Combined Acoustic and Geotechnical Scour Measurements at Bridge Piles in the New River, Virginia

Nina Stark, Dennis Kiptoo, Reem Jaber, Julie Paprocki and Samuel Consolvo

Charles E. Via, Jr., Department of Civil and Environmental Engineering, Virginia Tech, 750 Drillfield Drive, Blacksburg, VA 24061, USA; e-mail: [email protected]. Corresponding author.

ABSTRACT

Scour assessment and monitoring can be time- and cost-intensive. In this preliminary study, the combined use of acoustic and geotechnical in-situ instruments was demonstrated through a graduate class field experiment at the Vietnam Veterans’ Memorial Bridge crossing of the New River in Fairlawn, Virginia. Acoustic Doppler current profiling was conducted for flow velocity measurements, chirp sonar was deployed for acoustic sub-bottom scanning, and rotary side scan sonar was used for acoustic river bed imaging. A portable free fall penetrometer was deployed to estimate geotechnical properties of the river bed surface sediments, and sediment samples were retrieved. A scour hole was clearly detected and measured in depth. Differences in surficial river bed stiffness were mapped, and boulders and were detected. Through the combined use of acoustic and geotechnical method, some key information relevant for scour assessment were determined within few hours and without larger support infrastructure or divers.

INTRODUCTION

Scour focused site investigations at bridge piers represents a time- and cost-intensive task for bridge managers, owners, and surveyors. About 84% of all bridges within the United States stretch over waterways, and thus, require scour assessment and mitigation (FHWA 1989). The quality and reliability of scour prediction and risk assessment highly depends on the availability of high quality scour observations (Melville 2000; Briaud et al. 2007, 2013). Therefore, despite numerous studies on the development, enhancement, and evaluation of scour observation methods, there is still a need for improvement, specifically regarding an optimization for site specific conditions and needs. Advances in subaqueous sensors may offer novel pathways for scour investigation. The offshore industry has demonstrated novel capabilities through the combination of geotechnical and geophysical site investigation methods, representing nowadays a standard in offshore site investigation for projects involving the seabed (ISO 2014). This study represents a demonstration of a combined use of acoustic sensors and geotechnical measurements to conduct scour measurements at bridge piles at the Vietnam Veterans’ Memorial Bridge of Virginia State Route 114 crossing the New River in Fairlawn,

– 1 – Fig. 1. Google Earth satellite image from October 03, 2017, of the Vietnam Veterans’ Memorial Bridge of Virginia State Route 114 in Fairlawn, Virginia, crossing the New River. The river flow direction is to the North. White water visible south of the bridge indicates rocky shoals. At the southwestern part of the bridge a small boat ramp allows access to the river. The center of the bridge is approximately located at N37.16173° and W80.55197°. Virginia (Fig. 1). This section of the New River is fairly wide (~ 100 m wide) and shallow (on the order of meters depending on water level). Some rocky shoals are located near the bridge, and flow velocities can vary strongly depending on stage. The nearest flow gauge located in Radford, Virginia, (USGS 03171000) has recorded discharges ranging from about 20 m3/s to 1700 m3/s throughout 2019 and measured gauge heights from < 0.6 m to > 2 m from September to December 2019 (no data was available for earlier in 2019). It follows that the river can only be navigated by vessels with little draft. Therefore, methods to be considered would allow measurements from small vessels and during medium flow conditions. Methods were explored that would neither require support through divers nor measurements from the bridge, since both would restrict surveying time, increase surveying costs, and may represent increased risks for the surveying team. The study represents a preliminary test of the combined use of highly portable geotechnical and geophysical devices and was carried out in the framework of a graduate class field experiment. The bridge was chosen for reasons of site conditions (representing features typical for many bridges), access, and known susceptibility to scour based on communications with the Virginia Department of Transportation (VDOT) who also enabled site access and supported team logistics.

– 2 – METHODS

The study was limited by the following restrictions: An approximately 12-foot-long skiff (~ 4 m) with outboard engine but without winch or other infrastructure supportive in lifting/lowering equipment was provided by VDOT for the measurements. The measurements needed to be completed within one work day. Finally, instruments should allow safe deployment from this vessel and provide useful information for scour assessment.

Acoustic surveying instruments. An acoustic Doppler current profiler (ADCP) was used to measure flow velocities near the bed. Here, a Nortek AquaDopp HR was deployed mounted to a weighted platform that was lowered to the river bed (Fig. 2). In the mode used, the device has a blanking distance of 9.6 cm and measured a profile of 90 cm in length. This way the device measured flow velocities along a vertical profile at a distance from about 20 cm to 110 cm above the riverbed with 3 cm resolution. It was assumed that the river flow conditions would remain approximately constant throughout the deployment time. Chirp sonars transmit a frequency-modulated acoustic sound pulse of which the different frequencies penetrate into the riverbed or seabed to derive information on soil stratigraphy from the reflected signals (Schock and LeBlanc 1990). Chirp sonar has been applied in riverine environments mostly for geological studies and the investigation of depositional dynamics (e.g., Klingbeil et al. 2005). It has also been utilized to investigate fluid mud dynamics in estuarine environments (Schrottke et al. 2006). With regards to scour, chirp sonar may provide a pathway to address the issue of scour hole infill. Scour infill can bias post-hoc scour hole measurements, but chirp sonar could provide insight into maximum scour hole depth despite infill by distinguishing the infill from the stable substratum (Placzek and Haeni 1995). However, shallow water depths and small vessels can be a challenge for chirp sonar systems. In this study, the SyQwest StrataBox HD chirp sonar was tested as the system is highly portable. The transducer (speaker-like cone of ~ 40 cm diameter) was mounted to a pole that was clamped to the side of the skiff, while the communication box (about 30 cm) and a laptop were in the skiff. The system lists operational water depths of < 2-150 m with a blanking distance of ~ 1 m and reports a top resolution of 6 cm with up to 40 m bottom penetration at 10 kHz. Side scan sonar offers sea and riverbed imaging from acoustic return signal intensities. In riverine settings it has been applied most commonly for habitat mapping and imaging of sediment bedforms (e.g., Kostaschuk and MacDonald 1988; Manley and Singer 2008; Kaeser et al. 2013;

Fig. 2. Nortek AquaDopp HR ADCP mounted to weighted river bed platform facing upwards.

– 3 – Powers et al. 2015). With regards to scour, side scan sonar has the potential to display the scour hole in its context of surrounding infrastructure (Whitehouse et al. 2008, 2011). In this study, a rotary side scan sonar system was used. The sonar head of the Kongsberg Mesotech MS 1000 rotary side scan sonar is usually connected to a stationary tower deployed on the seabed for temporal monitoring of sediment remobilization processes and bedform evolution (e.g., Rubin et al. 1983). Due to logistical limitations and for mobility reasons, the rotary sonar head was attached to the skiff and the skiff was held approximately in position for the duration of the scan.

Geotechnical in-situ testing. Traditional cone penetration testing (CPT) would be the method of choice to derive a detailed vertical soil profile. However, in this setting this would require a larger CPT rig configuration to be able to maneuver in the deeper waters, active flows, or from the fairly high bridge deck. Additionally, the soil conditions would represent a high risk for early refusal and damage to the cone, while the scour assessment would not even benefit from a deep soil assessment. Nevertheless, it would be beneficial to assess surficial soil conditions, and particularly variations in surficial soil conditions associated with the scour process and hole evolution. Portable free fall penetrometers (PFFP) have come to the fore for surficial geotechnical seabed and river bed investigation in areas of difficult access including strong hydrodynamics, vicinity of infrastructure, potential presence of shallow bedrock, as well as deployment from small vessels of opportunity (Stark et al. 2014). They have also been utilized for the monitoring of surficial soil conditions with scouring at offshore monopiles (Stark and Kopf 2011). In this study, the PFFP BlueDrop was used (Fig. 3). The device measures its own deceleration and ambient pressure behind the tip, both at a sampling rate of 2 kHz. No sensors are housed in the tip, making the device rugged against impacts with rocks, and it can be deployed by hand without winch support. A direct interpretation of measured deceleration is possible, but in most cases the measured deceleration is utilized to estimate an equivalent of quasi-static bearing capacity or undrained shear strength. Additional details on the device and data processing are provided in, e.g., Stark et al. (2016) or Albatal (2018). The goals for using the PFFP in this study was to investigate variations in soil strength around the bridge piers and specifically in relation to possible scour.

