ABSTRACT:

This paper summarizes the first complete hydrofracturing stress measurements at the Hanford Site in one of the basalt flows considered for the construction of a nuclear waste repository. The results indicate a strongly anisotropic stress field suggestive of thrust faulting along east-west striking planes. These findings are supported by local geology and seismicity and by evidence of core discing and borehole spalling. The measured in situ stresses were instrumental in the rational design of the future rock caverns with respect to site suitability, cavern orientation and shape, and support requirements.

RESUME:

Cette publication presente les premiers resultats complets de mesures de contraintes par hydrofracturation au site de Hanford. Une couche de basalte y est prevue pour la construction d'un depôt de dûchets radioactifs. Les resultats suggèrent une anisotropie importante dans les contraintes. Ils sont confirmes par la geologie locale et d'autres mesures à partir de sondages. Les mesures des contraintes in situ ont ete prises en compte dans l'etude des differentes cavernes prevues.

ZUSAMMENFASSUNG:

In diesem Artikel sind die ersten Kompleten Hydrofractur-spannungs messungen aus einer der Basaltfluesse, die fuer den Bau einer nuclearen Abfallaufbewahrungsstelle in Hanford in Betracht gezogen wurden, zusammengefasst. Die Ergebnisse zeigen ein stark anisotropes Spannungsfeld, das eine Schubfaltung ent lang von Ost-West verlaufenden Ebenen vorschlaeght. Dieser Fund wird von der oertlichen Geologie und Seismizitaet, dem Vorfinden von Scheibenbildung an Bohrungen und Bohrlochreissen unterstuetzt. Die Gemessenen in situ Spannungen waren behilflich fuer den vernuenftigen Entwurf von zukuenftigen Felskavernen mit Ruecksicht auf Eignung des Ortes, Kavernen orientierung und-form, und Stuetzungsanforderungen.

1 INTRODUCTION

A unique requirement of underground nuclear waste repository design is a significantly longer service life than would be expected for common civil structures. The long term stability requirement makes the knowledge of local in situ stress conditions an essential element in establishing the feasibility of a site for nuclear waste disposal and in rationally designing the appropriate subterranean openings. At the Hanford Site the state of stress is a particularly critical parameter since local micro seismic events (Malone et al. 1975), core discing (Moak 1981) and borehole spalling (Paillet 1985) indicate high horizontal stresses which could have considerable impact on the construction, shape, orientation, and support of excavations, as well as on the layout of waste emplacement. Early in the site investigation at Hanford the decision was made to conduct a comprehensive series of stress measurements in the vicinity of the proposed location and depth of the planned underground waste repository. After some initial in house feasibility tests at an intermediate depth (343 m), we conducted the first complete set of stress measurements in a test hole that reached the depth of the lowest candidate horizon (the Umtanum flow). This paper describes these tests and their implication regarding the repository design.

2 LOCATION AND GEOLOGIC SETTING

The reported stress measurements were conducted in vertical borehole DC-12 (76 mm in diameter) located near the geographic center of the Hanford Site (lat. 46°28'6" N, long. 119°32'26"), some 24 km northwest of Richland, Washington (Figure 1). The hole is situated near the axis of the Cold Creek syncline, which is part of the Pasco Basin, one of the several basins in the western Columbia Plateau. In the Cold Creek syncline the Columbia River Basalt Group occupies much of the top 1500 m of the earth's crust, and within it several basalt flows have been identified as potential repository rock hosts (candidate horizons). At the time of our measurements the Umtanum flow was considered the preferred horizon because of its thickness (about 75 m), depth (979–1051 m in hole DC-12), lateral extent (present throughout the Cold Creek syncline), and thick uniform entablature (13 m in DC-12). All our tests were conducted in the Umtanum flow.

3 TESTING PROCEDURE
3.1 Selection of test intervals

The method used for the stress measurements was hydro fracturing (Haimson 1974). Test intervals were selected after studying the condition of the extracted core from color photographs. Six depths were identified within the Umtanum flow as being suitable for hydrofracturing (i.e. intervals, at least 1 m long, which appeared free of discontinuities).

3.2 Hydrofracturing

Hydrofracturing equipment setup, and procedure were kept the same in all tests. The hydrofracturing tool consisted of two inflatable rubber packers straddling a 60 cm interval. The tool was lowered into the test hole using a slim high-pressure drillrod which also served as the hydraulic conduit to the straddled interval. The packers were independently pressurized via a slim hose strapped to the outside of the drillrod.

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