Predicting geologic formations and the hydrogeology to be encountered along a tunnel is the major task of the geologist.
(1) Lotschberg (Switzerland). The 14-6-km-long railway tunnel (1913) was designed to cross the river Kander well beneath the normal water level.
Construction ran straight into alluvium which filled the old river bed well below the present river; several workers were killed. The gallery was sealed off and was by-passed by a tunnel about 1500 m long. Lack of systematic geologic prospection before the construction was the direct cause of this disaster.
(2) Mauvoisin (Switzerland). The site of the proposed 237-m-high arch dam of Mauvoisin, at that time the highest arch dam in the world, was examined most carefully from the geological point of view. The depth of alluvium filling the narrow gorge at the site was measured seismologically.
To enable a direct geological survey to be made, it was decided to drive a gallery deep into the rock abutment on the left-hand side of the gorge. This gallery was planned to pass beneath the gorge and to extend into the right- hand abutment. In addition to a first-hand inspection of the rock, water from several springs was analysed. Later, the geological gallery was used for grouting a deep grout curtain under the dam. When the narrow gorge was reached the gallery unexpectedly penetrated into alluvium, which suddenly gave under the pressure of the water. The gallery was flooded and four engineers were killed.
It is most likely that wrong interpretations were placed on the wave reflec- tions in the narrow triangular-shaped gorge.
(3) Malgovert (France) (Pelletier, 1953). Immediately after the Second World War French power-stations were nationalized. Electricite de France was created to administer the whole French electric generating system, with the exception of the power-stations on the river Rh6ne itself, which were entrusted to the Compagnie Nationale du Rh6ne, in Lyons. Electricite de France immediately realized that there was an acute shortage of power in post-war France, and possible schemes were hastily examined. A plan proposed for the upper valley of the Isere was found to be too modest. A new scheme was designed for a 180-m-high arch dam at Tignes and the first power-house at Les Brevieres (3 X 44 000h.p., gross head 168-6 m). The
Typical case histories 15 Malgovert tunnel, 9 | miles long, was driven from the portals and through twelve adits. The effective cross-sectional area of the tunnel designed for a discharge of from 47-5 to 50m3/s was 160 ft2, while the excavation ranged from 215 to 290 ft2 (20 to 27 m2). The head available between the pool at the Brevieres power-station and the end of the Malgovert section was 2463 ft (750 m). The Malgovert power-house is equipped with 4 x 105 000 h.p. Pelton wheels.
On the upstream section, between the intake and adit no. 12, three different methods were used: (a) 65-ft2 pilot tunnel in invert, (b) 65-ft2 pilot tunnel in crown, (c) full-face (in a few places). The ground behaviour did not give rise to any difficulties, except for the last 1000 ft, where heavy steel supports were needed.
On the downstream section (started in January 1948), between adits 13 and 16, it had been decided to try the full-face method suggested by the geologists. Two large jumbos, both carrying 8 DA-35 Ingersoll-Rand drifters, were available and an American Conway 75 shovel. Forty-eight holes were needed for the full heading. As work proceeded (1949), the quality of the ground steadily deteriorated. Polygonal steel supports were erected at 4-ft centres along the heading. Between adits 13 and 14, it became necessary to revert to the so-called 'Belgian' tunnelling method, using top heading and successive enlargements on part of the tunnel length. Between adits 14 and 16 a 100-ft2 invert gallery using drifters for full-face operation was started;
the widening of the small pilot gallery was carried out in either one operation or in a number of steps, depending on the type of ground. Heavy longitudinal reinforced concrete beams were used in places as a footing for the polygonal steel arches which maintained the tunnel crown. With these methods it took nine months to drive only 1531 yd.
Difficulties were encountered all the way from adit 13 downwards (Permian-Carboniferous and Carboniferous rock formations). The tunnel caved in with a consequent inflow of sand and water and collapse of the overburden. The ground pressure was so great that the steel ribs used to support the tunnel had to be spaced closer and closer together. When only one foot apart the ribs twisted and the reinforced concrete beams sheared through. Severe damage occurred in one adit where a lateral concrete wall moved inwards by 5 ft and the rail track was lifted by 4 ft in a few hours.
At one point, the tunnel ran into swelling ground, and a stratum of badly crushed shales was located near a fault. These shales were comparatively dry, and driving was easy. A few days after excavation this ground became wet, swelling and developing such a pressure that the steel supports failed, even where they were set flange-to-flange. Work on the heading was stopped and a diversion tunnel driven. But a failure in the roof near a fault caused a heavy inflow of water (nearly 4000 gal/min) which flooded the tunnel. Yet again work was stopped and several pumps brought into action. Progress was held up for several months.
