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1 1.1 Background The importance of the filter component of a countermeasure for stream instability or bridge scour should not be underestimated. Filters are essential to the successful long-term perfor- mance of countermeasures, especially armoring countermeasures. There are two basic types of filters: granular filters and geotextile filters. Some situations call for a composite filter consisting of both a granular layer and a geotextile. The specific characteristics of the base soil determine the design considerations for the filter layer. The filter must retain the coarser particles of the subgrade while remaining permeable enough to allow infiltration and exfiltration to occur freely. It is not necessary to retain all the particle sizes in the subgrade; in fact, it is beneficial to allow the smaller particles to pass through the filter, leaving a coarser substrate behind. The filter prevents excessive migration of the base soil particles through the voids in the armor layer, permits relief of hydrostatic pressure beneath the armor, and distributes the weight of the armor to provide more uniform settlement. Guidance for the design of both granular and geotextile filters is provided in NCHRP Report 568: Riprap Design Criteria, Recommended Specifications, and Quality Control (Lagasse et al. 2006) and is found in Volume 2 of Hydraulic Engineering Circular No. 23 (HEC-23) as Design Guide 16 (Lagasse et al. 2009). Volume 1 of HEC-23 also provides some limited guidance on placing filters underwater. However, experience and a survey of practitioners indicate that few countermeasure installations in water actually include a filter even though a filter is shown on the design plans and is recommended in the technical guidance publications cited above. The reasons given for the omission of a filter are most commonly related to constructability issues or environmental concerns. Consequently, research on filter selection, design, and installa- tion techniques for scour and erosion countermeasure systems in various underwater conditions is a high priority requirement if the most recent design guidance on armoring counter measures (e.g., HEC-23) is to be implemented effectively. Moreover, an effective implementation plan will be necessary so that department of transportation (DOT) design, construction, and mainte- nance personnel (along with their consultants and contractors) receive training on how counter- measures function and the overriding importance of the underlying filter. The purpose of this document is to provide specific guidance for design, construction, and maintenance personnel on the function of filters and their installation. The supporting research considered various depths and velocities of stream flow for placing geotextiles and granular filters under countermeasures, with an emphasis on underwater installation. This document addresses â¢ Selection â¢ Design â¢ Construction C H A P T E R 1 Introduction and Results
2 Guidance for Underwater Installation of Filter Systems â¢ Maintenance â¢ Specifications â¢ Quality Control/Quality Assurance 1.2 Approach This guidance document is based on a review of the technical literature that was combined with a survey of practitioners to produce a detailed synthesis of the current state of practice. The supporting research included development of selection criteria in the form of flow charts for granular and geotextile filters in relation to requirements and constraints for underwater place- ment. Prototype-scale laboratory testing using certified divers was implemented to document underwater installation procedures and equipment requirements, to validate diver-supported installation techniques, and to establish limits on conditions where diver assistance could be employed safely. Findings from these activities supported development of design, construction, maintenance, testing, recommended specifications, and quality control guidance for underwater placement of filters as an essential component of bridge scour and other erosion control armor- ing systems. Two case studies of DOT projects involving underwater installation of filters illustrate the techniques adopted by these agencies for geotextile filter placement and summarize the lessons learned. Environmental and permitting issues are also addressed. A listing of references cited is also included. 1.3 Results 1. Although it is not a primary purpose of this document, it is necessary to clearly establish the functions of, and need for, granular and/or geotextile filters as an essential component of any armoring system. These are established in Chapter 2, together with accepted filter design procedures. 2. A survey of practitioners across the United States revealed that some underwater installation practices in other countries are still new and largely untried in the United States. The use of self-sinking geotextile composite fabrics (e.g., a sandmat) is one such practice. Another example is the use of a flexible tremie hose to deliver a slurry of coarse granular filter material to divers for placement underwater. When conditions allow divers to safely access the work area, they can, under moderate flow conditions, place granular material loosely on the bank or bed. 3. A proof-of-concept demonstration of underwater filter placement by divers was conducted as part of the research supporting this document. The divers were able to install a variety of filter materials around a prototype bridge pier in flowing water. The trials were conducted in the large outdoor river engineering flume at Colorado State University under different flow rates and velocities. The various trials are summarized in Chapter 3 of this document. 4. For both granular and geotextile filters, recommended design procedures, specifications, material testing requirements, installation alternatives, and QA/QC checklist items are provided in Chapter 4. In addition, filter selection guidance (in the form of flow charts) is provided based on site-specific conditions that include flow depth, flow velocity, access for construction equipment, and overhead clearance. 5. Geotextile filters can also be installed as bags filled with sand or gravel. The geobags can be filled prior to placement, sewn shut, and dropped through the water column. Alternatively, empty geobags can be placed by divers and filled in place with a flexible tremie hose. Both approaches are commonly used in Europe, but are relatively unknown in the United States. Wider use of the geobag or geocontainer approaches would constitute a significant advance in the state of practice for placing filters in flowing water in the United States.
Introduction and Results 3 6. Practitioners in the United States indicate that underwater inspection of a filter installation is not usually required prior to placing the armor layer on top. This is an undesirable practice that must be rectified. Recommendations for filter inspection, as one component of a quality control program during construction, are provided in Chapter 4. 7. In summary, the current state of practice for filter installation provides a sufficient variety of filter material types and placement techniques to accommodate most underwater filter requirements. Indeed, there should be very few instances where a filter cannot be placed as an integral part of a properly designed and installed scour or erosion countermeasure.