When blasting operations are carried out in urban areas, special care must be taken to avoid any serious damage t o nearby structures, such as buildings, tunnels, etc.
In order to assess the safety of the nearby structures, it is essential that their dynamic behaviour under blast excitation firstly be investigated. For this purpose, some field investigations were performed. This paper presents the results of these investigations, and some discussions are given on revealing the dynamic behaviour of the structures.
When blasting operations are carried out in an urban area for excavating in rock to build various types of underground structures, such as railway/road tunnels, water supply tunnels, sewage treatment plants, car parking facilities, etc., it is of great importance that the magnitude of vibrations to the existing nearby structures be evaluated, and if necessary, that special steps be taken t o avoid any serious damage.
In order to assess the safety of nearby structures, it is essential that their dynamic behaviour under blast excitation firstly be investigated. For this study, some field investigations were performed on a five-story reinforced concrete building, a two-story wooden house and a concrete tunnel lining, and their vibrations under blast excitation were measured. Particle velocities, in particular, were focussed upon.
This paper presents the results of the above case studies, and some discussions are given on revealing the dynamic behaviour of the structures.
Field investigations were performed by measuring the dynamic response of a school building to blasts fired at the construction site of a subway tunnel located underneath the building's foundation. The plan and section views of the test site are shown in Fig. 1. The school building is a five-story reinforced concrete structure and its foundation is located directly on a rock base, except a t the middle part of the south side of the building where 4-m long piles are installed. The subway tunnel is located a t a depth of approximately 50 m beneath the building, and in the plan view, the tunnel crosses almost at right angles t o the longitudinal axis of the building, as shown in Fig. 1 (a). Dynamic responses of the building to blasting were recorded when the face of the pilot tunnel (excavated prior t o the main subway tunnel) was located almost directly underneath the building's foundation.
Time-delay shots consisting of fifteen stages were used. The total amount of charge was approximately 6–7 kg and passed the tunnel face by about one meter.
The underground medium around the test site consists of slightly weathered granite and the propagation velocity of the longitudinal wave was approximately 3.0 km/sec. Instrumentation It is known that the frequency of ground vibrations caused by blasts generally falls in the range of 10–300 Hz, which may coincide with the higher natural frequencies of building.
(figure in full paper)
