Evaluation and comparison of different detection technologies on simulated voids near buried pipes

Highlights

• Assess performance of different technologies for detecting erosion voids near pipes.

• Compare void detection results for these technologies.

• Give recommendations for selecting the most appropriate technologies.

Abstract

Erosion voids near buried pipes may reduce the service lives of buried infrastructure assets and present a concern to municipalities as they are difficult to detect accurately and effectively. Various technologies exist to detect erosion voids but limited testing has been performed to evaluate their effectiveness. In order to evaluate and compare different existing erosion void detection technologies, a blind experiment (i.e., without operators having prior information about void locations and geometries) was conducted. Two kinds of pipes were used for the evaluation: a corrugated steel pipe and a reinforced concrete pipe. A total of five artificial voids were made near the pipes. Four commercially available void detection technologies were evaluated: (i) conventional Backscatter Computed Tomography (BCT), (ii) handheld BCT, (iii) Ground Penetrating Radar (GPR) and (iv) Pipe Penetrating Radar (PPR). Additionally, Infrared Thermography (IRT) was also employed by the authors although this was not a blind study. Each technology and the data provided by that technology are presented. Based on the results, Handheld BCT and IRT were recommended for initial detection of the voids near corrugated steel pipes. To obtain higher accuracy and geometry detail, conventional BCT and PPR were suggested for the corrugated steel pipe and concrete pipe, respectively. GPR was able to detect a large void simulated between the two test pipes.

Introduction

Erosion voids can develop near pipes due to leaks at the joints or holes due to corrosion of the pipe, and can result in loss of soil support to the pipe. Loss of soil support can result in excessive deflection, shape distortion, joint opening in flexible pipes, and increase of moment in rigid pipes (Moore, 2008). For example, Tan and Moore, 2007, Meguid and Kamel, 2014, Shaver and Moore, 2021 have studied the relationship between geometries of the voids near pipes and moment change in rigid pipes using finite element analysis, and quantified how moments in the pipe increase as the contact angles between the void and pipe increase, potentially causing the pipe to crack. Moore (2008) introduced a Soil Deterioration Index (SDI) to capture the relationship between the increase of deformation of the pipes and the decrease of soil support around the pipes. El Taher and Moore (2009) investigated the impact of backfill soil erosion on corrugated metal culvert stability against buckling. The work demonstrated that increases in the erosion void volume (or culvert perimeter without soil support) and the extent of invert corrosion led to a decrease in elastic buckling strength. Recently Peter et al., 2018, Peter and Moore, 2019 conducted full-scale experiments to simulate erosion voids near a reinforced concrete pipe and corrugated steel pipe, respectively, and measured strains around the pipe using optical fiber strain sensors. In each of the studies, the relationship between increasing size of the erosion void and increasing stresses in the pipe structure was demonstrated, indicating the need for accurate approaches for detecting voids and measuring their extent. The formation of erosion voids can also lead to ground collapse that endangers stability of infrastructure above the pipe. For example, Davies et al. (2001) reported that the development of defects in a sewer pipe allowed the migration and wash-out of the backfill material, resulting in the loss of ground support and excessive pipe deformation, which eventually caused the pipe to fail. Peter and Moore (2019) quantified how a large void reduced the load capacity of an unpaved roadway above the culvert below its design requirements.

However, it is difficult to detect and characterize erosion voids in the supporting soil envelope around pipes. This is because conventional pipe inspection methods are primarily based on visual inspection, meaning that anything that cannot be seen is often missed completely or not characterized accurately. The method cannot provide the size of an erosion void, and may even fail to note the possible development of voids forming in the vicinity of pipe joints. Various researchers have employed different technologies to detect and characterize erosion voids behind pipe walls, including Backscatter Computed Tomography (BCT), Ground and Pipe Penetrating Radar (GPR and PPR) and Infrared Thermography (IRT).

Using backscattered radiation, McCormick et al. (2015) reported that the image produced permits verification whether it is soil or a void behind the pipe and visualisation of the void dimensions and shape. Anderson and Bowles (2012) conducted a pilot project using BCT to quantify undermining of several culverts in the City of Toronto and the results showed that BCT was able to quantify voids behind a culvert wall and provided essential information for further replacement or rehabilitation. Another project was completed by McCormick et al. (2015) using BCT to assess supporting soil conditions around a corrugated metal storm-water pipe. The work again verified the feasibility of utilizing BCT to image erosion voids behind corrugated metal pipe walls. However, to date the system has not been trialled in a situation where the exact extent and location of the void next to a pipe was known, allowing for the accuracy of the technology to be determined and compared relative to other technologies.

