Influence of weathering and pre-existing large scale fractures on gravitational slope failure: insights from 3-D physical modelling

Using a new 3-D physical modelling technique we investigated the initiation and evolution of large scale landslides in presence of pre-existing large scale fractures and taking into account the slope material weakening due to the alteration/weathering. The modelling technique is based on the specially developed properly scaled analogue materials, as well as on the original vertical accelerator device enabling increases in the 'gravity acceleration' up to a factor 50. The weathering primarily affects the uppermost layers through the water circulation. We simulated the effect of this process by making models of two parts. The shallower one represents the zone subject to homogeneous weathering and is made of low strength material of compressive strength &sigma;_l. The deeper (core) part of the model is stronger and simulates intact rocks. Deformation of such a model subjected to the gravity force occurred only in its upper (low strength) layer. In another set of experiments, low strength (<i>&sigma;_w</i>) narrow planar zones sub-parallel to the slope surface (<i>&sigma;_w<&sigma;_l</i>) were introduced into the model's superficial low strength layer to simulate localized highly weathered zones. In this configuration landslides were initiated much easier (at lower 'gravity force'), were shallower and had smaller horizontal size largely defined by the weak zone size. Pre-existing fractures were introduced into the model by cutting it along a given plan. They have proved to be of small influence on the slope stability, except when they were associated to highly weathered zones. In this latter case the fractures laterally limited the slides. Deep seated rockslides initiation is thus directly defined by the mechanical structure of the hillslope's uppermost levels and especially by the presence of the weak zones due to the weathering. The large scale fractures play a more passive role and can only influence the shape and the volume of the sliding units

Numerical simulations of an ocean/continent convergent system: influence of subduction geometry and mantle wedge hydration on crustal recycling

The effects of the hydration mechanism on continental crust recycling are analyzed through a 2D finite element thermo-mechanical model. Oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method. Hydration is accomplished by lawsonite and serpentine breakdown; topography is treated as a free surface. Subduction rates of 1, 3, 5, 7.5 and 10 cm/y, slab angles of 30o, 45o and 60o and a mantle rheology represented by dry dunite and dry olivine flow laws, have been taken into account during successive numerical experiments. Model predictions pointed out that a direct relationship exists between mantle rheology and the amount of recycled crustal material: the larger the viscosity contrast between hydrated and dry mantle, the larger the percentage of recycled material into the mantle wedge. Slab dip variation has a moderate impact on the recycling. Metamorphic evolution of recycled material is influenced by subduction style. TPmax, generally representative of eclogite facies conditions, is sensitive to changes in slab dip. A direct relationship between subduction rate and exhumation rate results for different slab dips that does not depend on the used mantle flow law. Thermal regimes predicted by different numerical models are compared to PT paths followed by continental crustal slices involved in ancient and recent subduction zones, making ablative subduction a suitable pre-collisional mechanism for burial and exhumation of continental crust.Comment: 10 figures, 3 table

Three-dimensional numerical modeling of hydrostatic tests of porous rocks in a triaxial cell,

International audienceIt is known that there is stress concentration at specimen ends during experimental rock tests. This causes a deviation of the measured nominal (average) stresses and strains from the actual ones, but it is not completely clear how strong it could be. We investigate this issue by numerical modeling of the hydrostatic tests using a reasonably simple constitutive model that reproduces the principal features of the behavior of porous rocks at high confining pressure, PcPc. The model setup includes the stiff (steel) platens and the cylindrical model rock specimen separated from the platens by the frictional interfaces with friction angle ϕint. The whole model is subjected to the quasi-statically increasing normal stress PcPc. During this process, the hydrostats PcPc(ε) are computed in the same way as in the real tests (ε is the average volume strain). The numerical hydrostats are very similar to the real ones and are practically insensitive to ϕint. On the contrary the stresses and strains within the specimen, are extremely sensitive to ϕint. They are very heterogeneous and are characterized by a strong (proportional to ϕint) along-axis gradient, which evolves with deformation. A strong deviation of the stress state at the specimen ends from the isotropic state results in inelastic deformation there at early loading stages. It follows that the nominal stresses and strains measured in the experimental tests can be very different from the actual ones, but they can be used to calibrate constitutive models via numerical simulations

An experimentally constrained constitutive model for geomaterials with simple friction–dilatancy relation in brittle to ductile domains,

International audienceComplexity of the mechanical behavior of geomaterials makes it very difficult to formulate constitutive models (both at micro- and macro-scales) valid for different loading conditions and deformation regimes. To make progress in understanding this complexity, we take advantage of the large set of data for the synthetic rock analog GRAM1, a granular, frictional, dilatant, and cohesive material formed of bonded rigid particles. We use also data from literature for two real rocks. All data are from conventional triaxial tests conducted for a wide range of confining pressures covering the material behavior from brittle fracturing to ductile flow. The data processing allowed to define both the yield function and the inelastic volume strain as functions of the mean stress σmσm and the accumulated inelastic strain View the MathML sourceγ¯p. The internal friction coefficient and dilatancy factor calculated from these functions were shown to be different but evolving very similarly with σmσm and View the MathML sourceγ¯p for all the three materials. This allowed to relate the yield and plastic potential functions and thereby to complete the constitutive formulation within the framework of the classical elastoplasticity theory. The obtained results are also used to elucidate the relation between the yield surface and failure envelope as well as the meaning of the internal friction coefficient derived from the failure envelope which is routinely used in geomechanical applications and which is very different from the internal friction coefficient derived from the yield function