Soil sampling for geotechnical laboratory testing. Subaquaeous soil sampling equipment is often heavy or requires heavy support equipment to ensure sufficient penetration and extraction of material. Being limited to a small vessel without winch support and facing in many locations too deep water depths for easily pushing Shelby or other tube samplers, a small Ponar grab sampler

Fig. 3. PFFP BlueDrop after deployment into muddy sediments (not at the study site).

– 4 – was chosen. Due to the hard and often coarse nature of the sediments, this sampling technique was only successful in about 50% of the attempts, resulting in three samples of sufficient volume for further testing of which one was collected close to a bridge pier and two close to the river banks. In this preliminary study the sample analysis was limited to a grain size analysis through sieving following ASTM D6913.

RESULTS AND DISCUSSION

Field measurements were completed within five hours by a team of eight graduate students supported by a boat pilot and additional field personnel from VDOT. Many of the students used these types of equipment and/or worked from a boat for the first time.

Acoustic Surveying Equipment. The ADCP was successfully deployed on the riverbed between the western riverbank and the westernmost bridge pile (N37.161476°, W80.552550°) at noon and retrieved at 14:10 local time the same day. The device recorded continuously, but little variations were observed over time so that a representative flow velocity was derived from averaging the measured velocities along the profile over time. The resulting flow velocity profile exhibits a logarithmic trend with slower velocities closer to the device (i.e., closer to the river bed). This is expected from boundary effects and interaction between the bed and the flow. The near-bed flow velocity (~ 20 cm from the riverbed) equaled approximately 0.14 m/s (Fig. 4). Following the Hjulstrom diagram (Hjulstrom 1935), this near-bed velocity would be sufficient to initiate sediment transport of sediments with a particle size of < 1.2 mm (i.e., medium and fine sands).

Fig. 4. Time averaged flow velocity profile measured using an ADCP with distance of 0.1 m from the riverbed.

– 5 – The deployment location would suggest that the flow velocity is larger compared to the free stream river from the presence of the bridge piers and streamline contraction.

Chirp sonar measurements were carried out as transects circling the chosen bridge piers as a result of navigational limitations of the skiff and the lack of a real-time kinetic GPS tracking system. It should also be noted that data processing of the chirp data has been limited to the system software output in this preliminary demonstration study. Figure 5 shows an example of the chirp data output around the pier closest to the ADCP location. Effects of the blanking distance from the sonar (~1 m) are clearly visible and affecting measurements at shallow water depths. Nevertheless, the strong change in seabed elevation between the downstream side of the pile (denoted as “DS” in Fig. 5) and the upstream side (denoted as “US” in Fig. 5) reaching a maximum of about 4 m in scour depth is clearly visible. Additionally, the backscatter intensity (expressed by color in Fig. 5) varied along the transect from highly reflective sediment (hot colors) to less reflective (cool colors). It should be noted that this result has not been corrected for effects from topography, yet. Nevertheless, the trends can still be considered valid, indicating changes in riverbed sediment properties along the transect circling the bridge pier. It can be envisioned that such data can be further processed to depict spatial and vertical differences in sediment properties.

