Rehabilitation of Urft Dam - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 774

The spillway is located on a natural ridge at the right slope, seen from upstream (Fig.
24.4). It consists of an overfall sill followed by a cascade. The cascade composed of
concrete steps is founded on the foundation rock consisting of sandstone layers and
alternating sequences of silt-, sand- and claystone layers of the Lower Devonian.
Since the raising of the Schwammenauel dam in 1959 the Urft dam on its down-
stream side is impounded up to a level of 12 m. This reservoir at the downstream side
of the Urft dam is called the “Obersee” (Fig. 24.2). As a consequence, since that time
the bottom outlets have not been accessible, and seepage water could no longer be
measured.
Since the Urft dam is a gravity dam, two-dimensional FEM analyses were carried out
for a preliminary stability check of the dam. As a result, vertical tensile stresses at the
upstream side of the dam were calculated which were not admissible according to DIN
19700-11 (2004). Therefore a rehabilitation of the dam became necessary.

24.3 Rehabilitation Concept

Because the Urft dam is curved, its load-bearing behavior exhibits a certain arching ef-
fect. In order to take advantage of this arch action it was decided to conduct the stabil-
ity proof by means of three-dimensional FEM analyses. An integrated model including
the dam and the foundation rock was used to account for the interaction of the dam
and the rock foundation. In this way, the stability proof according to DIN 19700-11
(2004) should be conducted to avoid comprehensive rehabilitation measures.
Since the Urft dam was originally designed as a gravity dam, the supervising authority
requested a proof that the dam is able to carry the loads with the aid of a certain arch ac-
tion, that is, that the loads to some extent can be transferred towards the abutments and
the slopes of the valley, respectively. For this purpose, reliable parameters for the masonry
and the rock mass had to be evaluated and the load-carrying behavior of the dam needed
to be verified by monitoring. In addition, for adjustment to today's technical standards,
the excavation of two inspection galleries was required. One inspection gallery should be
constructed at the foundation level and the other above the water level of the Obersee
(Fig. 24.5). To prevent permeation through the dam and leakage at the downstream side
and to reduce uplift induced by the pore pressure acting on the dam's base, the replace-
ment of the existing, not fully functional, drainage system was also required.
Site investigation, laboratory and in-situ testing of the masonry and the foundation rock
were carried out, and a monitoring system was established (Fig. 24.5). The monitoring
results, after a certain measuring period, should allow a calibration of the three-di-
mensional analysis model by means of back analyses. On this basis the final stability
proof was planned to be conducted and, if successful, to be accepted by the supervising
authority. The monitoring system also serves for observation of the long-term behavior
of the dam.