Problem | Bridge Failure Due to Scour
In the United States, bridge scour — or the erosion of soil or sediment around abutments and piers — is one of the leading causes of bridge failure. During the construction of a bridge, countermeasures are taken to prevent this erosion. However, small soil particles can still pass through voids and gaps in the countermeasure structure.
To prevent this additional erosion through the countermeasure, a filter should be placed between the countermeasure and the underlying soil. Despite the importance of including filters in the construction of countermeasures, a survey of bridge projects showed that few bridges in the U.S. include them due to constructability or environmental concerns.
Solution | Installation of Filter Systems
The goal of the original research undertaken by Ayres Associates on behalf of NCHRP was to provide guidance on the process for installing these filter systems, addressing common concerns and issues that prevent their inclusion (NCHRP Project 24-42, Research Report 887).
In it, the research outlines the ways in which filters— both granular and geotextile filters — can be installed under different construction conditions. The research examined a number of approaches using these two methods:
Loose granular filter (placed either by clamshell or tremie, not dumped)
Geotextile fabric by itself (typically placed by divers and temporarily secured by sandbags, steel frame, or pins)
Self-sinking mat (machine-placed)
Self-sinking mat (diver-placed)
Geotextile affixed to armor
In cases using the loose granular filter and geotextiles, many approaches used certified divers to conduct the installation, giving greater flexibility and control over the process. Divers were responsible for filling geocontainers, guiding self-sinking mats, or placing granular materials using a tremie or clamshell depending on the method.
The main considerations in determining which materials and methods to use were the access to the site and clearance (e.g. if the filter to be placed under a pier), the depth of the placement, and the velocity of the flowing water. After installation of any filter type and the armor layer is placed on top of the filter, there is no maintenance required unless the armor layer is damaged (for example during a flood event).
Implementation & Impact | Training Workshop
As part of the project, Ayres Associates produced an implementation document, Training Manual for Underwater Installation of Filter Systems which formed the basis of the implementation work conducted as part of this project 24-44.
Using this training manual as the foundation, Ayres Associates worked with NCHRP to plan a one-day workshop on October 10, 2019, in Fort Collins, CO. Over 60 different state departments of transportation (DOTs) were invited to participate in the workshop, and ultimately representatives from 16 DOTs attended along with two consultants, and a representative from the Federal Highway Administration (FHWA).
Over the course of the workshop, participants learned about the purpose and importance of filters, approaches for their installation, and best practices and recommendations from NCHRP research. At the conclusion of the educational sessions of the workshop, participants had the opportunity to work through a problem-solving session using a real-world case study, where they could apply the knowledge learned earlier in the workshop. Attendees were provided the resources (the Instructor’s Guide, the Power Point® presentation, and a brief video) and the knowledge to return to their agencies and provide essential training on underwater installation of filters to their associates, consultants, and contractors.
After returning to their respective DOTs, participants said that the workshop offered unique growth opportunities in their state contexts. Parts of the United States face specific environmental, logistical, and budgetary circumstances for which the workshop is able to provide added value and knowledge. In Wisconsin, for example, extreme precipitation and flooding events have shown to be detrimental since 2017, emphasizing the potential impact of bridge scour.
Furthermore, the workshop provided a knowledge-sharing opportunity through the convening power of the National Academies. Most participants shared that one of the most helpful parts of the workshop, beyond the training, was the ability to discuss common challenges, solutions, and experience with other DOTs from across the country. Participants expressed that developing these relationships during the workshop gave them a long-term opportunity to continue learning what other departments will be doing in the future and allowed them to share best practices.
Participants also noted that access funding to attend the Workshop in-person was a key appeal, giving them this opportunity to network outside of their own state. The opportunity to learn in person, see demonstrations, and build these relationships was highlighted as a tangible improvement in learning and retention over virtual seminars.
Additionally, the workshop empowered participants to tackle challenges strategically. Although cost was one of the most common concerns, the participants said that different approaches shared in the workshop provided innovative and cost-effective ways to implement solutions. For departments that have not yet installed underwater filters, they are looking forward to doing so in the future and learning how to navigate challenges from the experience of other departments who have started the process. Looking ahead, the novelty of the knowledge shared in the workshop has shown to be galvanizing for participants, who have also shared that this novelty presents creative opportunities for open-minded engineers.
Other Research & Resources