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66 3.1 Overview A variety of means and methods are available for placing filter materials underwater and in flowing water. The selection of any particular technique depends on the materials to be used and the flow conditions at the time of placement (e.g., flow depth and velocity). NCHRP Proj- ect 24-07(2), on pier scour protection, demonstrated the viability of placing sand-filled geo- containers around a bridge pier in flowing water to achieve a filter prior to placing the armor layer (riprap in that case). The research report on NCHRP Project 24-07(2), NCHRP Report 593: Countermeasures to Protect Bridge Piers from Scour (Lagasse et al. 2007), documents the successful placement of geocontainers using a hydraulic excavator with thumb to drop individual geocontainers from above the water surface. The geocontainers were filled with filter sand prior to placement and weighed 200 lb each. Figure 3.1 illustrates the placement technique used in that project. Refer to Figures 2.20 through 2.26 and related text for a more thorough discussion of that research effort. For NCHRP Project 24-42, the research on which the guidance herein is based (see NCHRP Web-Only Document 254, Volume 1 for the full research report), diver-assisted underwater filter placement was performed in the large outdoor river engineering flume at CSU. Both granular filters and geotextile filters were installed in flowing water under prototype-scale conditions. A variety of filter materials and placement techniques were investigated. Of particular interest was placing filters where access for construction equipment is limited (beneath a bridge deck with little clearance above ordinary low water, for example). Certified divers placed both granu- lar and geotextile filter materials at a pier to demonstrate placement of filters in flowing water as a proof-of-concept demonstration. Given the wide range of capabilities of the large-scale outdoor river engineering flume at CSU, the following filter materials were selected for diver- assisted placement around a prototype-scale pier in flowing water: For granular filters, the selected materials were the following: â¢ Pea gravel mixed equally with 0.6 mm masonry sand and placed with a 2-inch diameter rigid tremie pipe at various elevations above the bed (d50 = 4.0 mm). â¢ Nominal 3/8-inch (9 mm) pea gravel placed loosely around the pier using sand pump and 2-inch diameter flexible tremie hose (d50 = 6.0 mm). For composite filters, the selected material was empty geobags placed on the bed adjacent to the pier and filled by divers underwater with pea gravel using a sand pump and 2-inch diameter flexible tremie hose (d50 = 6.0 mm). C H A P T E R 3 Documentation of Underwater Filter Installation Techniques
Documentation of Underwater Filter Installation Techniques 67 For geotextile filters, the selected materials were the following: â¢ Nonwoven, needle-punched geotextile sheets (buoyant) unrolled in the direction of flow adjacent to and downstream of the pier. â¢ Double-sided drainage composite sheets (negatively buoyant) unrolled in the direction of flow adjacent to and upstream of the pier. 3.2 Diver-Assisted Filter Placement 3.2.1 Testing Facility The river engineering flume at CSU is 180 ft long, 20 ft wide, and 8 ft deep. In the floor of the flume, approximately halfway along its length, is a deep pit that is 30 ft long by 8 ft deep, referred to as the sediment recess. A prototype-scale bridge pier measuring 4 ft long by 1.5 ft wide was constructed of timber and plywood and bolted into place in the center of the sediment recess. The recess was filled with 0.8 mm uniform concrete masonry sand, which acted as riverbed sediment around the pier. Figure 3.2 shows the installation. A 20 ft wide moveable data collection carriage spans the width of the flume and traverses its length on rails. It was used to collect point velocity data during the tests and, while the divers were in the water, it was stationed directly above the pier to simulate a bridge deck with limited overhead clearance. The lower part of the data collection carriage can be seen in Figure 3.2 (b). 3.2.2 Testing Approach and Documentation A certified dive team was used to place the filter materials around the bridge pier as a âproof-of-conceptâ demonstration to determine the advantages and limitations of various filter materials and underwater placement techniques. The flume was filled to a 4 ft depth with water, and two steady flow rates were established. (a) 200-lb sand-filled geocontainer placed from above. NCHRP Project 24-07(2). (b) Geocontainers partially covered by riprap armor around bridge pier. NCHRP Project 24-07(2). Figure 3.1. Sand-filled geocontainers placed as a filter around a bridge pier.
68 Guidance for Underwater Installation of Filter Systems The first flow rate was held steady at 50 ft3/s, resulting in a modest approach velocity of about 0.6 ft/s along the flume centerline. The second flow rate was established at 200 ft3/s, which resulted in a centerline approach velocity in excess of 3.5 ft/s upstream of the pier. The dive team consisted of two scuba divers and a topside line handler who managed the teth- ers. Underwater communications were established via communication lines in the tether ropes. Figure 3.3 shows the test setup with flowing water (a) and divers in the flume (b). Underwater and above water video and still photos were taken before, during, and after each individual dive to document the placement of the various filter materials. 3.3. Underwater Installation of Granular Filters 3.3.1 Critical Velocity vs. Particle Size Placing a granular filter in flowing water requires knowledge of the flow velocity that would be expected at the job site during the construction season. Granular filters should not be used alone if the local velocity (e.g., at a pier) exceeds the critical velocity for particle movement. As a rule of thumb, local velocities at a pier are estimated to be 1.5 times the approach velocity at circular or round-nose piers and 1.7 times the approach velocity for rectangular or blunt-nose piers. Critical velocity for a given particle size of granular filter material can be estimated from Arneson et al. (2012): V = K y d (3.1)c u 1 6 1 3( ) (a) Construction of lower section of pier within the sediment recess (looking downstream). (b) Completed installation (looking upstream). Note data collection carriage above pier. Figure 3.2. Installation of prototype-scale bridge pier in the river engineering flume at CSU.
