Flood Fragility of a Cavity Wall

Abstract

Because of the fortunate absence of great flood events in The Netherlands during the second half of the twentieth century, no physical data was added to the structural understanding of flood fragility on walls. Considering that during the same period the cavity wall became standard in the construction of houses, a certain inconsistency can be recognized between the expected and observed (structural) behaviour of a modern wall during flood events. Nevertheless, failure, or collapse, of residential buildings is included in the Dutch flood protection standards as an important cause of damage and one of the main reasons for loss of life. It is generally believed that the developments in terms of building materials translate to a safer residence i.e. less prone to flood actions. This consensus is also found in the trade-off for certain mitigation measures, especially in areas where horizontal evacuation is difficult. An improved understanding of the flood fragility of a modern cavity wall is thus needed to prove the general consensus or to adjust the current mitigation measures to comply with the allowed fatality risks.

A window-featured cavity wall section was constructed at the Flood Proof Holland facility, using calcium silicate bricks, fired clay bricks, and a weaker mortar to partly account for the virgin effect of any newly-built wall. Both inner and outer walls were connected with adequate wall ties. The wall section was subjected to several hydrostatic pressures at both sides. These experiments were performed to physically grasp the deformations corresponding to certain flood scenarios affecting cavity wall sections from ordinary terraced houses. Additionally, the effect of the window was investigated; both on the stability and its contribution to the water height inside a residence.

Computations showed that the cavity wall in a one-way bending configuration starts to show significant cracks between 1.3 and 1.6 meters of outside water level. Because of the brittleness of the masonry, this would imply failure. It was further found that the non-linear deformations would reach 4 millimeters. Considering that for these water levels the internal moments were still far from their maximum capacity, the results suggest that failure occurred due to cracks that were forced to form because of the deformations. This indicates that modern cavity walls are still quite vulnerable for floods and their flood actions. The influence of the hydrostatic pressure, however, can be decreased considerably by a water level inside the residence that acts as a counter force. Contrary to what was expected, the window does not contribute to this inside water level, since its leakages turn out to be negligible. To keep the damages to a minimum and preserve the overall stability, it is advised to seal the residence to a height of 1.0 meter; floods that exceed this sealing should not be countered anymore and rather be allowed to enter the residence.