Lindisfarne Castle a Rock Mass Stability Assessment
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COOLING PRIZE PAPER Lindisfarne Castle A Rock Mass Stability Assessment Katherine Jones, Dunelm Geotechnical and Environmental Abstract Figure 1: The Lindisfarne Castle is built on an outcrop of the Locations of FIGURE 1 Whin Sill, which has a strong joint sequence due to the discontinuity WESTERN END slow cooling of the igneous intrusion. The orientation surveys © 2003 DS4 Gentles Limited, and distribution of these discontinuities may lead to the Low Level Aerial structural integrity of the castle becoming compromised. Photography This paper uses discontinuity survey data to assess the DS3 stability of the historic monument, which has long been an iconic symbol of the region. Walkover survey observations are described and locally DS5 DS2 surveyed discontinuity data is used to characterise jointing patterns within the Whin Sill outcrop. This is used to provide a representative record of the rock condition, DS1 together with a visual assessment of the slope stability. During the assessment, consideration was made of the impact of plant growth on the slope, in particular red DS6 valerian, and its effect on the stability of the rock face. 1.0 Introduction DS7 1.1 General Background The Lindisfarne Castle, situated on Holy Island, Northumberland, was constructed in the 16th century, long before engineering standards became common practice. Now a National Trust-owned property, the castle is situated on a steep outcrop of the Holy Island dyke, an offshoot of the Whin Sill (Goulty et al 2000), which has been noted in recent years to be disintegrating. Discontinuity characteristics within the rock mass play an DS8 important role in the stability of the faces of the outcrop, and the security of the structures founded above them. Discontinuity survey data was used to assess the stability of the rock mass supporting the historic EASTERN END monument. 1.2 Objectives During the assessment, consideration was made of the The paper described the use of walkover survey impact of plant growth on the slope, in particular red observations and locally surveyed discontinuity data valerian, and its effect on the stability of the rock face. Red to characterise jointing patterns within the Whin Sill valerian is a perennial shrub commonly found on coastal Lindisfarne castle outcrop. This provides a representative slopes which can produce woody stems and has roots with record of the rock condition, together with a visual a propensity to cause damage by jacking. assessment of the slope stability. Where accessible, discontinuity survey data has also been collected, allowing 2.0 Geology stereonets of the joint sets to the plotted. The Holy Island dyke is oriented E-W through the 26 GROUND ENGINEERING July 2014 island. The rock outcrop comprises dolerite of Upper areas across the slope. Carboniferous age, and is part of the Whin Sill igneous Large boulders up to 0.6m in diameter were noted at outcrop (Miller and Mussett, 1963). The principal joint the base of the slope, having fallen from the cliff. Further, pattern observed within the outcrop, consists of two loose boulders were noted within the cliff face, which may main sub-vertical orthogonal sets (Spears, 1961) induced come dislodged. This area has open access to the public, by the slow cooling of the magma intruded within the and although no path is present directly along the base of Carboniferous sedimentary rocks of the region. this cliff side, there is a grassed area where members of the public could sit. 3.0 Field measurements Planar and wedge failures were observed as blocks 3.1 Method have clearly dislodged along the discontinuity sets. The In November 2013 a discontinuity mapping survey was persistence of the discontinuities was not observed to be undertaken to collect field data from accessible rock large (typically 1 to 3m) and with joint spacing medium exposures, at eight locations within the Lindisfarne on average therefore, potential rock failures are generally Castle rock exposure, labelled DS1 to DS8 on Figure small, with visible blocks up to 2m in length having 1. The purpose of this survey was to obtain local become dislodged from the rock face (Figure 2). discontinuity data from the outcrop to determine, where Around the location of DS4, large areas of the cliff face possible, regional trends in the orientation and nature of were noted to have been remediated using masonry. This the discontinuities to assist in the assessment of the rock was identified to be in generally good condition, however, face stability. the stability of the rock beneath is unknown. Between A systematic approach was adopted in recording the areas of remediation rock outcrops were noted to be discontinuity data, in compliance with industry guidance heavily weathered, possibly acting as channels for water (BSI 2003) which entailed recording of the following data, flow. These areas were also noted to be heavily vegetated, where appropriate, from