Behaviour of the sediments in the Storegga Slide interpreted by … · 2016. 1. 8. · Later Poulos...

IntroductionSubmarine slides have been reported from passive,active and sheared continental margins and they occuron all scales (Locat 2001; Mienert et al. 2003). Subma-rine mass movements have been classified by manyresearchers (Varnes 1958; Mulder & Cochonat 1996;Shanmugam 1996; Locat 2001). In the submarine envi-ronment acoustic methods are needed to image the seafloor. It is normally difficult however to determine thetypes of submarine mass movement from acousticimages alone (Mienert et al. 2003), particularly sinceone type of mass movement can lead to another.

Landslides normally possess two essential features; aslide surface and a displaced mass of sediment (Hamp-ton et al. 1996): The slide surface is where failure tookplace and along which downslope movement origina-ted. The displaced mass is the material that travelleddownslope. It usually rests partly on the slide surface,but it may have moved completely beyond it. More-over, the displaced mass may remain intact, be slightlyor highly deformed, or it might break up into distinctslide blocks, or be completely disintegrated as a flow.

Generally speaking, the properties of materials, theinclination along the slope and factors involving fluidmechanics determine whether the displaced massmoves a short or long distance. Leroueil et al. (1996)divided the development of a slope instability into fourstages: pre-failure stage, the failure stage, the post-fai-

lure stage and the re-activation stage. During the fai-lure stage, soil mechanics plays an important role, andthe stage is characterized by the formation of a shearzone through the entire sediment mass. During thepost-failure stage, visco-plastic flow and fluid mecha-nics control the behaviour of the sliding mass, and thisstage includes movement of the sediment mass fromjust after failure and until it essentially stops. Whitman(1985) defined slope failure in two ways. "Disintegra-tive failure" describes the condition where a sedimentmass can deform continuously under a shear stress lessthan or equal to the static shear stress applied to it.This failure is a flow failure. "Non-disintegrative fai-lure" involves unacceptably large permanent displace-ments or settlements during failure, but the earth massremains stable and internally undeformed. This failureis a less mobile type of landslide, but the undeformedmass may also be moved for a long distance alongother sliding planes. These two failure terms were usedby Kayen et al (1989) and Lee et al. (1991) to characte-rize the morphology of the slides. Disintegrative andnon-disintegrative failures can be used to describe thesubmarine slides and it is also a relatively simple wayto classify slides into two failure types geotechnically.One type has the potential to travel a long distance,and the other one only involves a limited movementafter the initiation of the slide.

The Storegga Slide has been studied intensively due tothe development of the Ormen Lange gas field in theupper slide scar (Figure 1). The morphology of the

NORWEGIAN JOURNAL OF GEOLOGY Behaviour of the sediments in the Storegga Slide 243

Behaviour of the sediments in the Storegga Slideinterpreted by the steady state concept

ShaoLi Yang, Anders Solheim, Tore J. Kvalstad, Carl F. Forsberg & Michael Schnellmann

Yang, S. L., Solheim, A., Kvalstad, T. J., Forsberg, C. F. & Schnellmann, M.: Behaviour of the sediments in the Storegga Slide interpreted by the steadystate concept. Norsk Geologisk Tidsskrift, 86, pp. 243-253. Trondheim 2006. ISSN 029-196X.

Two different sediment types were involved in the Storegga Slide. Marine clay sediments show contractive behaviour, and glacial deposits (tills andglacial debris flow deposits) were observed to be dilatant. In this study, the steady state concept is used to interpret the behaviour of the sediments inthe Storegga Slide. Disintegrative failure took place in the marine sediments with a positive state parameter, and non-disintegrative failure occurredin the glacial deposits with a negative state parameter. This provided a favorable condition for long run-out distances of the material along themarine sliding planes during the post-failure stage and limited displacement in some parts of the Storegga Slide in glacial deposits.

ShaoLi Yang, Anders Solheim, Tore J. Kvalstad, Carl F. Forsberg & Michael Schnellmann, International Centre for Geohazards, Norwegian GeotechnicalInstitute, Oslo, Norway.

