Introduction
The embankment dam, completed in 1990, is part of a hydro power plant in Austria. First design studies were undertaken in 1964, initially with two embankment dams and two reservoirs with a pumped-storage facility. The project finally selected for construction, comprised one dam with a diversion system from another catchment area, without the pumping facility.
The 85-m high Feistritzbach Dam was finally designed with a central water barrier and sound rock as fill material. The available rock turned out to be partly severely weathered and crumbled badly during processing. Because of the different soil mechanics properties the embankment settled at least three to four times as much as originally calculated. The dam settlement in the central part during construction and first impoundment reached a maximum of about 0.9% to 1.0% and up to now a range of 1.1% to 1.5% of the total dam height. The bituminous concrete membrane followed all deformations of the embankment without cracking and is watertight.
Rock material investigations
Field tests
The shell material was subjected to large-scale shear tests in two different soil mechanics laboratories and placing tests in a trial quarry at the site were carried out additionally. The field tests included the following program:
● Placing of a 70-cm-thick layer and compacting the material using 300 kN vibratory rollers. The field density was determined following a varying number of roller passes (1 to 10); the properties of the fill material were determined and the settlement resulting from compacting was measured.
● Placing of l-m-thick layer, with a maximum particle size of 700 mm. A 600 kN roller (dynamic load) was used for compacting the material. Densities were determined after 1, 3 and 6 roller passes, with and without addition of water (500 1/m³). The irrigation was seen to accomplish generally higher deformation modulus, in particular however higher initial loading modulus. Another compaction test was made on a layer of the same thickness using 300 kN rollers. However, this did not give optimal fill placing results [1].
The preliminary testing program comprised a multitude of trials using different fill depths, compacting energies, addition of water and different numbers of roller passes. The quarried material available was found to give optimal results under a dynamic compacting energy of about 300 kN for a layer thickness of 60 cm and a maximum particle size of 40 cm. Five or six roller passes were recommended from the preliminary field tests as the optimal number. The required density of 22.5 kN/m³ was reached during the compaction of the dam after three or four passes.
In fact, tests using 600 kN rollers had led to substantial breaking of particles, with the addition of water being considered as particularly advisable for the upstream shell (see Fig. 1).
Laboratory tests
In addition to the field investigations, several standard tests were carried out to determine all normal soil mechanics values such as angle of internal friction, density, material strength, etc. In large-scale shear tests, the angle of internal friction for sound material up to 30 cm maximum particle size was determined. Based on the results of the preliminary triaxial tests, it had been expected that the angle of internal friction would range between 39° to 44°, depending on the magnitude of the normal stress and the applied s3 values (up to 10 N/cm2). The large-scale direct shear tests with a shear surface of 1 m×1 m showed a significantly lower angle of internal friction (s1 is 20 N/cm22 to 100 N/cm2, j = 34°, c = 5 N/cm2).
Further triaxial laboratory tests on soft and partly crumbled rock gave for low s3 values a substantial cohesion (between 3 N/cm2 and 7.3 N/cm2), which had not been found during the test on solid rock material. The angle of internal friction of the weathered soft rock decreased to 34° and 41°. On the other hand the crumbled soft rock had a lower modulus of deformation and tended to higher settlements.
Finite element analyses
Material properties obtained from laboratory tests (see Fig. 2) were used to perform the finite element analyses of the embankment. The tests were carried out on sound rock material and consequently relatively high modulus of deformation and relatively small settlements were obtained and used in the calculation. The upper part of the water barrier (bituminous concrete core) should be inclined to increase the lateral earth pressure acting on the membrane to afford better support of the core.
Because of different and changing material properties of the available fill material (soft rock with a much lower deformation modulus) the result was that the settlements measured during the construction by horizontal and vertical gauges were more than twice as high as had been expected and preliminarily calculated. The total settlements of the dam during construction, first impounding and more than 15 years of operation are several times higher compared to the results of the finite element calculations with sound rock. These results indicate difficulties in defining and finding adequate soil mechanics parameters representing the natural variations of the available fill material properties. The laboratory tests are basically carried out on some 100 m³ and the total fill volume of the dam ranks in this case study in a range of about 1.6×106 m³.
Dam design
The case study for a comparison of calculated and monitored dam settlements is based on an embankment dam with a central vertical bituminous concrete membrane, inclined over its upper portion (see Table 1, Figs. 3 and 4).
