A reconsideration of the Bladon Landslide of 1839

John Pitts and Denys Brunsden

PIITS, J. & D. BRUNSDEN 1987. A reconsideration of the Bindon landslide of 1839. Proc.Geol. Ass. 98(1), 1-18. The Bindon Landslide is situated on the coast of south-east Devonbetween Axmouth and Lyme Regis and is a complex block slide with a rear graben whichproduced a short-lived offshore toe reef in 1839. No evidence has been found to support earlieropinions of a major rotational component or liquefaction. The geology of the slope comprises aseries of Mesozoic rocks ranging from Penarth Group on a stable Mercia Mudstonesbasement, through Lower Lias, unconformably overlain by Upper Greensand and Chalk. Thetriggers of the slip movement were a combination of high rainfall and strong marine erosion.Shear strength parameters for the bedding partings within the black shales of the WestburyFormation have been obtained by back analysis and laboratory testing. The failure surface isinterpreted as being in these materials. The dip of the beds is very low. The main driving forcewas generated by water pressure in a tension crack and on the failure surface. Groundwaterdata have been obtained from a nearby well. Results of stability analyses support the view thatfailure of the undercliff was the initiator of the main phase of landsliding.

J. Pitts, Harry Stanger Limited, The Laboratories, Fortune Lane, Elstree, Herts, WD63HQD. Brunsden, Department of Geography, University of London, King's College, Strand,London WC2R 2LS

1. INTRODUCTION

On Christmas Day 1839 there occurred 'a mostextraordinary and terrific explosion of nature'. With asound like 'the rending of cloth', accompanied by'flashes of fire and a strong smell of sulphur' the cliffsat Bindon, Devon, subsided towards the sea.Attendant rumours of volcanoes and earthquakes onlyserved to enhance interest in what was in fact one ofthe largest landslides ever to occur in Great Britain.

The landslide at Bindon (SY 276895) is one of aseries of major slope failures within late Triassic,Lower Jurassic, and Cretaceous strata along the coastof south-west Dorset and south-east Devon. Bindonlies approximately 2 km east of Seaton and is situatedwithin the Axmouth-Lyme Regis Undercliffs NationalNature Reserve, a coastal strip of about 8 km in theextreme south-east corner of Devon (Figs. 1 & 2).Landslides in the region have previously beendescribed by Arber (1940, 1941, 1973) and details ofslope stability within the Reserve by Pitts (1981a).

Within the Axmouth-Lyme Regis Reserve, themain topographic features are a rear scarp cut into aChalk upland, which on the coast reaches about 170ma.D., and a broad undercliffvarying in width betweenabout 100 and 500 m. At Bindon itself, the rear scarpreaches an elevation of 124 m a.D. and the undercliff,a width of 415 m. The undercliff is generally broadlystepped in profile, with a gently seaward slope losingheight mainly in a series of 'steps and risers'. AtBindon however, the undercliff is much more complexwith major interruptions to the general seawarddecrease in height. This configuration was establishedby the slip of 1839 and much of the profile has beenmodified only in detail.

Descriptions of the landslide were published shortly

1

after the event (e.g. Conybeare, Buckland & Dawson,1840; Buckland, 1840; Roberts, 1840; Hutchinson,1840) but since then, relatively little work has beenundertaken, and none until now, of a quantitativenature. The geological setting of the Bindon landslidehas, in the absence of boreholes, been established bycareful field mapping. Sampling was undertaken fromexisting exposures using specially formed portableequipment consisting of a UIOO sampling tube anddriving equipment. From these samples, a laboratorytesting programme was implemented. Stabilityanalyses were carried out on the basis of the survey ofhistorical data, field mapping and the results of thelaboratory testing. The aims of this paper are toattempt to clarify the mechanisms involved in theBindon landslide, to clarify and assess the relativeimportance of the geological formations involved, andto present a quantitative assessment of the changingstatus of the stability of the landslide since 1839.

The accounts of the Bindon landslide of 1839 havebeen reviewed elsewhere (Pitts, 1974, 1981a, b), butthe major events may be reiterated here. The failureproper started in the early hours of Christmasmorning, although signs of fissuring in the cliff top hadbeen observed a week earlier. This episode was theforerunner of the main failure which occurred onChristmas night. A chasm opened at the rear of theslide (Fig. 3) which was reported by Conybeare et al.(1840) as being 'more than 300 feet (92 m) broad, 150feet (46 m) deep and three quarters of a mile (1200 m)long'. Coastguards reported that the remainder of theevents, mainly the upheaving of the sumarine beach(reef), the alteration of the shore and the protrusionof the cliff (Fig. 4) occurred three hours aftersubsidence in the chasm first started. Hutchinson(1840) stated that 'a pyramid of rock of great size and

2 JOHN PITfS AND DENYS BRUNSDEN

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remarkable outline sank in the order of 150 feet(46m)'. Movement of the existing undercliff had beenexperienced on December 23rd , when one of thecottages built on the undercliff began to suffersubsidence. At 4.00 a.m . on Christmas morning, theoccupier, on trying to leave the cottage, found greatdifficulty in opening the door because of distortion ofthe frame and walls.