Storegga Slide has been discussed in several recentpapers (e.g. Kvalstad et al. 2005; Solheim et al. 2005).Enormous amounts of debris have passed through thelower part of the slide scar, and the material was trans-ported for a long distance. In the upper part of theslide, the slide material was only displaced a short dis-tance (a few km at the most), and intact blocks can beseen inside the slide masses from the seismic data.Both disintegrative and non-disintegrative failures canbe observed in the geometry of the Storegga Slide.With this in mind, the main focus of this paper will bethe behaviour of the sediments during the failure andpost failure stages. The specific objectives are to:• Apply the steady state concept to the behaviour of

the sediments involved in the Storegga Slide.• Interpret the slide mechanism geotechnically.

244 S. L. Yang et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig 1: Upper part of the Storegga Slide

Fig 2: Contractive and dilatant behaviour (Been and Jefferies 1985)Stress of the sample moves to the right at undrained conditionStress of the sample moves to the left at undrained condition

Characterization of the behaviour ofsediments from the literatureTerzaghi (Terzaghi & Peck 1948; Terzaghi et al. 1996)was the first person to regard the void ratio as one ofthe most important state variables to characterize thebehaviour of sediments. Later Roscoe et al. (1958,1963) selected p′ (mean effective stress), q (deviatoricstress) and e (void ratio) as the state variables in Criti-cal State (CS) soil mechanics and the critical state wasdefined as the state at which the sediment continues todeform at constant stress and constant void ratio (Ros-coe et al. 1958). Later Poulos (1981) proposed the ste-ady state (SS) concept, which was defined as the state atwhich the sediment continues to deform at constantvolume, constant normal effective stress, constant shearstress and constant velocity. Been and Jefferies (1985)proposed the state parameter ψ to characterize thestress-strain response of sediment based on the steadystate concept.

(1) ψ = e – ess

where e is the void ratio of the sediment, ess is the voidratio of the sediment at the steady state. If the state ofsediment is above the steady state line (SSL), ψ is posi-tive and the sediment shows contractive behaviour. Ifthe state of sediments is below the SSL, ψ is negativeand the sediment shows a dilatant behaviour (Fig.2).

At undrained conditions, three different types of typi-cal stress-strain behaviour have been observed for sedi-ments in the laboratory (Fig. 3). For specimen A, thesediment exhibits a peak strength at a small strain andthen collapses to flow rapidly to large strains at loweffective confining pressure and low strength. This hasbeen consistently observed in the laboratory (Sladen etal. 1985; Lade and Yamamuro 1997; Yang 2002), and iscalled strain softening (flow liquefaction for sand). Forspecimen B, the peak strength at a small strain is follo-wed by a limited period of strain softening behaviour,and strain hardening occurs at intermediate strains. Ahigher large-strain strength can be approached, andthis was termed limited strain softening (limited lique-faction for sand) (Kramer 1996). Specimen C initially

245NORWEGIAN JOURNAL OF GEOLOGY Behaviour of the sediments in the Storegga Slide

Fig 3: Stress-strain (a), stresspath (b) and excess pore pres-sure-strain (c) during undrainedtest for clean sand (Kramer 1996)

contracts, followed by strain hardening at small strainlevel, and large-strain strength can be reached. Alltested specimens can reach the SSL at large strains instress path (Fig. 3). Failure line is the SSL, which isobtained from drained or/and undrained tests.

Granular sediment may become unstable even beforethe stress state reaches failure (Lade 1992; Chu et al.1993; Leong et al. 2000). This type of instability is akind of pre-failure flow instability, and has been consi-dered as one of the failure mechanisms that lead to flowslides or collapse of granular sediment slopes in a num-ber of case studies (Lade 1993; Olson et al. 2000).

