More than 100 hundred large reservoir dams were constructed in the Canary Islands (Spain), between 1900 and 1990 (78, in the island of Gran Canaria and 20, in the island of Gomera). Most of the dams were built by means of cyclopean masonry, a few as rockfill dams and one as a vaulted dam. The historical and cultural value that these great sites hold must be acknowledged, but their state must be checked. In the Canary Islands, there isn’t a Dam Safety Department.
Construction of large dams in the Canary Islands: lime and cement mixed mortar
The second stage of great reservoir dams in the Canary Islands arises when the lime and cement mortar starts to be used in the body of the walls of the dam. The difficulty of acquiring the necessary cement and the circumstance that the lime was of excellent quality was what motivated the binder to be used preferably. Los Pérez dam in Gran Canaria, with mixed mortar up to 30 metres high and lime mortar until the top, was for some years the islands' tallest dam (45 metres).
One of the most important dam manufacturers in the Canary Islands, the engineer Julio Alonso Urquijo, explained in the reports of the Projects that “the use of mixed mortar was meant to accelerate the forged wrought of the lime mortars in walls of great thickness, as well as the increase of resistance”. He also wrote that “it adopted the gravity dam profile ordinarily used, which must resist, under its own weight, exclusively, to the force of water. And which with such calculation basis, the fact that the walls have a straight or curve form in plan, but which I preferred projecting them in circular arch, obtaining as well an additional factor of safety.” This explains why there is a very high number of dams with curved plan in the Canary Islands, especially in Gran Canaria. Only some old masonry dams were constructed with a straight plan, although they had previously been designed with curve plan (e.g. Cuevas de las Niñas).
After the construction of the little dam of Los Rajones, in 1942, as a trial, in 1943, a dam wall started to be constructed in the top of Tamadaba’s massif, according to the typical stone attached rockfill dam profile in its body and a hydraulic masonry and plaster coat impermeabilization screen with bastard mortar. Tamadaba’s dam stopped being constructed in 1954 when there was one metre left for its levelling. This large dam is 17’70 metres high on the bed, 19’70 on the foundations and 163 metres of length on top per 6, wide. A massif of stone craft.
In 1950, engineers José María Valdés y Díaz Caneja, José Luis Fernández Casado and Manuel Lorenzo Blanc, as members of the Public Works Geology Consultancy, made a visit to Gran Canaria in view of the construction of El Caidero de la Niña’s dam, in the most important gully of all the Canary Islands, by far. This visit to La Aldea’s Gully was made, in order to pass judgement about the conditions of the land, where a reservoir dam, which was going to be constructed, was going to become a very clear example of the indirect and substantial benefits that a hydraulic use work well conceived produced. With the construction of the dam, not only did the builders expect to broaden noticeably the cultivation lands and to improve La Aldea de San Nicolás’ outgoing communications, by means of the subsequent construction of a “little pier” or the “refurbishment or substitution of the Andén Verdes road”, but also cleaning up effectively by regulating and lessening the impetuous avenues of the gully. The sanitary situation was truly pitiful, due to the fact that the gully’s ponds were the cause of endemic malaria and of the propagation of typhic infections. The infant mortality reached the proportion of the 40 per cent.
In 1964, the engineer Manuel Alonso Franco highlighted that “the dam of El Caidero de la Niña stepped aside the classical construction of dams in the Canary Islands, because it met a more modern conception.” Thus, it was constructed with mass concrete with cement binders Portland (brick concrete) It had transverse contraction joints and three longitudinal visit and inspection galleries. Its firm limp spillway has three openings and it is situated in the centre of the dam, equipped with a launching springboard. The type of this great dam straight plan gravity and height of 46’40 metres on the bed.
In a report dated in 1962, about a project of three stepped dams for water storage coming from Tejeda’s Tunnel (Gran Canaria), the engineer José Luis Fernández Casado mistrusted cyclopean concrete, because “he had seen several times how it became a bad concreting masonry that could almost be described as dried masonry.”
Between 1902 and 1961, 41 great masonry dams had been constructed in Gran Canaria. The banana trees, the great consumers of water, were mainly responsible for this high number of great dams. The supplying of Las Palmas city and of Puerto de la Luz remained in the second place. However, the number of great dams constructed in Gran Canaria in only 60 years, acquires more importance, when we combine it to the number of ponds (some 5.000), wells (some 800 drilled 78 km), galleries (some 420 with 140 km), diversion channels or tunnels (153 km) and distribution pipes (534). Numerous hydraulic ventures that say a lot in favour of the insular diligence, specially the private initiatives.
In the 60s, not only were many masonry dams with bastard mortar (lime and cement) finished, whose works had began in previous decades, but in 1962 the construction of the vaulted dam of Soria started (concrete) and some masonry dams were constructed only using mortar cement, concrete screen and drainage pipes. In the islands, there was a great tradition in the construction of masonry dams, especially in Gran Canaria. Towards 1964, in La Gomera, 7 great old dams had been constructed by privated builders and 7 dams by the Hydraulic Services. The private dams with curved plans and lime masonry. The more modern dams with straight plan and cement mortar masonry. All of them without drainage in their body and in their foundation.
According the reports of 1964 by the Dam Safety Department (engineer Manuel Alonso Franco), “in all the masonry dams in the Canary Islands it is supposed that the density of their compositions has a relatively low value. We do not think that it is exaggerated believing that many of this constructions have densities that can range from 2’10 to 2’25 Tn/m³.” “A more favourable hypothesis than that possible”, said the engineer José Luis Fernández Casado.
The study made between 1970 and 1971 by the engineer José Sáenz de Oiza (SGOP), about the state of Las Cuevas de las Niñas Dam resulted in a very low density of the composition, lower than 2’0 Tn/m³ and close to 1’8 Tn/m³. After a visit to the Canary Islands, the engineer Federico Macau Vilar (SGOP) wrote in a note “I do not like the Cueva de las Niña at all.” It still is the only real scale model for the theoretical study of stability of other dams in Canary Island’s Archipelago.
The engineer Fernando Sáenz Ridruejo says that “as a result of the reports of 1964, in which Alonso Franco revised their safety conditions, all these dams entered in way of rationality". In fact, the first critical judgement do the stability fo the great masonry dams constructed in the Canary Islands, by the dams builders Fernández Casado and Alonso Franco, influenced very much in the development of some projects of great dams, as well as in the subsequent construction of the brick concrete with facings finished in concrete (Parralillo, Gambuesa or Fataga by Gran Canaria’s Inter-island Council). But it was more determinant for the overlay with masonry composition in old dams, due to the incentives granted by the State to the owners. The overlay of the dams of San Lorenzo or la Umbría, both in Gran Canaria, constitute a good example. However, the overlay El Mulato’s dam was with concrete.
According to the overlay projects and works in some reservoir walls, and due to the absence of inspection galleries, of composition’s drainage and bottom dewatering in the dams constructed with lime or bastard mortar with rough ashlars with a reduced density, the engineers explained in Madrid in 1966 that “the civil service should feel concerned with the safety of all the constructions with masonry compositions, especially in Gran Canaria and La Gomera due to the high number of dams which entered in the classification of great dams.” The general solution then proposed by the Dam Safety Department was that “the State should take charge of its structural guarantee, being met by the necessary auscultation and reinforcement expenses in the works whose owners lacked resources and suitable technical staff.”
A personal thought.