Zones of embankment dam
The upstream ⓖ and the downstream ⓒ dam shell (see Fig. 3) were basically designed and calculated to use solid sound rock as fill material. As quarrying proceeded, the rock turned out to be partly severely weathered, rich in mica and crumbled badly following quarrying by blasting, placing and compacting (see Fig. 5).
In terms of rock mechanics, a large proportion of the fill material, especially after quarrying, had to be classified as soft rock. A sufficient quantity of sound and high-strength rock was, however, available for the filter and other zones subjected to major stresses. Therefore, better and harder rock with lesser tendency to crumble was used for the upstream shell. In the downstream shell, the soft and partly badly crumbled rock had to be used.
The gradation curves of the rock-fill material obtained during construction are shown in Fig. 6. The much finer-grained material with the relatively small portion of major sizes as compared with the original design was mainly used in the downstream dam shoulder.
Better and less crumbled material was also used in the downstream shell above berm 1050 m as rock quality improved as quarrying advanced.
Downstream zone ⓕ was constructed as a drainage toe using rockfill of good quality. The material was placed in layers with a maximum thickness of 130 cm and a maximum particle size of 100 cm. Over the whole downstream dam foundation a filter (zone ⓔ) with a maximum particle size of 60 mm was placed to protect fine particles at the dam foundation from piping. The material was placed in layers with a maximum thickness of 50 cm, over a total thickness of 1.5 m.
The same grading was used for filter zone ⓗ on the upstream side to protect the fine-grained zone in front of the membrane from being piped towards the shell. The material was placed in layers up to 20 cm thick. The zone is 2.0 m wide.
Very tight soil mechanics criteria were applied to the downstream filter ⓓ behind the bituminous concrete membrane. These included a limitation of the fine particles content smaller than 2 mm to a maximum of 5 percent. This 1.5 m wide zone was placed in 20 cm thick layers in a single operation with the bituminous concrete membrane and the fine-grained zone in front of the membrane by means of a special finisher. Final compacting of this very important zone was accomplished by means of 12 kN vibratory roller. The moist density ranges around 24.5 kN/m³. The voids content is about 16 percent.
Fine-grained zone ⓘ in front of the bituminous concrete membrane is a fairly unusual feature, which was provided because sufficiently impervious, partly cohesive material was available at the site. Stability tests demonstrated this material to show properties similar to those of the downstream filter zone, making it a suitable fill material. As mentioned above, this zone was placed in a single operation with the impervious core, in a width of 1.5 m and in layers 20 cm thick. Initial compacting was carried out by the finisher. Final compacting was by means of immediately following 12 kN vibratory rollers. In order to stabilize the material during or after rainfalls, 3 percent of lime was added using the mixed in plant method. The moist densities reached for this fine-grained zone ranged around 21 kN/m³.
On the upstream embankment slope riprap with a thickness of 1.5 m was provided as a protection against destructive wave action (maximum rock diameter 70 cm, D50 = 30 cm). A limitation was imposed on the percentage of fine particles.
The bituminous concrete membrane ⓙ as a central impervious core was placed over a width of 70 cm, 60 cm and 50 cm, in layers 20 cm thick together with both adjacent 1.5 m wide zones in a single operation, using a special modern finisher. Following comprehensive finite element analyses, a vertical bituminous concrete membrane inclined to the downstream over its upper 15 m was selected. Before construction work was commenced, a comprehensive investigation program was undertaken on aggregates from several sources and on various bitumen grades, mainly to obtain optimal conditions for imperviousness, stability, plasticity, durability and minimum embrittlement for use as an impervious material (see Fig. 7).
The high requirements to be met by the aggregates regarding strength, uniformity, petro-graphical, composition and affinity with the bitumen were matched by the equally high requirements to be met by the bitumen. Penetration, softening point, paraffin content, decrease in penetration following heating and placing and several other parameters were defined and subjected to tighter limit values than normally practiced in bituminous concrete construction. Furthermore, no additives for improving adhesion to the rock surface, nor artificially modified bitumen grades were permitted. In tests of long duration on a B70 bitumen (penetration according to Austrian Standard : 80/10 mm to 50/10 mm, softening point according to ring and ball method 47°C to 54°C), the bitumen content necessary for reaching optimal deformation, stability and impervious conditions was found to be 6.5 percent.
Basically, a softer mixture with a higher bitumen content was used and guaranteed that the bituminous concrete core was able to follow the larger settlements of the fill material without cracking and leaking.
Stepping of the membrane from 70 cm to 60 cm thickness was provided at El. 1020 m, and from 60 cm to 50 cm thickness at El. 1050 m. To identify the origin of potential seepage flows, drainage compartments were created constructing horizontal bituminous seals at El. 1050 m. Sections were also provided along the gallery, with separate pipes leading into the drainage gallery.
