The IRT review of geological conditions in the target area is based on SKB reports (primarily TR 11-01
and R-02-43), SKB responses to IRT questions, and SKB presentations provided during IRT site visits on
December 14, 2011.
Data Base: The candidate area
3at Forsmark was explored in detail during a two-staged investigation
program during the years 2002-2007. The investigations included geological and structural surface
mapping (natural outcrops and trenches), magnetic surveys, reflection (with 10 m shotpoint/receiver
spacing) and borehole VSP seismics, with 25 cored drillings (> 16 km, 60 mm core, upper 100 m large
diameter percussion drilling), most of them (19) reaching the repository level. Based on these
investigations the northern part of the candidate area was selected as target area
4, i.e the location of the
proposed repository.
Rock Types and Rock Domains: SKB subdivides the host rock in the target area at Forsmark into two rock
domains called RFM029 and RFM045. The main part, forming the core of the target area, is rock domain
RFM029 which is composed of medium-grained metagranite, pegmatitic granite or pegmatite, and
subordinate amphibolite and other mafic to intermediate rocks. This steeply SE dipping rock domain is
surrounded by aplitic granite, medium-grained granite and felsic volcanic rock belonging to different rock
domains (inside the target area mainly RFM049). All these rocks belong to the Fennoscandian Shield,
formed between 1.89 and 1.85 billion years ago during the Svecokarelian orogeny and show high grade
metamorphic overprint.
Tectonic History: Tectonic deformations lead to ductile and brittle overprint of these rocks. A
comprehensive 3D structural model was developed based on surface and borehole mapping, and seismic as
well as magnetic surveys. Early ductile deformation has resulted in large-scale, high-strain belts and more
discrete high-strain zones mainly striking NW-SE. Tectonic lenses, in which the bedrock is less affected by
ductile deformation, are enclosed in between the ductile high-strain belts. The candidate area is located in
the north-westernmost part of one of these lenses, called Forsmark tectonic lense. Whereas the northern
part of the candidate area shows predominantly steeply dipping foliation (in the form of a large sheath
fold), the foliation in the southern part of the candidate area is gently SE dipping. Cooling into the brittle
field started about 1.75 billion years ago. Most of the ductile structures were reactivated and four stages of
fracture infillings, from epidote, laumontite, pyrite to clay minerals, are observed. The latest mineral
infillings with clay minerals are considered to be about 500 million years old (Phanerozoic). Several
phases of sediment loading (up to 2 km, resulting in loading of about 50 MPa) and uplifting, erosion and
unloading occurred until the Mesozoic. This suggests that stress released fractures were set up to form
during the Mesozoic.
Deformation Zones: Seismic investigations in the southern part of the candidate area show a large number
of gently dipping highly persistent reflectors. Verification by core drilling showed that these reflectors are
mainly ductile-brittle fracture zones with significant thickness (damage zone thickness of typically 20
meters), composed of dozens of sealed fractures, often showing old hydrothermal alterations but no gauge.
It is suggested that the gently dipping foliation and layering has favoured the formation of gently dipping
deformation zones. Single and cross-hole hydraulic testing demonstrate that these deformation zones have
3 The area of the detailed 2002-2007 site investigations, including the target area and extending to the SE 4 The target area corresponds to the smoothed footprint of the proposed repository layout
relatively high transmissivity over long distances (1000 m). In the northern part of the candidate area, i.e.
the target area which has been selected for the repository location, such gently dipping deformations zones
occur only rarely and mainly in the hanging wall of the proposed repository level (470 m). Here
deformation zones are mainly steeply dipping and ENE and NNE striking fracture zones, including crush
zones, composed of fractures filled with a large variety of minerals. These steep deformation zones are not
visible on seismic surveys and are mainly based on ground and airborne magnetic surveys verified by core
drillings.
Deformation Zone Model: A deterministic 3D structural model constructed for the target area shows
twenty-two steeply dipping zones that either show a trace length at the ground surface between 1,000 and
3,000 m or form minor splays or attached branches to such zones, at 400 to 600 m depth. Five gently
dipping zones are also present at 400 to 600 m depth inside or immediately above the repository volume
5.
According to SKB TR 11-01 the majority (> 60%) of these deterministic deformation zones with trace
length longer than 1000 m are judged to have a high confidence of existence, and the occurrence of
undetected deformation zones longer than 3,000 m is judged unlikely.
Fracture Types and Domains: Statistical fracture properties in rock volumes between deterministically
modeled deformation zones are described by fracture domains. The target area shows a strong decrease in
the frequency of open (non-mineral filled) fractures down to about 400 m depth, differing from the depth-
independent distribution of open fractures in the SE part of the candidate are. This is mainly attributed to
the observation that high fracture frequencies correlate with deformation zones, and that gently dipping
deformations carry most of the open fractures. Outside deformations zones the most important open
fractures are large subhorizontal sheet fractures mainly occurring in the uppermost 150-200 meters below
ground surface. This observation correlates with a strong reduction in seismic velocity below 200 m (R-02-
43). Up to 60 m depth open sheet fractures with sediment infillings (presumably from Pleistocene glacial
periods) are observed. Based on these observations SKB subdivided the rock mass in the target area in two
deep fracture domains FFM01 (in rock domain RFM029) and FFM06 (in rock domain RFM045) showing
a low frequency of open and partly open fractures, and a shallow fracture domain FFM02 characterized by
a complex network of sub-horizontal or gently dipping, open and partly open fractures forming a well
connected permeable network. At repository depth (entire candidate area) the frequency of open fractures
ranges between 0.5 and 0.75 (1/m), while in the upper 200 m the open fracture frequency ranges between
0.8 and 1.7 (1/m).
According to SKB the most important uncertainty concerns the size-intensity relationship of fracturing in
the potential repository volume, i.e. fracture domains FFM01 and FFM06. The main reason for this is the
lack of data on fracture sizes in these sub-surface domains (see also Section 4.1.2). Direct data on fracture
sizes in these domains can only be obtained from underground mapping, i.e. when the repository
excavation has reached relevant depths.
IRT Evaluation of Geological Conditions in Target Area
The IRT considers the geological model of the target area as well supported by a large amount of high-
quality field data, interpreted by state-of-the-art methods and a comprehensive conceptual model.
Of highest importance for the performance assessment of a KBS-3 repository at Forsmark are the
geological conditions in the target area at repository depth (470 m). The predicted conditions at repository
5
Although not explicitly defined in TR 11-01 we consider the repository volume (target volume) to be the volume around the target area between 400 and 600 m depth.
level are mainly based on the results from 12 deep core drillings supported by findings from magnetic and
seismic surveys. The IRT considers that the deterministically-modeled deformation zones (having an
extent of more than 1000 m) have the uncertainty specified in TR 11-01, even though the SKB and IRT
assessments are only expert opinions. The IRT suggests that SKB also assesses the uncertainty of unknown
deformation zones smaller than 3000 m trace-length in the repository volume.
Hydrothermal alterations in quartz-feldspar-rich basement rocks can strongly reduce rock mass strength
and alter flow and transport properties through dissolution (and precipitation) processes. While
hydrothermal fluids might initially flow along brittle deformation zones, zones of hydrothermal alterations
are often found at large (>100 m) distances from these zones. They appear as large clouds of chemically
altered rocks without significant macroscopic shear deformation. As such their location and dimension is
nearly impossible to predict. The IRT recommends assessing the spatial distributions (relative to the
locations of deformation zones) and fabrics of such hydrothermal alterations systematically.
In document
Revista ILLAPA nº 7, julio 2010.
(página 118-124)