The Storegga slide: evaluation of triggering sources and slide mechanics

Tore J. Kvalstada,*, Lars Andresena, Carl F. Forsberga, Kjell Bergb,Petter Brynb, Magnus Wangenc

aNorwegian Geotechnical Institute, P.O. Box 3930, Ullevaal Stadion, N-0806 Oslo, NorwaybNorsk Hydro, N-0246 Oslo, Norway

cInstitute for Energy Technology, P.O. Box 40, N-2027, Kjeller, Norway

Received 2 December 2003; received in revised form 15 June 2004; accepted 15 October 2004

Abstract

The Storegga slide occurred about 8200 years ago. The enormous slide was released in an area where the average slope inclination is in the

order of 0.6–0.78. Slide mechanisms and possible triggering sources that can cause large scale slide events at such low inclinations have been

studied. The morphology of the upper part of the slide scar shows lateral spreads along marine clay layers indicating a retrogressive slide

mechanism. Mechanical models of this slide mechanism have been developed considering the stain softening behaviour typical for marine

clays. Numerical modelling of the rapid sedimentation during glacial periods and field measurements show that excess pore pressure existed,

and still exists in areas adjacent to the slide. The three factors; slide mechanism, excess pore pressure and strain softening, have been

incorporated in a retrogressive slide model, which demonstrates that large scale slide processes are possible, even at the low slope gradients

in the Storegga area.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Slope stability; Soil strength; Pore pressure; Gravity; Strain softening

1. Introduction

The Storegga slide is located in the Norwegian Sea 70–

150 km off the West coast of Norway (Fig. 1). The slide

mass involved was about 3000 km3 and affected an area of

90,000 km2. The upper headwall has a length of about

300 km and the NW boundary of the deposition zone

extends about 800 km towards NW (Berg et al., 2005; Bryn

et al., 2005).

The slide area has been extensively mapped and

investigated in connection with petroleum exploration and

especially in connection with the slide risk evaluations for

the Ormen Lange gas field. The slide base has an average

inclination less than 18 and in the deepest part sliding along

soil layers more than 700 m below the present seabed can be

seen on seismic profiles. The slide left a deep slide scar and

headwalls with gradients in the range 20–308 and locally

even steeper. The headwalls have been stable since the slide

0264-8172/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpetgeo.2004.10.019

* Corresponding author.

E-mail address: tk@ngi.no (T.J. Kvalstad).

event about 8200 years ago, and the present stability of the

steep headwalls and the risk associated with slope failure

have been evaluated (Kvalstad et al., 2005; Nadim et al.,

2005). This required also understanding of possible

triggering sources and slide mechanisms involved in the

Storegga slide and a methodology for evaluation of the

possibility of renewed large scale sliding in the Ormen

Lange area. The main objective of this paper is to present

results from this work.

2. Bathymetry and slide scar morphology

As can be seen from the bathymetry (Fig. 2), the slide

scar consists of large, slightly inclined terraces forming

steps up the slide scar over a series of steeper scarps. These

scarps or headwalls have heights in the range 50–350 m and

the distance between the scarps in downslope direction

varies from 5 to 10 km along the Northern Flank to more

than 50 km in the central deep part of the slide (Fig. 3).

Marine and Petroleum Geology 22 (2005) 245–256

www.elsevier.com/locate/marpetgeo

Fig. 1. Location of the Storegga slide.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256246

The distance between the upper and the lower headwall in

the Ormen Lange area varies between 15 and 25 km.

The slide scar is typically draped with debris flow

material and large areas are covered with slide blocks.

Locally, windows in the debris can be seen where the slide

base is covered only by a thin drape of post-slide Holocene

sediments (Fig. 4). The average slope inclination from the

shelf edge at 250–350 m water depth to the Møre Basin at

2800–3200 m depth is in the order of 0.6–0.78. Along the

Northern Flank and in the lower part towards the basin

somewhat steeper areas are found with inclinations in the

range 1–28, but in large parts of the slide gradients are less

than 0.48. The major headwalls have inclinations typically

in the range 10–308.

