REMR TN CS-ES-2.28/87

COSTS : The items required and their approximate cost are:

Zeiss N12 level $2,000Zeiss autocollimator 1,000Optical vernier 2,400Mirror installation (es) 175

Total $5,575

Mirror installations are permanent. Therefore, the total cost for a specificsite will be dependent on the number of mirrors required. The level, auto-collimator, and vernier can all be stored at a central location and used at anumber of sites.

FIELD PERFORMANCE: This system has been successfully tested at sites inseveral Corps Districts.

BACKGROUND: Normally, tilt measurements of structures are made throughinstallation of a plumbline, an inverted plumbline, or an optical plummet.Tilt is inferred from the displacement and length of the plumbline. Theseinstallations are expensive, and normally only a few monoliths within a struc-ture would be instrumented. The method of tilt measurement outlined in thereference uses modern survey equipment combined with some additional opticalequipment to reliably measure long- or short-term tilt of a structure to~2 Seconds of arc.

DESCRIPTION: The equipment used for this method of tilt measurement is anautomatic level, an autocollimator, an optical vernier, and a mirror. Themirror is used as a sight and is mounted to the wall of a monolith or, forshort-term measurements, could be mounted on a surveyor’s tripod. The auto-collimator will attach to the eyepiece of the automatic level, which will bemounted on a tripod. The automatic level maintains the projected line-of-sight normal to the gravitational force from one measurement to the next. Theautocollimator is a device that projects a collimated beam of light onto themirror. The reflected light re-enters the autocollimator, and the differencein angle between the projected light and reflected light may be measured. Bypassing the line-of-sight through an optical vernier, the displacement of theprojected light rays can be measured precisely.

REFERENCE : An instrument to monitor the tilt of large structures.K. Robertson. US Army Engineer Topographic Laboratories,Ft. Belvoir, VA, Apr 1983. Technical Report ETL-0313.