The Magheramorne Landslide, Northern Ireland Mechanics and Remedial Measures

Technical report ■The Magheramorne Landslide, Northern Ireland mechanics and remedial measures Richard J. CHANDLER Department of Civil Engineering, Imperial College of Science Technology and Medicine Key words: landslide case study; partial progressive failure; partial first-time failure ; factors of safety; movement observations. 1. Introduction 2. The Magheramorne Landslide Case studies describing landslide investigations are a The Magheramorne landslide is located a few kilo- valuable resource. They provide information on the metres south of the town of Larne, on a coastal slope type and mechanics of the landslides of their geological running down to Larne Lough, a coastal inlet which area, and hence are a useful guide for studies of simi- opens to the Irish Sea; see Fig. 1. At the foot of the lar landslides in the same area or in similar geological slope are a major road (the "Coast Road") and a rail- conditions. Case studies dealing with remedial works way, both of which run on level ground about three to are also of interest, particularly where the effective- four metres above sea level. Away from Larne Lough, ness of the remedial works is reported. to the west, the slope steepens and rises to about 38 m This paper presents a landslide case study which de- above sea level (a.s.l.; Belfast datum) over a distance scribes the investigations and remedial works for a of 150 m, to the foot of a large man-made tip of rock landslide on the east coast of Northern Ireland, high- waste. This tip has side slopes inclined at about 35•‹, lighting those aspects of the study that are thought to and rises to about 100 m a.s.l. It is referred to subse- be of particular interest. It can be demonstrated that, quently as either the "Works Tip", or simply "the tip". though the landslide was partly a reactivation of a The rock waste is from a quarry excavated through much older landslide, it also involved a significant mass Tertiary Basalt to Cretaceous Chalk, a particularly of material that appeared not to have been previously pure limestone that was excavated for cement manu- part of the landslide. This resulted in the landslide be- facture. The Basalt had no economic value, and was re- ing, in part, a first-time event, involving the simultane- moved to allow the Chalk to be excavated. The ce- ous development of more than one sliding surface, and ment works is now closed. raising fears of a possible accelerating movement as a Downslope from the tip the natural ground varies in result of progressive failure of the brittle, strain- inclination, being about 12•‹ immediately below the tip, softening stiff clay involved. Additionally, movement observations show displacement rates to be much greater at the crest of the slope than at the toe. An- other interesting result relates to the remedial measures, which were straightforward the removal of the load placed at the crest of the slope that had caused the movement of the landslide. The effect of this load removal was monitored by inclinometers, and the observed movements are reported, allowing the effective- ness of the remedial measures to be assessed. Substantial movements of the slope took place in 1977-78, and it is these movements and other events during the period 1977-80 with which this paper is con- Fig. 1 Plan of the Magheramorne landslide, and the loca- cerned. tion of the site. The Line of Section is that shown in Fig. 3. J. of the Jpn. Landslide Soc., Vol.38, No. 4 327 (2002) 27 Chandler, R. J.: The Magheramorne Landslide, Northern Ireland: mechanics and remedial measures Table 1 Summary of the geological succession in the Magheramorne area Fig. 2 Oblique aerial photograph of the Magheramorne landslide area, date not known, but before tip was raised in 1973. B: bungalow; PO: post office; T: toe of landslide. Landslide outline shown. reducing to around 8•‹ near the Coast Road. Much of quence. Alternatively, the superficial deposits may be the area below the tip is hummocky grassland. An a random, intimate mixture of broken bed-rock frag- oblique aerial photograph showing the general layout ments set in a matrix of softer material. The former of the area is shown in Fig. 2. deposit is termed "Landslip debris", the latter "Head". In 1973 it was suggested that operations at the Often there is a graduation between the two deposits, quarry were causing damage to two buildings, a Post and no firm distinction between the two can be drawn. Office and a bungalow, and also to a road ("Ballylig To complicate the problem, fill derived from the Road") that formed the downslope boundary to the quarry workings, and thus composed largely of basalt area of the