|Nenskra Dam - Dynamic analysis by means of numerical modelling||References_4939|
Lombardi Ltd was commissioned by Salini-Impregilo SpA to perform the Basic Design of the Nenskra HPP (installed capacity = 280 MW), which will be built on the Nenskra river in the Svaneti region (Georgia), some 260 km north-west of Tbilisi.
The HPP foresees the construction of a 130 m high Asphaltic Face Rockfill Dam (AFRD) which has a crest length of 940 m and rests completely on alluvial soils at the valley floor (max. depth = 140 m) and alluvial fan deposits at both abutments (max. depth = 60 m).
Due to the seismicity of the Caucasian region, the stability of the dam has been analysed in dynamic conditions. The behaviour of the dam body and its foundation, the asphaltic face, the dam crest and the cut-off wall has been evaluated performing fully nonlinear dynamic simulations, by means of FLAC2D and FLAC3D software. The execution of large scale laboratory tests and in-situ measurements allowed the proper definition of the mechanical parameters for the modelled materials.
The dynamic behaviour of the dam was checked for both MCE and OBE earthquakes. The maximum permanent displacements at the dam crest have been evaluated (below 1% of the height of the dam for a MCE earthquake). The permanent displacements under an OBE event have been analysed to verify the serviceability conditions.
A detailed analysis of the bituminous facing was also performed by means of the 3D model, to evaluate if the maximum asphalt deformation was concentrated either close to dam abutments or within the dam body. The maximum tensile strain of the asphalt allowed evaluating the potential of cracks onset and the consequent risk of saturation of the dam body. The behaviour of the jointed connection between the head of the cut-off wall and the inspection gallery at the upstream toe of the dam embankment has been verified during both the quasi-static phase (dam construction) and the dynamic loading.
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.
|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.
|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.
|Arenal RCC gravity arch dam||References_6401|
The hydroelectric project Arenal, which uses the water of the Yaguala River, is located at the districts of Arenal and Olanchito, department of Yoro, in northern Honduras.
The project comprises the construction of a RCC arch-gravity dam with a height of 93.5 m from the foundation to the spillway crest (H=100 m to the dam crest). The dam has a RCC volume of 270’000 m³ and impounds a reservoir with a volume of 72’000’000 m³.
The works also include the construction of an upstream intake structure, a diversion tunnel of 4.52 km length with a cross section of 28.5 m², an external steel penstock consisting of two tubes with D = 2.40 m each and L = 110 m, a powerhouse with two Francis turbines with vertical axis and two bypass tunnels for the Yaguala river during construction, one of them to be used as the bottom outlet after construction. The central spillway of the dam has a capacity of more than 6’000 m³/s.
The average annual production will reach 230 GWh with an installed power of approximately 60 MW for an available net head of 129 m and a design discharge of 51 m³/s. In addition a dotation turbine with a design capacity of 1 MW will be installed.
|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.
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.