Instability Line (IL) is a line which connects the peakpoints (peaks of the deviatoric stresses, q) of a series ofeffective stress paths obtained from undrained com-pression tests (Chu & Leong 2002; Leong et al. 2000). It

originates from zero point in the p′-q plot for sand withno cohesion (Fig. 4). Yamamuro and Covert (2001)showed that the stress condition required to initiateliquefaction under cyclic loading corresponded to theinstability line established from the monotonic undrai-ned triaxial compression tests. Yang (2004) carried outseries of static and cyclic triaxial tests for sandy andsilty sediments, and these results also indicated thatliquefaction occurs when the cyclic stress path reachesthe position of the instability line from the staticundrained triaxial tests. The instability line is also defi-ned as the line that separates potentially unstable stressstates from stable stress state (Chu and Leong 2002).

The concept of the steady-state of deformation can alsobe applied to predicting the ultimate form a landslidewill take. This has been used both qualitatively andquantitatively by Schwab and Lee (1988) and Kayen etal. (1989) to discuss progress of sediment flow, evaluateslide initiation, transformation of slumps into flow andsubmarine landslide morphology. The steady stateapproach is useful for understanding how differenttypes of submarine slides mobilize and evolve.

Geological setting of the Storegga SlideareaComposition and distribution of the sediments in the Sto-regga Slide area reflect the changing depositional environ-ment during several glacial-interglacial cycles. Very highsedimentation rates persisted during periods of maxi-mum glacial coverage (Solheim et al. 2005). The thickdeposits resulting from these relatively short periods con-sist of tills and debris flow deposits. During the major partof glacial-interglacial cycles, glacial marine and normalmarine depositional processes were predominant on themargin, resulting in fine-grained marine deposits (Sol-


Fig 4: Terminology in stress path and definitions (Chu and Leong.2002;Yang et al. 2006. *Instability Zone is defined as the zone bet-ween the steady state line and the instability line

Fig 5: State I and II from effective stress path (Kayen et al., 1989)

heim et al. 2005), such as the marine layers O3 and R2which are discussed in this study. As a result of the highsedimentation rate, high excess pore water pressure mayhave been built up in the clayey sediment in this area.With regard to physical properties, the glacial sedimentsare characterised by generally low water content, low plas-ticity index and relatively low clay content, whereas themarine sediments have high water content, high plasticityindex and high clay content (Kvalstad et al. 2005). The gla-cial deposits can show a strain hardening behaviour, whe-reas the marine sediments may show a strain softeningbehaviour. This will be discussed later in this paper.

Initiation of submarine slide and mecha-nism for its failure typesVarious triggering mechanisms for submarine slideshave been reported in the literature. The initial trigge-ring of the Storegga slide is believed to have been causedby the high excess pore pressure caused by rapid deposi-tion in the adjacent North Sea Fan possibly combinedwith earthquake loading in a locally steeper slope in thelower part of the slide area (Kvalstad 2005).

The downslope gravitational shear stress, qs, is less thanthe peak static shear strength of the sediment, su, beforefailure (Fig. 5). The steady state shear strength must beless than or equal to qs for disintegrative flow to occur.The sediment state in which su>qs>qss and no drainage


Fig 6: Stress-strain (a), stress path (b) andpore water pressure- strain (c) plots formarine sediments of slide unit O3 at NorthFlank (NF) of the Storegga Slide. For loca-tion of site NF, see Fig.1.

Fig 7: The determination of the normally consolidated line (NCL)from oedometer test for marine sediments O3 at North Flank.

occurs is referred to as state I (Whitman 1985). Thedownslope gravitational shear stress, qs, is less than thesteady state stress in state II. The sediment is stable ifthere is no external downslope stress.

If there is an external downslope stress, a positive porewater pressure builds up, the effective stress path moves

to the left (Fig. 5a), and the steady state strength isreached at large strains. Even though the external load isstopped, the original driving stress is still more than thesteady state strength. Therefore the sediment may travela long distance underwater. If a negative pore water pres-sure builds up, the effective shear stress path moves tothe right (Fig. 5b) and the shear strength is increased. Ifthe external load is removed, the original driving stress isless than the steady state strength, and the movement ofthe sediment will stop. The material can only be trans-ported a limited distance. A disintegrative failure canonly occur when the in-place driving shear stress or stressthat is exerted by static downslope gravitational loading,exceeds the value of the steady state shear strength thatcorresponds to the appropriate sediment porosity.

