Photogrammetric Joint Mapping for the Relocation of T.H. 53
The Minnesota Department of Transportation (MnDOT) relocated a portion of T.H. 53 between Eveleth and Virginia, MN. The preferred alignment crosses the existing Rouchleau iron ore pit on a 1,100-foot bridge. Two abutments and one pier of the bridge will be founded on bedrock. Future iron ore mining adjacent to the alignment will create rock slopes up to 500 ft high. After mining, the new roadway will be atop a trapezoidal cross-section 300 ft wide at the top and with downslopes of about 53 degrees. MnDOT wished to ensure that the future rock slopes will be stable. These factors require characterization of the rock discontinuities present, including bedding, jointing, and faulting. The lateral extent, difficult access, and ongoing sloughing of the pit walls prevented hand-on mapping.
Photogrammetry field work involved one day for site characterization and one day for flights. Joint mapping was completed using JMX Analyst, the software module used for virtual mapping of the structural features. The joint surfaces were easily identified in the software's interface, and point measurements, joint traces, and polygonal surfaces were captured directly on the 3D image. Once joint identification was complete, stereonets provided joint orientation analysis and clustering of the joint sets. Joint spacing, large-scale roughness, and continuity analyses were conducted using the geometry data from JMX Analyst.
The extensive joint set data developed by these methods were invaluable in understanding and characterizing the rock discontinuities. Among the greatest benefits, solely from the large number of features logged, is confirmation that a joint set, on average, dips to the southwest at about 79 degrees. The southwest dip of this set leads to a flexural toppling mechanism of slope failure that is not possible for vertical jointing. This dip was confirmed when joints were exposed during excavation for the east abutment and pier.