Geotechnical Challenges in the Caspian Sea

Hill, Andrew J.; Prakash, Anant; Hampson, Kevin M.; Gray, Thomas E.; Rushton, David; Mackenzie, Brian; Puzrin, Alexander M.

doi:10.4043/25988-ms

Cited by 14 publications

(3 citation statements)

References 6 publications

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“…The giant Storegga slide off the coast of Norway received particularly intensive multidisciplinary investigations, because of potential threats to the Ormen Lange natural gas development [4,10]. Risk assessment of submarine landslides has also been conducted for offshore developments in the West Nile Delta [7], the Caspian Sea [9,11,23], the Mozambique continental margin [1], the Nankai Trough [31] in addition to the Gulf of Mexico [13].…”

Section: Introductionmentioning

confidence: 99%

Dynamic propagation criteria for catastrophic failure in planar landslides

Zhang

Wang

Randolph

et al. 2016

Num Anal Meth Geomechanics

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Quantitative assessment of the risk of submarine landslides is an essential part of the design process for offshore oil and gas developments in deep water, beyond the continental shelf. Landslides may be triggered by a reduction in shear strength of subsea sediments over a given zone, caused for example by seismic activity. Simple criteria are then needed to identify critical conditions whereby the zone of weakness could grow catastrophically to cause a landslide. A number of such criteria have been developed over the last decade, based either on ideas drawn from fracture mechanics, or considering the equilibrium of the initial weakened zone and adjacent process zones of gradually softening material. Accounting for the history of the weak zone initiation is critical for derivation of reliable propagation criteria, in particular considering dynamic effects arising from accumulating kinetic energy of the failing material, which will allow the failure to propagate from a smaller initial zone of weakened sediments. Criteria are developed here for planar conditions, taking full account of such dynamic effects, which are shown to be capable of reducing the critical length of the softened zone by 20% or more compared with criteria based on static conditions. A numerical approach is used to solve the governing dynamic equations for the sliding material, the results from which justify assumptions that allow analytical criteria to be developed for the case where the initial softening occurs instantaneously. The effect of more gradual softening is also explored.From the solution, the maximum value of ΔP is 1 À r whenx ¼ω 0 . Therefore, the critical process zone length for the static state is ω = ω 0 , and the critical length l c,s of the pre-softened Figure 8. Displacement distributions at quasi-critical conditions. 2322 W. ZHANG ET AL. Case 2.1 r ≥ (1 À r) / βThis case cannot occur, in that for high gravity loading the process zone cannot develop to the extent that the shear strength falls to τ r at the junction with the pre-softened zone, hence Δl fs remains at 0 (hence Case 1.1).Case 2.2 r < (1 À r) / β andω ¼ω f s ¼ω 0 þ βarcsin βr 1Àr À ÁThe shear stress at the interface between the process and fully softened zones reaches the residual value and propagation of the fully softened zone occurs, i.e. Δl fs > 0. Therefore, the length of the Figure 12. Profiles of shear stress and lateral force at the failure condition.

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Section: Introductionmentioning

confidence: 99%

Dynamic propagation criteria for catastrophic failure in planar landslides

Zhang

Wang

Randolph

et al. 2016

Num Anal Meth Geomechanics

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“…Geotechnical data show that currently, very shallow excess pore pressures at the landslide failure surface do not exist (and were also not likely to exist at the time of failure 5.2 ka BP; Hill et al 2015b). Although uncertainties exist, the most likely mechanism of failure is that of steep slopes which initially failed and subsequently initiated a limited zone of spreading back upslope along the base of the steep scarp.…”

Section: Discussionmentioning

confidence: 99%

Application of the Shear Band Propagation Method to Slope Stability Analysis of a Palaeo-Landslide in the Caspian Sea

