Electrical Time Domain Reflectometry (TDR) and Optical Time Domain Reflectometry (OTDR) were utilized to monitor a zone suspected to have a slope stability problem on SR 690, 7.8 miles east of the city of Athens, in southeast Ohio. Two pairs of cables – one coaxial (electrical TDR) and one fiber optic (OTDR) in each pair – were installed at the suspected location. The cables were installed in pairs to facilitate a comparison of the results from the two methods. The data from the coaxial cables were acquired using TDR100 unit, whereas a Yokogawa AQ7275 OTDR system was used to acquire that data from the fiber optic cables.
After assessment of the electrical TDR and OTDR results and no evidence of movement of the slope at the suspected zone, the site was further investigated to determine the possible causes of the embankment failure. A comprehensive geostratigraphic profiling of the site was conducted using a cone penetration test (CPT). Weak layers of soils that are prone to consolidation, at depths of up to 6 ft, and layers of over-consolidated fine-grained soils, at depths of below 6 or 7 ft, were seen that potentially contributed to settlement of the road embankment and development of cracks in the pavement structure.
A new method of cable installation using a heavy-duty CPT truck was developed and practiced successfully in this study. The coaxial and fiber optic cables were pushed along with the cone rods by the hydraulic system integrated with the CPT truck. A disposable tip--unable to carry tension along the axes of the rods--for the cone rods was designed and built to stay at the desired depth of installation holding the cables after the cone rods are pulled out.