Determination of the steady state line ofmarine sediments and glacial depositsin the Storegga SlideA considerable number of undrained triaxial tests werecarried out for marine sediments and glacial depositsfrom the Storegga Slide. The undrained triaxial tests onthe marine sediments of slip plane unit (O3) at the North


Fig 8: Stress-strain (a), stress path (b) and pore waterpressure-strain (c) plots for marine sediments of slideunit R2 at Site 31 of Storegga Slide. For location ofsite 31, see Fig.1.

Fig 9: The determination of the normally consolidated line (NCL)from oedometer test for marine sediments R2 at Site 31

Flank (NF) site outside the Storegga Slide (Fig.1) showedbuild-up of excess pore water pressure, decrease of effec-tive stress and shear stress (Fig.6). It is assumed that thesteady state is reached at an axial strain of around 20%.The steady state points plotted in e-logp′ space can notdefine the SSL very well because the difference of thestresses at steady state is not big. Since no drained triaxialtests were carried out, the oedometer tests on the samemarine sediments are considered in this study. In v-logp′space (v, the specific volume, v=1+e), the normally conso-lidated line is parallel to the steady state line, and the slopeof the SSL can therefore be found from the oedometertests (Fig.7). Based on the three SS points and the slope ofthe SSL, the SSL in e-logp_ space can be obtained. Thesame procedure was applied to determine the SSL for themarine slip unit R2 at site 31 (Fig.8-9) and glacial depositsat site 19 (Fig.10-11). All the SSLs for the above sedimentsare given in Figure 12.

The instability zone can only be observed for themarine sediments (O3) at the North Flank. When thestress path from the cyclic triaxial tests and the statictriaxial tests following the cyclic ones were plotted inthe stress path of the marine sediments in Figure 6a, the

marine sediments did not show the same behaviour asthose observed by Yamamuro and Covert (2001) andYang (2004). The instability or failure (liquefaction forsandy or silty sediments) can not be observed when thestress path reached the instability line. The stress pathended on the steady state line under the static loading


Fig 10: Stress-strain (a), stress path (b) and porewater pressure-strain (c) plots for glacial deposits(GD) at Site 19 of the Storegga Slide. For location ofsite 19, see Fig.1.

Fig 11: The determination of the normally consolidated line (NCL)from oedometer test at site 19

following the cyclic tests (Fig.13). Since the marinesediment is a clay-dominated material, the instabilityconcept is not suitable in this case.

The ultimate strength of the failed mass is an importantparameter to control the evolution of the post-failurebehaviour, and to estimate the rheological properties offlows. Normally, this ultimate strength can be regardedas the remoulded strength of the sediments. This valuecan be much smaller than the large strain shearstrength obtained from undrained triaxial tests, and isconsidered to give a lower bound estimate of strength.It was found that remoulded strength from index tests(fall cone and UU triaxial), the sleeve friction fromCPTs and the residual strength from rapid ring sheartests on "intact" material fell in the same range (Kval-stad et al. 2005). These values can be used as the lowerboundary estimate of the ultimate strength of the sedi-ments. The sensitivity from ring shear tests is the ratiobetween the peak shear stress and the residual shearstress under rapid shearing, and the values for themarine sediments are between about 1.7 and 2.9 (NGIreports 2000, 2002). This means that the shear strengthfor the marine clays can be reduced considerably atlarge strains. The ratio between the intact and remoul-ded shear strength can reach more than 5 from fall conetests (Yang et al. 2005), therefore, the strength of thematerial can be very low after remoulding in the flow.This provides a favourable condition for the failed massto be transported over long distances.

