Building damage due to vibration from rock blasting

Construction activities such as blasting, piling, compaction, excavations, and construction traffic can produce vibrations of sufficient strength to cause damage to neighbouring buildings and structures. Therefore, many countries have national limit values for construction vibration in standards. However, building damages assumed to originate from vibrations are seldom observed. This may indicate that today's limit values are unnecessarily strict. In this field little newer research has been undertaken to scientifically observe the onset of cracking, and there is a particular lack of information about which role the frequency content of the vibration plays. In this study the onset of blast induced cracking was observed in two instrumented test structures located in a rock quarry. Two buildings were constructed, one in cast-in-place concrete without reinforcement and one made of lightweight construction blocks in expanded clay aggregate (LECA). The buildings were instrumented with geophones and Fiber Bragg Grating Sensors (strain sensors). In addition, vibrations on the ground surface and air blast overpressure were measured. Test blasts were designed to produce increasing vibration values, starting with peak particle velocities (PPVs) around 20 mm/s and ending with PPVs above 250 mm/s. No visible cracks were found on any of the two buildings. However, the last blast, which produced PPVs above 260 mm/s, resulted in a residual displacement of 0.05 mm across the 110 mm strain gage length above the door of the concrete building. The results of the test indicate that the limit values of most national standards include a large safety margin for buildings founded on rock. Further, the dominant frequency was determined by different methods and the results show a considerable deviation, with a distinct difference between methods which determine the frequency in a short time interval around the highest peak and methods which are using the entire vibration time series. In addition, methods which determines the frequency in short time intervals show a large spread in the frequency between the different vibration cycles.