Documentation of Underwater Filter Installation Techniques 69 where Vc = Critical velocity above which particles of size d50 or smaller will be transported, ft/s (m/s) y = Flow depth, ft (m) d = Particle size ft (m) (typically taken as the median size d50) Ku = Coefficient for system of units = 11.17 English units (ft-lb-s) = 6.19 SI units (m-kg-s) Equation 3.1 can be rearranged to calculate the critical particle size, dc, at the threshold of motion for a given velocity and depth. Figure 3.4 is a graph of critical velocity vs. particle size for granular filters having a specific gravity of 2.65 and ranging in size from d50 = 1 mm (coarse sand) to 100 mm (cobbles). The pea gravel used in the flume tests at the prototype-scale pier had a d50 of 6.0 mm and a critical velocity of 3.9 ft/s at a flow depth of 4 ft. 3.3.2 Granular Filter Placement with a Flexible Tremie Placing a granular filter under water is relatively easy to accomplish with a standard solids- handling pump (also known as a trash pump) and a flexible tremie pipe or hose; thus, even lim- ited access or clearance beneath a bridge deck is no impediment to granular filter placement. The solids-handling pump can be used to suction up a slurry of gravel and water with up to about 40% solids by volume and deliver it to divers working underwater at the site of filter placement. Figure 3.5 illustrates the setup at CSUâs river engineering flume in May 2017. In Figure 3.5 (a), the solids-handling pump and 40-hp engine are shown. The pump is capable of passing solids up to 3 inches in diameter. The pump has 4 inch diameter suction and discharge capacity; the research team reduced both suction and discharge hoses to a 2 inch diameter for ease of handling by the divers. The wooden bin containing the pea gravel and suction hose can be seen in the foreground in Figure 3.5 (b). A composite filter consisting of empty geobags placed on the bed adjacent to the pier and filled by divers underwater with pea gravel using a sand pump and 2 inch diameter flexible (a) Flume test at 4 ft depth and 200 ft3/s, approach velocity 3.5 ft/s. Note bow wave around nose of pier. (b) Tethered divers preparing to place filters during 200 ft3/s test at 4 ft depth. Figure 3.3. Prototype-scale bridge pier test setup with (a) flowing water and (b) divers in the river engineering flume at CSU.
70 Guidance for Underwater Installation of Filter Systems 0 2 4 6 8 10 12 14 1 10 100 Cr iti ca l v el oc ity , ft /s Particle size, mm 8 ft 20 ft Particle S.G. = 2.65 depth = 4 ft Figure 3.4. Critical velocity vs. particle size (particle specific gravity = 2.65). (a) Solids-handling pump. (b) Submerged bin with pea gravel (looking upstream). Note 2 inch diameter suction hose. Figure 3.5. Test setup for granular filter placement.
Documentation of Underwater Filter Installation Techniques 71 tremie hose was also tested. Figure 3.6 shows the geobag with fill port (a) and divers filling it underwater (b). Figure 3.7 shows the pea gravel delivered to the pier during and after the 50 ft3/s run. For this demonstration, the tremie hose approach was successful in placing the material adjacent to the pier, as well as for underwater filling of 3 ft by 3 ft square geobags having an integral fill port. During the 200 ft3/s demonstration, the loose pea gravel could not be uniformly placed due to the turbulence around the pier; however, the divers were able to fill the geobags successfully, even at the higher flow rate. Figure 3.8 shows the placement after the 200 ft3/s run. 3.4 Underwater Installation of Geotextile Filters Both buoyant and self-sinking panels of nonwoven needle-punched geotextiles were placed around the pier, first at a flow rate of 50 ft3/s and again at a flow rate of 200 ft3/s. A standard 8 oz per square yard polypropylene geotextile (Mirafi 180N) was used as the buoyant fabric. (a) Nonwoven needle-punched geobag with fill port. (b) Filling geobag with pea gravel underwater. Note 2 inch diameter flexible tremie hose. 3 ft 3 ft Fill port Figure 3.6. Geobag with fill port (a) and divers filling the geobag underwater at the pier (b). (a) Underwater photo of diver placing loose pea gravel around pier at 50 ft3/s flow rate. Note gravel exiting the tremie hose. (b) After 50 ft3/s test, showing loose pea gravel and gravel-filled geobags, filled in flowing water. Loose pea gravel Geobags filled underwater Gravel Nozzle Figure 3.7. Tremie-placed granular filter during and after the 50 ft3/s demonstration.
