slope failure. To predict the timing of a major event however is extremely difficult and unreliable and therefore it is essential that, together with the monitoring, processing and review strategy of the data, a response procedure is put in place to react to circumstances when movement becomes a risk to public health and safety of a property. At Barton this has taken the form of: · Addition/maintenance of appropriate warning signs on the site including emergency

contact numbers of the public to inform NFDC of any major movement; · Fencing off of areas considered to be of high risk; · Informing the property owners in areas considered to be at risk; · Establishing evacuation procedures from properties; · Establishing lines of communication should an event take place, ie. 24 hour contact

telephone numbers of appropriate NDFC staff and if necessary with the police; · Consideration has been given to the installation of an automatic early warning monitoring

system at high risk areas. In view of the strong correlation between rainfall and slope movement, the integrity of the drainage system at Barton is also continuously checked and appropriate measures carried out to repair any defects that may develop as a result of ground movement. The Council is presently looking at what is good value for money in order that they can optimise their programme of monitoring. Ultimately the monitoring aims to provide data for the management and future protection of the coastline. The site has contributed to this LIFE project as an example of a location where a well-organised local authority has established and managed an effective monitoring strategy. The quality of the emerging data and the experience of the staff has allowed the data to be interpreted over time to assist towards predictions of slope instability events. The way that the data is presented in the appendices to this study report is intended to illustrate groundwater levels and movement trends in a readily understandable way.


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7. REFERENCES

Barton M.E. 1973. The degradation of the Barton Clay Cliffs of Hampshire, QJEG, 6, 423-440). Fort D.S., Clark A.R., and Cliffe D.G. 2000. The investigation and monitoring of coastal

landslides at Barton-on-Sea, Hampshire, UK. In E.N. Bromhead, N. Dixon and M-L. Ibsen (eds.) Landslides: In Research, Theory and Practice. Thomas Telford, London.

Figure G12.1 Barton-on-Sea location map.

Figure G12.2 Stratigraphy at Barton-on-Sea.

Figure G12.3 Geological cross-section through Barton-on-Sea.

Figure G12.4 Relationship of the start and end of significant ground movement with rainfall, Barton-on-Sea, Hampshire, UK.


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Plate G12a View looking west, Barton-on sea

Plate G12b House on cliff edge, Barton-on-sea


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Plate G12c Aerial view of cliff instability at Barton-on sea

geotechnical study area g12 barton-on-sea, … · geotechnical study area g12 barton-on-sea,...

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