Structures constructed by man such as dams, building, bridges, etc are subject to loads which can deform the structure and can lead to failure. A deformation survey is a survey to determine if a structure or object is changing shape or moving. The three-dimensional positions of specific points on an object are determined, a period of time is allowed to pass, these positions are then re-measured and calculated, and a comparison between the two sets of positions is made. Deformation surveys are performed at periodic intervals to monitor the stability of structures and to give early warning of any unexpected behavior. Terrasurv has performed more than 350 deformation surveys on more than 40 locks and dams operated by the US Army Corps of Engineers. These surveys are custom designed for each structure to best monitor potential movements and take into account the type of structure (concrete, earthfill) as well as the past history of each. A typical monitoring project consists of a site visit to determine existing conditions and to plan the network. Pre-Analysis of each structure is preformed using the simulation function in least squares programs to design the most economical network which meets project specifications. Typical design accuracies are ±3 mm for concrete structures and ±1 cm for earthen dams. This information is then used in the office to plan a scheme of observations that will meet the required accuracy. Next, the field observations are performed. This data is then processed and adjusted in the office to yield data that can be used as a baseline for future observations or to compare against previous surveys.
Another type of deformation survey is the monitoring of ground subsidence above mining operations, such as salt and coal mines. These surveys are done using a combination of precise leveling, high accuracy traverse/triangulation, and GNSS Surveys.
Example Project: Braddock Locks and Dam: