The Belesar arch dam, owned by Gas Natural Fenosa SLU, was built between 1957 and 1963 on the Miño River, north-west of Spain. The dam is 129 m high and impounds a reservoir of 655 hm³, used mainly for power generation. It is equipped with a bottom outlet (160 m³/s) at the central part of the dam body and two gated spillways, located at both gravity abutments (total capacity 4’000 m³/s). The total length of the dam crest is about 500 m, considering the arch (268 m) plus both abutments.
During the 1970’s, shortly after construction, permanent displacement towards upstream as well as vertical deformation of the dam body were noticed. In addition, a peripheral crack near to the foundation appeared at the downstream face. This phenomenon has been caused by and ARR reaction, producing an expansion of the dam body and a concrete swelling. The irreversible deformations have been increasing up to now with values of 10 cm in the radial direction at the dam crest, without reaching an equilibrium state so far.
After 50 years in service, Lombardi Ltd has been commissioned by Gas Natural Fenosa SLU to carry out a safety assessment of the dam, focussing on the expansive phenomenon due to the concrete alkali-silicate reactions. The expertise includes the evaluation for the medium-long term of the safety and operation conditions and the proposal of potential restauration measures, consisting probably of vertical slots using diamond wires to release the compressive stresses induced by the ARR reaction.
|Pavana dam rehabilitation||References_8082|
The Pavana mixed type dam was built in the years 1923-1925.
The central part consists of a multiple arch dam on buttresses, the left part is a caisson type and the right is a concrete gravity structure.
The spillway is an independent structure, located upstream of the dam on the right bank and consist of two vanes, one with a sector gate and the other with an automatic flap gate. The spilled discharge is collected in a basin, and by means of a vertical vortex shaft is evacuated into the outlet tunnel. The same outlet tunnel receives the discharge from the intermediate outlet.
The bottom outlet is located between the dam and the spillway structure. Its inlet is regulated by two hydraulic gates and the water is released downstream by an independent tunnel.
The concrete of the structure is subject to expansive alteration phenomena due to alcal aggregate reaction (AAR), with visible cracks on the arches and buttresses.
Lombardi has been awarded with the revision of the final design and the construction design of the rehabilitation works. After completion, the historical arch-buttress static scheme will be converted in a gravity dam, preserving part of the original concrete. Structural joints will be realized to avoid adverse transmission of loads between existing and new parts. A new drainage scheme will be implemented.
The design activities include the verification and seismic enhancement of the dam and its appurtenant structures.
Lombardi Ltd. is also in charge of environmental assessments, support for authorizations, site supervision and related technical assistance.
|Frankonedou and Kogbedou Dams||References_6122|
The Frankonédou and Kogbédou hydropower complex on the Milo river in Upper Guinea involves the construction of two cascade dams with the following characteristics:
Frankonédou dam: composite dam with concrete core, height 40 m and length 305 m, provided with outlet structures and power house equipped with two Francis groups with installed capacity of 18 MW and two side rockfill dams with clay core, 403 respectively 242 m long.
Kogbédou dam: composite dam with free concrete threshold in the central part, maximum height 11 m and total length 216 m, and two rockfill dams with clay core on the sides, 267 respectively 324 m long.
|Les Marécottes - Rehabilitation of the Regulating Basin||References_4804|
The compensation reservoir Les Marécottes, which holds the turbined water of the Châtelard hydroelectric plant for further power generation at the Vernayaz hydroelectric plant, is owned by the Swiss Federal Railways. The downstream multiple arch dam is the characteristic element of the compensation reservoir, which was designed in 1926 by the engineer A. Sarrasin. The multiple arch dam consists in 43 thin, inclined reinforced concrete arches supported by buttresses. The total length of the structure reaches 200 m. Although no unusual behavior of the dam has been detected since commissioning, a general rehabilitation of the plant aims to increase the service life of the fairly sensitive structure.
In close collaboration with the expert Prof. E. Brühwiler (EPFL), the feasibility study was completed and optimized in order to cautiously preserve the historic monument. The first intended step is the removal by high-pressure water jetting of the pore sealing layer currently covering the surface of the buttresses. This water vapor impermeable layer caused various spalling phenomena in concrete and corrosion on the original steel reinforcement. After local treatment of the damaged zones and reprofiling with a color-matched repair mortar, the entire concrete surface is saturated with a deep impregnation. The purpose of the depth impregnation is to ensure the corrosion protection of the existing reinforcement. The upstream surface of the buttress dam is sealed by an application of synthetic PUR-based layers. The damaged zones of the ground slab of the basin and on the upstream retaining walls are restructured locally.
The rehabilitation is planned for 2019 and 2020 and includes the motorization of the vertical gates at the intake of the compensation reservoir as well as the reconstruction of the bottom outlet.
|La Punilla multipurpose scheme||References_4258|
The Punilla multipurpose project will be built on the Ñuble river, about 70 km East of Chillán in the Ñuble region (Central-South of Chile).
