Geotechnical study in Arratzua-Ubarrundia

In the municipality of Arratzua-Ubarrundia, substrates of limestone, dolomite and marble predominate, accounting for almost two thirds of the ground. These materials, which are of a compact nature, usually offer a high bearing capacity, which favours shallow foundation solutions where bedrock outcrops and is competent. The secondary presence of sandstone, conglomerates, flysch and, to a lesser extent, fine sands and silts should be considered only in specific areas; however, these lithological units generally do not negatively condition the bearing capacity at most sites. It is important to note that sand and silt layers may reduce the bearing capacity locally and, therefore, require detailed investigation if foundations are to be constructed on these materials.

With regard to additional geotechnical aspects, Arratzua-Ubarrundia has seismicity classified as high, with 200 events over 30 years, although the maximum recorded magnitude is moderate (Mmax 4); therefore, seismic action must be assessed in structural design in accordance with the applicable regulations. The fact that the dominant substrate is not clayey implies that there is no significant risk of expansivity, ruling out this problem for foundation safety. There are also no relevant karst phenomena in the area, so global stability depends mainly on correctly characterising the rock and on considering local seismicity as the main conditioning factor.