The performance of GPR to detect subsurface anomalies is determined primarily by three factors: (1) the speed of degradation of radar energy, (2) the size and depth of the anomaly, and (3) the degree of contrast in electrical properties between the anomaly and the surrounding material (Dawson et al., 2016). The penetration depth of the electromagnetic wave is dependent on the frequency of the transmission antenna selected. In addition, other factors can decrease wave penetration depth and/or the resolution of the radar image, such as existence of water dissolved by salts and other minerals (which increases the electrical conductivity of soils) and pavement (e.g. asphalt or concrete). Therefore, the frequency of the transmission antenna must be carefully selected to produce the highest resolution possible for a given subsurface condition within the target penetration depth of the survey (Dojack, 2012). Although Chen and Scullion, 2008, Xu et al., 2010, Cassidy et al., 2011, Kofman et al., 2006, and Jeng and Chen (2012) have employed GPR to detect voids directly beneath pavements, inside dams and reinforced concrete sections, the authors know of no studies where erosion voids around buried pipes were detected using GPR from the ground surface.

Pipe penetrating radar (PPR) is the in-pipe application of GPR for nonferrous pipes. PPR has been used to detect and characterize erosion voids in the soil envelope around the pipe in several studies. For example, Crowder et al. (2011) undertook a pilot project to develop a multi-array PPR inspection tool to locate voids around large diameter sewers (i.e., greater than 1500 mm in diameter) in the City of Hamilton. The study demonstrated that using PPR as an inspection tool can help municipalities determine if voids are present behind pipe walls, a result which had never been achieved using traditional inspection techniques. Dawson et al. (2016) conducted PPR scans along the inner crown and select locations of the upper interior liner surface in the Tiawah Tunnel in the City of Tulsa. Fifty-seven potential voids, extending up to 19 in. in depth and 12 feet in width, were detected with 1000 MHz PPR and confirmed through probe drilling with approximately 80% accuracy.

Infrared Thermography (IRT) is a non-destructive testing technique that measures infrared radiation emitted from objects with temperatures above 0 K (i.e., −273 °C) (Kouridakis, 2007). In general, good backfill has the least resistance to energy conduction, poor backfill has increased resistance, and air-filled voids allow essentially zero conduction. Therefore, a temperature difference should be apparent in regions with different backfill conditions. Weil (1995) conducted field tests using IRT directly on the pavement surface above underground sewers both during the day and at night to find temperature anomalies. Fourteen truth borings were also drilled within the project area to compare with the IRT results. The work indicated that IRT could detect the voids with a success rate of 80%. Voids of varying sizes, from a few centimeters to more than a meter, were discovered using IRT. No study has been reported where erosion voids around the pipe were detected by applying IRT on the inner surface of the pipe.

To the authors’ knowledge, no study has been conducted to assess and compare the performance and accuracy of these technologies. In addition, the performance of various inspection technologies designed to identify erosion voids around pipes has not been evaluated nor have the various technologies been compared. Consequently, the investigation presented in this paper aims to provide an evaluation and comparison of the performance of these technologies for detecting erosion voids. In this study, a testbed involving two buried pipes (a reinforced concrete pipe with 1.2 m internal diameter and a corrugated steel horizontal ellipse culvert with a span of 1.6 m and a height of 1.35 m) with prefabricated erosion voids of known dimensions next to them was set up in a pit within the Geoengineering Laboratory at Queen’s University. Backscatter Computed Tomography (BCT), Ground and Pipe Penetrating Radar (GPR and PPR), and Infrared Thermography (IRT) were applied for detecting and characterizing those voids. It is the first time that the effectiveness and accuracy of existing void detection technologies has been evaluated under controlled conditions, and the results and comparisons between technologies will help utility owners improve their asset management of their sewer and culvert systems.

The objectives of this paper are to (1) assess the performance of BCT, GPR, PPR and IRT for detecting the simulated erosion voids and (2) compare the void-detection results for these technologies and give recommendations for selecting the most appropriate technologies. The next section provides details of the experimental setup then the testing procedure, and results of the void detection surveys for each technology are described and comparisons are discussed.