The specifications now require that the washing be performed by directing a stream of water at the rock surface from a distance of four ft through a ¼-in. diameter nozzle at pressures between 50 and 100 lb./sq. in. Washing of the rock clean of rock dust not only ensures that no significant details are missed but also enables the inspections and assessments to be made more rapidly and confidently.

After the rock is washed by the contractor, it is systematically logged by an engineering geologist at a scale of 1 in.=20 ft. A condensed log at a scale of 1 in.=200 ft is then prepared together with a schedule of all areas considered to require treatment. This is usually done in sections of about one mile in length soon after excavation. An inspection by a group of senior engineers and geologists is then made to determine the final treatment. Members of this group have examined all other unlined tunnels in the area including tunnels dewatered for inspection after periods of service, and established consistent standards of treatment. The proposed treatment may be amended by the resident engineer if conditions disclosed during clean-up preparatory to treatment vary from those previously assumed, or if some new features are found.

The primary objective is to prevent fallout on a scale which could interfere with the designed operation of the tunnel, leading to shut–downs and costly maintenance. It is recognized that some fallout in unlined tunnels is inevitable and rock traps are incorporated in the tunnel floor at key points to limit the transport of rock fragments by the flowing water. However, provided fallout is limited to small quantities of the order of a cubic yard or so and does not lead to progressive deterioration of the rock mass, it is acceptable in unlined pressure tunnels.

Choosing between steel ribs and rock bolts

Before the introduction of rock bolts and pneumatically applied mortar, steel ribs were the only form of support during excavation, and concrete lining practically the only form of permanent treatment. Rock bolts and pneumatically applied mortar now replace steel rib supports and concrete lining to a considerable extent, and also are used extensively to treat smaller local defects thereby contributing to the increased reliability in service of many miles of unlined tunnels

Determination of the extent to which rock bolts should replace steel ribs as support during excavation is still largely a matter of judgement, and considerable differences of opinion often arise between experienced persons over particular areas. This is largely because there are no objective methods for accurately measuring rock quality in the marginal range, between obviously bad and obviously good.

In practice the condition of the rock itself may limit the use of rock bolts. For bolts to be used satisfactorily

1. The rock must have a sufficiently long stand-up time after blasting to allow bolts to be installed safely.
2. The rock must be capable of providing good anchorage for the bolts, suiiicient for full tension in the bolts to be obtained on installation and maintained indefinitely.
3. It must be possible to drill satisfactory holes for the bolts because some very closely jointed hard rocks ravel away under the action of the drill producing craters which are unsuitable foundations for the bearing plates.
4. The rock must not ravel away around the bolts after they have been installed to the extent that the foundations of the bearing plates are undermined.