Rehabilitation of Urft Dam - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 788

24.5.2 Inspection Galleries
The excavation of the inspection galleries, as requested by the supervising authority,
originally was tendered as “excavation by hand” using hydraulic chisels. Soon it became
clear that this excavation method yielded insufficient, very slow advance rates in the
lower inspection gallery of only some 0.3 m/d (Fig. 24.21). A continuation of these
advance rates would have led to an extension of construction time of some 15 months.
Thus, the feasibility of a drill-and-blast excavation was checked and proven by means of
trial blasts and corresponding analyses. After that, the inspection gallery was construct-
ed by smooth blasting. As a consequence, higher advance rates were achieved, and the
emerging extension of construction time could be reduced decisively (Fig. 24.21). A de-
tailed description of the excavation of the inspection galleries can be found in Polczyk
et al. (1996) and Wittke et al. (2012).
Figure 24.21 Rates of advance, lower inspection gallery (Polczyk et al. 1996)
The lower inspection gallery was provided with a 20 - 30 cm thick lining of reinforced
concrete (Fig. 24.22). To ensure drainage, joints of 20 cm width were placed between
individual concrete blocks. In the upper inspection gallery, only the invert was lined by
cast-in-place concrete.
A few years later, another old German masonry dam, the Diemel dam, was also pro-
vided with an inspection gallery using the drill-and-blast excavation method. For the
feasibility study, the experience gained from the Urft dam was valuable and useful
(Wittke & Schröder 2003).