AbstractThis case history pertains to three horizontal directional drilling bores that were advanced under streams and are in close proximity to or beneath three bridges. During construction, optical surveys were performed to assess movements of the bridge structures, pavements, and other features above and near the bore alignment. The survey documented downward movements of as much as 19.8 and 12.8 mm heave, and 10.1 mm lateral displacement. The mud pressure in the bore was not measured, but the calculated minimum mud pressure needed for circulation varied from 32 to 219 kPa. The limiting mud pressures to avoid soil failure and unintended release of mud varied from 57 to 561 kPa. Differential movements in both vertical and lateral directions between points on the concrete deck, abutments, and wing walls of the three bridges were observed. Differential movements were also observed between points on pavements. During the advancement of the bore, the direction of the vertical movements fluctuated between heave (upward movement) and downward movement. The lateral movements in the N-S and E-W directions fluctuated as well along the N-S and E-W axes. Because some points moved in different directions at practically the same time, the differential movements in both the vertical and lateral directions exceeded the total. Published analytical relationships are summarized to establish a framework for predicting movements. While the validity of such relationships was not confirmed, as actual measured mud pressure was not known, the relationships seem to be approximate predictors based on the assumed minimum and maximum mud pressures required for circulation and the empirically determined strength properties. It is concluded that movement during construction is at least as critical to bridge performance as is postconstruction settlement. It is also concluded that to test predictive equations, actual mud pressures in the bore must be monitored. The collected data will be useful in future research on developing methods for estimating the movements and response of structures such as bridges to directional drilling.