Comparison of Ground Vibrations vs. Frequencies to Strain Rates from Blasting Operations at the Tennessee Capitol Corridor in Nashville, TN

摘要

The Tennessee Capitol Corridor connects the Cordell Hull Building (CHB) to the State Capitol, which isa 232-foot-by-124-foot (71 × 38 m) structure in Nashville that includes porticoes built from local massivelimestone. This connection was effected by the Capitol Connector Pedestrian Tunnel (CCPT) project,which is part of the overall Cordell Hull State Office Building and Central Services Demolition, NewConstruction and Renovation Project (Cordell Hull Renovation Project). To construct this connection, thetwo existing elevator shafts and a new emergency stairwell in the State Capitol were extended down to anew lobby area—the elevators by about 50 feet (15.2 m) and the stairwell by about 60 feet (18.3 m). Thislobby was connected to the CHB by a 435-foot-long (133 m) pedestrian tunnel. It was predicted that mostof the tunnel would be excavated in a relatively strong limestone; therefore, considering the projectfeatures and constraints, drill-and-blast techniques were selected as the primary excavation method.Because of restrictions on blasting in urban areas and, more importantly, the proximity of this constructionto national historic landmarks, blasting operations were extensively monitored to measure and correlateground vibrations as peak particle velocity (PPV), frequency, and strain—these being the major factorsthat would impact the surrounding buildings. As a result, copious amounts of data were gathered andinterpreted within a short time to estimate blasting parameters and optimize the subsequent blast rounds.To calculate the ground response factor, the scaled distance of each shot was compared to the groundvibration as PPV, generating a subsequent regression curve with an R~2 value of about 0.7. Using the bestfit curve equation, the estimated ground response factor was about 150 for the best fit line. Other thansome cases with relatively high (> 2 in./sec [51 mm/sec]) recorded PPV values, the blasting operation wassuccessfully conducted as planned and the observed strain on the monitored buildings was very low,mainly within ±2 με (0.0002% elongation). (In general, the recorded PPV was lower than 2 in./sec atfrequencies of more than 50 Hz.). The strain gauges at key locations (cold joints and old fractures) in theCapitol and on other critical structures demonstrated strain was insignificant in relationship to groundvibrations from the blasting operations. The apparently small vibration-induced strains were related to themassive limestone construction of the Capitol and ancillary buildings, as the induced strain plus theexisting strain did not exceed the critical strain of the massive structures.