The offshore reef thrown up at the toe of the slipwas short-lived, being quickly destroyed by waveaction. It is described by Conybeare et al. (1840) asbeing composed of Greensand and Foxmoulddisplaying landward dip of 30° to 45° and having avery fractured nature. The change in level attained bythe strata forming the reef was estimated byConybeare to be 15 m, from 3 m below the sea to 12 mabove it .

Between June 1839 and the date of the landslide,50% more rainfall than normal had fallen (Arber,1939), and totalled 15.59 inches (396 mm) (Roberts,1840). Roberts also comments that, 'near the shore ,the remarkable dryness of the upper cliffs, consideringthe wetness of the season could not fail to be contrastedwith the cracks full of water or overflowing, the greensand saturated and everything resembling a wetsponge.' At this time , much comment on the effect ofwater on the Foxmould was forthcoming, althoughthere was relatively little written about the clay otherthan that it would form a barrier to the downwardpercolation of water from the Foxmould (Anon,1840). High spring tides were also experiencedimmediately prior to the landslide. The maximum tidewas recorded on December 23rd , immediately prior to

the commencement of major movements. Although itis now difficult to evaluate the significance of marineeros ion in 1839, it is known that considerable changeshad taken place in the height of the beach, as accountsexist of exposures of the Rhaetic Bone Bed atCulverhole Point. .

The Bindon landslide appears to conform mor­phologically (for the most part) to a block slide(Varnes, 1978) with a graben (The Chasm), a major,largely intact block (Goat Island) , and a pressureridge at the toe (The Reef). It possesses an induratedcapping material, with shear failure along a clay layer,and subsidence at the rear.

Goat Island is not however an intact block of rock(Figs. 5 & 6) , the slip leading to a degree of break-upof the mass into a series of units, most of whichsubsequently came together at the termination ofmovement. Some of the pinnacles however remaineddetached and their degraded remains exist mostnotably at the eastern end of the Chasm (Fig. 7) .

Goat Island itself contained parts of fields belongingto both the Bindon and Dowlands Estates, the cropsin which were harvested during the following season.A painting depicting these events hangs in the LymeRegis (Philpot) Museum. The fame of the slip evenbecame the inspiration for a piece of music composedby Ricardo Linter, and called 'Th e LandslipQuadrille' (Fig. 8) .

2. GEOLOGICAL SETIlNG

The Bindon landslide originated from a Chalk-cappedplateau dissected by approximately southerly flowing

THE BINDON LANDSLIDE OF 1839 3

Fig. 2. Vertical aerial photograph of the Bindon slip. Cambridge University Collection: copyright reserved.

rivers, and truncated on its southern side by the sea.The succession consists of rocks of Triassic ageshowing variable dip in an easterly direction (average11°), unconformably overlain by Cretaceous bedscomprising Gault, Upper Greensand and Chalkdipping south-south easterly (seawards) at a low angle(maximum 5.so). This 'solid' succession is capped byabout 1.0 mof clay with flints. The late Triassic, andin adjacent slopes, Liassic strata, are repeated alongthe coast as a result of faulting. Their variable dip isthe result of small-scale folding.

(a) Penarth Group (Rhaetic)

The Penarth Group (Warrington, 1980) constitutes aseries of variable passage beds between the MerciaMudstones (Keuper Marl) and the Lias. Unfortun­ately, no accessible exposure of truly in situ WestburyFormation (Rhaetic Shales) exists within the Reserve(to the authors' knowledge). However, it does occurin a quite intact, albeit slightly weathered state in aslipped block at beach level at Culverhole Point(SY 274892), and has been described by Richardson

4 JOHN PITfS AND DENYS BRUNSDEN

Fig. 3. View of the Bindon Chasm from the west. From Conybeare et al. (1840).

(1906). Althouth variable within the formation, theWestbury Formation may best be described as verystiff and hard, grey to black, laminated andoccasionally fissured, variably carbonaceous pyriticand selenitic shales, with occasional interbeds of thin,fine, dirty limestones. Some polishing and rareslickensiding was apparent on bedding laminae andfissure faces. Illite and kaolinite are the main clayminerals present, in association with both calcite andminor quartz. A range of index properties wasmeasured for each sample, and the results are shownin Fig. 9. The average bulk density of the material wasdetermined as 2.045 Mg/rrr' for an average moisturecontent of 23%.

