During the design stage of the Alto Vague del Norte Hidro Project in the Domi nican Republic. a cost analysis was made comparing lined and unlined sections for the - construction of two tunnels with a total length of 9.6 km. This analysis concluded - that the unlined tunnel allowed an important reduction in time and construction cost. This posibility is frequent in the hydro projects with a design discharge up to 50 m3/s and with tunnels excavated with the drill and blast method because. in these cases. the rock is usually competent and the internal pressure is not high or is compensated by ex ternal pressure. In the article are anlysed both solutions from the hydraulic and economical points of - view. comparing a lined tunnel with an unlined tunnel with the shape of an inverted U With a concrete slab on the invert built on top of compacted muck. The construction and cost analysis reveals that the unlined tunnel allows an important reduction in the cost and construction period. against a modest increment in the operation and maintenance costs.
During the design of the Alto Vague del - Norte Hydroelectric Project in the Domini can Republic. it was made a cost analysis comparing lined and unlined tunnels with a total length of 9.6 km. This analysis concluded that the unlined tunnel alterna tive allowed an important reduction both in time and costruction cost.
This is a frequent possibility in the high and medium head hydroelectric projects which often involve long head or tailrace tunnels driven in competent rock that 0- could without problem, be left unlined. Furthermore, most of these projects have a relatively small design dicharge (less than 50 m3/s),and large rock cover (more than the design pressure) that are appropiate for the unlined tunnels, because they required small cross-section and the static head is co~pensated by the water pressure.
The purpose of this article is to compare both alternatives from the economical point of view to asses the relative conomical 1 advantages.
Probably, the most common section in use for the lined pressure tunnel is the circular section, that for the range of discharges considered here, it would have an internal diameter between the economic mi nimum (around 2,50 m) and 4,5 m.
The linning thickness will be around 0.25–0,35 m and the excavated section ("A" line) will usually have a modified horseshoe shape to facilitate the tunnel construction. The paid overexcavation (i.e. the distance between "A'land "B" lines) is usually 0,10 m. that, make up a paid linning thickness of 0,35–0,45 m and a diameter for the "B" line of 3,20–5,40 m (fig. l.a) 289
The most convenient cross-section in - the' case of unlined tunnels is inverted U form, that more often than not, has the same width than height. In this case, to facilitate the maintenance and eventual repair works, it is convenient to provide a pave d invert in the form of a concrete slab 0,15 m thick.
