RD&D-Programme 2004 - SKB

A thorough overview has been done of stress measurement methods and methods for evaluation
of stress measurements in the light of the work with stress description in the site models /19-24
to 19-28/.
The strategy for developing site descriptive models was first tested on data from Äspö /19-29/,
and was then applied to Laxemar /19-30/ and in the so-called version 0 reports for the site
investigations /19-31, 19-32/. The strategy is also applied in the continued site modelling.
The importance of the stress paths for e.g. loading and movements in the intact rock has been
described systematically in an overview study and compared with criteria for different modes
of failure that can occur around underground openings in the repository /19-33/. The study
particularly shows that light support, for example from the tunnel backfill, can be of importance
in preventing the occurrence and growth of brittle failure.
Detailed characterization and modelling of stresses and deformations has been carried out in
the ongoing Apse experiment in the Äspö HRL /19-34/. The results are being compared with
measurement data.
During blasting of the Apse tunnel, detailed convergence measurements were performed in
nearly fracture-free rock. A set of back calculations of the convergence movements has been
carried out /19-35/. If it is assumed in the numerical model that the elasticity properties of the
rock are close to those that have been measured for intact rock, the back calculations confirm
the predicted stresses: σ 1 = 30 MPa, σ 2 = σ 3 = 10 MPa, where σ 1 is horizontal and perpendicular
to the tunnel. Figure 19-3 shows that the agreement is good and thereby also verifies that
the response to the rock excavation is primarily elastic, which is expected in view of the low
fracture density in the measurement section.
0.006
0.005
Measured displacements
plus calculated
convergence before
measurement
Convergence (m)
0.004
0.003
0.002
Examine 3D
E-modulus 55 GPa
Poisson’s ratio 0.26
Calculated deformation
before first convergence
measurement
Measured displacements
0.001
0
−11
−9.8
−8.8
−7.8
−6.8
−5.8
−4.8
−3.8
−2.8
−1.8
−0.8
0.17
1.17
2.17
3.17
4.17
Distance to tunnel front (m)
Figure 19-3. Convergence measurement of the Apse tunnel. The pins were installed at a distance
of between 0.3 and 0.5 metre from the tunnel front. The principle of convergence measurement
and the positioning of the pins in the measurement section are shown at the left. The decrease
in the distance between studs 3 and 4 as the distance to the tunnel front increases (Measured
displacements) is shown at the right. The calculated deformations, “Examine3D”, apply to a
case where stress field and elastic properties have been determined by back calculation to provide
the best possible total agreement with the measurement values, corrected to include calculated
convergence movement before the first measurement.
250 RD&D-Programme 2004