The Simplon Pass (2005 m above sea level) was used by the Romans. In the Middle Ages it became a vital route for commercial exchanges between Germany and Northern Italy; German Emperors crossed the pass. The present road was built on Napoleon's orders (1801-5) for military reasons, after he won several battles in north Italy. Since 1968, the road has been systematically improved and enlarged on the Swiss and on the Italian sides, to cope with increasing traffic.
On the south side, near Gondo (Switzerland) the dangerous 50 m high rock spur of Baji-Krachen (at 990 m above sea level) had to be partially cut for a length of about 60 m to achieve straightening and the widening of the road from about 7*70 m to 12 m.
The geologists reported that the rock mass forming the spur is very hard gneiss, cut by three sets of joints, some of them being potentially dangerous.
15.1.2 The structure of the spur
Figure 15.1 shows a typical cross-section of the rock with the three main sets of joints. The joints of set I, inclined at 38° over most of their length, with a steeper dip to 45° at the lower limit, are the most dangerous. The vertical joints belonging to set II could also cause trouble, but mainly where they occur as discontinuous structure underlying more gently dipping joints at the base, and near the face, of the rock mass. The subhorizontal set of joints III are not seen as a source of trouble in themselves, although in relationship with those of set II, some instability could result.
The diagram of fig. 15.2 refers to a typical shear test on a plane (a, r) carried out in two opposite directions, after reversing the force. The test is on a core sample from a boring, along a joint of set I. The test shows clearly that a first shear level occurs for c = 0 and <j>' = 32°, in the two directions. After increasing the T-value, a second shear level is reached for </>' = 44° in one direction and <f>' = 36° in the other direction. These higher <f>' values are reached after the joint faces have moved in a relative displacement.
On the right hand side of sketch fig. 15.1 it is shown how the stability of the upper mass of the rock spur was obtained by friction along fissures I,
[424]
A rock spur on the Simplon Pass 425
1060
1040
1020
1000
0 10 20 30
i j i
excavation line
Fig. 15.1 The Baji-Krachen rock spur (Simplon Road). Typical cross-section.
I, II, III sets of joints (after Lombardi, 1975).
- 0 . 5 - (13.0)
- 1.0 - (26.0)
= 32°
TV in tonnes (aN in kg/cm2)
y = 2.65 t/m3
ffrupture= 1170 kg/cm2 (D/L = 1)
Fig. 15.2 Baji-Krachen rock spur. Rock shear tests on bore hole core (after Lombardi).
inclined at a = 38°, after the rock has been displaced, so that condition
</>' = 440 is obtained. The fissures remain tightly closed. This displacement of the upper block causes an opening of the lower end of the joints I, where a = 45° > ft and also of the set of vertical joints II. Designers expected that when excavating the rock spur as required by the design of the new Simplon road section, the lower block of rock might slide. Inspection of the rock spur in situ after the deformations had taken place proved that it was still stable.
15.1.3 The stabilization scheme
Lombardi (1975) in charge of the consolidation work analysed two possible reinforcement schemes. In the first (fig. 15.3a) anchors (1) were to secure the
(a)
Fig. 15.3 Baji-Krachen rock spur. Two alternative proposals for rock support (after Lombardi).
upper part of the inclined rock before the rock mass (2) was excavated. A second set of anchors (3) were to be installed before finalizing the excavation (4). This reinforcement scheme was rather expensive as it required the cable anchoring to be done in two phases.
The alternative scheme (b) was adopted whereby all anchors (1) were placed before excavation of the rock mass (2).
Figure 15.4 shows the final design for the rock mass support. Stability calculations for such a design are straightforward. Interesting is the table of calculated 'safety factors' rj = (retaining forces/active forces) calculated for different values of the angle </>', varying angle a and a series of anchor forces
F(in t/m). For V = 0, r\x is obviously less than 1, when </>' < a. But the most important result of these calculations is the fact that, even with very powerful anchorages (anchors of 140 tonnes were used at Baji-Krachen) it is not possible to raise the 'safety factor', rj, above 1-3 to 1-5. Designers concerned with the rock supports for large underground excavations using cable anchorages
Stabilization of a very high rock face
0 10 20 30
prestressedj;
anchors
Fig. 15.4 Baji-Krachen rock spur. Final design for rock support (after Lombardi).
(1) Active force; (2) natural friction force; (3) negative force due to prestressing;
(4) Friction force due to prestressing. Safety factor r\ — (resisting force/active force).
V = prestressing force, a = angle of dipping joint, <j>' = angle of friction.
arrive at similar conclusions, as explained in the chapters dealing with under- ground works. Forces in cables remain small compared with the weight of rock masses and other natural forces to be dealt with. Lombardi remarks that these factors refer to the safety against small rock deformations along the fissures. Large deformations for which the safety factor would be higher (according to the shear tests) are no more likely when stiff anchorages with cables give rigidity to the reinforced mass.
The sketch on the right of fig. 15.4, shows, in addition to the main system of cables which are inclined at 27° to the horizontal, a second system of cables at a = 65°, nearly at right angles to the first system of cables. These cables are supposed to pin blocks of rock in danger of sliding down the rock face after excavation. It was considered that drilling work on the nearly vertical unstable rock face would entail unacceptable dangers to the workers. This explains the reason for this second family of anchorages.