The main purpose of the scheme is to secure and improve the irrigation in the valley. Additionally, the scheme is equipped with a powerhouse which uses the irrigation water releases to produce power (approx. 500 GWh/yr).
The project main features are:
- a 32 m high upstream rockfill cofferdam;
- two diversion tunnels (D=10 m, L=850 m and 1'015 m) designed for an AEP
1:30 return period flood discharge of 1’800 m³/s;
- a 137 m high CFRD (Concrete Face Rockfill Dam) which impounds a
625 Mm³ reservoir;
- a gated spillway located on the left bank equipped with three radial gates
(BxH=11,0 x 17,5 m) and a chute ending with a ski jump, designed for a
discharge of 4'800 m³/s (AEP 1:1’000);
- a bottom outlet (B=2.5 m, H=3.5 m, radial gate) located in a diversion tunnel;
- a 410 m long penstock in tunnel and buried (D=5.2 to 2.2 m);
- an external powerhouse equipped with two 47 MW Francis turbines
- a bottom outlet with two Howell Bunger dissipating valves (D=1.55 m)
installed in dissipation chambers;
- a substation of 220 kV.
The Francisco Morazan dam, known as El Cajón, located on the Comayagua River in Honduras, was built between 1980 and 1985 on a limestone karst site.
With a maximum height of 226 m and a crown length of 382 m, it creates a basin of about 1’470’000 m3 with the characteristic of never having been filled to its highest level of exploitation. In fact, since its commissioning in 1985, significant percolations and abnormally high pressures have been detected in the foundation of the dam.
Since then, various injection rehabilitation works have been carried out.
The services provided by the Lombardi office include:
- Calculation of the dam’s stability in 1996
- Study of a new access to the tunnel of the "El Nispero" power plant in 1996
- Periodic safety analysis of the dam by the experts G. Lombardi from 1996 to
2011 and R. Bremen from 2016 onwards
- Configuration of an interpretative model of the dam's behavior
- Implementation of MIC auscultation software in 2018.
|ATG - Gotthard Base Tunnel - Surface deformations and their impact on reservoirs||References_253|
Dewatering effects due to tunnel excavation in saturated jointed rock mass lead to a lowering of the groundwater table and might cause substantial surface deformation.
This presented a particular challenge also in the construction of the Gotthard Base Tunnel, which was driven through a zone close to the 3 arch dams Nalps, Santa Maria & Curnera. With a minimum horizontal distances between the GBT axis and the dams of only 600 m and a vertical distance of 1'400 m, the risk of affecting the safety and serviceability of the dams by tunneling was already recognized at an early stage. Serious damages to the dams would have resulted in a hindrance of tunneling work with corresponding high costs and delays.
It was up to all involved parties to develop a corresponding measure planning as well as possible rehabilitation for different deformation scenarios on the short and long term.
Structural sensitivity analyses by calibration of monitored dam deformations were performed to evaluate the allowable valley deformation. Installation of geodetic measuring systems already before tunnel excavation allowed to distinguish seasonal surface deformations due to groundwater fluctuations from deformations attributed to the excavation. Information from continuous surface and dam monitoring was then used to perform numerical simulations of corresponding deformation developments in the affected zone. These simulations were performed by means of a hydro-mechanical model (FES-model) specifically developed in-house.
The Toachi-Pilatón hydroelectric power plant uses the water from the homonym rivers, which catchment area stretch out on the provinces of Pichincha, Cotopaxi and Santo Domingo de los Tsáchilas, in the north-west part of Ecuador. The cascade plants have a total installed capacity of 255 MW and are interconnected at the right bank of Toachi dam.
The upstream part includes the following main works:
- Water intake on the Pilatón river, with desanders (Q=40 m³/s);
- Headrace tunnel (D=4.10 m, L=5.9 km);
- Upstream surge shaft (lower part: D=4 m H=110 m; upper part: D=12 m,
- Sarapullo power cavern (L x B x H=49.5 x 14 x 31 m, equipped with 3 Francis
units (3x17 MW));
- Tailrace tunnel (D=3.80 m, L=470 m).
Toachi-Alluriquín scheme includes the following main works:
- Concrete gravity dam, 59 high, including a water intake (Q=60 m³/s);
- Gated spillway (Q=1'200 m³/s);
- Bottom outlet controlled by segment gates (Q=3'200 m³/s);
- Small powerhouse at the toe of the dam equipped with a 1.5 MW Francis unit
to turbine the ecological discharge (Q=4 m³/s);
- Headrace tunnel (D=6.00 m, L=8.7 km);
- Upstream surge shaft (lower part: D=4 m H=125 m; upper part: D=15 m,
- Alluriquín power cavern (L x B x H=66 x 24 x 45 m), equipped with 3 68 MW
- Downstream surge chamber (L x B x H=41 x 10 x 36 m);
- Tailrace tunnel (D=5.60 m, L=500 m).