A visitor to the tunnel site could have seen typical signs of a slow rock slide on the mountain slope; not a tree stood upright! Site engineers, com- menting on the difficulties encountered, said that because of the pressing demand for electric energy, geological investigations had relied too much on work done before the Second World War. The obvious rock creep was considered to be a superficial movement; rock in the depth was declared to be sound. Facts proved this opinion to be over-optimistic!
Rock slides are still not fully understood. The dynamics of creeping rock masses have yet to be worked out and better methods for determining their depth have still to be devised. (See the Vajont rock slide, chapter 14.)
As a result of this pressurized rock zone, the lining of the tunnel had to be delayed until the creeping rock had stabilized. It was found that the tunnel soffit had been lowered on average by one or two feet over a distance of several kilometres and the cross-sectional area of the tunnel proportionately reduced. Additional pressure losses would have occurred in the water flow with a corresponding loss of electric power. It was decided to restore the tunnel and cut the crown to the planned height before lining the tunnel.
Some poor rock characteristics in the Malgovert tunnel area below adit 13 were not due to creeping movements of the rock and could have been foreseen in a more detailed geological investigation.
In 1946, while driving adit 13, a wet crushed-quartzite seam was encountered at about 426 ft from the portal. The same seam was found again near another portal. In both cases the heading collapsed, the tunnel was filled with wet crushed-quartzite sand and the water pressure found to bell51b/in2.
A specialist contractor was called in to carry out grouting operations to strengthen the crushed rock. A cylinder of rock, about 81 ft diameter and coaxial with the tunnel (19-6 ft diameter bore) to be excavated, was con- solidated. A reinforced concrete bulkhead was built to provide protection against fissures in the ground which might be formed as a result of the grouting pressure. After consolidation, the tunnel was excavated, but not to the end of the treated section. Another bulkhead was built and further grouting carried out (2-8-in boreholes were drilled).
The powdery quartzite sand was treated in four stages (Pelletier, 1953) (fig. 2.5).
(a) Sodium silicate solution grouting to prepare the ground.
(b) Cement grouting was continued until the quartzite was compressed at 1-432 lb/in2.
(c) Sodium silicate and phosphoric acid solution grouting. A grouting pressure of up to 142 lb/in2 was used for this low-viscosity solution.
When the grout sets, after about 15 min, it forms a coherent gel, filling the voids in the consolidated rock. The void ratio is 30%.
(d) Cement grouting keys the ground perfectly by further compression.
Typical case histories 17
boreholes
reinforced concrete bulkhead
overlap
Fig. 2.5 Malgovert tunnel: method of grouting for tunnelling in quartzite.
Do = tunnel diameter, 19-6 ft; D1 = consolidated quartzite, 81 ft (after Pelletier,
1CKTI
1953).
The usual methods were used for the excavation, although the consolidated ground was so hard it was necessary to use jack hammers and explosives instead of pneumatic drills. The tunnel was worked in 115-ft stages. No further difficulties or falls occurred. Altogether 3800 tons of cement and 5240 yd3 of gel were used in 16 085 ft of tunnel and 3 J miles of reboring.
(4) here-Arc (France) (Olivier-Martin & Kobilinsky, 1955). Immediately downstream from the Tignes-Malgovert scheme, the same team of French engineers designed and constructed the Isere-Arc power development. It is interesting to see how experience gained from the Malgovert tunnel was used in the next step of the Isere Valley development.
The Isere-Arc tunnel, from the Isere to the Arc Valley, was designed for a 100 m3/s discharge, double that of Malgovert. A detailed geological survey indicated that the tunnel would cross a weak inclined strata of schists, for which the geologists were able to predict the approximate position and thickness.
The problem facing the engineers and the contractor was how to cross the weak zone of crushed rock. About 60 m were excavated full-face and the tunnel supported with heavy steel arches (I-type differdinger steel weighing 36 to 44 kg/m). The tunnel was lined immediately behind the heading before the rock mass started squeezing inwards. Daily progress, which under ordinary conditions was about 8 to 12 m per day, dropped to 1 m per day.
As the difficulties increased it was decided to change to a bottom pilot tunnel 14 m2 in cross-sectional area; a top heading pilot tunnel might have been easier. The bottom heading was chosen because the rail track and the whole equipment was at the level of the tunnel invert. The soffit was excavated laterally as the bottom heading progressed. It took about eight months to cross the 260-m zone.