Above the Westbury Formation comes the LilstockFormation , Cotham member, comprising 1.5 m ofmainly poorly fossiliferous calcilutites.

(b) Lower Lias

The Lower Liassic strata do not appear in outcropanywhere at Bindon, and its existence is inferred fromstructural and bed thickness considerations. TheLiassic strata consist of a lower sequence called BlueLias (Torrens, 1969) and possibly also the overlyingShales with Beef. The Blue Lias comprises calcareous

shales and calcarenites with occasional largeammonite-bearing nodules. The Shales with Beef aremainly composed of dark pyritic calcareous shaleswith inter-bedded fibrous calcite (beef) .

(c) Gault and Upper Greensand

The sub-Cretaceous unconformity between the Lil­stock Formation and the Gault is exposed in the sameslipped block at Culverhole Point. At this locality, theGault bears the characteristics of a basal littoraldeposit being conglomeratic. However, in general it isa black to greyish-green sandy and clayey silt.

The Upper Greensand at Bindon consists of 55 m ofvariable arenaceous deposits containing glauconite,chert, limestone and phosphatic nodules . A generalsuccession through the Upper Greensand may bepresented as:Chert Calcareous sandstones with numerous

phosphatic nodules.Beds Calcareous sandstones with grey and

honey coloured chert .Foxmould Dense, green glauconitic slightly cal­

careous silty sand.Dense green, glauconitic , slightly cal­careous silty sand with calcareousconcretions.

THE BINDON LANDSLIDE OF 1839 5

Fig. 4. The toe of the Bindon slip showing the reef and lagoon. Engraving on stone by G. Hawkins Jnr.

The presence of the overlying Chalk capping haspreserved the Upper Greensand beds in a relativelyunaltered and competent state.

The Foxmould forms the basal member of theUpper Greensand succession, and at Bindon, the6.0 m which are exposed comprise dense, green toyellow-green (when weathered), friable, calcareous,glauconitic very uniform silty sand, the quartz andglauconite being in approximately equal proportionsin some cases. It is apparently structureless, often withno sign of bedding. The calcium carbonate content isusually up to about 1% by weight and when water isintroduced, the Foxmould disintegrates. The glaucon­ite includes particles of silt and clay size and impartssome cohesion to the Foxmould although insufficientto render the material truly plastic.

The overlying indurated Upper Greensand faciescontribute much to the accumulations of scree at thefoot of the backscar of the landslide.

(d) Chalk

The Chalk proper lies above about 1.0 m of rubblysandy limestones of Cenomanian age. At Bindon, theChalk is of Middle Chalk (Turonian) age, and is up to11.8 m thick. For the most part it is white and purewith local variations in hardness generally reflected bychanges in the fracture frequency. Towards the baseof the sequence, the Chalk may be nodular or evensandy and glauconitic.

Fracturing generally tends to increase towards theground surface, probably a reflection of periglacialprocesses during the Pleistocene. However, bands ofwell recrystallised Chalk retain a massive structurewith a wide bedding spacing. The face of the Chalkcliffs tends to be quite fresh, a reflection of itsrelatively recent formation.

The Chalk and Upper Greensand contain adominant sub-vertical lineation striking between

6 JOHN PITTS AND DENYS BRUNSDEN

Fig. 5. View of the Bindon slip from the east showing the partial break-up of the Goat Island block. From Conybeare et al.(1840) .

NE-SW and ENE-WSW. The main significance ofthis seems to be, in the context of the orientation ofthe backscar at Bindon, that the joints are likely tocontrol both the origin of tension cracks, possibly to agreat depth, and also the orientation of the backscaritself.

3. PRESENT GEOMORPHOLOGY OF THELANDSLIDE, AND ITS DEVELOPMENT

A geomorphological map (Fig. 10) of the Bindonlandslide (Pitts, 1979, 1981a, b, 1983a, b) wasconstructed to clarify the current state of the featureswhich are, in parts , now heavily overgrown . An earlyplan (Fig. 11) was made and a section (Fig. 12) of thelandslip has now also been constructed . The backscaris composed of Chalk and indurated facies of theUpper Greensand. Some of the early engravings anddrawings of the Chasm indicate that the Foxmouldwas also exposed in the lower part of the backscar.Hutchinson (1840) commented on the extremeirregularity of the backscar and that irregularity is stillapparent . This would tend to indicate that thebackscar was not fault bounded (c.f. Anon , 1966).

