Geotechnical Study Area G12 Barton-On-Sea, Hampshire, UK
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Geotechnical Study Area G12 Barton-on-Sea, Hampshire, UK 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 1 Geotechnical Study Area G12 Barton-on-Sea, Hampshire, UK 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. 2 Geotechnical Study Area G12 Barton-on-Sea, Hampshire, UK 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 3 Geotechnical Study Area G12 Barton-on-Sea, Hampshire, UK 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.