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Technical Report NTB 87-14

Grimsel Test Site Geology

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This report discusses the results of geological and petrographic investigations which were carried out in the Juchlistock and the Grimsel Test Site (GTS) areas between 1980 and 1987. The investigations basically consisted of:

  • surface geological mapping of the Juchlistock
  • geological mapping of around 840 m of tunnel
  • geological logging of around 1476 m of cored boreholes · statistical evaluation and interpretation of around 3500 structural data points (underground)
  • hydrogeological investigations such as measurement of the discharge of formation water into tunnels and boreholes.

Geology

The GTS is located in the southern part of the Central Aar Massif, around 400 m below the surface of the Juchlistock. The rocks in this area are almost exclusively granitic, the northern part of the GTS being located in Central Aare granite and the southern part in Grimsel granodiorite. These rocks are penetrated by sets of lamprophyres and, to a lesser extent, by aplites. The Variscan age of solidification for the granites is approximately 290-300 My and that of the aplites around 250 My.

The whole Aar Massif was subjected to strong alpine deformation and metamorphism. The dominant textural overprinting of the rocks occurred coevally with the development of the greenschist facies metamorphism some 20-25 My ago. The P-T conditions during the metamorphism were 400-450 °C for temperature and around 3kb for pressure, corresponding to a depth of around 10 km. The deformation affected all the rocks but overprinted them to varying degrees, so that both ductile and brittle structures occurred. The former include schistosities, shear zones and mylonites and the latter cross- and shear fractures. Metamorphic alterations accompanied, and to some extent promoted, the ductile deformation, e.g. the alteration of potassium feldspar to micaceous minerals.

In addition to these structures, tension joints were formed during a later phase of the alpine textural overprinting, mainly along significant mechanical discontinuities such as granite/lamprophyre contacts. The minimum age of these tension joints is 15 My. The formation of these joints was accompanied by strong hydrothermal activity. Some areas of rock were leached, leading to a marked increase in porosity (to a max. 18 Vol-%). This hydrothermal fluid circulation also sealed most of the brittle structures.

The alpine orogeny was followed by the uplift of the Aar Massif, which is still occurring to some extent today. During this uplift, the most significant zones of weakness were reactivated, the result being the formation of the majority of the open fractures which today represent potential water flow-paths.

Structural geology

For the purpose of statistical evaluation, the brittle structure data points were divided into categories depending on the mineral coating on the fracture faces. The data come from the tunnel sections and from 12 boreholes. Three discontinuity systems with a ductile origin and preferred mineral orientation could be recognised:

  • the NE-SW-striking main schistosity S2 which dips steeply to the SE and
  • the steep shear surfaces conjugate to S2, i.e. S1 (often subparallel to S2) and S3 (E-W strike).

Since S1 can barely be distinguished from S2 on the basis of azimuth and angle of dip alone, these two discontinuity systems were treated together (S1 + S2).

Six brittle structure systems are also covered (generally described by K, except for the nomenclature S4 and S5 originating from STECK, 1968a). These are:

  • S4/K4, K2/L (L = lamprophyre direction), K1, K3, S5 and
  • a sub-horizontal tension joint system ZK.

These systems generally form conjugate pairs to S2. S4/K4 and K2/L were treated as one system since, in most cases, there is no clear distinction due to extensive overlapping of the orientations.

The GTS was sub-divided into areas corresponding either to a borehole or a tunnel section. There are marked differences in the frequency distribution of the systems and deviations in direction and changes in the angle of dip (10 to 20°) are common. In general, the most widely-spread fracture systems contain the highest percentage of open fractures; these dominate mainly in S1 + S2. However, since individual areas are characterised by different discontinuity populations, systems such as S3, K2/L and S5 can also be rich in open fractures.

The surface mapping programme allowed the data acquired underground to be extended spatially, although fewer systems were recognised at the surface. Besides the main schistosity S2, only S1, S3, K2/L and K3 were identified.

Surface-, tunnel-, and borehole measurements are based on the same one-dimensional data acquisition process (logging of drill-cores, measurements in tunnel sections and along profiles). Analyses of data acquired underground allow comparative studies of discontinuity locations and frequency distributions to be performed, while surface mapping completes the picture by providing data on the extent of the systems and spacing of the discontinuities. surface observations also give an indication of movement directions.

On a small scale, the Juchlistock-GTS area appears to be very complex; extensive unfractured areas are either non-existent or very rare. The large-scale picture is much simpler since fewer discontinuity systems (basically only S1 + S2, S3 and K2/L) can be recognised.

Water circulation

The GTS was purposely located outwith the sphere of influence of the large, water-bearing disturbed zones which can be mapped in the main access tunnel to the Oberhasli power plant (KWO). It therefore lies in a relatively dry area of rock. Disturbed zones with a marked water-outflow were encountered only in the north (fracture system flow test zone) and in the south (decompressed zone test).The water present in the rock circulates mainly in the discontinuities, in this case in the systems S1/S2 and along lamprophyre contacts. These systems extend over considerable distances and often penetrate right to the surface.

In the case of water-flow into the main access tunnel, climatic factors (snow-melt, changes in water-level in the lakes) affect discharge down to a depth of 250 m. In the GTS area with an overburden of more than 400 m, such external influences are no longer recognisable.

In general, water-flow into the GTS is low. The amounts are

  • 0 - 12 ml/min·m' in the borehole
  • 0.1 - 3 ml/min per open discontinuity in the borehole.

There is a marked local increase in water discharge in strongly fractured areas (BK, US, MI).

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