The floor of the chasm is extremely uneven.Subsided blocks of Chalk showing tilt of strata at avariety of angles and directions occur virtuallythroughout. At the seaward side of the Chasm,several blocks show a seaward tilt of strata havingoriginated as failures from the rear edge of GoatIsland.

The degraded pinnacles which remain in the chasmnow display rounded tops and rarely exceed 6.0 m inheight . The extent of their degradation since 1839 isreflected in the large amount of scree nowaccumulated around their bases.

The exposed rear face of Goat Island is composedalmost totally of Middle Chalk and varies in heightbetween approximately 12.0 rn, and zero where it isnow totally obscured by scree. On Goat Island itself,vegetation masks the ground detail in many places,although mapping (Pitts, 1979) has indicated theposition of some of the degraded fissures remainingafter the blocks forming Goat Island coalesced. Therocks forming Goat Island dip gently seawards atvirtually the same angle as the corresponding rocks inthe backscar.

Lateral movement of Goat Island certainly occurred

THE BINDON LANDSLIDE OF 1839 7

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8 JOHN PITIS AND DENYS BRUNSDEN

Fig. 7. View of the Bindon slip from the east in about 1910 showing the degraded pinnacles in the eastern part of the Chasm.

since space was required into which the blocks in thefloor of the Chasm could move. A reconstruction ofthe slope indicates that the magnitude of the lateralmovement amounted to a few metres only. Thereconstruction also indicates that no vertical displace­ment of the main part of Goat Island took place, aswas suggested in many historical accounts. Localsubsidence, notably in the Chasm did take place, thatof the rearmost blocks being of the order of 40 m.

The precipitous seaward edge of Goat Island ismainly composed of the indurated facies of the UpperGreensand with a thin Chalk capping. It possesses avirtually identical (directional) structure to that of thecomparable formations within the backscar, althoughit has a higher fracture frequency. The face is verysteep and occasionally overhanging, producing acomplex array of features. One indication that thefailure of large blocks from the seaward face of GoatIsland occurs quite frequently is that the scree slopesare poorly developed at the foot. Elsewhere, debrisslides appear to have resulted from the severedisturbance of an established scree slope by latermovements of large blocks behind them.

The area in front of Goat Island is occupied by anon-circular rotational slip, that is, one having a

rotational mechanism but in which the failure surfacedoes not describe part of the arc of a circle. It doeshowever, possess certain surface peculiarities. Moststriking of these are the very large crags, 'walls'(Arber, 1973, p. 130 & Plate 9a) and pinnacles of rockupstanding from the general surface (Fig. 13). These'walls' of rock reach up to 15.0 m in height withalmost sheer faces and knife-edged tops. Each 'wall'consists of flat-flying indurated Upper Greensand, onealso having a capping of Cenomanian Limestone andMiddle Chalk. It is believed that each 'wall' began aspart of Goat Island, being a sheet of rock bounded bystress relief joints and/or fissures created during the1839 slide. They appear to have been subsequentlyrafted downwards by small movements of therotational slip upon which they sit. No seawarddipping strata have been found in between the blockswhich might have been expected had the 'walls'constituted the remains of units which had failedtowards the land. The land in between the 'walls' isrelatively flat lying, sloping slightly to the east, and islittered with large blocks of chert.

Seaward of the 'walls' and to their west, the groundis open with a gentle seaward slope. The products ofdebris slides cover the surface along with some larger

THE BINDON LANDSLIDE OF 1839 9

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Fig. 8. The front cover of "The Landslip Quadrille " by Ricardo Linter.

blocks of sandstone. Despite the superficial depositscovering this part of the landslide, and its generallygentle and open aspect, it is in detail composed oflarge, heavily fissured, backtilted blocks of Chalk andindurated Upper Greensand rocks . This structure, aswell as being identified in the deep open fissures, isalso observable in the faces of the sea cliff, where thecapping of now crumbling sandstone with chertoverlies the Foxmould, the latter forming most of thecliff. The blocks have been brought down to this lowlevel by successive failures of the rotational slip, andby block disruption (Brunsden & Jones , 1976).

