With overgrowing population and scarcity of water looming large in most countries, water preservation and management has become a key issue to consider. Dams will play an increasingly crucial role in the years to come, hence they must be maintained in safe and efficient operating conditions. Many dams suffer from deterioration and seepage that can be attributed to different factors. Uncontrolled water infiltration in concrete, RCC and masonry dams can deteriorate the cementitious material and cause increase in uplift pressures, while in embankment dams it can trigger erosion, piping, and embankment instability. The functional life of the dam can be reduced, and ultimately, its safety can be threatened.
Having and maintaining an efficient water barrier over time is therefore crucial for dams. Geomembranes, used as waterproofing elements since more than 60 years, are an established technology that has proven its efficiency and reliability in the field in hundreds of hydraulic projects; many papers and articles published in international conferences and specialised magazines, engineering manuals, and ICOLD bulletins, provide extensive information on the subject. Main technical advantages of geomembranes over traditional materials are their computer-controlled and controllable constant properties, including pre-established permeability that is not altered during field installation and by weather conditions, and flexibility and tensile properties that allow accommodating differential movements and deformations that would destroy more rigid traditional materials. The success of geomembrane systems depends on appropriate design, on adoption of high-quality durable materials, and on the skill, experience and conscientiousness of the specialty contractor executing the installation. When adequately designed and installed, they contributed to the safety of dams, reservoirs, canals, hydraulic tunnels, and shafts, by stopping seepage in existing structures, or by preventing seepage in new structures. This paper focuses on the applications of geomembranes in dams' rehabilitation.
In rehabilitation, regardless of the type of application, the geomembrane is installed at the upstream face, and in exposed position, unless specific site conditions (e.g. instability of slopes with fall of rocks, vandalism) require a cover layer. All types of dams can be waterproofed, all types of subgrades accommodated (concrete, RCC, bituminous concrete, granular subgrades). Installation is quick and can be staged following the needs of the owner. Geomembrane systems can be used to restore watertightness over the entire upstream face of the dam, or only over the area/s causing most seepage or can be used to waterproof cracks/failing joints. Starting in the 1990s, such technologies conceived for dry installation were further developed to provide systems that could achieve the same objectives while being installed underwater, with the aim of reducing practically to zero the impact on the operation of the reservoir. The following chapter present a recently completed cases on full face rehabilitation of a large all available options, i.e. total re-facing/staged installation, partial re-facing, and repair of failing joints/cracks, discussing recent dry and underwater projects in India and Africa.
FULL FACE REHABILITATION: UPPER BHAVANI MASONRY DAM, INDIA
Background, scope of works and planned schedule
Upper Bhavani is an example of a full-face rehabilitation executed in the dry and, due to site-specific constraints, staged over separate campaigns. The dam, of the masonry gravity type, built in 1959-1965, is located on the Bhavani river, near the border between Tamil Nadu and Kerala, in an uncontaminated wildlife habitat, at elevation 2278.68 m. Owned by Tamil Nadu Generation and Distribution Corporation (TANGEDCO), the dam is a storage & forebay for Kundah powerhouse 5, one of the biggest electricity generating schemes in Tamil Nadu. and is the main source of water for Five hydro power houses that are constructed further down the hills of Nilgiris. The dam, founded on hard granite rock, is 80 m high and 419 m long at crest. It has a 28.95m long central gated spillway, and a 1.52 x 2.13m scourvent gate . 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 minimum elevation 2210.00, and a grout curtain. The maximum water level is at El. 2276.86 m, minimum draw down level at El. 2249.42, sill level of scour vent 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 growth of small plants provided evidence of persisting leakage. High leakage was found in more than 8 shafts, increasing every year: the total leakage was recorded as 8,168 LPM l/minute at EL 2267.74 m., measured at 9.14 m meters below the FRL level and the two shafts surrounding the spillway contributed to the maximum leakage. Discussions with client and the engineers indicate the leakage at FRL would be approximately 16,000-18,000 LPM apart from the leakage through the downstream side of the dam on both the flanks which goes unmeasured. TANGEDCO intended to restore imperviousness to the dam with an exposed geomembrane system that they had already used on two of their dams very similar to Upper Bhavani, Kadamparai 67 m high masonry dam (2005) and Servalar 57 m high masonry dam (2018) As for Servalar, the tender for rehabilitation works was under the World Bank funded DRIP (Dam Rehabilitation and Improvement Project) initiative.
