9.7.1 Supervision of potential rock slides
Preceding chapters have emphasized the need for scientific investigation and survey of large potential rock slides, to define their characteristics and estimate their actual danger. Potential rock slides can sometimes be detected by the surface appearance of the slope: some signs are trees growing at an angle, isolated rocks beginning to roll or slide, and the downstream edge of the slope becoming unstable
To survey a creeping rock mass requires precise measurements of the position of lines of fixed points on the surface and checking their displacement in horizontal and vertical directions. These measurements must be recorded versus time and then systematically analysed. Some slow rock slides in the Alps have been traced back seventy years with the help of early topographic maps of the area which have been compared to new triangulations. Others have been observed by direct measurements for over fifteen years.
The permissible errors should not be greater than 10% of the expected displacement to be measured. For example a displacement of 5 cm should be measured to an accuracy of about ± 5 mm (Kobold, 1968). The position of
222 Rock slopes and rock slides
a point on the map can be determined by angular measurements within
± 5 mm over a distance of 1 km if the basis of the triangulation is known and the fix points of the basis of the triangles are not moving. The measure- ment of a length is more difficult. Electronic methods allow an accuracy of
± 11 mm over 1 km, the accuracy of a laser is far greater. Surface measure- ments must be supplemented with those from inside the mass of moving rocks (boreholes and galleries). The depth of the sliding surface should be estimated and the volume of the moving masses calculated on the basis of the measurements.
An interesting example of measurements made inside moving rock masses was at the semicircular railway tunnel of Klosters (Alps), excavated in slightly unstable rocks. Accurate geodesic measurements were made along the rails inside the tunnel and at its two portals in 1952, 1956 and 1966.
The portals moved 15 to 20 cm in fourteen years. Inside tunnel displacements were lower, about 5 to 10 cm, maximum 12 cm, and 18 cm near the portals.
Any acceleration of the displacements must be interpreted as a possible warning of ruptures inside the mass. Differences in the velocity profiles may indicate discontinuous creep of rock masses and may possibly point to the formation of a discontinuous rock slide (Jaeger, 1968^, b; Miiller, 1964).
During the Fifteenth Symposium on Rock Mechanics at Salzburg (Sep- tember 1964) displacements varying from 0-25 cm to 50 cm a day were described by several experts.
Seismic tests may detect rock fissuration or fracturation at different depths in the creeping masses. At Vajont a sharp drop in wave velocities was observed about two years before the rock burst. Other experts measure the noise level, which increases before a rock fall occurs and decreases as stabiliza- tion progresses. Measurements of the water-table and its variations and correlating this with rock displacements are an easy method to check what changes are occurring deeper inside the rock.
9.7.2 Stabilization of potential rock slides
(a) Minor rock slides. Potential minor rock slides can be stabilized with a concrete retaining wall or buttress, with anchor bolts or stressed cables anchored in sound rock or with a combination of both. But proper drainage of the rock is always a first requirement, and sometimes cleaning and con- creting fractures may be useful. The design of any reinforcement must be based on a detailed study of the rock masses; their fissures and fractures and potential sliding surfaces. The final choice of the retaining system is then a matter of statics of forces and weights.
Proposals for concreting a rock fracture are shown in fig. 9.16 - there are many possible alternative methods. The statics of a retaining wall are shown in fig. 9.17. The weight W of the rock mass has a component FFsin a in the direction of the potential slip line. The friction force, F = fFcos a x tan <j>9
rockfracture concrete block cleaned and
concreted fracture
rock fracture cleaned and
widened concreted
fracture
Fig. 9.16 Cleaning and concreting of rock fractures (after Miiller, 1963a).
(b)
Fig. 9.17 Retaining wall (a) and polygon of forces (b). W9 weight of rock mass;
A, reaction from the wall; R, total reaction force on slip line; F, friction component due to Wonly; <£, angle of friction.
would be unable to withstand this component without a reaction force A from the retaining wall, which has a component F* = A cos a in the direction of the slip. The reaction A from the wall becomes an active force only when there is an incipient sliding movement of the rock mass. Post-tensioned cables are immediately active. P is assumed to be the force in the cable after being jacked up (fig. 9.18). Possible combinations of retaining walls and
Fig. 9.18 Steep slope reinforced with anchored cables.
224 Rock slopes and rock slides
Fig. 9.19 Anchored retaining walls.
post-tensioned cables
Fig. 9.20 Anchored buttress.
buttresses with anchored cables are shown in figs. 9.19 and 9.20. An interest- ing reinforced structure with cable anchorage is shown in fig. 9.21.
Proper drainage of the rock mass is always vital.
Force Z applied to an anchored cable. The cable is prestressed by applying a force F which causes the steel or reinforced concrete plate to compress the
Fig. 9.21 Reinforced concrete structure with cable anchorage for protection of the Passan-Wegscheld road (after Miiller, 1963a).
rock surface (fig. 9.22), q0 is the unit load, s the side of the plate, F = q^
the force, L the cable length and A its cross-section, the surface displace- ment being I - aF = * , / ! A force Z is applied to the anchorage; whereby
reinforced concrete
plated >. ( ^ anchored cable
Fig. 9.22 Tensile force, z% on an anchored cable.
a force Z2 is transmitted to the cable, causing a cable lengthening A2 = bZ2= Z2L/EA. A force Z-Z2 = Z± causes a relief in the rock compression by the plate; the surface displacement decreases by Ax = aZx = A^/^r.
We must have
Ax = A2 = aZ1 = *Z2; Zx = - Z2 andZ2 = kZ.
The force in the cable is now
When Zx reaches a value * the plate gets loose from the rock and the whole force Z is transmitted to the cable. A short calculation yields k = 1/(1 +
and R = F/(l - k).
(b) Stabilization of large rock masses. Chapter 15 is entirely devoted to the analysis of the stabilization of the Baji-Krachen rock spur and the 300 m high rock abutment at the Tachien dam.
10 Galleries, tunnels, mines and underground excavations