No argillaceous material of greater than Foxmouldage is exposed in the lower slopes at Bindon , and theseabed geology is virtually wholly of indurated UpperGreensand. This presents a problem of interpretation

of the position of the shear surface of the rotationalslip and the block slide behind it. The position of theblack shales of the Westbury Formation overlying thetopmost beds of the Mercia Mudstones Group is suchthat a marked local steepening of the dip from justwest of Culverhole Point takes these formations downbelow beach level. It seems probable that sliding hastaken place at such a depth and the shear plane isbelieved to exist within the Westbury Formation. Atno point in the Reserve is any part of the MerciaMudstones Group known to be involved in large scalemass movements. At Culverhole Point , undisturbedMercia Mudstones Group rocks displaying thestructural characteristics of the rocks forming the cliffsjust to the west , can be found in the offshore zone atlow tide. The landslip at Culverhole Point has a shear

10 JOHN PITIS AND DENYS BRUNSDEN

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surface at approximately beach level within theWestbury Formation and the Rhaetic Bone Bed,which overlies the Mercia Mudstones Group, has beenfound as an in situ outcrop (Richardson, 1906).Further east within the Reserve, the same sequence ofrocks occurs at Charton Bay (SY 299900). The slopefailures there have also developed shear surfaceswithin the Westbury Formation, the complete MerciaMudstones Group succession up to the Rhaetic BoneBed, being traceable within the in situ rocks of the seacliff (Pitts, 1986).

The anomalous geological succession exposed in theReef in 1839 requires some explanation. Consideringthe maximum dip of the unconformity observed in thearea, it would still fail to take even the lowestmembers of the Upper Greensand succession to thedepth required for inclusion in the Reef. The twomost likely explanations of its occurrence in the Reefare faulting or previous landslide activity. Since nofaults of the orientation required to produce such aneffect are known to exist in the Reserve, it seemslikely to be attributable to prior slope failures.Furthermore, there are descriptions of a pre-existingundercliff at Bindon upon which the cliff cottages,

damaged in the 1839 event were built (e.g. Conybeareet al. 1840; Hutchinson , 1840).

The Reef may reflect not only the presence ofdisturbed strata above an aquiclude/failure surface,but also, the terminal stage of failure beyond thebeach. In this area, the present beach structureconsists of a three-tiered cobble storm beach 40 to50 m in width and about 10 m high, producingsignificant toe weighting. Over this portion of theprofile, the shear surface had been established duringearlier periods of landsliding. The offshore profilebeyond the abrupt termination of the cobble beach isof a low gradient, with a mantle of sand ofapproximately 0.2 m thickness, covering sandstone.

In summary then, the mechanisms of the 1839Bindon landslide are proposed as an initial non­circular rotational failure of the pre-existing undercliffreducing the toe support of the block behind . Thisresulted in planar slippage on the WestburyFormation of the mass known as Goat Island.Pinnacle-like masses of Middle Chalk and induratedUpper Greensand rocks subsided into the Chasm fromboth the rear of Goat Island and the rear cliff. Thishypothesis is diagrammatically represented in Fig. 14.

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THE BINDON LANDSLIDE OF 1839

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Fig. 10. Geomorphological map of the Bindon slip.

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4. GROUNDWATER CONDmONS

Unfortunately, the quality and quantity of ground­water data are very poor for all parts of the Reserve,including Bindon. Only one well exists for whichrecords are available and that is at Dowlands Farm(SY 288903) about 1.0 km to the east and 0.5 kmbehind the cliff top. The records only cover the periodJanuary 1969 to August 1973 in the form of monthlyreadings up to the well going dry. The well itself is117m deep. The water level in the well variedrelatively little over the period of measurement (Fig.15). The high levels of February and March 1969coincide in the record with a change in the method ofmeasurement, returning to the normal use of a Stowindicator in April 1969. For the period of measure­ment excluding February and March 1969, the rangeof levels varied between a maximum of 71.87 m and aminimum of 70.45 m. The peak consistently occurredduring the winter months. The gradual rise in thepeaks may relate to the cause of the well going dry inAugust 1973, when the bed was found to be only 86 m

below ground level. A later attempt to record a waterlevel in April 1974 reached 95 m without success, thebottom of the well being filled with mud.

5. LABORATORY INVESTIGATION

Samples of the black shales of the WestburyFormation were obtained for determinations ofresidual shear strength parameters. The samples wereobtained from the slipped block at Culverhole Pointbut were too weathered and disturbed for peakstrength determinations to be reliably undertaken.

Residual shear strength parameters were deter­mined using a 100mm square shear box. Reconsti­tuted samples of the Westbury Formation shales wereformed by consolidation under high loads. A planewas then cut in the sample and the faces of the cutplane smoothed and polished on a glass plate. Thesample was then re-consolidated and sheared severaltimes until a fairly consistent value was obtained . Thefinal pass was made at a low rate of shear . Values ofnormal stress were increased and the shearing process

12 JOHN PITTS AND DENYS BRUNSDEN

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THE BINDON LANDSLIDE OF 1839 13

Fig. 13. Oblique aerial photograph of the Bindon slip showing the "walls" and pinnacles in front of Goat Island, BritishCrown Copyright Reserved.

