The vertical stress S_v is estimated by integrating the bulk density of the drilled formations. Micro­hydraulic fracturing (MHF) tests were performed in vertical boreholes to provide quantitative information on the S_Hmin magnitude, and in a few cases also served to independently verify values of S_v from logs (Nagra 2024o). In comparison to previous stress testing campaigns (e.g. Nagra 2001), dry sleeve reopening (SR) tests on previously induced fractures were also performed to derive values of the S_Hmax magnitudes (Nagra 2024o for details).

From the 139 successful MHF tests in eight of the nine new boreholes and the following SR tests, 121 reliable S_Hmin magnitudes and 65 reliable S_Hmax magnitudes were estimated. In addition to the new tests, two measurements from the Benken borehole were also re-analysed (Nagra 2024o). This unique dataset from the three siting regions has increased the number of stress magnitude data by an order of magnitude since 2019 (Fig. 2‑3).

A representative example of the stress data obtained is shown for the STA3 borehole in Fig. 4‑77. The figure shows that values of S_Hmin are close to values for S_v in the shallow subsurface but become increasingly lower than S_v with greater depth. Similarly, S_Hmax values are greater than S_v closer to the surface while becoming lower than S_v as depth increases. This can be explained by topographic effects (Nagra 2024o).

Fig. 4‑77:Stress magnitudes and comparison to clay-mineral content (proxy for layer stiffness)

Shown are data for the STA3 borehole (Nagra 2024o). Continuous line of average clay-mineral content is from Becker & Marnat (2024), and ranges per formation in grey represent the P5 to P95 interval according to Nagra (2024m). Fm.: Formation, DAO: Dogger Group above Opalinus Clay.

Hence, depth exerts an important control on the absolute stress magnitudes. It is noted that the value bands for S_Hmax (blue bars) are always wider than those of S_Hmin, because the method used to determine S_Hmax implies a range at least three times that of S_Hmin (Nagra 2024o).

Another important control on the stress magnitudes stems from mechanical stratigraphy. This is illustrated by the clay-mineral content in Fig. 4‑77 showing clear variations between formations. Formations with lower clay-mineral content, e.g. the limestones of the «Felsenkalke» and «Massenkalk» or the anhydrite-dominated Zeglingen Formation have a different gradient of S_Hmin (closer to S_v) than the clay-mineral-rich units of the Opalinus Clay and adjacent formations.

MHF and SR tests sample stresses of the formation on a metre scale. Hence, variability of material properties within the formation and relative contrasts between formations also impact the resulting stress magnitudes. This is highlighted in Fig. 4‑78, which summarises all obtained S_Hmin values as gradients of S_Hmin, i.e. magnitudes normalised by depth per formation and siting region. The figure indicates a relatively large variation of S_Hmin gradients in the Klettgau, Bänkerjoch and Zeglingen Formations. This is also consistent with the larger variation in stiffness compared to other formations (Fig. 4‑77). For the Bänkerjoch Formation two ranges were estimated to account for the bimodal nature of the data because of strongly contrasting bands of weak clay-rich rocks and stiffer anhydrites. The latter is also responsible for the Bänkerjoch and Zeglingen Formations standing out with some S_Hmin gradients above typical gradients for the vertical stress (24 to 25 MPa/km), whereas the Schinznach Formation exhibits consistently very low values. In the shallower formations above the Opalinus Clay, S_Hmin gradients tend to increase significantly, as effects of shallow burial superimpose the effects of mechanical layering, especially in the Malm limestones in the NL region, and the Hauptrogenstein and Wildegg Formation in JO.