transects at each location: with red valerian noted in many of the joint sets. 1. Location This area was also accessed from above using rope 2. Rock description access to further assess its stability. Iron supports 3. Discontinuity type including: were noted within the mortar; their date is unknown. i. Orientation (dip and dip direction) Many fresh cracks were noted within the rock, forming ii. Spacing potentially unstable blocks. It is considered likely that iii. Persistence this instability is aggravated by the extensive red valerian iv. Roughness and shape growth identified in this area. v. Aperture No seepages were noted during the investigation, Figure 2: vi. Infill Columnar however, this may vary depending on the season. In addition, a visual inspection was undertaken of the structures rock faces in order to identify possible weak areas which formed by 4.1.3 Discontinuity Summary may be jeopardising the stability of the rock face and discontinuity sets Measurements were obtained from the three principal castle above. observed at DS3 joint sets that were evident within the north face Q The positions of the locations surveyed are illustrated in Figure 1, and can be categorised between the northern, southern and western rock faces 4.0 Summary of visual observations and discontinuity data 4.1 North face 4.1.1 Locations Exposures of the Whin Sill Dolerite were surveyed at four locations along the northern face of the Lindisfarne Castle outcrop, shown on Figure 1. The slope was noted to be up to 20m high in the west of the site with an overall slope angle of approximately 70o. 4.1.2 Observations The rock face was noted to be generally stable, with large areas having previously been stabilised using masonry. The masonry was noted to be in generally good condition, however, cracks have been noted in the mortar in some areas. In the location of DS1, an area of almost completely disintegrated masonry support was noted. The cliff face was noted to be heavily vegetated, with red valerian noted in some areas across the face, especially the upper areas. Root jacking was observed in isolated July 2014 GROUND ENGINEERING 27 COOLING PRIZE PAPER P outcrop. The three joint sets were noted to create FIGURE 3: NORTH FACE DISCONTINUITY SETS columnar structures dipping almost parallel with the slope, and stereographic projections show that planar/ DS1 DS3 W P Discontinuities show a Discontinuities show a steep wedge failures are possible along the rock face, W P planar failure (P) and planar failure (P) and assuming an angle of friction of 35˚ based on the rough steep wedge failure (W) in steep wedge failure (W) in nature of the discontinuities. Stereographic plots for each the orientation shown the orientation shown discontinuity survey location are shown in Figure 3. The joints were noted to be generally of tight aperture, however, near the surface, joints have been weathered with spacing of around 20mm between planes. DS2 DS4 Discontinuities show a Discontinuities show a P W planar failure (P) and W P planar failure (P) and 4.2 South face steep wedge failure (W) in steep wedge failure (W) in 4.2.1 Locations the orientation shown the orientation shown Four sections were surveyed along the southern facing cliff wall, above the existing public footpath. The rock face reduces in height from 15m in the west to 2.5m in the east and dips at an angle of approximately 60˚. This section is of particular concern, as loose blocks falling could potentially pose a safety issue to the public using the FIGURE 4: SOUTH FACE DISCONTINUITY SETS access track below. The slope was noted to be partially covered with DS5 DS7 vegetation, mostly comprising grasses. Discontinuities show no Discontinuities show a failures, however toppling steep wedge failure (W) in failure was noted, due to the orientation shown. the dimensions and Toppling failure was also 4.2.2 Observations orientation of the blocks noted Observations along the southern face encountered several apparently unstable blocks toppling away from the cliff W face (DS5). It was evident that blocks had recently fallen away from the cliff, and further loose block areas were DS6 DS8 visible in the west and central areas of the section. Three Discontinuities show a The slope is shown to be steep wedge failure (W) in stable at this location discontinuity sets were clearly visible creating columnar the orientation shown. Toppling failure was also structures (a common cooling feature of sills). noted An area with significantly closer spaced discontinuities was noted in the centre of the section (DS6). This could be due to a slight change in mineral composition grain size W or a possible fault zone. Discontinuities were also noted to change orientation about this feature. The rock surface appears to be highly weathered, especially in the western section, with fresh discontinuity 4.2.3 Discontinuity Summary surfaces exposed indicating that blocks have recently The rock face primarily comprised three joint sets, fallen away from the cliff.