A rotary side scan sonar scan was performed at four locations with some being more affected by the lack of stability of the small vessel than others. An excerpt of the scan closest to the position of the ADCP and the shown chirp sonar is shown in Figure 6. While in this case the distance to

Fig. 5. Chirp sonar image showing a vertical distance of 5 m from the sonar head, being located about 0.5 m under the water surface along a transect circling the western bridge pier. “DS” denotes downstream locations and “US” denotes upstream locations. Colors represent backscatter intensity, indicating acoustic reflectivity (associated with hardness) of sediments.

– 6 – Fig. 6. Rotary side scan sonar image showing an excerpt of a scan displaying a large boulder. the pier was too large to display the pier and scour hole and in other cases strong flows around the piers prohibited a stable scan without deployment of the seabed tower, the image demonstrates capabilities of displaying and measuring objects such as in this case a larger boulder. In the future, a stronger anchoring system or the use of the system’s seabed tower should be considered to achieve the stability needed to achieve high quality imagery of the piers and the scour holes

Geotechnical in-situ testing. The PFFP was deployed in total at 22 locations in the vicinity of the bridge piers. Seven of the deployments were discarded due to impact with hard objects, likely boulders as visible in Figure 6. This also highlights the advantage of conducting acoustic seabed imaging prior to physical testing and sampling. However, the device experienced no damage from these impacts. The PFFP achieved penetration depths of only up to 13 cm and recorded maximum decelerations ranging between 9-150 g (with g being gravitational acceleration), suggesting hard sediments overall. The resulting estimated maximum quasi-static bearing capacities (QSBC) yielded ~ 12-1500 kPa near the riverbed surface supporting the previously mentioned presence of hard sediments. Nevertheless, the resulting profiles shown in Figure 7 display a noticeable variability in sediment strength between the locations. Very hard impacts (Fig. 7 dashed profiles) were found at the sides and downstream side of bridge piers, likely representing riprap that was deployed as scour mitigation in the past. This is also in line with the chirp data. However, the chirp data indicates that some of the riprap was covered with a thin layer of sediment. Medium hard profiles were found more at upstream locations around the piers, suggesting stiff sediment (potentially from removal of looser material) but no surficial presence of riprap. Softer sediments

– 7 – Fig. 7. Quasi-static bearing capacity (QSBC) versus sediment depth estimated from deceleration measurements by a portable free fall penetrometer at different locations in the vicinity of the bridge piers. were observed further away from the bridge. However, it should be noted that penetration depths and measured resistance still suggests relatively hard coarse-grained sediments.

Soil sampling. Only three sediment samples were successfully extracted due to hard sediment conditions. All samples suggested a median grain size of d50 = 0.5-0.6 mm and classified as poorly graded following the Unified Soil Classification System (USCS) (ASTM D2487-17).

CONCLUSION

A preliminary study was carried out by a team of graduate students and supported by VDOT to test a combined approach of acoustic methods, portable free fall penetrometer testing, and soil sampling for scour investigation at the Vietnam Veterans’ Memorial Bridge crossing the New River in Fairlawn, Virginia. All methods were deployed from a small motorized skiff and without further winch support. The acoustic methods allowed to visualize and measure the depth of the scour hole, as well as identify variations in sediment properties and the presence of large boulders. The ADCP provided a clear data set of local flow conditions. The portable free fall penetrometer complemented the acoustic data by locating hard impacts with likely slightly buried riprap, as well as variations in surficial sediment strength. Soil samples confirmed coarse grained sediments in the area. Overall, the measurement suite appeared to be a promising strategy for scour assessment of existing structures without requirements for larger deployment infrastructure or divers. More research is needed to refine strategies for the most effective deployments of instruments and correlation between the geophysical and geotechnical data.

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ACKNOWLEDGEMENTS

The authors would like to thank the Virginia Department of Transportation, and specifically, Carl Benson, Travis Higgs, Donald Martin, David Kelley and team, for supporting this study through logistics, field support, information, and feedback. The authors would also like to thank Harsh Patel and Taylor Sisti who contributed to data collection as participating students, and Matthew Florence (Virginia Tech) who provided support with the ADCP.

REFERENCES

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