An Innovative Waterproofing Solution For Pumped Storage Reservoirs and Ageing Dams To Enhance Power Generation

Jagadeesan S , Business Unit Manager, Carpi India Waterproofing Specialist Private Limited


ABSTRACT

Many countries are implementing Pumped Storage Schemes (PSS) to even electricity shortages and surpluses in electricity grids. In PSS, leakage is an issue for the structural safety of the reservoirs, and for the profitability of the scheme. Concrete facings and bituminous concrete facings require periodical maintenance having significant impact on operation, since generally long outage is needed, involving heavy revenue losses. Geomembrane liners are an advantageous alternative because thanks to their elongation properties they are more performing in respect to settlements and differential displacements, grant durable watertightness, require no maintenance, and if damaged can be repaired underwater, with no outage. The paper discusses about a project executed in 2019 in a new pumped storage reservoir in Portugal.


The construction and maintenance of the dam directly contribute towards the speed of aging of dam. Upper Bhavani Dam (2021) is a typical example of dam whose service life has been extended by another 4 decades. At one point of time in 2005, the dam was declared a distressed dam by the dam owner (TANGEDCO) and the Geomembrane system has come to their rescue and helped them extend the service life of the dams.  In new dams (Kohrang, Sar Chesmeh) besides achieving large savings in earth and civil works, flexible geomembranes maintain water tightness in presence of settlement and seismic events and are not subject to cracking, which makes them durable and maintenance-free.

 

A Geomembrane waterproofing system is an effective method for seepage control and plays a very important role in the safety of the dam and reservoir which will also increase their lifespan. Geomembrane waterproofing systems are an established technique for long-term waterproofing. In India, Cases of Kadamparai Dam, (2005) Servalar Dam (2018), and Upper Bhavani Dam (2021) are typical examples of dams where the service life has been extended.  A Geomembrane waterproofing system apart from improving the life of the dam and reservoir also provides the engineers with ease of repair as the system is installed in an exposed condition.

1. UPSTREAM GEOMEMBRANE FACED FOR NEW PUMPED STORAGE RESERVOIR AT PICO DA URZE, RESERVOIR PORTUGAL

1.1       PROJECT DATA AND ORIGINAL DESIGN

Pumped storage reservoirs are built in a variety of site-specific configurations, connecting existing lakes and dams, or using an existing reservoir and connecting it to a new reservoir, or constructing a totally new scheme with new upper and lower reservoirs. Another option is using the ocean as lower reservoir and building the upper reservoir up above the coastline

The Calheta PSS (Pumped Storage Scheme) in the island of Madeira in Portugal aims to optimise the operation of the grid, combining wind generation with hydropower generation. The new upper reservoir formed by Pico da Urze dam and by an excavated reservoir, with a total volume of 1,000,000 m3, is part of a set of infrastructures that will allow expanding the power generation; the lower reservoir, formed by Calheta pond, has a volume of 67,000 m3. Pico da Urze dam is a 31 m high compacted rockfill embankment with extensive grading, inclination 1V:1.4H. The crest is at elevation 1354 m, the excavated bottom of the reservoir at elevation 1329 m. Due to deformable foundations having variable characteristics, a maximum estimated 0.67 m settlement was foreseen for the construction phase, a 0.054 m settlement for the first phase of reservoir filling, and a settlement at crest of about 0.50 m for the exploitation phase.

Since the soil at site is permeable (weathered granite and breccia), a geomembrane was chosen as water barrier on the dam and on the slopes and bottom of the reservoir. The tender design foresaw a 2.5 mm thick high-density polyethylene (HDPE) geomembrane installed over a 1000 g/m2 anti-puncture geotextile; the two resting on a 4 m thick granular transition zone. The geomembrane was to be left exposed (maximum design wind velocity 120 km/h) and anchored by embedment in peripheral trenches placed at crest, at the berms, along the slopes at elevation 1338 m, and at the toe of the slopes. Based on the elevation of the bottom outlet, it was assumed that the anchorage on the bottom of the reservoir would be provided by the ballasting action of < 1 m of water permanently acting on the bottom part of the reservoir.

1.2 INSTALLATION

The watertightness of a geomembrane system can be monitored over its service life by measuring the water from the drainage system installed behind the geomembrane, and by additional more refined monitoring systems. In PSS this event is unlikely to occur due to the generally remote location and inaccessibility of the reservoirs to the general public. Accidental damage can be repaired during the low-level phase in a very short time even by the personnel of the operator, or underwater by specialised divers, without outage of the scheme

The vertical anchorage trenches formed in the compacted slopes of the reservoir spacing provide a resistant and regular surface for installation of the mechanical anchorage by batten strips. At the location of stronger winds shown in figure 1a &1b, additional trenches were formed at the top part of the slopes. The shotcrete at the dam consisted of two layers of 50 mm each, reinforced at the interface by a geogrid shown in figure 2. The overall surface of the shotcrete was covered by a 1000 g/m2 anti-puncture geotextile. The main purpose of the geotextile was to increase the drainage capacity between the shotcrete and the geomembrane. 












Figure 1a & 1b: At left the vertical trenches for anchorage system on the slopes – note the closer spacing in the top part; at right the reinforcement geogrid placed over the first layer of shotcrete.