The waterproofing system is the one widely adopted for masonry dams' rehabilitation and discussed in the most important guidelines for geomembrane systems in dams (ICOLD 2010). The waterproofing liner is a flexible composite geomembrane, SIBELON® CNT 4400, consisting of a 3.0 mm thick SIBELON® geomembrane heat-bonded during extrusion to a non-woven, needle punched 500 g/m2 polypropylene geotextile. Before installation of the waterproofing liner, at all anchorage lines discussed below the surface was regularized by a layer of mortar. A strip of high-transmissivity drainage geonet was placed over the mortar and under the vertical tensioning profiles, and a 2000 g/m2 nonwoven geotextile over the upstream stone masonry facing to protect the geocomposite from puncture as well as to provide some drainage capability.
Face anchorage of the waterproofing liner is obtained by vertical stainless-steel tensioning profiles adopted by Carpi in several similar projects and allowing continuous linear fastening and pre-tensioning of the liner. The tensioning profiles were placed at 5.70 m spacing and waterproofed with a cover strip of SIBELON® C 3900 geomembrane (the same 3.0 mm thick material composing the SIBELON® CNT 4400 geocomposite, but without geotextile). All welding of the waterproofing liner and geomembrane cover strips was done by “hot air” one-track welding guns. The waterproofing liner is confined at all peripheries by mechanical seals that are watertight against water in pressure where submersible (along bottom peripheries, scourvent, spillway, at connections between horizontal sections) and against rainwater and waves at crest. Submersible perimeter seals are made with 80x8 mm flat stainless-steel profiles tied to the dam with anchor rods embedded in chemical phials. The perimeter seal at crest is made with 50 x 3 mm flat stainless-steel batten strips tied to the dam with mechanical anchors..
A double 1 m high band of drainage geonet along the bottom periphery of each horizontal section constitutes the bottom drainage collector, conveying water to pipes embedded in transverse holes drilled to reach the gallery. Anti-intrusion stainless steel plates in front of each discharge pipe, and ventilation pipes at crest and to the gallery, complete the drainage system.
An Optical Fibre Cable system was installed like at Kadamparai dam, to implement the monitoring of the geomembrane system done by measurement of drained water.
Development of works and Challenges Faced and Mitigated
The works were affected by two unpredictable events already mentioned, the Covid and a peak monsoon season in 2020 with an exceptional rainfall (more than 160 cm recorded in a span of 48 hours). The Covid imposed restrictions to crews, and particularly impacted on the presence of expatriate technicians, who were either unable to access the country, or had to undergo quarantine, and face long re-patriation trips. Against the two seasons, the entire installation had to be carried out in three seasons.
Since the central section of the dam below El. 2225 contributed only to a very minimal extent to leakage, it was agreed with the owner to leave unlined the central gorge portion (area shaded in grey in 5).
All possible combination of difficulties and challenges were experienced in this entire project: delayed handing over of the dam all 3 campaigns, Covid 19 first and second wave, and in 2021 forest fire in the camp area, State election disturbances, Covid 19 spread at site, 2 major cyclones and the need to change the design to accelerate installation works, last minute mobilisation of Optical Fibre Cables experts, etc. Despite facing all the above odds, the target of completing works within the 30th of June 2021, including the installation of Optical Fibre Cable monitoring system to assess the performance of the geomembrane liner, was achieved. This project confirmed the capability of an exposed geomembrane system to accommodate unforeseen events that require prompt adaptation in design and schedule.
As always, challenges are also an opportunity to improvise, to develop and to gain precious experience. Carpi Site Engineers and Indian technicians are now fully equipped to work independently, good local stainless-steel suppliers, Optical Fibres Cables technicians, underwater diving teams have been identified and experimented.
Results and Performance
The dam started impounding since 1st July 2021 and with water level fast rising the results are very satisfying to the client TANGEDCO as well as Carpi. The shafts which were contributing to the maximum leakage are completely watertight now. With water level on 28th July at around El 2258m, the two shafts which was leaking at the order of 6000 Lpm & 3000 LPM is now reduced to less than 20 Lpm. The downstream of the dam appears dry and there is no major discharge of water inside the gallery as on 28th July 2021. Carpi and the client TANGEDCO would be eager to see if the results exceeds the performance of Kadamparai Dam (Successfully in service for more than 16 years now). Thanks to TANGEDCO for placing trust on the latest technology on waterproofing and surely being a role model for many other dam owners in India.