Damage by blasting (continued)

Many slabs cannot be barred off and remain permanently. Where the rock is jointed it breaks to varying degrees along existing joint faces, rather than across fresh fractures through joint blocks, and the outer layers of joint blocks are loosened by the blasting. The percentage of the surface of the excavation formed of fresh fractures as distinct from joint faces is a useful index of the cornpactness of jointed rock, This is easily assessed and is recorded routinely by the geologists when mapping excavations.

Spalling and popping rock

Rock may also be fractured as a result of the opening causing stress concentrations around it which exceed the strength of the rock. This leads to failure by popping and spalling in which slabs of rock, one or two in. thick and many sq. in. or ft in diameter become detached suddenly from roof and walls. This has occurred to a small extent only over long distances of the Snowy·Geehi tunnel under high cover of about 2000 ft or more and locally in other tunnels. It is confined to the soundest massive granite with widely spaced tight joints.

Ground water

The rock material through which the tunnels are driven is practically impervious but ground water may exist in connected systems of open joints and fractures through the rock mass. Exploratory drill holes from the surface invariably encounter ground water within a hundred feet or so.

Flows of ground water into tunnels are encountered at irregular intervals. Long sections of most tunnels are dry, or show only dampness or slight seepage from joints. Some sections show areas of drips or localized small flows. A few places have large flows of hundreds of thousands of gallons per day. Flows are usually greatest when first encountered, decreasing to a steady rate or drying up altogether with time, and as the heading advances.

The large flows come from open joints. In some places these are in the vicinity of faults, in others they are simply open joints through sound rock The latter was the case where the largest inflows were encountered. Flows of up to 700,000 gallons per day occurred through open joints in sound granite in one section of the Snowy–Geehi Tunnel under 2200 to 2400 ft of cover. The peak aggregate flow was about 4 million gallons per day reducing to about 2 million gallons per day within a week. Flows from individual joints diminished rapidly as the face advanced and usually dried up almost completely. The seams of crushed rock and clay in fault zones are characteristically impervious, although ground water may flow out of adjacent joints and irregular fractures. This is particularly fortunate since the fault zone material when dry or only damp often stands quite well in small spans whereas saturation with water converts it to flowing ground.

TYPES OF SUPPORT AND FINAL TREATMENT

Support during construction

This is the rock support required to create an maintain a sufficiently stable and safe tunnel behind the advancing face. However, although this is the primary function of support at this stage, the requirements for final treatment of the tunnel to fit it for service as a water tunnel, naturally influence what is done. The two main forms of support during construction are steel ribs and rock bolts

Steel rib supports

These are conventional horse-shoe shaped steel ribs. In the larger diameter tunnels they are fabricated from rolled steel joists 6 in. deep x 5 in. wide weighing 25 lb per lineal ft, or 8 in. deep and 6 in. wide weighing 35 lb per lineal ft.

They are installed by conventional methods (Andrews, McIntyre and Mattner, 1964). Particular attention is paid to their alignment and grade to ensure that they occupy the correct position when finally embedded in concrete in the finished tunnel.