Figure 2: In the foreground the dam with shotcreted upstream slope, in the background the vertical anchorage trenches, at closer spacing in the top right corner, and the first geocomposite sheets under installation.


 

Before installing the waterproofing geocomposite, a 1000 g/m2 anti-puncture geotextile was placed over the most irregular surfaces (Figure 3a). Each sheet of geocomposite had a specific length and was placed at the exact position of the Geocomposite Layout. Adjacent sheets were watertight joined with double track seaming (Figure 3b).  All field seams are controlled for watertightness with standard methods.

Decades of experience in lining of hydraulic structures have demonstrated that flexible PVC geomembranes with a bonded anti-puncture geotextile exhibit a better puncture and burst resistance and a friction angle against the subgrade higher than rigid HDPE geomembrane placed on a separate geotextile, because they better adapt to an irregular subgrade requiring less surface preparation, distributing on a larger surface the load exerted by water and reducing the risk of damage due to concentrated loads at protrusions (puncture) and cavities (burst). A flexible, low thermal expansion composite geomembrane (PVC geocomposite) would reduce the forces transferred to the anchorage lines and would not form large wrinkles (Giroud, 2005). Accordingly, the HDPE geomembrane and 1000 g/m2 geotextile of the original design were substituted by a single geocomposite SIBELON® CNT 3950, consisting of a 2.5 mm thick flexible thermoplastic PVC geomembrane heat-bonded during fabrication to a 700 g/m2 polypropylene geotextile.


Figure 3a & 3b: At left an anti-puncture geotextile roll under placement before being covered by the waterproofing geocomposite, at right seaming of adjacent geocomposite sheets.

The batten strips fastening the geocomposite to the concrete of the trenches are secured with chemical anchors at appropriate spacing, and are equipped with watertight fittings preventing water infiltration where the anchors cross the geocomposite. The batten strips fastening the geocomposite to the shotcrete are secured with mechanical anchors and are waterproofed with cover strips of the same geomembrane composing the SIBELON® geocomposite (Figure 4a &4b).




Figure 4a & 4b: At left the vertical batten strips fastened on the concrete trenches, at right waterproofing of the vertical batten strips on shotcrete. 


Face anchorage of the geocomposite at the berms, at the spillway platform, and at the feeding canal platform is made by ballast with reinforced concrete slabs. Before casting the slabs, a protection geotextile was placed on the geocomposite to prevent any possible damage.

1.3 RESULTS

The waterproofing liner is confined at all peripheries by a continuous seal that is watertight and designed to prevent water infiltrating underneath the liner. The top perimeter seal is watertight against water at low pressure (rain) and is made by embedment in the top trench, as shown in Figure 4b. The submersible perimeter seals at all concrete structures, watertight against water in pressure, consist of stainless-steel batten strips secured to the concrete with chemical anchors at appropriate spacing, and conceived to be able to accept differential displacements, with a special “loop” configuration that allows reducing the stresses that can be created by a large displacement. Figure 5 shows the operation of the reservoir.

The contract schedule of Pico do Urze reservoir was 90 days which includes installation plus a 2-week allowance for bad weather. Waterproofing works started on July 8, 2019 and were completed in 99 days, of which 21 days of bad weather, for a surface of about 82,400 m2. Using a geocomposite was a main factor allowing such fast installation.



Figure 5: Pico da Urze reservoir in operation

2 WATERPROOFING OF UPPER BHAVANI MASONRY DAM

2.1 PROJECT DATA AND ORIGINAL DESIGN

Upper Bhavani Dam is a masonry gravity dam built between 1959-65 and is located on the Bhavani river, near the border between Tamil Nadu and Kerala at an elevation 2276.88 m.  The Dam is Owned by Tamil Nadu Generation and Distribution Corporation (TANGEDCO) and is one of the biggest hydro-electric generating schemes in Tamil Nadu and is the main source of water for eight hydro powerhouses that are constructed further down the hills of Nilgiris .Figure 6 shows the geographical location of the upper bhavani dam. This is located at the border of Kerala and Tamilnadu.




Figure 6: Geographical location of the Dam

The dam, founded on hard granite rock, is 80 m high and 419 m long at crest. It has a 19.81 m long central gated spillway, and a 1.52 x 2.13m scourvent tower. The 13 vertical construction joints are spaced at 30.48 m. The upstream face, vertical from crest to El. 2264.70 in the non-overflow section and to El. 2258.60 in the overflow section, and 1H:10V in the lower part, is formed by random rubble masonry with raised pointing. The dam has a drainage gallery in the central part, with a minimum elevation of 2210.00, and a grout curtain. The maximum water level is at El. 2276.88, minimum drawdown level at El. 2249.42, sill level of scourvent at El. 2221.99.The masonry facing deteriorated over the years, resulting in decreased imperviousness, cavities in the rubble masonry pointing, and seepage through the dam, which emerged at the downstream face where the growth of small plants provided evidence of persisting leakage. High leakage was found in more than 8 shafts, increasing every year: about 8,000 l/minute were recorded as leakage coming only from the two shafts surrounding the spillway.