Enormous amounts of debris have passed through the

lower part of the slide scar and left little evidence of the

slide mechanism. However, along the upper headwall and in

the vicinity of the Ormen Lange field, the debris material

has not been overrun by slide material, and the slide pattern

is well preserved (Fig. 4).

In the shallow part of the slide, at Storneset to the north

of the Ormen Lange field, the slide material seems to have

undergone limited downslope displacement. Ice berg

plough-marks can be traced from the shelf into the slide

area, which demonstrates that surficial material has not been

Fig. 2. The Storegga slide area showing locations of the Ormen Lange field, the bath

the profile (6) applied in simulations of excess pore pressure in the North Sea Fa

lost. The lowering of the surface in the slide area is solely

due to lateral spreading.

The zone at the interface between the shallow and the

deep part of the slide shows a similar pattern. A series of

graben structures and ridges can be seen and the lateral

spread pattern in the shallow slide zone is well preserved in

the graben structures as shown in Fig. 4. Again no direct loss

in material occurred, but a further lowering of the seabed

was caused by lateral spreading along a deeper slide base.

Sections of seismic profiles crossing the upper part of the

slide reveal a pattern of nearly intact triangular ridges and

graben structures (Fig. 5). The reflection is severely affected

by the rugged seabed leading to spreading and attenuation

and hence loss in resolution and clarity.

Based on morphology and seismic data, the slide

mechanism was interpreted to be retrogressive sliding

over a sensitive base layer, where the slide activity

generates progressive softening of the sensitive base layer

as the slide retrogressively spreads upslope and also

laterally.

3. Modelling of progressive failure and retrogressiveslide mechanisms

Analytical and numerical methods for analysis of

retrogressive slide mechanisms were sought in an early

phase of the project. However, no such tools were found to

exist, and a stepwise development of methodology was

carried out during the project.

3.1. The infinite slope model—slab slides

The explanation of large-scale slab slides in slopes with

slope angles in the order of 0.5–28 requires a very low

resistance. For the soils investigated in the Ormen

Lange/Storegga area the undrained and drained strength is

much too high. Fig. 6 shows the relationship between the

safety factor against undrained sliding, slope inclination and

excess pore pressure ratio ruZDu/g 0z, where Du is the

excess pore pressure above hydrostatic pressure relative to

ymetric profiles (1, 2 and 3) in Fig. 3, the seismic profiles (4, 5) in Fig. 5 and

n area.

Fig. 3. Bathymetric profiles through the slide area (location; see Fig. 2).

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256 247

seabed, g 0 is the submerged unit weight of the soil and z is

depth below seabed. A normalised strength ratio between

the undrained direct simple shear (DSS) strength, suDSS, and

effective consolidation stress, s 0vc, of 0.2 is applied.

As can be seen, excess pore pressure ratios of about 0.9

and a corresponding reduction in shear strength is required

to bring the factor of safety below 1.0 at slope angles of

interest. This means that only 10% of the overburden stress

is left, and either an extremely high initial pore pressure

must exist or collapsible of a highly sensitive soil layer must

be triggered. Potential sources for excess pore pressure were

investigated;

–

Fig

diff

earthquake induced shear strains generating excess pore

pressure

. 4. Bathymetric views of the upper part of the Storegga slide at the intersectio

ers from Fig. 2; red about 300 m to blue about 1000 m).

–

n b

melting of gas hydrates releasing methane gas and water

–

shear strain induced contraction with pore pressure

generation and softening during the slide process causing

progressive failure and retrogressive sliding

–

rapid deposition

The materials investigated do not exhibit highly brittle

behaviour, and extreme earthquake loading with very long

duration (Lindholm et al., this issue; Bungum et al., this

issue) would be required to remould the material comple-

tely. It was considered unlikely that this process could take

place simultaneously over thousands of square kilometres.