The in situ effective overburden stress of the sedimentsat various locations and depths (Fig. 14 and 15) and thein situ corresponding void ratio are calculated andshown in the e-logp′ plot (Fig. 12). The shallowermarine layer O3 as well as the deep marine layer R2show a positive state parameter. These sedimentsbehave as contractive material and have potential fordisintegrative failure (A and B on Fig.12). The marinelayer R2 under the slide scar at site 31, and the glacial


Fig 12: In-situ stress state in the e-logp’ space A: depth, 22.32m, marine sediments, O3, at North

Flank (Fig. 14) B: depth 240m, marine sediments, R2, above upper

part of the slide scar (Fig.15) C: depth 92.25m, marine sediments, R2, under slide

scar at site 31 (Fig.15) D: depth 86.25m, glacial deposits, at Site 19 (Fig.15)

Fig 13: Cyclic stress path and the static stress path (a) d=22.8m, (b)d= 23.32m at North Flank


Fig 14: Seismic data across North Flank and location for the marine clays, O3

Fig 15: Seismic dataacross sites 31and19, and location forthe marine sedi-ments, R2 and theglacial deposits

deposits at site 19 show a negative state parameter (Cand D on Fig.12), and the deposits behave as dilatantmaterial. Thus sediments do not show a potential fordisintegrative failure. However, the sediments of R2under the slide scar at site 31 lost considerable overbur-den pressure during the slide, and the in-situ effectiveoverburden stress before the Storegga Slide was muchhigher than the present one. The sediment is presentlyoverconsolidated, but a contractive behaviour mostlikely occurred during the slide as observed in testsfrom sites 19 and 20 and on tests consolidated to theassumed stress states prior to the slide event.

The present stress state is used in the analysis of thebehaviour of the marine and glacial deposits. It can beextended to the stress conditions close up before theStoregga Slide occurred by taking the loss of overbur-den into account.

Morphology of the Storegga SlideThe main sliding planes for the Storegga Slide are loca-ted in the marine layers. The lowermost slip plane isonly exposed in the deepest part of the slide scar area.Further upslope, the slide planes get shallower resultingin a step-like morphology. This morphology is inter-preted as the result of a retrogressive sliding process(Kvalstad, et al. 2005). The sediment travelled a longdistance in the lower part of the slide area. Some intactblocks were also observed in this area. They were pro-bably transported along the sliding plane without inter-nal disintegrative failure. In the shallowest part, close tothe shelf break, the steps occurred in very hard glacialdeposits. Here, local shear planes with a listric geome-try cut through the glacial deposits. The fact that thesedeposits show a dilatant behaviour, may explain whymany of these blocks were not able to move far.

The mechanisms for the morphology of the StoreggaSlide are complicated, and depend on many factorssuch as material properties, topography and visco-plas-tic fluid dynamics. However, if the state parameter for amaterial is positive, the material shows a contractivebehaviour, and as a consequence, the shear strength willbe reduced considerably at large strains. Such failedmaterial is characterized by disintegrative failure andmay thus be transported for a long distance.

If a future submarine slide is initiated in the Storeggaslide area again, new sediments will have compensa-ted for the overburden that was removed by the lastslide and a disintegrative failure will occur in themarine sediments. These masses therefore could havea potential to be transported for a long distance if theslope is sufficiently steep. However, with the presentpore pressure conditions, the steady state strength ishigher than the downslope gravitational shear stress,

and sliding will be localized to locally steep escarp-ment areas only.

Conclusions• The state parameter for marine sediments O3 at North

Flank and deep marine layer R2 at Site 19 is positive,and the material is contractive. Positive excess porewater pressure built up, and the shear strength decrea-sed at large strains under undrained loading for thesemarine sediments in the Storegga Slide.

• The state parameter for shallow marine layer R2 atSite 31 and the glacial deposits at Site 19 is negative,and the material dilatant. Negative pore water pres-sure built up, and the shear strength increased atlarge strains under undrained loading.

• Failures in marine layers were disintegrative and pro-vided a favourable condition for the transport ofmaterial over a long distance. In contrast, glacialdeposits showed non-disintegrative failure, and thesliding material could only travel over a limited dis-tance.

• The steady state concept can be used to interpret thebehaviour of the sediments in the Storegga Slide, andthe mechanism for the slide can be explained to somedegree.

Acknowledgements: This is publication number 111 of the Internatio-nal Centre for Geohazards (ICG), and was prepared under Euromarginproject (European Science Foundation 01-LEC-EMA14F). NorskHydro and the other Ormen Lange licence partners, who have provi-ded access to sediment data, are also acknowledged.


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