. UpperGreensand

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Fig. 14. Diagrammatic reconstruction of the development of the Bindon slip. F refers to the factor of safety against sliding,and M Type failure is a Multi-Rotational Slide (Skempton & Hutchinson, 1969).

suggest that levels of a much higher magnitude areperhaps unlikely to have been present in 1839. Theaquifer tapped by the East Devon Water Board about4 km to the east, shows similar stability, quite smalldecreases in discharge being recorded even during thedrought conditions of 1975-76. An average value forunit weight of 20 kN/m3 has been used in analyses offirst-time slides. In analyses of slips incorporatingpreviously slipped material, a 10% reduction indensity was assumed (Walton, pers. comm.)

The slope profile prior to the 1839 slip has beenreconstructed using data from contemporary sources,and changes in slope facet positions based on rates oferosion determined from the recession of high watermarks between 1888 and 1972, the dates of the first1: 2500 plan and the latest aerial photographs of thearea. The assumption made is that this portion of theAxmouth-Lyme Regis Undercliffs was previouslyoccupied by a small coastal slip extending offshore.The landward development of the slip at that time wastaken as being up to what became the seaward edge ofGoat Island in its pre-slipped position. The shearsurface was taken to be within the WestburyFormation and is most likely to have developed onbedding partings. It is probable therefore thatcohesion values would be very-low.-In order to obtainmore realistic values of the shear strength of theWestbury Formation, a back analysis was performedon the simple planar slide at Charton Bay in June,1969 (Pitts, 1986), for which conditions are quitewellestablished. The original profile of the slide wasreconstructed from aerial photographs. This provideda peak value for the angle of shearing resistance of thebedding of cp' = 13°. This value was then used in theanalyses for Bindon where peak strengths wererequired, the cohesion being considered to be zero.

The use of zero cohesive strength throughout theanalysis of the failure of Goad Island reflects theprobability of sliding along a discontinuity, ofprogressive strength loss within the WestburyFormation shales by shear creep during the pre-failureperiod, perhaps during progressive erosion of the toe,and the use of drained strength parameters.

The stability of the slope in front of Goat Island wasthen investigated using a modem profile (Fig. 16). Asingle unit weight was used to represent the slippedmaterial, in conjunction with the residual strength ofthe Westbury Formation shales along the shearsurface. Finally, analyses were undertaken toinvestigate the state of the stability of the currentslope. This considered the three main components ofthe modem slope, namely The Chasm, Goat Island,and the slipped mass in front of Goat Island.

A slab slide was assumed despite the existence ofsome (although not consistently) backtilted blocks inthe floor of the Chasm. A perfectly planar shearsurface at the base of a deep tension crack sited at therear of the Chasm was assumed. The results of the

tdry

19731972

JOHN PIns AND DENYS BRUNSDEN

1971

Well Depth' 117 m

1970

different method of measurement used

1969741----+-----+--:--+--:":"::--+~:::_-+

was repeated. The results obtained for the effectiveresidual cohesion were c; = 4 kN/m2

, and for theeffective residual angle of shearing resistance,cp; = 4.5°. There must be some doubts as to thevalidity of such an unusually low value for the angle ofshearing resistance, and the rate of shear may havebeen too great to generate truly drained conditions.Nevertheless, similarly low values were obtained forthe Westbury Formation in Somerset by Hawkins andPrivett (1985).

6. STABILITY ANALYSES

No stability analyses of any description are knownto have been carried out on the 1839 Bindon slip.Analyses were therefore undertaken in an attempt toclarify the actual mechanism of failure, ambiguitieswithin the geomorphological configuration, and thecritical shear strength parameters at failure in 1839.An attempt has also been made to evaluate thechanges in the stability which have occurredsubsequent to 1839. Analyses were carried out interms of effective stresses using the method of Janbu(1973), and also Hoek and Bray (1981) for a blocksliding on a clay layer.

Values of pore pressures on the slip surface werebased on water levels recorded in the Dowlands Farmwell. Although well levels were taken for a shortperiod only, the very small variations observed

Fig. 15. Water levels in the Dowlands Farm well.

14

70

70.5

71

71.5

....~~ 72..sJ:....0.oJQ

72,5

7J

73.5

THE BINDON LANDSLIDE OF 1839

rear of chasm

Layer 2. Upper Greensand

-~------

15

Layer 4. shear ed Rhae t ic shales

Layer 3. Fox mould

layer 5. intac t Rhaetic sha les.:.....- a

50 100 L50 200 250 300

me t res

350 400 450 500 550

Fig. 16. Reconstructed cross section of the Bindon slip for use in stability analyses. Within the figure, the letter (a) refers tothe top of Westbury Formation assuming a strike section; and the letter (b) refers to the top of Conybeare et al. 's (1840)"argillaceous medium" from sections of 1840.

back analysis for the failure of Goat Island and theChasm produced a value of ep' of 20.80 mobilised atfailure, a value much higher than that obtained atCharton Bay for the same material. This disparity mayrelate to the proposed trigger of the failure.