Infinite slope response analyses carried out with 0.3 g peak

ground acceleration (PGA) earthquakes and cyclic soil

models, showed very little accumulation of strains and pore

etween the deep part and the shallower part at Storneset. (Colour scale

Fig. 5. Bathymetry and seismic profiles 4 and 5 in the upper headwall in the Ormen Lange area. Interpreted morphology of slide block/debris is indicated with

black lines.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256248

pressure generation. However, more severe earthquake

loading than a 0.3 g PGA earthquake is considered a

potential trigger source for initiating a retrogressive slide

process in locally steeper areas, especially when combined

with existence of excess pore pressure/underconsolidation

as discussed later.

Also gas hydrate melting was dismissed as a major

contributor. Although bottom simulation reflectors (BSRs)

are observed in some areas adjacent to the slide scar, no

signs of gas hydrate have been found in the geoborings in

the area. When subjected to temperature increase and/or

pressure reduction, gas hydrates may dissociate and release

gas and water with an increase in volume, a reduction in soil

strength and a potential reduction of the stability of a slope.

Temperature changes in the seabed caused by warm bottom

water influx after the last deglaciation may have triggered

dissociation, but were limited to water depths shallower

than about 700 m and could only have affected a small part

of the slide area. The water pressure increase caused by the

rising sea level following deglaciation has further counter-

acted the dissociation process. The distribution of gas

hydrate is non-uniform and the potential melting front along

the base of the gas hydrate stability zone does not follow the

stratigraphy and hence not the observed slip surfaces. Gas

hydrate melting could locally have weakened the soils in the

upper part of the slide area and contributed to the extension

of the slide, however, the possible spreading and extension

of the melting zone is not consistent with the observed slide

morphology.

The two latter sources for pore pressure generation,

contractive soils with strain softening causing retrogressive

sliding and rapid deposition, were considered to have higher

potential.

Fig. 6. Infinite slope analysis, (a) geometry and stresses, (b) factor of safety

vs. base inclination angle and excess pore pressure ratio, ru.

3.2. The retrogressive slide model

The slide scar and the debris formations seem to have

similarities with a retrogressive quick-clay slide. However,

the sensitivity (i.e. the ratio denoted St between the intact

and the remoulded strength) of the clays investigated is

considerably lower than typical quick clays found onshore

in Scandinavia and Canada. A model for evaluation of

retrogressive sliding was developed (Fig. 7) and can be

described as follows:

–

an initial slide is developed in the lower and possibly

steeper part of a slope, and the mobility of the slide

Fig. 7. Retrogressive slide model.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256 249

material is sufficient to more or less completely unload

the earth pressure against the initially developed head-

wall (Fig. 7a)

–

the unloading of the headwall causes undrained lateral

expansion of the soil and strain concentrations develop in

the toe area of the headwall

–

the large shear strain in the strain concentration zones

causes strain softening, primarily in the marine clay base

layer, and progressive failure develops along this layer

–

the factor of safety decreases below unity and the failing

soil mass (Fig. 7b) starts to accelerate downslope. A

triangular front wedge is formed, being pushed along the

slide base by a gradually distorted rhomb and triangular

wedge creating a graben behind the front wedge, and in

this way forming a new headwall (Fig. 7c)

–

the released potential energy is partly consumed as

friction along the base and circumference of the slide

mass, and partly in remoulding of the slide material along

the slide base and internally in the distorted slide mass

–

excess potential energy is transformed to kinetic energy

accelerating the slide mass further downslope

–

the reduction in strength gives sufficient mobility to

unload the next headwall and the process repeats itself

(Fig. 7d) until soil strength, layering or geometry change

sufficiently to reduce mobility and decelerate the sliding

process

–

if the mobility is too low, the slide mass will block

further retrogression along the base layer and the process

will, if possible, continue along shallower marine clay

Fig. 8. Distorted mesh from finite element analysis of 100 m high, steep slope with strain softening base material, showing shear band formation.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256250

layers, creating steps in the slide base (Fig. 7e) as

observed in the Storegga slide (Fig. 2)

The failure model is kinematically admissible and

creates morphologic features in correspondence with

observed bathymetry (Fig. 4) and seismic profiles (Fig. 5).