At the time when the Malgovert and the Isere-Arc tunnels were built, rock bolting was not known to Continental tunnel engineers. Commenting on the Isere-Arc tunnel, the chief resident engineer, Kobilinsky, expressed
the opinion that rock bolting would have substantially accelerated the work on this tunnel.
Comparing the two tunnels built by the same team of engineers shows how the more precise geological survey for Isere-Arc allowed the engineers to change from full-face heading to bottom heading with a minimum loss of time.
(5) Kemano (Canada) (Libby, Cook & Madill, 1962). Geologists at the Kemano tunnel in British Columbia had noticed a fault crossing the tunnel in hard, medium crystalline quartz-diorite. It ran at approximately right angles to the tunnel and with a nearly vertical dip of 80°. The fault contained mylonite \ in to 2 in in size (microbrecciated rock) and gouge (a soft clayey material). At some places the wall rock was chloritized and softened for up to several feet on either side of the fault. Two sets of closely spaced fractures, probably related to primary jointing, also occurred on either side of the fault.
Fracture members were 4 to 7 ft apart for a distance of 30 ft from the fault.
It is known to geologists that faults filled with mylonite and gouge erode deeply to depths of several feet, whereas faults filled with only mylonite or with only gouge erode less.
The engineers in charge of the construction decided not to line it as the tunnel was 16-7 km long and the lining would cost about 12 million dollars.
About two years after completion, pressure losses along the tunnel increased; a rock fall inside the tunnel was suspected. Piezometric measure- ments made through boreholes from the surface down to the tunnel located the rock fall in the region of the fault. The exact location of the rock fall was confirmed by the reflection time of water hammer waves created by suddenly closing the turbines.
Repairing the damaged tunnel was a major job. The rock fall formed a natural dam inside the tunnel and the water held behind it had to be pumped out. A 500-ft-long, 12-in pipe was used to pump 4000 gal/min (300 litre/s).
A timber bulkhead was built upstream of the huge cavern formed in the tunnel roof by the fall. The space between the natural rock fall and the bulk- head was filled with crushed debris to a level just above the original tunnel crown and an attempt was made to shape the surface of the fill correctly.
A ventilation duct 4 x 6 ft was provided at the top of the fill. The cavern walls were washed and 1000 yd3 of high-slump concrete was deposited by a continuous operation on top of the fill, over the entire cavern area, to form a protective arch, 6 to 12 ft thick, 35 ft wide and 65 ft long. A layer of f to
| in crushed rock, 3 ft thick, was blown on top of the concrete as a protection against falling rocks. The fill was then excavated and steel ribs positioned to support the protective concrete arch. Finally, the protective arch was blocked and the tunnel lined with concrete. The whole repair operation had been difficult and dangerous because pumping out the water had caused further rock falls.
Typical case histories 19 Minor rock falls along the tunnel were stopped by the conventional method of pneumatic concreting to about 24 in thick. The cost of the repair work was 2-1 million dollars less than that of lining the whole tunnel, but far more than preventative action (local treatment of fractured area) would have been.
(6) Kandergrund (Switzerland, 1910) (Jaeger, 19636). Accurate information from the geologists may not be sufficient to trace the real cause of damages.
The Kandergrund tunnel (4 km long) was designed and concrete-lined as a free-flow tunnel with a large reservoir chamber excavated in the rock. Later, the tunnel was put under pressure and the reservoir chamber used as a surge tank.
Twice, the concrete lining was severely damaged a few hundred metres from the tunnel inlet. Geologists pointed to very poor rock conditions. The damaged section was relined and the poor rock blocked. The tunnel burst a third time at the same place. Water, seeping through wide fissures in the concrete lining, accumulated over a curved, impervious rock formation and caused a landslide. A forest was destroyed and two people were killed when a house at the foot of the slide collapsed. Geologists again pointed to the obviously poor quality of the rock. For the third time, the rock was grouted and blocked and the tunnel relined.
Three 30-m-long and 5- to 7-cm-wide fissures had formed in the soffit of the tunnel lining and there was evidence that it had been lifted by extremely high hydraulic pressures. Poor quality rock could not explain these conditions.