Several simple stability analyses were carried out toinvestigate further the order of failure at Bindon. Thisparticularly concerned the hypothesis that the triggerof the main failure of the Goat Island mass was anon-circular rotational falure in front (seaward) ofthat mass (Fig. 17). An analysis of the forces acting onGoat Island when it was unsupported seawards, usingthe shear strength parameters from the back analysisof the Charton Bay slip and the water levels inDowlands Farm well, in each case produced factors ofsafety of less than 1.0.

An attempt was therefore made to analyse thecontribution of the seaward support to the stability ofGoat Island. No direct method of analysis seemed to

exist which dealt with this contingency. A method wasadopted which had been outlined by Hoek and Bray(1981) as a part of a stability analysis procedure forrock masses subject to toppling failure (Fig. 18a). Theformula presented by Hoek and Bray (1981) forcalculating the propensity of any of the blocks to sliderather than topple was:

P. = P. Wn(Tan ep cos a - sin a)n-l n I-Tan2 ep

where ep is the angle of shearing resistance and thevarious forces acting on the block are as shown inFig. 18b. For the situation prior to the 1839 failure, afactor of safety of 1.15 was obtained for Goat Island.

It is difficult to be sure at what stage of failure thetoe block was required to be in order to produce afactor of safety of 1.0, that is, the factor of safety atwhich sliding just begins to occur, for Goat Island.

Goa t Islandsubside d Chasm bloc ks

,,layer 1. Chalk and Upper Green sand :,, Laye r 1

100

met res[A.a.oJ

layer 2

approximate e xte nt of ree f

50

sea lev e l

modern beach

100 150

Layer 3. she red Rhaeti c sh Ices

200 250 300

metr es

350 400 450

,II1,1

500

Fig. 17. Reconstructed sections of the profile after the 1839 slip and the modern section for use in stability analyses. Thesheared Rhaetic Shales may now be read as Westbury Formation in the terminology of Warrington (1980).

16 JOHN PIlTS AND DENYS BRUNSDEN

a

~iItr--\---=- Zone of toppling

Zone of sliding

b

Fig. 18. (a) Model for limiting equilibrium analysis oftoppling failure on a stepped base. After Hoek & Bray(1981). (b) Forces acting on a toe block liable to failure bybasal shearing. After Hoek & Bray (1981).

The indication is however that the failure of the toemass would have been almost completed before theslip of Goat Island occurred, a factor of safety of 0.99being obtained for the pre-failure geometry. Unfortu­nately, no calcuation has been attemped to assess theeffect of the subsiding masses in the Chasm on thestability of Goat Island. Too little is known about theprecise course of events to quantify their effectsadequately.

One analysis was carried out on the slope geometryshown in Fig. 17 to determine the gain in the factor ofsafety resulting from the lengthening of the profile andthe formation of the offshore reef. Although residualstrengths operated throughout the length of the failuresurface, the factor of safety had increased to 1.49.

The slope geometry in front of Goat Island as itexists today has suffered the erosion of the reef, about140 years of marine erosion and crown loading fromthe degradation of the seaward facing slope of GoatIsland. The factor of safety is very low, around 1.0,and the slope is in a quasi-stable state, if theassumptions made, particularly about shear strengthsand groundwater are realistic.

Finally an analysis was undertaken of the wholeslope using the present day geometry (Fig. 17). Avalue of F = 1.23 was obtained, compared to that ofF = 1.49 for the immediately post-failure situation, areduction of 17.5% in about 140 years. In view of theapparently less stable situation in front of Goat Island,some decrease in support by a failure of the toe massmay dramatically decrease the stability of Goat Islandin a way similar to 1839, except that now, lowerstrengths obtain beneath Goat Island.

7. DISCUSSION AND CONCLUSIONS

It would normally be the practice to compare theBindon landslide with others which had a shearsurface in the Westbury Formation shales. However,apart from two further landslides within theAxmouth-Lyme Regis Undercliffs, very few otherslope failures are known to exist within this material.The data are very sparse, but some confirmation existsin the solution of the stability analyses for theexistence of very low strengths for the WestburyFormation shales, although perhaps not quite as lowas those recorded in the laboratory. This perhapssuggests that slow shear creep had occurred over along period during which time orientation of clayparticles along the shear surface may to some extenthave occurred. Alternatively, and more realistically,the failure occurred on a pre-existing structure whichmay have possessed such a fabric. Fabric laminationscharacteristic of certain mudrocks, and the basis offissility, (Lundegard & Samuels, 1980) may be of thistype. The exposure of the Westbury Formation shalesat Culverhole Point shows the material to possess anexcellent bedding plane fissility in weathered ex­posure. It may therefore be that when dealing withlaminated mudrocks, bedding strengths are moreimportant measures of criticalshear strength than arethose which, through limitations of laboratoryprocedure and apparatus, are produced by causingfailure oblique to existing lineations. This point issupported by analyses of other landslides in theReserve where bedding strengths determined in thelaboratory conform closely to those obtained fromstability analyses.