3.3. Numerical modelling of undrained progressive failure

A prerequisite for the process is that the unloading of the

headwall causes strain concentrations and strength loss in

the base layer sufficient to reduce the factor of safety below

1 and thus initiate a dynamic slide process. This was

investigated through a series of static finite element (FE)

analyses of a 100 m high and steep slope to evaluate the

effect of sensitivity and also the rate of softening on the

development of progressive failure (Andresen, 2001). A

non-linear strain-softening material model was applied. The

slope material was simulated as non-sensitive normally

consolidated clay resting on a sensitive base clay layer over

a strong base. The front headwall was unloaded by applying

pre-excavation effective stress tractions on the exposed

headwall. Fig. 8 shows the distorted mesh for a simulation

with a sensitivity, StZ3, i.e. strain-softening to a residual

strength 1/3 of the initial strength. Strain softening caused a

considerable drop in the factor of safety, and as can be seen

from Fig. 8, shear bands developed along the base and also

in the overburden slide mass. The analysis confirmed

progressive spreading of shear bands and thus softening

along slip planes in good agreement with the assumed

failure model (Fig. 7).

Fig. 9. Dynamic wedge

3.4. Modelling of the initial run-out phase

The next step in the process was to model the following

dynamic instability process. The simple 2D geometric

failure model shown in Fig. 9 was developed to evaluate the

dynamics in the initial run-out phase. The tri-block or wedge

model consists of an intact front wedge (triangle) being

forced out on the base slip plane by the weight of a rhomb

and another triangle sliding down a steeply inclined slip

surface. The two latter elements are gradually distorted to

adapt to the change in flow direction along the base layer.

The wedge (or block) type failure mechanism defined in

Fig. 9 has a simple geometry, which closely resembles the

observed shear band formation from the FE analyses.

The deformation pattern associated with small as well as

large displacements is kinematically admissible, and if the

failed zone can accelerate and slide down to unload the

slip surface sufficiently, then a repeatable sliding process,

i.e. retrogressive sliding, may develop.

The model is based on an energy approach, and is as such

an upper bound method. The model is deformed incremen-

tally in small steps (Fig. 9). The release of potential energy,

Ep, from the sliding mass consisting of the frontal wedge

and distorted zone ðEpZW1d1 sin aÞ and the drop zone

(EpZW2$d2z) is compared with the friction energy, Ei,

dissipated along the slip surface (EiZt1l1d1Ct2l2d2) and in

the distorted mass (EiZtgVdist). The difference will be

transformed to kinetic energy, Ekin. During the sliding

process the changes in geometry, volumes and relevant

shear strengths are updated and accounted for in the energy

balance allowing velocities to be calculated. As long as

(tri-block) model.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256 251

excess kinetic energy is produced the slide mass will

accelerate, otherwise retardation will take place. The motion

will continue until the consumed energy equals the released

potential energy.

The run-out distance is strongly dependent on the

remoulded strength of the material in the slip surface.

High velocities might lead to a high frontal water pressures

and intrusion of water or remoulded soft material from the

previous slide block into the slide base causing further

reduction of the shear strength. With increasing travel

distance the soil mass will pass major steps in the slip

surfaces and break-up of slide blocks and intermixing with

water may take place. These processes are not well

understood, but might help explain excessive run-out on

low inclination slide bases.

