New types of time domain reflectometry sensing waveguides for bridge scour monitoring

Abstract

Scour is a major threat to bridge safety, especially in harsh fluvial environments. Real-time monitoring of bridge scour is still very limited due to the lack of robust and economic scour monitoring device. Time domain reflectometry (TDR) is an emerging waveguide-based technique holding great promise to develop more durable scour monitoring devices. This study presents new types of TDR sensing waveguides in forms of either sensing rod or sensing wire, taking into account of the measurement range, durability, and ease of field installation. The sensing rod is composed of a hollow grooved steel rod paired up with a metal strip on the insulating groove, while the sensing wire consists of two steel strands with one of them coated with an insulating jacket. The measurement sensitivity is inevitably sacrificed when other properties such as the measurement range, field durability, and installation easiness are enhanced. Factors affecting the measurement sensitivity were identified and experimentally evaluated for better arranging the waveguide conductors. A data reduction method for scour-depth estimation without the need for identifying the sediment/water reflection and a two-step calibration procedure for rating propagation velocities were proposed to work with the new types of TDR sensing waveguides. Both the calibration procedure and the data reduction method were experimentally validated. The test results indicated that the new TDR sensing waveguide provides accurate scour depth measurements regardless of the sacrificed sensitivity. The insulating coating of the new TDR sensing waveguide was also demonstrated to be effective in extending the measurement range up to at least 15 m.

A bundled time domain reflectometry‐based sensing cable for monitoring of bridge scour

“…As expected, it is difficult to identify the water/sediment reflection if not impossible. This is why previous studies relied on the total travel time analysis for calculating scour depth . As shown in Figure b, the measured waveforms are greatly simplified by simply changing the measurement direction from top‐down to bottom‐up.…”

“…Poorly graded quartz sands (particle size ranging from 0.1 to 1.0 mm) were then stepwise put in the acrylic tank at an interval of 0.1 m thickness up to 1.0 m. At each step, the corresponding TDR response was measured for further waveform analysis. The sensitivity of TDR measurement to scouring is reflected in the amount of travel time (Δ t ) change in response to the change in sediment thickness . For the equivalent amount of change in sediment thickness, the sensing cable with higher measurement sensitivity would result in a longer travel time (Δ t ) change.…”

“…The key component in this system is the scour sensing wire, which was composed of one stainless steel strand and one polyvinyl chloride (PVC)‐coated steel wire, in a way similar to TDR inspection of soil‐nailing works . The TDR scour sensing wire can be placed into a predrilled standard NQ or HQ borehole and grouted at the bottom . Above the grouted base anchor is the detecting section with the sensing wire and backfilled sands.…”

“…In the proposed case, the mediums include the PVC insulator and the surrounding sediments. The propagation velocity, effective dielectric constant, and travel time are related by the following equation: where V is the propagation velocity, K eff represents the effective dielectric constant, c is the speed of light (2.998 × 10 8 m/s), L is the actual section length, and ∆ t is the round‐trip travel time in that section. Because the change of ∆ t associated scour and water level change can be measured from TDR signal, water level and scour depth can be estimated once the propagation velocities in each medium section are known or properly calibrated in advance.…”

“…31 The TDR scour sensing wire can be placed into a predrilled standard NQ or HQ borehole and grouted at the bottom. 30 Above the grouted base anchor is the detecting section with the sensing wire and backfilled sands. The top of the sensing wire is anchored by a steel plate on the pier cap, bridge girder, or bridge slab.…”