The rise in pressure inside the tunnel was caused by water hammer waves forming standing resonance waves inside the tunnel. This complex hydraulic phenomenon has since been observed in other hydro-power schemes. The cause of all the trouble was a defective air valve on the pressure pipe line (fig. 2.6). The standing pressure waves caused other minor cracks in the
intake surge tank
i \ fissures -=M vibrating
valve
Fig. 2.6 Kandergrund tunnel. Resonance of an odd harmonic (pressure wave) causes severe damage to the tunnel (Jaeger, 1963).
concrete, and where the rock was very weak, the cracks had opened to wide fissures. The geologists had traced the local poor rock conditions but not the real cause, which was a problem of fluid dynamics.
(7) Mantaro (Peru). High in the Peruvian Andes at about 2400 m there is a huge bend in the river Mantaro. Consulting engineers designed a scheme utilizing the 1000-m gross head between the arms of the bend along the line I-I (fig. 2.7). Alternatives to this basic scheme were being considered when
news came that aerial surveys had discovered a major error in the mapping of the river course. On official maps, the river penetrated the Peruvian jungle along the course a in fig. 2.7. Aerial surveys had shown that there was
a false official mapping
!*" correct Mantaro j second bend . '
Fig. 2.7 The two bends in the Mantaro river (not to scale) showing the three possible lines for a trans-Andes hydro-power tunnel and the possible power develop- ment IV-IV through the second river bend.
another big bend in the Mantaro and that a further fall of 1000 m could be developed. Alternative schemes along the sections II—II or III—III were considered, all equally attractive to the design engineers.
A team of experienced geologists, led by Professor Falconnier (Lausanne), were on the spot for several months. They described the extremely involved geology of the high Andes as a 'mass of granite rock, covered by an older formation of folded and fissured metamorphic rocks'. Springs along the edge of the impervious granite mass indicated the level of the water-table which was inclined from south to north. A cross-section through the tunnel proposed along III—III (fig. 2.8), indicated that both ends would be in
springs w v ' water table ^^_ Jt springs
impervious igneous rock impervious water table rock
Fig. 2.8 Section III—III. The tunnel would have been under 400 m water pressure.
Fig. 2.9 Section II—II. The tunnel is above the water-table in fissured pressurized rock (overburden about 2000 m).
compact granite but that the central section would be in fissured rock where the water-table was about 400 m above the tunnel. Along the section II—II the tunnel would be almost entirely through fissured pressurized rock but above the water-table (fig. 2.9).
Peruvian engineers had just completed the construction of another, far smaller tunnel to divert water from the upper Mantaro catchment area to
Typical case histories 21 the west coast (Pacific) crossing the First Cordillera. They had experienced very severe difficulties due to the number of springs cut by the tunnel. In order to discharge about 2 m3/s of spring-water, the area of this tunnel had to be doubled.
The whole Mantaro problem had to be reconsidered. Route I-I was discarded because development of the second Mantaro bend would have been far more costly, or even impossible with the tunnel in this position. In addition the east end of the tunnel I-I ran into gypsum, and anchorage of the pressure pipe on indifferent rock would have been very difficult. Route III—III was discarded because of the difficulty of driving a tunnel through fissured rock 400 m below the water-table. Route II—II was acceptable to the geologists, in spite of fissuration of the pressurized rock. Route Il-IIa was an adaptation for local conditions and towards the possibility of develop- ment of the second Mantaro bend.
This is an example of engineering geology because the geologists' findings decided the basic design of the hydroelectric power scheme.
(8) Santa Giustina. Italian engineers had similar problems to solve when designing the Santa Giustina power-house, which was to be sited in an underground cavern. Starting from the high-arch Santa Giustina dam, a pressure tunnel crosses hard fissured limestone. The Santa Giustina power- station could have been located in a cavern excavated in this limestone, but the engineers preferred to site it further downstream in pressure-developing clayey schists. The sharply inclined pressure shaft leading to the power- house required an elaborate design, to cross the contact line between the limestone and plastic rocks. The cavern is oval-shaped with a heavy reinforced-concrete lining to withstand the rock pressure. A reinforced- concrete floor at the level of the generator floor is supposed to take the horizontal thrust from the pressurized rock.
(9) Tunnel lining. Most rail and road tunnels are lined. Hydro-power engineers are accustomed to balancing the cost of the concrete lining against the cost of energy losses caused by rough unlined tunnels. In some cases the final decision lies with the geologists who have to decide on the stability of the rock or on its capacity to withstand erosion by the water-flow.
French geologists and rock mechanicists (Mayer, 1963) have devised a method by which the resistance of some rock types to erosion is correlated with the porosity of the rock and to its perviousness to air under pressure.
The method will be described in detail later.