The work described here refers to the Genesee River Interceptor Southeast Project. This project is a part of the Combined Sewer Overflow Abatement Program, Phase I, for the Rochester Pure Water District, Rochester, New York. Our contract required the design of a 13,200-feet long main tunnel, 14 feet finished diameter, approximately 100-120 feet below ground; drop shafts and diversion tunnels to deliver surface flow to the main tunnel; a gate chamber whereby the main tunnel is dropped 35 feet, and provision made for connection to the future tunnel in Phase II, as well as a host of other surface structures. Cross sectional dimensions for the structures involved varied from 16 feet circular to a 20'x50' horseshoe, in a competent rock formation consisting of Lockport Dolomite and Rochester Shale. In the construction of a rock tunnel, the problems of the tunnel Designers are as follows:
To evaluate stresses in the rock as a result of excavation.
Design a temporary support system to prevent any failure during the period of construction. Design of this temporary support system is empirical. Design loads given by empirical relations are dependent upon the rock quality and size of opening. Temporary support system may either be steel frames closely spaced along the length of tunnel and tied together, in the case of poor rock, or, an even spacing of rock bolts in the case of competent rock.
Design a final lining for the service loads. Service loads, in our case of a water-carrying tunnel, are hydrostatic pressure due to ground water, dead-weight of lining, pressure of contained water and time dependent rock loading. The constitutive properties of the rock are extremely complex because of discontinuities, pre-existing stresses due to overburden, material symmetries and changes of initial state associated with the process of excavation. The changes of initial state may be changes of stress fields along with the consequent yield of the material or the extension of existing discontinuities.
Make a comparative study of the available methods of analysis and design of concrete lining for an underground opening of any desired shape in rock. It was expected that by taking into account the procedure of construction and the integral action of rock with concrete lining, a mathematical model will be developed to represent the real problem. It was thus felt that this mathematical modeling with certain idealizations, (based upon the available geological data), will predict stresses in the concrete lining( support system) and the surrounding rock (medium), thereby resulting in a safe and economical design.
1) Lining Design, Assuming no Rock Interaction
There are various methods of analyses available for the design of tunnels in rock. The simplest and most common method assumes no interaction between concrete and rock whereby a circular tunnel can be designed as a pressure vessel. When the thickness of lining is less than about 1/10 the radius, then, under uniform pressure, meridional and hoop stresses are practically uniform throughout the thickness of the lining.