The Test of Time

When stabilizing a channel or protecting a structure, it’s the long-term results that count.

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Credit: KYOWA
In channel stabilization, time is the test of the resiliency of a chosen solution. Several projects worldwide have demonstrated mitigation approaches that are making the grade.

Case in point: the Akashi Kaikyo Bridge is a 3.91-kilometer-long suspension bridge linking Kobe, the capital of the Hyōgo Prefecture on Honshu Island, to Iwaya on Awaji Island, also within the Hyōgo Prefecture.

Its longest span measures 1.991 kilometers. It took nearly 10 years to construct and was open to traffic on April 5, 1998. At the time, it was the world’s longest suspension bridge.

During its 1989 construction, the tower foundations were to be laid in significantly deep waters where they would be subjected to strong tidal currents in the Akashi Strait, which is 3.6 kilometers wide with a maximum depth of 110 meters situated between the Japanese islands of Honshu and Awaji and connecting Seto Inland Sea and Osaka Bay. The narrow strait has a maximum tidal current of 7 knots in spring tide.

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“This high speed tidal current was a cause of difficulty of construction,” notes David O’Callaghan, company spokesperson for Kyowa, which manufactures Filter Units.

Monitoring data from the time of installation (left) and after 11 years

Because outcrops of granite basement rock are located at a considerable depth, the foundation of the tower on the Kobe side (2P) is supported by the gravel bed known as the Akashi layer, and that of the tower on the Awaji Island side (3P) is supported by the Kobe soft rock layer.

Consequently, the occurrence of major local scour in the peripheral soil of the tower foundations was predicted early on, posing a major challenge, according to an engineering report prepared by Tsutomu Takazawa, manager for the engineering management section of the Engineering Works Department for the Honshu-Shikoku Bridge Authority.

“Large riprap was selected as the main scour prevention technique, but a filter layer was needed to prevent long-term draw-out scour under the riprap layer and to provide interim scour protection during the period of caisson construction before riprap could be installed,” says O’Callaghan.

In constructing the foundations, the use of an installed caisson base—a zero setting as far as scour is concerned—was planned, requiring scour protection measures from construction to post-completion.

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Major scour was predicted for the 2P tower foundation of the Akashi Kaikyo Bridge.

At the point where the 2P tower was to be founded, the water is 46 meters deep and the maximum current speed is 7 knots. The bottom sediment is a firm gravel layer with a median diameter of 2 centimeters and a maximum diameter of 10 centimeters.

The 2P tower is a cylindrical spread foundation with a diameter of 80 meters. The structure of the scour protection method was characterized by three factors: prior excavation, riprap coverage, and the use of Filter Units.

The prior excavation was designed to ensure a level seabed where the caisson was installed without creating a sandbank and to ease rapid scour after the caisson had been installed.

It also had been necessary to have the ability to anticipate the stability of surrounding topography at the time of installation, the extent of initial scour, and the development process of the scour pore.

The engineering team conducted a hydraulic model study, which came to three conclusions.

The first: prior excavation holes dug at three different depths had several waves of the maximum current that could occur. “We then compared the stability of the seabed by checking the condition of the sand bank that occurred on the sea bed,” notes Takazawa.

“This experiment concluded that by selecting the depth of prior excavation according to the current speed and conditions of the bottom sediment—10 meters or deeper for the 2P tower—it is possible to ensure a level sea bed where prior excavation is conducted for a comparatively long time under strong currents,” he adds.

Second, when installing a caisson into a prior excavation hole with a depth of 10 meters when weak currents exist, a slight scour pore can occur directly beneath the front edge of the caisson.

“However, no sediment was dragged underneath the bottom of the caisson, and no major disturbance was observed on the face of slope, so the caisson could be installed safely,” says Takazawa.

Placing roll stock

Third, “when we had currents that are expected to occur after the caisson’s installation act successively on the two prior excavation holes with depths of 10 meters and 20 meters, the rate of local scour development can be relieved by deepening the prior excavation hole,” he notes.

“Nevertheless, a scour pore still occurred near the separation point of the current when the first strong current hit. As the current speed subsequently increased, the scour region expanded to the upper reach side and the maximum scour depth increased as well.”

In the case of prior excavation with a depth that is shallower than the final scour depth, it is difficult to control the occurrence of local scour through prior excavation only, Takazawa notes.

This effect of prior excavation on decreasing of local scour can be explained in that, at the ordinary sea bed, when a flow with mostly uniform flow velocity distribution in the water depth direction comes near the outer edge of the bore hole, its flow velocity distribution becomes considerably deformed, resulting in a significantly lowered flow velocity near the sea bed, Takazawa points out.

“This rapid lowering of flow velocity distribution recovers gradually downstream, but the flow velocity at the bottom of the bore hole is very small compared to that at the ordinary sea bed,” adds Takazawa.


The hydraulic model study addressed the riprap coverage work for long-term scour protection. The 2P tower’s riprap covering work is laid out in a concave shape. The riprap diameter was 1.0 to 1.3 meters, notes O’Callaghan.

The design was chosen for several reasons. Riprap covering work that remains stable when a 7-knot current occurs must be approximately 1 ton or heavier. Secondly, riprap is most easily moved on the structure’s upstream lateral sides where accelerated flow separates followed by the downward flow route of the separated accelerated flow.

