Abstract
The study is a good example of the application of engineering geology to an operational-maintenance problem, a usage not often recognized. The Clear Creek Tunnel, a key feature of the U.S. Bureau of Reclamation (USBR) Trinity River Division of the Central Valley Project, is 17.7 km long, circular, 5.33 m in diameter, and concrete lined. It conveys stored Trinity River water into the Sacramento River Basin. Power availability at a hydro powerplant at the downstream end of the tunnel is subject to seasonal fluctuations of flow resistance losses, with maximum head (energy) losses during the summers. Annual electric power generation losses of some $80,000 have been estimated.
The losses are related to numerous transverse calcareous “ridges” up to 4 cm high observed on the lining during several inspections of the tunnel. These ridges grow over minute irregular shrinkage cracks in the lining and are composed of calcium carbonate interlaced with fungal mycelium. During winter seasons the ridges are partially dissolved and have denuded crests. No carbonate deposits were noted at weep holes and at strong seeps.
The ridges are developed by carbonates precipitated from CO2-rich ground waters which dissolve calcium carbonate from the concrete lining and grout before seeping into the tunnel. Inside the tunnel, the seeps release CO2 into the tunnel water and precipitate carbonate. Fungal growth in the cracks slows seepage rates and encourages deposition of carbonates. Winter increases of CO 2 content in reservoir waters related to the seasonal decrease of photosynthesis and decay of organic detritus, and some decrease in water temperature, reverse the process and lead to partial destruction of the ridges. Control by periodic scraping of the ridges is suggested. Construction joints in future tunnels may prevent cracking and the related development of ridges.