The sketch in Fig. 4‑79 highlights how absolute depth and mechanical layering can exert a fundamental control on the stress field at different stages of tectonic loading in a highly simplified, alternating limestone and claystone sequence. With only gravitational loading (uniaxial strain condition) and assuming an elastic model, the horizontal stress S_h increases by u/(1- u)*S_v, where u is the Poisson’s ratio (e.g. Thiercelin & Plumb 1994). Since u is greater in the claystone than in the limestone, the gradient of S_h is smaller in the latter, leading to initially lower S_h values than for claystone at the formation boundaries (Fig. 4‑79a). However, with lateral tectonic loading (e.g. Miocene times, Section 3), S_h grows proportionally to the Young’s moduli in each layer, leading to much larger stress increments in the limestone than in the claystone for the same amount of lateral shortening (Fig. 4‑79b). With further tectonic loading, this initially leads to a homogenisation in stress magnitudes across the different lithologies, and eventually to a reversal of the initial situation, where the S_h values in the limestone are greater than those in the claystone. Fig. 4‑79c also highlights that S_h can exceed S_v in the shallow subsurface, as is observed with the high gradients in the Malm limestones in the NL siting region and in the Hauptrogenstein in the JO siting region (Fig. 4‑78). The simplified model also illustrates that S_h in competent layers at greater depth remains lower than S_v, even with stronger tectonic loading, as the depth effect still dominates, i.e. the shift of S_h by tectonic loading is less than the difference from initial gradients.

Fig. 4‑78:Control of mechanical layering on stress magnitude data

This figure displays the gradients of 121 reliable measurement of S_Hmin from MHF tests from the TBO campaign and two reanalysed MHF tests for the Opalinus Clay from the Benken borehole. Data are grouped by the three siting regions and sorted according to the distance to the top of each formation. Grey vertical bars show the range between the medians of the upper and lower bounds of the S_Hmin gradients for each formation. For the Bänkerjoch Forma­tion two ranges are estimated. The vertical axis does not show absolute depth, just relative depth. The very high gradient of one measurement in JO in the Upper Dogger comes from a very stiff section of the Hauptrogenstein and is not representative of the entire formation.

Fig. 4‑79:Conceptual sketch of the evolution of the horizontal stress magnitude S_h in a mechanically layered limestone – claystone sequence

(a) Overburden stress (S_v) is translated laterally through Poisson’s ratio, which is higher in claystones than in limestones (ν2 > ν1), leading to a steeper gradient (higher S_h values) in claystones in comparison to limestones. (b) Uniform horizontal tectonic loading leads to larger S_h increments in limestones because of larger stiffness (E1 > E2), and eventually to homogenisation of S_h values across the layers. (c) Further tectonic loading can lead to higher S_h values in limestones than claystones, and to S_h > S_v in the shallow subsurface.

In the Opalinus Clay, the stress measurements show a consistent linear trend with increasing depth, irrespective of the siting region (Fig. 4‑80). In other words, the gradient of S_Hmin is similar for all sites. The magnitude of S_Hmin in the Opalinus Clay is clearly lower than S_v, and the ratios of S_Hmin/S_v range between approximately 0.6 and 0.9. Measurements close to or outside the bounds depicted in Fig. 4‑80 can be explained as local outliers and are discussed in Nagra (2024o). Note that MHF and SR tests sample on a metre scale and thus S_v is generally the largest of the principal stress magnitudes in the Opalinus Clay, followed by S_Hmax and S_Hmin. Given the larger bandwidth of S_Hmax, it cannot be ruled out that in some cases S_Hmax reaches a value similar to S_v or, in rare cases, even larger than S_v. Overall, this translates into a stress regime that is dominated by normal faulting to strike-slip faulting in the Opalinus Clay.

Fig. 4‑80:Compilation of stress magnitude data in the Opalinus Clay across siting regions

Bars denote values of S_Hmin (a) and S_Hmax (b) for the various stations including the value range from test interpretation. Lower and upper bounds refer to the median of the lower and upper values of the estimated range, and lowest and highest bounds to the P10 and P90 percentile range of all measurements. Pore fluid pressure and vertical stress S_v are approximated with constant densities (1'000 kg/m³ and 2'450 kg/m³, respectively).