Fig. 10 shows development of velocity, run-out distance

and geometry vs. time for an example run. The model

assumes instant application of remoulded strength at the

base slip surface and will thus overestimate the acceleration

in the initial phase. The displacement of water caused by the

sliding mass will generate a surface wave (tsunami), which

will contribute to consumption of slide energy and reduce

the initial acceleration as well as the average velocity and

the run-out. The effect on the initial acceleration can be

modelled by introduction of added water mass. Applying an

added water mass equal to 1–2 times the volume of the

slide mass leads to a reduction of the initial acceleration of

about 15–25%, but has little effect on average velocity and

run-out.

The fraction of slide energy transformed to wave energy

during the slide event depends primarily on the thickness of

Fig. 10. Example of front velocity and travel distance vs. time (upper figures) and

black, final configuration in red.

the slide mass relative to the water depth and the slide

velocity. In the distal parts of the Storegga slide with large

water depths, the effect is considered small (!10%), while

in the shelf edge area, the effect will increase and may

introduce considerable interaction between slide mass and

water.

3.5. Modelling of retrogressive spreading

The single 2D tri-block model described above was

extended to a large number of tri-blocks. In this model a

predetermined final shape controlled by the drop height, Dh,

and the inclination, b, of the frontal wedge, is applied to all

tri-blocks as shown in Fig. 11.

In the same way as above, released potential energy and

consumed energy along slip surfaces and in distorted

volumes are calculated. The number of tri-blocks is

increased until consumed energy exceeds released energy.

The number of tri-blocks and the predetermined geometry

determines the run-out length and the cut-back length of the

slide and also allows assessment of average slide mass

velocity from start to end.

In order to evaluate the volume, which is a central

parameter in tsunami analysis, an assumption regarding the

width of the slide front has to be made. Karlsrud and

Lillevik (1984) presented data from a large number of slides

showing the width to cut-back length ratio being less than 2

for nearly all cases evaluated. This applies also to the overall

Storegga slide as well as for local embayments in the upper

headwall, and a value of 2 was applied in the estimates of

slide mass geometry at different time steps (lower figure). Initial shape in

Fig. 11. Slide development with retrogressive multi-wedge model.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256252

slide volumes involved in potential renewed slide activity in

the area.

By varying the assumed drop height of sea bed surface

and inclination of front wedge, the critical geometry can be

found which gives the maximum run-out length, cut-back

length or other parameters like slide mass momentum or

slide mass energy which can be related to tsunami

generation as described by Løvholt et al. (this issue).

Numerical simulations based on computational fluid

mechanics (CFD), where the soil strength is modelled as a

Bingham fluid with strain softening yield strength, demon-

strates the same type of behaviour (see Fig. 6 in Gauer et al.,

this issue). However, the CFD analyses are computationally

very heavy and not suited for parametric studies.

The input parameters for the retrogressive models are

primarily geometry, intact and remoulded normalised shear

strength and in situ pore pressure conditions. The geometry

is controlled by the stratification and inclination of the base

slip plane and the assumed geometry of the failure model.

The soil conditions and the pore pressure situation at the

time of the Storegga slide are discussed in the following

sections.

4. Stratigraphy and soil conditions

The interpreted seismic stratigraphy shows that the soil

profiles in the Storegga area are composed of slightly

inclined interbedded layers of glacial debris and marine/-

glacimarine clay (the Naust formation) deposited during the

major Pleistocene glaciations. These deposits are overlying

thick siliceous Tertiary oozes of the Kai and Brygge

formations. The geology, the stratigraphy and stratigraphic

nomenclature (Naust, Kai, Brygge) and the soil conditions

are described in more detail in companion papers by Berg

et al. (2005) and Kvalstad et al. (2005).

The Storegga slide developed along the marine/glaci-

marine clay layers. In the shelf edge area the shallow part of

the slide (to the north of the Ormen Lange field) follows the

Naust O3 layer and in the deeper part of the slide, where the

Ormen Lange field is located, the R2 layer forms the slide

base. With increasing distance from the shelf edge, the slide

base follows increasingly deeper strata of the Naust

formation and finally cuts into the Brygge oozes in the

lowermost part of the slide scar (Bryn et al., 2005; and

Solheim et al., 2005).

