The 16 km long Ana1co-San Jose Tunnel is part of a comprehensive water supply system for Mexico City. Owing to a considerable overburden of up to 800 m there were only poor geological and geomechanical data available for the preliminary design. However, during construction a careful geological survey coupled with deformation measurements was carried out providing a basis for decisions on the temporary support and especially on the final lining. A particularity of this tunnel comes from the fact that a consistently enlarged excavation was produced by the drilling and blasting methods. Statistical evaluation of the actual profile along the whole length of the tunnel allowed the optimum final lining to be found with respect to hydraulic requirements, rock conditions and costs.
The 16 km long Analco-San Jose Tunnel forms a part of the Cutzamala System for water supply to Mexico City with about 80 km of pipelines, five pumping stations and one water treatment plant. Due to the considerable overburden of up to 800 m only a rough picture of the regional geology and that of the rock conditions were available when establishing the tender documents. From hydraulic considerations a uniform free surface flow with an inside circular section 4.00 m in diameter and, for constructional reasons, an outside horseshoe section allowing a minimum thickness of the concrete lining of 30 cm were initially chosen. The tunnelling operations started in June 1978 with three vertical shafts and the two portals using conventional blasting techniques. For the authors in Spring 1979 the problem arose of finding alternative solutions for the final lining which would be adequate for the actual rock conditions encountered. A particular aspect of the lining design was caused by the enlarged excavation which resulted from the drilling and blasting methods, Fig 1.
(Figure in full paper)
The Analco-San Jose Tunnel is being excavated through the Sierra de las Cruces, which belongs to the physiographic province of the Neovolcanic Chain in Central Mexico. This province is formed by andesitic and dacitic rocks originated from volcanic emissions following a fracturing of the crust in a general SE direction during the Upper Tertiary period. Thus, the tunnel crosses andesites and traquiandesites Fig 2, breccias and silty-clayey, tuffs, Fig 3, in other words, formations of alternating weak and hard rocks. The rock mass shows various degrees of alteration and jointing and is intersected by fault zones. The breccias are very alterable, while the tuffs are associated with talus deposits.
(Figure in full paper)
The main objective of such a classification was to provide a rational basis for the design of the final lining and to facilitate contractual relations. Therefore, a very pragmatic approach was followed. The undermentioned features were given consideration:
degree of rock alteration
deformability of the rock mass
initial rate of deformation
required time to reach complete equilibriumFor the rock mass classification it was fundamental to carry out a detailed geological survey throughout the whole length of the tunnel during excavation.