In the offshore zone, where a large proportion ofthe failure surface of the Bindon slip exists, the shearstrength of the Westbury Formation may have beeninfluenced by ion exchange resulting from theircontact with seawater (Hutchinson, Bromhead &Lupini, 1980). The process may lead to changes in theplasticity and strength characteristics of the material.

The investigation of the Bindon landslide hasutilised highly flexible methods of stability analysis toassist the interpretation of the feature as a whole.They have been used in conjunction with carefullyassembled historical and field data to produce analysesas reliable as possible within the given constraints.

THE BINDON LANDSLIDE OF 1839 17

Only when boreholes, properly obtained 'undisturbed'samples and good groundwater data are available willmore reliable stability analyses be produced.

Evidence from the morphology of the Bindon slipwas ambiguous in that backtilted blocks constitutedpart of its make-up. The graben-like chasm at the rearis a zone of predominantly downward movement, andproduced a feature somewhat similar to that atAnchorage where, in association with the 1964earthquake, liquefaction of the sensitive BootleggerCove clays took place (Hansen , 1965). Varnes (1978,Fig. 2.1) also shows local subsidence effects to beassociated with earth lateral spreads, founded on softsands, silts and clays. Neither of these mechanismsseem to provide an adequate basis for explaining thegraben at Bindon. No quick clay exists, and thematerials present are not soft. It is suggested thatindurated cap rocks subsided into the Chasm tocompensate for the forward motion of Goat Island,both from the rear of Goat Island and from the'mainland ' until a stabilised backscar was produced.

The row of displaced blocks in the rear of theChasm generally displays landward tilt of strata, andthis is consistent with a gradual settling of blocks fromthe 'mainland' . However , elsewhere in the Chasm, theorientation is far less ordered. A number ofillustrations exist (e.g. Figs. 2, 4, 6 & 7, and in Pitts,1974) which demonstrate this point and indicate thatsome blocks resulted from subsidence from the rearedge of Goat Island, which had moved seawards.Others appear not to have moved laterally at all, butsimply to have subsided.

The early accounts and explanations of Bindon (e.g.Conybeare et al., 1840; Buckland, 1840; Roberts ,1840; Hutchinson , 1840) made much of the quicksandcondition of the Foxmould. It seems unlikely howeverthat such a dense materialliquified and it is indeed notnecessary to invoke such a process to account for thesubsidence . However, their main emphasis was on thesubsidence of Goat Island, and no evidence has beenfound to confirm that (see also Arber, 1973). The

later accounts generally interpreted the event as beingeither a non-circular rotational slip (e.g. Ward , 1945),who compared its mechanism to the slip at FolkestoneWarren , or even a circular rotational slip (Macfadyen,1971).

The main change in interpretation here is that norotational element exists at Bindon behind theseaward edge of Goat Island. No tilting of the bedsforming Goat Island has occurred , which effectivelyprecludes rotational failure. The apparently rotationaleffects at the toe may be accounted for withoutproposing an overall rotational mechanism. The reefmay represent a pressure ridge, or perhaps morelikely, locally reactivated shears which boundedpreviously rotationally slipped and eroded blocks.Since the slip in front of Goat Island clearly includes arotational element and probably reflects a processlong in existence, such relict shear surfaces are verylikely to exist in the offshore region. The slip of GoatIsland itself is a block slide in the WestburyFormation, although the mass did break-up alongsub-vertical fissures and coalesce towards the end ofslipping.

It must be said finally, however, that the earlyworkers at Bindon showed themselves to beparticularly perceptive, and also illustrated the valueof being able to observe the freshly formed , newfeatures.

ACKNOWLEDGEMENTSThanks are due to Professor John Hutchinson ofImperial College for his most helpful criticisms andsuggestions for improving the original draft of thepaper; to Geoff Walton and Alan Cobb for assistancewith the stability analyses; to Yeo Chin Soon of NTIfor drawing the figures and to Jamillah S. of NT! andSusan Dawes of Harry Stanger Ltd. for typing thepaper. Thanks are also due to two un-named refereeswho made helpful suggestions which contributed tothe improvement of an earlier version of the paper.

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