The clays were investigated with respect to strength and

how the strength was affected by shear strains under

monotonic and cyclic loading as induced by deformations

during sliding and earthquake loading. The clays show

strength anisotropy, and the undrained shear strength along

the nearly horizontal slip planes is best described by the

undrained strength measured in direct simples shear tests,

suDSS. The intact strength is a function of the effective

overburden stress, and can be expressed as:

suDSS Z aDSSs0v Z aDSSðg

0z KDuÞ (1)

where s 0v, vertical effective stress, aDSS, normalised

undrained DSS strength, g 0, submerged unit weight, z,

depth below seabed, and Du, excess pore pressure. For the

marine clays aDSSZ0.2 was found to be a representative

value.

Along inclined slip surfaces, which cut through several

layers of glacial debris and marine clays, the compressive

strength from triaxial tests is more representative. For the

glacial deposits a normalized undrained compressive

strength, aC, slightly above 0.3 was found and for the

marine clays a value of 0.25 seems to be representative.

Dependent on the amount of glacial debris relative to marine

clay in the slide, a value between 0.25 and 0.3 should be

representative, with lower values for the more distal areas

with a lower content of glacial debris.

When subjected to large strains the marine clays shows

contractive behaviour causing pore pressure development

and reduction in strength, i.e. strain-softening behaviour.

The glacial debris layers are typically dilatant and thus

ductile. However, at large strains limited strain- softening

occurs. The behaviour is illustrated in Fig. 12.

The large strain, undrained residual strength can be

assumed to be equal to the remoulded strength of the

materials (Kvalstad et al., 2005). Based on interpretation of

sleeve friction from cone penetration tests, remoulded

strength from fall cone tests as well as residual strength

from rapid ring shear tests, the normalized remoulded

Fig. 12. Typical stress–strain development for marine clay and glacial

debris showing strain-softening from peak shear strength, tp, towards

residual shear strength, tres, with increasing shear strain.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256 253

strength, (surem/s 0v) was found to be in the order of 0.04–

0.10, increasing with depth for the marine clays and 0.10–

0.20 for the glacial clays. The ratio between peak strength

and remoulded strength is denoted sensitivity. If the intact

strength from triaxial compression is used as reference, the

sensitivity of the marine clays will be in the range 3–6,

while the glacial clays typically fall in the range 1.5–3.

Using DSS strength as reference will give somewhat lower

sensitivities.

Fig. 13. Parameters applied in simulation of pore pressure development

under rapid deposition. (a) Void ratio vs. effective overburden stress, (b)

permeability vs. void ratio. (the ‘modified glacial’ curve was applied in the

North Sea Fan simulations).

5. Overburden stress and pore pressure conditions

The submerged unit weight of the marine clays is in the

order of 6–7 kN/m3 in the upper metres, increasing towards

9 kN/m3 at larger depths below seabed due to increasing

compaction. The glacial debris has lower clay content,

resulting in higher density and shows typically submerged

unit weight in the range 10–12 kN/m3.

Deposition rates derived from the thickness, density

and age of the different strata showed rapid deposition in

the Ormen Lange area during glacial periods, and

especially in the adjacent North Sea Fan (Berg et al.,

2005; Nygard et al., 2005; Rise et al., 2005), and excess

pore pressure was suspected to exist in large parts of the

area.

Prediction of excess pore pressure requires information

about the compressibility and permeability of the sediments

and how these parameters change with increasing over-

burden stress and compaction. A large number of oedometer

compression tests were performed on material from the

different strata and again the materials could roughly be

divided in three groups, marine clays and glacial deposits of

the Naust formation and oozes of the Kai and Brygge

formations (Berg et al., 2005). Average relationships

between effective overburden stress, void ratio and

permeability were established (Fig. 13) and applied in the

sedimentation simulations.