Exhumation of UHP/LT rocks due to the local reduction of the interplate pressure: thermo-mechanical physical modelling

Earth and Planetary Science Letters, v. 271, n. 1-4, p. 226-232, 2008. http://dx.doi.org/10.1016/j.epsl.2008.04.011International audienc

Dilatancy factor constrained from the experimental data for rocks and rock-type material,

International audienceThe dilatancy is as important property of rock-type (granular, frictional, cohesive, and dilatant) materials as the internal friction, but it is generally ill-constrained and is rarely taken into account in applications, being set to zero. Theoretical analysis shows that the dilatancy factor β strongly affects the onset of the deformation instability (localization). Numerical models confirm this but also reveal that evolution of this instability, resulting in the deformation localization bands and fractures observed in the experiments, is defined by the evolution of the constitutive parameters and notably of β with inelastic strain View the MathML sourceγ¯p and mean stress σ . Therefore for the modelling of instability and rupture of rock-like materials, the knowledge (at least to a first approximation) of function β (View the MathML sourceγ¯p, σ ) and of how it can vary from one material to another is necessary. We constrain these functions from a large original experimental data set obtained for Granular Rock Analogue Material (GRAM1) as well as for hard rocks, Tavel and Solnhofen limestones, from the published data. All data are from axisymmetric compression tests conducted under different confining pressures. The data processing has shown that in spite of very different (orders of magnitude) values of elastic modulii and strengths, the aspect of the β (View the MathML sourceγ¯p, σ ) functions for all the three materials is similar: For all the materials β reduces with σ and increases with View the MathML sourceγ¯p until certain View the MathML sourceγ¯p value after which it reduces approaching zero. At intermediate σ values, β changes a sign from negative to positive with View the MathML sourceγ¯p. A relatively simple analytical expression of β (View the MathML sourceγ¯p, σ) grasping this behaviour is proposed

Physical Modeling of Arc-Continent Collision: A Review of 2D, 3D, Purely Mechanical and Thermo‐Mechanical Experimental Models

International audienceIn this chapter we present a review of 2D, and 3D, purely mechanical and thermo‐mechanical experimental models of arc-continent collision obtained using the modeling technique pioneered by A. Chemenda. Also presented are earlier models of oceanic and continental subduction which led to the development of the arc-continent collision experiments. Physical models of continental subduction revealed the existence of two principal subduction regimes defined by the interplate pressure, which is inversely proportional to the slab pull‐force. In both high and low compression regimes, the subduction of a continental passive margin generates a horizontal compression of the overriding plate that can produce failure of the overriding plate in the arc area or near the back‐arc basin spreading center if the arc-continent collision was preceded by oceanic subduction in the extension regime. Failure of the overriding plate can lead to subduction reversal or the subduction of either the fore‐arc block or the entire arc plate. Evolutionary scenarios including subduction of the fore‐arc block have been proposed for Taiwan and the Urals, where the fore‐arc block is presently subducting or is missing. The scenario including the subduction of the arc plate with total or partial subduction/accretion of the arc crust fits the geological data of the Oman Mountains, the western Variscan belt and Kohistan‐Ladakh arc in western Himalaya. Although these modeling results correspond well to the geological data, it was purely mechanical and did not consider any change in the mechanical properties during subduction. In nature, however, both pressure and temperature increase with depth causing the strength of the subducting crust and mantle to be reduced by about one order of magnitude when reaching 100 km‐depth. Thermo‐mechanical laboratory experiments revealed that such strong change deeply affect the subduction and exhumation processes. In the low compression regime, subduction of the continental passive margin does not produce failure of the overriding plate in the arc area and the continental crust can only subduct to ~120 km‐depth in the asthenosphere. By then, it has become too hot and weak and undergoes large deformation, including upward ductile flow of the deeply subducted portions and a localized failure of the upper crust at depth of a few tens of kilometers allowing the buoyancy‐driven exhumation of a crustal slice in between the plates. In the high compression regime, the subducted continental crust reaches greater depth (~150-200 km), remaining relatively cold due to the subduction of the fore‐arc block or the arc plate that occurs in this regime. However, the exhumation of the deeply subducted continental crust that reached UHP/LT conditions does not occur in 2D models in the high compression subduction regime. Such exhumation has been obtained in 3D thermo‐mechanical laboratory experiments where the geometry of the interplate zone causes a local reduction of the interplate pressure, which in turn allows a local buoyancy‐driven exhumation of UHP/LT material

Mechanical and Field Studies of Deformation Localization in Rocks,

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