60 N. Christie et al. 7.4 Vibration Survey Vibration surveys enable the vibration performance of a site to be evaluated directly and are a key aspect of the development of technical building projects from the planning stage right through to commissioning. In this paper, attention is focused mainly on survey work at the planning stage where a proposed project site requires evaluation. The site is in an urban location with a main highway to the east and west of the site boundary and with a smaller road to the south [4]. This borehole is at a location that will be indicative of the future characterisation suite performance. The instrumentation at a typical measurement location is shown in Fig. 7.4 and comprises a steel canister (see inset image) placed at the base of a borehole in the ground and containing a tri-axial accelerometer arrangement connected to the data acquisition system. In this way vertical and horizontal ground vibration, at the building foundation elevation, is measured over a 24 h time period. The vertical vibration signal output from this test is shown in Fig. 7.5. The main source of vibration is highway traffic comprising cars, vans, trucks and buses and is generated by the interaction of vehicle dynamics with highway surface irregularity. The two most common metrics, namely average and maximum would be reported in most if not all projects. However, average would not typically be selected unless consultation with the user and manufacturer established that this would give a good indication of the vibration that might be disruptive in research practice. The maximum is by definition the worst case and needs to be evaluated and reported, however, in cases where there a very few instances in time when this occurs it can be less useful. The exceedence metric is a useful way of expressing temporal vibration performance and the 1% exceedence vibration is shown in Fig. 7.4 and indicates performance of VC-E compared to VC-D using maximum. 1% exceedence as a metric needs to be agreed with the project in the context of the equipment and methods being used but it typically means that VC-E would be exceeded for just a few minutes in each day and may be acceptable. In cases where image capture is not a process that can be quickly and easily repeated then several minutes of exceedence might be a concern. Fig. 7.4 Borehole vibration survey
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