The deposition rate was estimated for two different

profiles. One profile crosses the shelf edge in the Ormen

Lange area and another crossing from the depocentre in

the North Sea Fan into the lower part of the Storegga slide

(Fig. 2). Fig. 14 shows the reconstructed stratigraphy and

the estimated ages of the seismic stratigraphy considered for

the latter profile.

5.1. Modelling techniques for pore pressures caused

by rapid sedimentation

A 1D model was used at NGI for prediction of excess

pore pressure in the shelf edge area. This model does not

Fig. 14. Reconstructed stratigraphy of profile from North Sea Fan into the Storegga slide. The location of the profile is shown in Fig. 2.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256254

consider lateral flow, and is suitable for areas where lateral

flow is of little importance. In areas with varying thickness

and permeability, lateral flow processes may affect the flow

field significantly, and a 2D basin simulator model BAS

(Wangen, 1993) was applied for modelling the profile from

the North Sea Fan into the Storegga slide.

The 1D-analyses indicated excess pore pressure ratios,

ru, in the order of 0.2–0.3 in the shelf edge area at the time of

the Storegga slide, which is in reasonable correspondence

with the measured excess pore pressure of 100–110 kPa at

65 m depth below seabed in a location 2 km behind the shelf

edge (Strout and Tjelta, 2005). A series of simulations were

run for the NSF profile, varying the vertical and lateral

permeability distribution. At the time of the Storegga slide

(and also today) high excess pore pressure ratios were

Fig. 15. Examples of results from BAS simulations in Profile 6 from the North Sea

excess pore pressure ratio, ru, at the time of the Storegga slide. (a) Best estimate ve

with low permeability marine clay over high permeability Brygge ooze.

predicted in the North Sea Fan (NSF) and in the adjacent

area towards the Storegga slide (Fig. 15).

The BAS basin simulator does not allow expansion (or

swelling) caused by reduction in effective stress. In a

profile with large differences in deposition rate, the pore

pressure increase in a section with low deposition rate, as

in the toe area of the NSF, can be higher than the increase

in overburden stress. This will result in unloading and

swelling. The swelling requires an influx of pore water

and will introduce a delay in pore pressure response and

may explain that the most critical conditions may develop

with a considerable time lag compared with the deposition

history. This effect was demonstrated numerically with

FE analyses of an idealised fan structure, and indicated

that thousands of years may have passed from end of

Fan to the toe area of the Storegga slide (see Fig. 2) showing distribution of

rtical permeabilities with 10 times higher horizontal permeability, (b) Case

Fig. 16. Examples of retrogressive slide analyses of the Storegga slide showing slide velocity and kinetic energy vs. frontal run-out distance. (PJZ1015J). (a)

Case in the vicinity of the North Sea Fan with 500 m deep slip surface and 60% excess pore pressure gradient. (b) Case in the upper part of the deep slide area

with 250 m deep slip surface and 30% excess pore pressure.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256 255

rapid deposition in the depocentre at the top of the fan,

till maximum excess pore pressure is generated in the

toe area.

Table 1

Parameters applied in example cases

Parameters Case A Case B

Geometry

Initial slope height (H) (m) 500 250

Height reduction (Dh) (m) 250 125

Frontal wedge angle (b) (8) 35 35

Slope angle (a) (8) 0.6 1.2

Excess pore pressure ratio 0.6 0.3

Slide mass properties

Avg. subm. unit weight (g 0) (kN/m3) 9 9

Normalised shear strength (aC) 0.3 0.3

Sensitivity St relative to suC 2 2

Slide base properties

Normalised shear strength aDSS 0.2 0.2

Strength anisotropy suDSS/suC 0.8 0.8

Sensitivity St relative to suC 5 5

6. Example cases

Two example cases of the retrogressive slide model

applied for simulation of the Storegga slide are presented

below. Slide velocity vs. run-out distance and kinetic energy

vs. run-out distance are shown in Fig. 16. (The latter

parameter is essential for tsunami evaluations). Soil

parameters were chosen within the ranges obtained from

the soil investigations and excess pore pressure ratios

considered realistic for conditions at the time of the slide

event were applied (Table 1).

