Geotechnical study in Ondarroa

In Ondarroa, substrates of limestones, dolomites and marbles predominate, together with a significant proportion of sandstones, conglomerates and flysch. This lithostratigraphic composition often gives the ground a good bearing capacity, although in areas where deltaic muds and peats outcrop it is essential to avoid these materials as a foundation base because of their low strength and high compressibility. Permeability is generally low, although shallow aquifers may exist in certain zones, which can influence ground stability and the most suitable foundation methods.

The seismicity recorded in the municipality is notable, with 94 events in the last 30 years and a maximum magnitude of 3.7; although it does not reach critical values, this justifies carrying out an analysis for sensitive buildings. In addition, the presence of karstic formations in the limestones makes it necessary to verify the existence of cavities or dissolution processes before designing any foundation, given the potential risk of subsidence. Since expansivity risk is ruled out due to the absence of clayey soils, the main geotechnical constraint for Ondarroa is verifying the possible karst associated with the carbonate rocks.