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El Mayés Reservoir

Territorial Framework and Management.

Environmental Information

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Geology and Seismology


Regional Geology

El Mayés Reservoir is located between the Sierras de Ricote and Cajal. The area corresponds to an anticline with a Triassic core, partly diapiric, featuring great masses of sandy limestone and loams from the Miocene epoch. Mixed with the Triassic soils, some loams and limestone probably from the Eocene can also be found.

To the north-west of the area lie the mountains of Sierra de Ricote, mainly consisting of limestone from the Jurassic period. The Sierra de Cajal forms the northern side of the great syncline in Miocene soils that stretches toward the South and whose basal sandy limestones, here very thick, rise vertically. Two other small synclines from the Miocene epoch appear as faulted to the south of Ricote.

The great masses of sandy limestone and loams from the Miocene epoch are cut by large faults, along which the Triassic in diapiric form sometimes appears. Special mention must be made of the faults on the northern side of Sierra de Cajal and the Rambla del Carcelín, of great longitude, stretching westwards and nearly reaching the Albacete-Murcia railway line (km 435). Through them, a strip of diapiric Triassic has risen, which, to the south of Villanueva del Río Segura, has overflowed onto the Miocene loams. This fault has significantly influenced the Miocene thicknesses, much thinner to the North.

Following the depositing, folding and erosion of the materials from the Triassic, Eocene (or Cretaceous) and Miocene periods, some not-very-consolidated sediments were deposited, with gentle horizontal slopes, from a recent age (Quaternary or Pliocene) that form a great alluvial fan. These alluvial fans are formed by erosion materials from the Ricote and Cajal mountains and have been deposited with gentle slopes over old erosion levels, very flat, in loamy soils. These erosion levels in the loams have consequently eroded (due to the descent in the base level) into narrow ravines that have pushed out the unaltered loams.

Geology and Geotechniques of the downstream boundary.

El Mayés reservoir is located in the wadi of the same name, which is made up of loams from the Miocene, which, in this area, have the same directions (ENE-WSW), parallel to the riverbed and dips to the South of about ten or twenty-five degrees (10º-25º), although not very visible.

The loams appear intact on the steep slopes that have recently been eroded by the river and are compact loams which are quite calcareous (55-60% of CaCO3) with conchoidal fracture. On the gentle slopes, intact loams cannot be seen, but their fluvial material can, consisting of yellow-ochre silty clays of medium to low plasticity. The thickness of this fluvial material is between one (1.00) and three (3.00) metres.

On the left embankment, over old levels of erosion of the loams, deposits appear with gentle slopes of gravel and sand, poorly cemented and quite permeable. Lastly, in the riverbed there are deposits of gravel, sand and silt sometimes covered in superficial white powder, produced by the salts swept along by the water.

The downstream boundary is totally impervious, with the exception of the possible presence of some vein of gypsum and the existence of Quaternary deposits on the left embankment, below the area of the spillway.

Geology and Geotechniques of the Storage Area Upstream of the Dam.

This area is formed by the same materials as the natural downstream boundary for the storage area, that is to say, by loams with altered surface areas, gravel, sand, silt and clay in the low areas and alluvial fan debris or terraces at the top of the platforms, mainly on the left embankment. Its structure is the same as that of the downstream boundary, i.e., the loams are parallel to the wadi and with gentle dips to the South. The same as the downstream boundary, the storage area upstream of the dam is totally impervious and the possible appearance of some vein of plaster does not present any significant problem.

When the location of the dam and its maximum reservoir level were defined, a survey was made of the reservoir surface which was going to hold the water. This survey concluded that the reservoir basin was completely impervious, although some areas of permeability were detected in a side valley and on the left abutment of the dam. Permeability tests were made regarding the atmospheric pressure - drill length - , and the pressure sustained by means of a pump. The latter test was carried out after having advanced every 5.0 m, and so the possible waterproofing that could be generated by the detritus caused by the drill when filling the small fissures was very limited since the water injected under pressure cleaned them out. The result of the permeability tests determined that the fissures were very deep, and so cement was injected.


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