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Arbeitsbericht NAB 13-88

Local stress field sensitivity analysis - Case Study Nördlich Lägern

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For the region of Nördlich Lägern an existing geological model of the subsurface, consisting of lithological interfaces and faults is used as a basis for a geomechanical numerical model to conduct a sensitivity study of the 3D in situ stress state. Due to uncertainties regarding geometries and rock properties in the subsurface, the model exhibits generic features. Accordingly, the focus of the model is not on the precise quantification of the stress state, but on the estimation of the impact of individual factors contributing to the stress state and its spatial variability.

Based on the chosen modelling approach the results indicate that the relevant stress ratios SH/SV, Sh/SV and SH/Sh are considerably reduced in the Opalinus Clay in comparison to the formations lying above or below. Apart from local exceptions the stress conditions in the Opalinus Clay are only moderately affected by the parameter variations considered in the modelling runs and typically result in stress ratio values of 1.1 < SH/SV < 1.3, 0.8 < Sh/SV < 1.0 and 1.2 < SH/Sh H/Sh by 0.1.

Of greater importance is the coefficient of friction on the faults. Higher fault friction increases stress ratios in the host rock, particularly horizontal stress anisotropy is increased (SH/Sh in-creases by 0.1-0.2 when increasing the friction coefficient from 0.2 to 0.6) because the fault’s ability to slip and thereby weakening the push from the far field is reduced at higher friction. Incorporation of a generic backthrust adjacent to the Siglistorf anticline reduces stress ratios in the surroundings. However, at the depth of the Opalinus Clay its influence is only minor. Whether faults reach below the Mesozoic sediments or not does not much alter the stress state within the sediments. If a detachment horizon is present, stress ratios are strongly reduced north of the E-W striking faults.

Topography influences the stress state due to the spatially varying weight acting on the subsurface. However, of greater importance are those stresses that are induced by the topography as a response to the northward directed push from the far field. These induced stresses are positive below topographic depressions and negative below topographical highs and appear to depths of up to several hundred metres. An ice cover significantly lowers stress ratios and their lateral variability, particularly the ratio of horizontal to vertical stress is reduced.

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