A method for estimating the accuracy of tunnel convergence measurements using tape distometers

摘要

Any measurement should be known with the error it involves, because the monitored physical quantity has changed only if the amount of change is larger than the error. Error usually refers to the accuracy of measurement, defined as "the closeness of approach of a measurement to the true value of the quantity measured". A procedure to evaluate the accuracy of convergence measurements by using a distometer is proposed in this work. It estimated the accuracy in changes of line lengths as small as 0.08 mm, usually closer to 0.25 mm. This accuracy took into account only the random errors, because measurements with gross errors were previously recognized and classified as suspect, and systematic errors due to calibration and temperature changes were evaluated and corrected separately. The systematic errors resulted in being close to 1 mm each, making calibration and temperature corrections important for performing accurate measurements. The procedure applied for both three- and five-bolt sections because the triangular meshes were not needed as for the method using the control length c. Moreover, by using the control length c the measurements seemed to have a worse accuracy, even though systematic errors due to calibration and temperature were taken into account. It should be noted that the c-method supposes that all bolts lie on a plane. The measurements with the distometer for 584 lines took eight days and two workers. This technique could seem time consuming and uncomfortable, especially if compared to the use of the Total Station or the digital camera. On the other hand, its accuracy is better, having generally half the error declared for the other two techniques. This means that measurements can be repeated more frequently, and the mechanism of deformation is known sooner.