Gray¹,

Puzrin

Hill

2015

All Days

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Geophysical observations of palaeolandslides in offshore settings include long and relatively thin failures that occur on very mild slopes. Back analysis of such failures using the limiting equilibrium approach and measured geotechnical soil properties is typically used to inform the likely causes of past failures. An assessment can then be made of the present day significance of causal factors (e.g. the presence of excess pore water pressure), with the limiting equilibrium approach used to assess factors of safety for present-day slopes and predict the potential size of any future failures. However, for long, thin slides, use of the limiting equilibrium approach can lead to the conclusion that either a significantly large seismic trigger or other, abnormal geological condition was required to initiate failure. A result has been the focusing of academic research into other failure causes and conditioning factors. Further, the stability of the present day seabed for offshore developments can be overestimated, given the rarity of large seismic events in many areas and geotechnical measurements that not always show abnormal soil conditions. The shear band propagation approach offers an alternative view, where an initial small failure can grow into a larger failure at the point of initiation. A result is that an explanation of the conditions leading to failure can be attributed to many submarine landslides. Further, accounting for the potential propagation of a shear band in the future prediction of submarine landslides can result in greater predicted failure sizes and lower factors of safety than using limiting equilibrium only. The application of the shear band propagation approach to back analyse a palaeolandslide in the ACG Field, Caspian Sea, is used to demonstrate the methodology and compare with results using the limiting equilibrium approach. Results show that the shear band propagation approach successfully predicts observed failure geometries and suggests triggering conditions from a lower magnitude seismic event compared to that predicted using limiting equilibrium. The future application of shear band propagation theory to submarine landslide analysis is also discussed.

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“…Submarine landslides are a major natural geohazard commonly occurring in offshore continental slopes. They present significant threats to offshore infrastructure, in particular in the North Sea (Bryn et al 2005), the Gulf of Mexico (Jeanjean et al 2005), the Nankai Trough (Yamamoto et al 2015), the offshore Northwest Australia (Hengesh et al 2013) and the Caspian Sea (Hill et al 2015) where offshore resource explorations are active. Some significant historical events have been reported to have triggered disastrous tsunamis, for example the giant Storegga landslide that occurred 8200 years ago, the case off Papua New Guinea in 1998 (Synolakis et al 2002) and the latest Sulawesi landslide in 2018 that might have caused the deadly Palu tsunami (Liu et al 2020).…”

Section: Introductionmentioning

confidence: 99%

Depth integrated modelling of submarine landslide evolution

Zhang

Puzrin

2021

Landslides

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Submarine landslides are a major geohazard among worldwide continental slopes, posing significant threats to offshore infrastructure, marine animal habitats and coastal urban centres. This study establishes an original numerical package for time-efficient modelling of the entire submarine landslide evolution covering the pre-failure shear band propagation, slab failure and post-failure dynamics. The numerical scheme is based on the conservation of mass and the conservation of momentum and combines the shear band propagation theory and the depth-integrated method, with the consideration of the drag force from the ambient water. Shear band propagation in the weak layer and slab failure in the sliding layer are controlled by the strain softening and rate dependency of the corresponding undrained strength parameters. The post-failure behaviour in the sliding layer, such as retrogression upslope and frontally confined and frontally emergent mechanisms downslope, is also simulated. The numerical results from the proposed method are comparable to the analytical solutions and the large deformation finite element analysis. Application of this method to a back analysis of the St. Niklausen slide in Lake Lucerne reproduced the observed shape of the mass transport deposits, the position of the main scar and the travel distance. Because of its easy implementation and efficiency, the proposed numerical method for modelling of submarine landslides seems promising for practical applications.

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Geotechnical Challenges in the Caspian Sea

Cited by 14 publications

References 6 publications

Dynamic propagation criteria for catastrophic failure in planar landslides

Dynamic propagation criteria for catastrophic failure in planar landslides

Application of the Shear Band Propagation Method to Slope Stability Analysis of a Palaeo-Landslide in the Caspian Sea

Depth integrated modelling of submarine landslide evolution

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