Waterproofing of the San Juan reservoir dam

Descripción sistema

The reservoir, located in the extreme south-west of the Community of Madrid and south-east of Ávila, was built in 1955 and has a total storage capacity of 138 million cubic metres. It covers an area of 650 hectares, distributed longitudinally along a narrow valley in the vicinity of the San Juan pass.

The San Juan reservoir is used to supply water to the south-western part of the Madrid region, as well as to generate electricity. In addition to these uses, it also has a recreational and sporting function, which has made it a popular place for tourists and hikers.

It is the only reservoir in the Community of Madrid where bathing and motorised water activities are allowed, as well as other water sports such as water skiing and wakesurfing. The reservoir has a total of 14 km of beaches.

It has 2 bodies of dam, the main one (1) following the course of the Alberche river and the lateral one (2), which is the one that has been worked on.

Reason for the intervention

The dam had cracks with very obvious water seepage from the back of the dam where a car park is located.

The Confederation had received numerous warnings from individuals alarmed by the water leaks observed in the car park area.

Challenges and constraints

One of the main limitations of the work was the essential emptying of the affected area of the reservoir in order to be able to carry out the intervention. This meant that the work had to be carried out in cold weather, as the reservoir had to be emptied in December or January, when the volume of water is at its lowest.
Another major challenge was the size of the body of the dam: with its approximately 5,000 m2 it represented a challenge in the coordination of the execution, and had to be carried out in sections.

Gravity dam

The lateral body of the dam is a gravity dam. The stability of this type of dam is determined by the action of its own weight (P): the impounded water exerts a hydrostatic thrust (E) on the body of the dam which tends to make it slide on its base and, on the other hand, to make it overturn downstream. The action of the self-weight compensates the thrust and prevents these movements. This is achieved by placing the resultant (R) of these two forces inside the base of the dam.

Molins Solution

The work was planned to be carried out in two phases:
“In the first phase, joints and spalling were repaired with PROPAM® REPAR 40 class R4 structural repair mortar in accordance with UNE-EN 1504-3 and cracks and singular points were treated with BETOFIBER GLASS fibreglass mesh embedded between two layers of FLEXITEC.
“The second phase consisted of waterproofing the body of the dam by spraying two layers of FLEXITEC flexible two-component cementitious mortar with a certificate of suitability for contact with drinking water in accordance with RD 140/2003 and classified as C-(PI)-(MC)-(IR) in accordance with UNE-EN 1504-2.

FLEXITEC waterproof mortar was chosen in order to achieve a high quality waterproof and flexible coating that would allow for the movements in the substrate caused by the thermal cycles and the emptying and filling cycles of the reservoir. It was also necessary to use a material that could be applied by spraying due to the dimensions of the project.

Execution

“1. Preparation of the substrate with a pressurised water jet until the concrete surface is rough and sound, with no loose parts.

“Repair of cracks and spalling with PROPAM® REPAR 40 structural repair mortar. This fibre-added mortar is chosen because its setting behaviour allows repairs to be carried out during the central hours of the day when the highest temperatures are achieved, and to ensure that the mortar has set before nightfall.
“3. Treatment of cracks, cut joints, half-joints and singular points with the alkali-resistant glass fibre mesh BETOFIBER GLASS embedded between two layers of the waterproof mortar FLEXITEC.
“Application of the FLEXITEC flexible cementitious waterproof mortar by airless.
For the mixing of the mortar, no additional water addition is necessary as it is supplied in adequate quantities for the mixture.

The mixing is done by an automatic mixer on top of the dam. The hose of the spraying machine is suspended from the top and the spraying is carried out from a basket.

The application can only be carried out during the central hours of the day when the temperature of the substrate reaches +5ºC. When the mortar is applied, the substrate is damp to saturation and a first coat is applied by airless with an approximate consumption of 2 kg/m2 which leaves a layer thickness of approximately 1 mm.

The application is carried out in sections as the total surface of the dam is approx. 5000 m2.

The second coat is applied the next day, when the first coat is dry. Temperatures close to 0°C in the evening and at night slow down the hardening of the first coat and prevent the second coat from being applied earlier.

The final consumption of Flexitec was between 4 and 4.5 kg/m2 which left a total waterproofing thickness of approximately 3 mm.

Technical note

Flexitec:
Highly flexible two-component cementitious mortar. The liquid component is an aqueous dispersion of an acrylic resin that provides additional waterproofing and gives the mortar high flexibility, allowing it to bridge cracks and withstand movements due to thermal cycles.
The acrylic polymer, during the setting of the mortar, forms a film in the pores of the cementitious matrix forming resin bridges between aggregates, hydrated cement and substrate.