In Case A the slope inclination was set to aZ0.58, the

thickness of the slide mass, HZ500 m and the excess pore

pressure gradientZ60% to simulate the observed deep

seated sliding on very gentle base layer inclinations in the

vicinity of the North Sea Fan. The retrogressive slide model

predicts run-out distances in the order of 35 km. The

associated cut-back from the starting point is of the same

size giving a slide length of about 70 km with a slide scar

width of 70 km. The analysis shows that extensive slide

masses could be mobilised.

Case B was run with aZ1.28, HZ250 m, and a pore

pressure gradient of 30%, simulating the Storegga slide

conditions in the Ormen Lange area between the upper and

lower headwall. The resulting slide run-out is about 8 km

and thus the total slide length (run-outCcut-back) is about

16 km corresponding reasonably well with the distance

between the upper and the lower headwall.

The predicted run-out distances are sufficiently long to

push large slide masses over the major steps in the slip

surfaces. This will cause high speed impact at the base of

these steep slopes. Most likely this will lead to break-up of

the slide blocks and mixing in of seawater into the debris

material, which will reduce the average strength of the slide

mass as well as in the slip surface and allow transport of

debris over long distances.

T.J. Kvalstad et al. / Marine and Petroleum Geology 22 (2005) 245–256256

7. Conclusion

Site investigations have revealed moderately sensitive

clays and identified the marine clay layers which form

the main slip surfaces of the Storegga slide. The

morphology and seismic profiles from the upper (and

most intact) part of the slide area show clear pictures of

lateral extension indicating retrogressive upslope spreading

of the slide. Based on this interpretation mechanical models

for retrogressive failure were developed, for single step

slide release and for a continuous series of retrogressive

slide steps. The models are kinematically admissible and

based on an upper bound energy approach and reproduces

slide morphology in reasonable agreement with obser-

vations. A basic assumption and requisite is that the soil is

moderately sensitive and that the unloading caused by one

step in the slide process is sufficient to generate progressive

spreading of shear bands with strain softening causing

initiation of the next step. This was confirmed by FE

analyses for a single step slide and by CFD analyses for

continuous retrogression.

Simulations of excess pore pressure caused by the rapid

deposition during glacial periods showed that high excess

pore pressure ratios were likely to develop in the North Sea

Fan and adjacent areas during the last glaciations with more

moderate development in the Ormen Lange area. Pore

pressure measurements behind the shelf edge show values in

reasonable correspondence with the simulations.

Applying soil strength parameters from the site investi-

gations with excess pore pressure ratios from simulations,

the mechanical models for retrogressive sliding demonstrate

that retrogressive failure is a possible and likely slide

mechanism for failure along slightly inclined sensitive soil

layers. The retrogressive slide analyses show that extensive

areas may fail in a lateral extension/retrogressive failure

mode. The predicted run-out distances are sufficiently long

to push large slide masses over the observed major steps in

the slip surface. This would drastically increase the kinetic

energy of the slide masses and cause high speed impact at

the base of these steep slopes with break-up of the slide

blocks and mixing in of seawater into the debris material.

The resulting heavy slurry could well be capable of

transporting blocky material over long distances.

Retrogressive failure requires triggering of an initial slide

in the toe area. The initial triggering of the Storegga slide is

believed to be caused by the high excess pore pressure

caused by rapid deposition in the North Sea Fan combined

with earthquake loading in a locally steeper slope in the

lower part of the slide area. In these areas enormous

amounts of debris have passed, and little evidence of the

initial slide mechanism is available.

Acknowledgements

The work presented was carried out under the Ormen

Lange project. The authors would like to thank Norsk Hydro

and the Ormen Lange partners (BP, ExxonMobile, Petoro,

Shell and Statoil), for permission to publish the results. This

is publication number 46 of the International Centre for

Geohazards (ICG).

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