A numerical analysis by finite element model is carried out to optimize the design of a consolidation rock mass scheme to be ejecuted prior to excavate a hydraulic tunnel very close (1.5 m under) to the subway twin tunnels. The objective of this rock treatement is to avoid the damages that could be cause due to asymetrical settlements of the lining subway tunnels. The rock mass treatment finally designed, allows to reduce from 1.5 cm to 0.5 cm the theoretical settlements calculated by numerical simulations.
At the present moment, two twin tunnels of approximately 6 m in diameter, along a kilometer of the Line III of the Metro of Caracas, are in final phase of construction. With the use of TBM, in a rock formation of schistose and metheorized rock mass. The future development of the urban aqueduct anticipates the construction of a tunnel of 5 m in diameter (INOS tunnel) whose allignment will intercept, with a deviation of approximately 40°, the twin tunnels under them at distance of only 1.5 m. Anticipating the possibility that such future excavation could affect the continuity of the transportation in the above tunnels, a numerical analysis was carried out to simulate the damaging effects of the future excavation and later, the beneficial effects of a possible improvement of the rock mass, prior to the new hydraulic tunnel construction. Finally the analysis permits to optimize the design of the works to be executed prior to the beginning of the subway transit: by grouting to fill the joints of the rocks mass and consequently increase the global stiffness so as to reach acceptable figures of deformations that will develop with future excavations. The excavations found at approximately 70 m depth in a rock mass of schistose graphitose and metheorized formation, hard and very jointed. The geomechanics parameters for the rock mass are: RQD = 15 % and RMR = 25; in consecuence it is possible to estimate by Bieniawski correlation: E=10(RMR-10)/40 a global deformational modulus of E=2500 MPa. This value has been confirmed by a plate load test carried out in a nearby rock formation with lithology, structure and general fisical state very similar to the mass in reference; and also by laboratory test where has been obtained a elastic modulus of El=30000 MPa for the intact rock (E/El=0.08). The twin tunnels of the Caracas subway with a 6 m diameter and 11 m between each other, has been excavated by TBM and lined with a prefabricated concrete rings of 22 cm in maximun thickeness and a global deformation modulus of Er=32000 MPa. The INOS tunnel of approximately 5 m in diameter will be built with a minimum separation of 1.50 m from its ceilins and the bases of the subway tunneles, with an angle of intersection of approximately 40° (figure 1). The numerical simulation process was carried out with a plane model of ishoparametric quadrangular finite elements (372 nodules and 312 elements) simulating an elastoplastic behavior for rock mass and an elastic behavior for lining ring of the subway tunnels. The above was achieved assigning null deformational parameters to the three groups of elements that in each case, represent the section of excavation of the INOS tunnel. In such matter it is simulating the critical conditions corresponding to the construction stage in winch there in not support for that tunnel. For each one of the three geometrical situations analized, the conditions corresponding to the rock mass without treatment were simulated, and later, conditions that include a consolidation treatment were simulated.