We have conducted a stress analysis of overcoring stress measurements undertaken at the Zedex Test Site (ZTS) in the Äspö HRL, Sweden. Two different types of overcoring cells have been used at the site: the 9-gauge Borre Probe (CSIR-type) and the 12-gauge CSIRO HI cell. Inversion programs based on the least-squares criterion were developed and calibrated. The primary and secondary stress field in the ZTS were back-calculated using strain data from individual cells as well as from combining data from both cells. We propose that the combination of strain data from the two cells significantly improves the determination of the stress field. A striking result is the good agreement between the re-analysed overcoring data and the existing hydraulic fracturing data. The deviations for the horizontal stresses between the re-analyzed overcoring data and the existing hydraulic fracturing data are approximately 3 MPa. The deviation in the orientation of maximum horizontal stress is approximately 10–20°.

Nous avons analyse les resultats de mesures de contrainte par surcarottage realisees sur le site Zedex, du laboratoire souterrain HRL d'Aspö, en suède. Deux types de cellule de mesure ont ete utilises sur ce site: la cellule à 9 jauges (du type CSIR) et la cellule à 12 jauges (du type CSIRO HI). Un programme d'inversion mettant en oeuvre une norme par moindres carres a ete developpe et valide. Les champs de contrainte primaires et secondaires du site Zedex ont ete evalues d'une part en considerant chaque mesure isolement, d'autre part en integrant l'ensemble des donnees de deformation dans une inversion unique. Nos resultats suggèrent que l'association des diverses mesures dans une integration unique ameliore nettement la determination du champ de contrainte. Un resultat interessant est le bon accord entre ces nouveaux resultats et les resultats des mesures par fracturation hydraulique. Les ecarts sur l'amplitude des contraintes horizontales est de l'ordre de 3 Mpa, les ecarts sur les orientations de l'ordre de 10–20°.

Auf dem Zedex Test Gelande im Äspö Festgesteinslabor (HRL) wurde eine Spannungsanalyse mittels Overcoring Spannungsmessungen durchgefuehrt. Hierzu wurden zwei verschiedene Methoden verwendet: das 9-kanal Borre Gerat (CSIR- Ausfuehrung) und die 12-Kanal CSIRO HI Zelle. Inversionsprogramme, die auf dem "least-squares"- Kriterium basieren, wurden entwickelt und kalibriert. Das primare und sekundare Spannungsfeld des Zedex Test Gelandes wurde sowohl aus Verformungsdaten individueller Zellen als auch aus Kombination beider Zellen ermittelt. Wir nehmen an, daß die Kombination der Messergebnisse beider Gerate die Bestimmung des Spannungsfeldes signifikant verbessert. Überzeugender Beweis hierfuer ist die gute Übereinstimmung der ueberprueften Overcoring Daten und Ergebnisse aus existierenden Hydraulic Fracturing Versuchen. Die Abweichungen der horizontalen Spannungen aus beiden Datensatzen ist etwa 3 MPa und Abweichungen der Orientierung der größten Horizontalspannung liegen bei etwa 10–20°.


The Äspö Hard Rock Laboratory (HRL) of the Swedish Nuclear Fuel and Waste Management Co. (SKB), Fig. 1, has been a geoscientific research area since 1986. Among the vast number of research projects conducted are a large number of rock stress measurements. In total, about 250 hydraulic fracturing, overcoring, and hydraulic tests on pre-existing fractures have been made in the region (e.g.2–6).

Existing studies indicate a non-linear stress distribution versus depth, which is strongly influenced by existing discontinuities (e.g.1;7–10). There is also a significant difference between the different measuring techniques used. Thus, despite the large number of measurement collected in this limited area, the stress field is not accurately known.

In this paper we will concentrate on the strain data collected at the Zedex Test Site (ZTS) using the Borre Probe (e.g.11) and the CSIRO HI cell (e.g.12). All Borre Probe data from the Äspö HRL were recently re-analysed using a new interpretation technique (13), which will be briefly presented here. The same analysis is currently being applied on the data from the CSIRO HI cell (14).

(Figure in full paper)

The Zedex Test Site

The ZTS is located at about 415 m depth in the HRL

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