Structural assessment of existing buildings subjected to induced earthquakes in the Netherlands

The Groningen area, in the Northern Netherlands, has been characterized by the extraction of natural gas since the 1950’s. The drop in pressure that usually occurs during this process resulted in the registration of seismic events, which appeared to be increasing both in terms of regularity and intensity over the last years. The induced earthquakes occur at a shallow depth, of around 3 km, and this characteristically results in high peak ground acceleration (PGA) values. During the period before 2000, the Netherlands did not take any earthquake engineering measures into account for the design of new buildings, or for the strengthening of existing ones, but since the earthquake of Huizinge in 2012 everything has changed. The number of buildings affected by the induced seismic events, coupled with the resulting social and cultural impact and their consequences on the country, is very high. Therefore, the need for the investigation and study about the seismic behaviour, and the knowledge of the maximum allowable seismic load of existing building in the region is growing more and more. In this regard, the challenging aspects are numerous and one of the largest problems is related to the construction methods employed in this area making use of very thin cavity walls. These walls, with thickness of only 100 mm for the inner as well as the outer leaves, often have poor and heavily corroded connections. Another important issue to address is represented by the influence of the sub soil layers, consisting of several weaker clay layers underneath poor strip footings, in the structural behaviour of existing buildings. An integrated modelling procedure taking into account the interaction between soil and structure is described, and numerical analyses, involving dynamic and non-linear procedures, are performed with DIANA focusing on the seismic performance of the buildings. © 2016 Taylor & Francis Group, London.