Kemano Penstock Tunnel Liner Backfilled with Prepacked Concrete


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Title: Kemano Penstock Tunnel Liner Backfilled with Prepacked Concrete

Author(s): R. E, Davis, Jr., G. D. Johnson, and G. E. Wendell

Publication: Journal Proceedings

Volume: 52

Issue: 11

Appears on pages(s): 287-308

Keywords: no keywords

Date: 11/1/1955

The 11-ft diameter underground penstock tunnel which, under a 2600-ft static head, supplies water to the Aluminum Company of Csriada power plant st Kemano, British Columbia, was backfilled with prepacked concrctc. Unique with respect to magnitude of operating head and size of penstock, the steel liner was designed on the assumption that a large percentage of the total water load would be transmitted through the backfill to the rock sur-rounding the tunnel. To achieve this condition it was necessary that the backfill be substantially free of voids and that the temperature rise produced by hydrating cement be low. Coarse aggregate for the backfill was placed down steeply inclined sections for distances up to 2000 ft by a tremie pipe so as to fill the space between liner and rock. In horizontal sections, coarse aggregate was pneumatically transported and placed for distances up to 600 ft by a "rock blower" and conveyor pipe. The voids of the coarse aggregate backfill were intruded with heavily sanded grout which was pumped horizontally for distances up to 3000 ft and upward nearly 1500 ft. By such groutming, prepacked concrete containing about 4 sacks of cement per cu yd was produced at a maximum rate of about 600 cu yd per day. After the prepacked concrete had hardened, 1-in. diameter cores were taken at many of the liner grout holes. These cores indicated that the backfill wus free of voids of significant size. The only holes which took any measurable quantity of neat grout under high pressure were those where the surrounding rock structure was water-bearing or badly fractured. Physical tests on cylinders and cores indicated that ultimate compressive strength of the prepacked concrete was generally in excess of 5000 psi and that, unlike coonvcntional concrete, it behaved as an elastic material even at early ages. Inspections by tapping with a hammer after the backfill had cooled and hardened have indicated that there was complete contact between liner and backfill. Penstock pressure tests made at two stations where the liner was 1 15/16 in. thick, under a maximum pressure of 1000 psi maintained for 8 hr, showed that the backfill was carrying 76 percent and the steel liner 24 percent of the water load. As clear evidence that there were no shallow voids between liner and backfill, such as might be produced along the invert by bleeding, as loading took place the stress in the liner was directly proportional to pressure within the penstock.