SEISMIC TOMOGRAPHY TO DELINEATE KARST STRUCTURES BELOW A HIGHWAY BRIDGE FOUNDATION

摘要

Trouble-free transportation requires continuous maintenance of the underlying infrastructure.Most of it was built decades ago.Bridges are of special concern.They typically cross rivers or valleys.In most cases valleys result from geological discontinuities where weak rock was eroded and the remaining hard rock forms the sides of the valley.Furthermore,bridge foundations must carry the load and vibrations caused by the bridge itself and heavy traffic.Thus,foundations within a karstic geology require a high maintenance priority.We present results from a seismic tomography investigation carried out below the foundation of a highway bridge in Southern Germany.A regular inspection revealed that the water level of a near-by collecting pond dropped unexpectedly.Borehole logs and drilling information from decades ago showed the presence of weathered limestone and clay-filled cavities.To access the risk to the four bridge pillars near the pond a total of 20 boreholes,each about 20 m deep,were drilled surrounding the pillars to carry out seismic tomography.The work was carried out during night and over weekends when the traffic noise was reduced.About 20000 travel times on 42 cross-sections were determined and used for tomographic inversion below and outside the footprint of the bridge foundations.Inversion results indicate the presence of clay-filled cavities and agree well with borehole information.