It is common to see banks or inclines on the roadside or at the side of railway cuttings. They are formed when a side of a mountain or other earth feature is cut through for the construction of various infrastructure. As they are unstable zones, many problems are created and, in the case of the Basque Country, the problems are aggravated by a number of factors: the complex orographic terrain, the high rainfall and the area being abundant in sedimentary material.
Risk and consequences
Instabilities arise on rocky banks due to the geomechanical features of the rocky mass, because of the state of conservation of the slope itself, and due to the conditions of water penetration.
Once the movement of rocks has started, the key factor in the movement is the inclination of the bank. Also, the size of the incline influences the probability of rock falls. Why? Because the higher and longer the slope, the greater the rock mass under risk of rock fall and, in the case of a landslide occurring, greater will be the damage done.
It should be taken into account that some locations have the material of the banks pointing directly towards the road or railway, due to the fact that it had this orientation originally within the rock mass. In other cases, the rocks are pointing in another direction and so do not present such a risk and any rock fall will probably not reach the road or the rail tracks.
The aim of all these geotechnic measures is to identify, control and correct elements that might be unstable. To do this it is necessary to establish protocols or programmes of action. By means of these programmes, the improvements that may be obtained with corrective measures are evaluated and the problems registered thus resolved.
Concretely, the group of hydrogeology-geotechnics at the University of the Basque Country has developed a programme based on 10 parameters: classification of rocks (classification undertaken for the rock masses in the Basque Country), orientation of cracks, hydraulic conditions, facility of meteorisation of the rocks, history of the incline or bank, volume of the unstable blocks/mass, the height of the bank, the likelihood of the rocks reaching the road, the distance of vision vs. the distance of reaction and the importance of the road. Through these parameters, the stability of the rocky bank and the damage from a rock fall estimated. Thus, 8 classes of rock are differentiated and information provided on the features and behaviour of the bank.
Finally, corrective measures are proposed for each type of rock. Amongst these measures is the use of steel bars as tensors, covering the slope with concrete, placing dynamic nets to catch the rocks, etc. In the last analysis, the aim of all these studies is to propose suitable corrective measures for each case and, thus, to minimise risks.
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