oa Application Of Gpr And Seismic Techniques To Investigation Of Bridge Abutments

A combination of ground penetrating radar (GPR) and seismic<br>transmission delay was used to determine the thickness of stone<br>blocks making up the abutments of a bridge crossing trolley track<br>of the Massachusetts Bay Transit Authority's (MBTA) Red Line in<br>Dorchester, Massachusetts. The work was performed for Barnes and<br>Jarnis, Inc. and the MBTA to assess the adequacy of the bearing<br>strength of the abutments for a planned new bridge deck.<br>GPR data were obtained on both abutments with a 500 MHz system<br>on horizontal traverses at 3 to 5 foot intervals, on vertical<br>traverses at 5 foot intervals, and on single blocks. The GPR<br>survey encountered three types of complications: (1)<br>interpreting multiple reflections of the radar signal from the<br>backs of irregular blocks; (2) differentiating signals reflected<br>from the sides of blocks from signals reflected from the backs of<br>blocks; and (3) limited penetration of the radar signal in the<br>central portions of the abutments due to infiltration of salty<br>water from the roadway above.<br>These difficulties were resolved with a second, independent<br>survey method. A 12-channel seismograph was used to obtain<br>seismic data for one of the abutments. The geophones were<br>attached to the face of the abutment and seismic signals were<br>generated by a source 30 to 80 feet behind the wall. Two seismic<br>profiles were obtained with geophones attached in horizontal<br>lines and four profiles with geophones attached in vertical<br>lines. Differences in transmission times were used to estimate<br>total thickness of the stone blocks.<br>Using the combination of GPR and seismic transmission delay<br>techniques, we found the thickness of the stones near the top of<br>each abutment to be approximately 5 to 6 feet, increasing toward<br>the bottom to a thickness of approximately 10 to 12 feet.