tip, see Fig. 1. An extensive investigation of and chalk, sometimes with a clayey matrix, also occurs the cause of the damage to the two buildings was car- on the slope. This fill material is difficult to distinguish ried out in 1978, and this is described below. from Head. 2.1 Geology At the foot of the slope, Glacial Till (fragments of A stratum of Basalt, up to about 110 m thick, is the predominately local material such as basalt and chalk highest deposit in the area, though it does not occur on set in a matrix of grey Lias-derived clay, deposited by the slope with which this paper is concerned. Under Pleistocene ice) was encountered, overlain by almost this lies the Chalk (27 to 12 m thick), below which is up three metres of Estuarine Clay of Recent age. This to six metres of Glauconitic Chalk, which is unsuitable stratigraphic sequence is summarised in Table 1. for cement manufacture, and was not worked in the 2.2 History of the slope. quarry. The level of the base of workable chalk is A direct consequence of the presence of the rela- shown by other geological studies to be at about 33 m tively weak Lias, which outcrops at the foot of many of a.s.l. The Glauconitic Chalk in turn overlies about 16 m the Northern Ireland coastal slopes, is that, where of Greensand (glauconitic sands and sandstones), which either coastal or glacial erosion has steepened the are particularly variable in thickness, and locally may slope sufficiently, landslides have developed. The vast be in total as much as 25 m or as little as four metres. majority of these landslides appear to be stable at the Below the Greensand lies the Lias (of Jurassic age), present day, suggesting that these landslides are of which is either a stiff clay or a soft, grey, silty mud- considerable antiquity. It is assumed that they origi- stone with calcareous horizons and thin limestones. nated as a result of the highly erosive environment of This is the lowest horizon with which we are con- the last glacial period (Stephens 1958). It seems prob- cerned. able that the Magheramorne slope, too, has been In addition to the "Solid" deposits discussed above, formed by either glacial or marine erosion, or by a superficial deposits also occur. These are of two types, combination of the two processes. either a fragmented version of the bedrock sequence, There is a record of a landslide at the site at the end though often less thick than when found in situ, but of the 19th century. This seems to have been a re- with the various strata still in correct downward se- newal of movement of a much earlier landslide. reacti- 28 J. of the Jpn. Landslide Soc., Vol.38, No.4 328 (2002) Chandler, R. J.: The Magheramorne Landslide, Northern Ireland: mechanics and remedial measures Fig. 3 Section of the Magheramorne landslide. vated either by the placement of quarry waste on the boundary of the landslide could be seen as an obvious slope, or perhaps by exceptional rainfall. This move- crack in Ballylig Road, and by corresponding displacement is probably responsible for the topographic fresh- ment of the adjacent wall. The eastern boundary of ness of the landslide toe, which forms an obvious fea- the landslide, also crossed by Ballylig Road, could be ture about a metre high (Fig. 1). There are no other re- inferred by reference to the limit of minor movements cords of movement of the slope until April 1973, when of the road surface. it was suggested "that blasting from the quarries had In January 1978 cracks were observed 20m back caused damage to private property [the Post Office from the crest of the Works Tip, which delimited an and bungalow] nearby" (Anon., 1973). Cracks and struc- area where subsidence had occurred. It was to be ex- tural movements of up to five millimetres were re- pected that settlement of the loosely tipped rock waste ported, indicating a further renewal of movement of would occur, but the coincidence of the area of greater the landslide. settlement of the part of the tip immediately above the The date of commencement of construction of the general landslide area showed that this part of the tip Works Tip at the crest of the slope is unknown. It ap- was involved in the overall landslide movement. parently stood at about 89m a.s.l. for some years prior Both rotary cored boreholes and air-flush percussive to 1973, when tipping recommenced, though aerial pho- drilling were used to establish the sub-surface details tographs taken in 1975 show that little further mate- of the landside, shown in Fig. 3. All the cores were split rial had then been added. By 1977 the crest of the tip for detailed examination, and were logged for both li- was at 98m a.s.l.

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