The amount of placed layers per day did not exceed 3 layers, so as to allow cooling of the new layers. A smaller on-site bituminous concrete mixing plant and a well functioning site laboratory for strict quality control proved particularly useful. Quality control comprised, apart from the daily aggregate check, subsequent testing in two different test laboratories of drill cores taken from the completed membrane. The voids content in the field was below 2.5 percent in all the samples, and water pressure tests on drill core slices showed the bituminous material to be completely impervious.
At the contact with the concrete gallery the membrane was widened, and the material was placed by hand. The contact surface at the top of the concrete structure was sand blasted, cleaned and provided with a prime coat and then with bitumen mastics. [2]
Dam instrumentation and monitoring results
Because of the fact that the fill material tends to break up during quarrying, placing and compacting the behavior of the shell was different from what had been expected. During the whole construction period settlements and deformations were observed at several horizontal and vertical plate gauges, clinometers and extensometers.
Measurements on the membrane and in the filter gave only minor lateral strains. There was practically no widening of the membrane during dam construction and reservoir operation.
The settlements of the dam taken during construction up to December 1990 are shown in Fig. 8 (in maximum about 80 cm). At that time deformations in the direction of the dam axis as well as displacements transverse to the axis had not exceeded a few centimetres. At the level of horizontal gauge H1, the modulus of deformation during construction derived from the settlements measured by the vertical gauge was about 62 MN/m2 to 67 MN/m2. Only very minor settlement differences were measured between up- and downstream shell.
In the following three years (up to Dec. 1993) the upstream settlements increased at vertical gauge V1 to about 106 cm and also higher saturation settlements were measured at the horizontal gauge H3 (about 102 cm). The maximum downstream settlement was observed at the same time with about 80 cm to 86 cm (H5, V3) (see Fig. 9).
From 1994 to 2004 the downstream settlements increased slowly but not significantly (at a maximum of approximately 10 cm to 15 cm) whereas the upstream settlement caused by saturation effects increased significantly (up to approximately 130 cm to 140 cm or 1.5 % of the total dam height) (see Fig. 10).
In addition settlements of the bituminous concrete membrane were measured by special devices. The about 13-m-wide zone between the membrane and the floating shaft settled fairly even. This means that the shell of the dam forces its deformation on the bituminous membrane.
Also changes in membrane thickness were observed. The lower instrument station for measuring membrane thickness, installed at El. 1045 m, serves to determine changes in the downstream half of the membrane by means of a hall and steel plate embedded in the membrane. Movements are transmitted via glass fibre extensometer rods ending in measuring heads in the inspection shaft. The upper instrument, around El. 1061 m, determines in addition variations in thickness of the upstream half of the membrane by means of a triple extensometer.
The instrumentation with earth pressure cells was mainly located in the downstream filter, near the membrane. Most of the stations were equipped for measuring both horizontal and vertical pressures. The sand embedded cells for horizontal pressure measurements being located just behind the membrane. By determining horizontal and vertical earth pressures occurring in the vicinity of the bituminous concrete membrane it is possible to obtain deformation moduli by back calculation for the downstream filter zone.
The development of the horizontal and vertical earth pressure as well as the ratio of horizontal to vertical earth pressure is illustrated in Fig. 11 [3]. The influence of the inclined water barrier in the upper part of the dam is clearly expressed in the higher ratio of horizontal to vertical earth pressure. The time-dependent change of the horizontal and vertical earth pressure as well as the ratio of both values clearly show an increase of the vertical earth pressure over the entire dam height and only a slight increase of the horizontal earth pressures in the lower third of the dam height. The ratio of horizontal to vertical earth pressure increased primarily in the lower dam area and improved the support of the water barrier (see Fig. 12).
From the 40 m deep floating inspection shaft, deformations of the embankment fill and the membrane can be observed and variations in membrane thickness measured. The increasing of the thickness of the membrane dam was less than 1 cm.
Apart from these measuring devices, seepage measuring stations and piezometers are available.
Conclusion
Although extensive laboratory and field tests were performed before the construction of the dam to determine the soil-mechanical properties, the assessment of representative properties for the deformation calculation proved to be highly difficult. The wide variety of available rock materials in the quarry could not be assessed properly. Even though the dam settlements reached a multiple value of the calculated range, the dam and in particular the bituminous concrete core have performed well for nearly 20 years.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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