72 Guidance for Underwater Installation of Filter Systems A 6 mm thick double-sided drainage composite (CETCO Aquadrain G25) with high density poly ethylene geonet core was used as the self-sinking fabric; it was made negatively buoyant by affixing metal strips at 4 ft intervals along its length to achieve a handling capability comparable to a sandmat commonly used in Europe (see Figures 2.9 and 2.10). Figure 3.9 is a close-up photo of the self-sinking fabric showing the drainage net core with geotextile on top and bottom. All geotextile panels measured 4 ft wide by 16 ft in length and were placed around the pier in water that was 4 ft deep. Satisfactory placement of both materials was accomplished during the 50 ft3/s flow rate, as shown in Figure 3.10. One gallon sand-filled Ziploc bags and 9 inch riprap stones were used to temporarily weigh down the fabrics while they were being unrolled at the pier in the direction of flow. During the 200 ft3/s test, the turbulence around the pier created significant difficulty for plac- ing the buoyant geotextile. The divers reported that the geotextile was âsailingâ and âtwistingâ in the current and that the rocks and sandbags could not effectively hold it down. The divers used heavier pieces of concrete rubble to weigh the fabric down. The installation of the buoyant fabric was unsatisfactory during the 200 ft3/s test (see Figure 3.11a). The divers reported that the self-sinking drainage composite was easier to handle because it was stiffer and was easier to unroll and control, although the resulting placement was still not ideal. Figure 3.11b shows the results of the self-sinking geotextile placement after the 200 ft3/s test. Loose pea gravel (partially disrupted) Geobags filled underwater (a) Loose pea gravel and geobags after the 200 ft3/s test. (b) After 200 ft3/s test. Figure 3.8. Tremie-placed granular filter after the 200 ft3/s demonstration. Figure 3.9. Self-sinking geotextile filter.
Documentation of Underwater Filter Installation Techniques 73 3.5 Appraisal of Underwater Installation Testing Results Placement of granular and geotextile filters at a prototype-scale bridge pier was successful during all tests conducted at 50 ft3/s, with an approach velocity of about 0.6 ft/s. The tests at 200 ft3/s were conducted with a centerline approach velocity somewhat in excess of 3.5 ft/s. At the higher flow rate, difficulties were encountered placing sheets of geotextile fabric in flowing water during the tests. Placing granular filter material (in this case, pea gravel) using a standard solids-handling pump and flexible tremie hose proved successful at both flow rates. Underwater filling of stan- dard 3 ft by 3 ft square geobags with integral fill port (also known as dewatering bags) with the pea gravel material was found to be rapid and efficient, even at the higher flow rate. The geobags were unrolled in the downstream direction with the fill port on the upstream side. One diver held the bag in place against the pier while the other filled it using the flexible tremie hose. Figure 3.11. Geotextile filter placement after the 200 ft3/s demonstration. (a) Unsatisfactory placement of the buoyant geotextile during the 200 ft3/s test run (flow from right to left). (b) Self-sinking drainage composite after the 200 ft3/s test run (flow from right to left). Mirafi 180N geotextile CETCO Aquadrain G-25 drainage composite (a) Rolls of geotextile filters. (b) After 50 ft3/s run, satisfactory placement (looking upstream). CETCO Aquadrain G-25 drainage composite Mirafi 180N geotextile CETCO Aquadrain G-25 drainage composite Mirafi 180N geotextile Figure 3.10. Geotextile filter placement after the 50 ft3/s demonstration.
74 Guidance for Underwater Installation of Filter Systems The divers noted that with this approach, âthe bags almost filled themselves,â even during the high-flow test. Given the rate of gravel delivery from the solids-handling pump, each bag could be filled in about 3 minutes. The practical limit for buoyant geotextile placement by divers was exceeded at an approach velocity of approximately 3.5 ft/s. Placing a self-sinking fabric was barely manageable at this approach velocity. Based on the diversâ first-hand reports, recommended limits on approach velocity for placing buoyant vs. self-sinking geotextiles are 2.5 ft/s and 3.5 ft/s, respectively. Dispersion of granular filter materials was investigated using a rigid 2 inch diameter tremie pipe with its bottom end placed at different heights above the bed in 4 ft of flowing water. At each height, a 5 gallon bucket of well-graded granular filter material was injected into the flow via the tremie. After slowly draining the flume, the resulting piles of filter material were analyzed for their grain size distribution. In general, the filter material landing on the bed became increasingly coarse and more uni- form with increasing height of placement, with the finer particles being winnowed out and swept downstream as the material fell through the water column. The dispersion tests were conducted with two velocities: 0.6 and 1.0 ft/s. The dispersion effect was significantly more pronounced at the higher flow velocity, even though the grain size distribution of the original stockpiled filter material indicated that less than 2% of the particles were smaller than the critical size for initia- tion of motion at this velocity. Safety precautions were taken during all phases of the installation demonstration tests. Divers were tethered at all times and maintained constant communication with the topside line handler. Also, care was taken to ensure that the divers were never located directly downstream of the filter materials during placement, minimizing the potential for tangling or entrapment by loose geosynthetic fabrics caught in the turbulence.