Also, when riprap has moved at the point where accelerated flow separates, the bottom sediment directly under the structure is subjected to scour due to draw-out. This phenomenon becomes even more noticeable if the layer of riprap is thin, notes Takazawa.

Additionally, when the range covered by riprap is small, a major scour pore occurs at the outer edge of the riprap covering work on the downward flow route of the accelerated flow with a high likelihood of its function being lost, such as lowered capacity to bear the structure, among other factors.

Creating cells of uniform size

In another observation, if the range covered by riprap is increased, the structure of the riprap and its surrounding topography all remain stable for an extended period, although a minor scour pore occurs at the outer edge of the riprap covering work on the downward flow route of the accelerated flow.

“When we took a case where prior excavation holes around the structure are all filled up with riprap and the crown of the riprap covering work is made flat, and compared it with a case where the covering riprap layer thickness necessary around the structure is secured and the layer is finished into a concave shape, it was found that the latter has less impact on the surrounding topography,” says Takazawa.

A third conclusion from the hydraulic model study focused on a Filter Unit that prevented initial scour from after the caisson had been installed up until the completion of a riprap cover and also prevented draw-out of the bottom sediment from between the gaps of large-diameter riprap.

In the area around a structure built in the midst of strong flow, the bottom sediment is subjected to fluctuating uplift force components created by horseshoe vortices and running water currents that contain accelerated flow and complex disturbances, notes Takazawa.

Filling the gabion cells

“If riprap covering work is used to protect the bottom sediment that contains fine-grained fractions, the fluctuating uplift force causes the bottom sediment to be drawn out through gaps among the riprap, possibly causing scour,” he adds. “Since the mechanism of scour due to draw-out is extremely complex, its theoretical elucidation has yet to be attained. Also, due to the limitations of experimental techniques for modeling fine-grained fractions on the bottom sediment, it is difficult to simulate this phenomenon in a hydraulic model study.”

To prevent scour due to draw-out, the installation of a filter layer between the bottom sediment to be scoured and large-diameter riprap was believed to be effective.

The engineering team considered the body of research that had been conducted on the design of filters and various proposed standards: stability standards that stipulate a particle diameter ratio so that particles on the bottom sediment do not penetrate the filter layer, permeability standards stipulating a particle diameter ratio to ensure that the filter layer is more permeable than particles on the bottom sediment and to prevent pressure from being concentrated, and uniformity standards that stipulate a particle size composition to prevent filter particles from being separated, as well as standards on layer thickness.

The stability standards provide that the filter’s particle diameter must be three to five times that of bottom sediment particles or smaller.

“If we follow these standards, the filter’s particle diameter would be 6- to 10-centimeter gravel, as the median particle diameter of the 2P tower’s bottom sediment is two centimeters,” says Takazawa.

“However, if we install as a filter gravel with a particle diameter of around 10 centimeters at a point where a 7-knot strong current exists, they might be swept away, scattered, and lost before the riprap work is completed on top of them.”

The bridge authority partnered with Kyowa to develop a net solution that would contain the filter layer gravel, thus enabling it to be precisely lowered onsite and prevent the gravel contents from being swept away by strong currents while performing a filtering function.

The unit was constructed from a chemical-fiber net shaped like a bag and reinforced by a frame rope. It contained gravel loosely packed to 50 to 80% of its volume and demonstrated to the engineering team good elastic behavior and adhesion.

To investigate the Filter Unit’s protection effects against local scour—including that due to draw-out—units 2 meters wide by 1 meter high were installed around the circumference of only one of the two concrete sinkers that were 4 meters high, 10 meters long, and 6 meters wide, with no measures being taken for the other sinker.

For nearly four months, the two underwent a field test at the sea bottom at a depth of 45 meters where the maximum current speed was 6.5 knots. For the structure for which no measure was taken, strong currents that go back and forth created a scour area at both ends of the structure’s front portion facing the currents, causing local scour of 30 to 50 centimeters, whereas no scour was recognized through the entire structure that had been equipped with Filter Units.

The results confirmed their “remarkable scour protection effects,” notes Takazawa.

Approximately 8,000 Filter Units were installed at the North Bridge Tower. The Filter Units were filled onshore and then transported by barge to the project site. Next, they were dropped into position using spreader frames.

Filling the Filter Units took about 22 days, and installation took about 13 days.

Because Filter Units are costlier than riprap, they were used in limited areas around the structure, but by promptly laying the units all together after a caisson has been installed, initial scour of the ground around the caisson was expected to be prevented as well as the drawing out of the bottom sediment from the ground around the caisson for an extended period.

In March 1989, a caisson for the 2P tower of the Akashi Kaikyo Bridge was installed, and scour protection work was completed in August. As a follow-up study, the sea bed was observed 6, 12, and 30 months later using a sounding machine and an unmanned submersible system, confirming that the riprap covering work and surrounding topography were in a stable condition.

Takazawa says he hopes the scour protection work used in the bridge project will be informative for future projects to construct large-scale structures under more severe conditions in terms of currents, water depth, bottom sediment, and other factors.

The Akashi Kaikyo Bridge was the original project for which Filter Units were developed, says O’Callaghan. Since then, more than 700,000 Filter Units have been used at 17,000 job sites in Japan and Europe.

Surveys performed over an 11-year period post-installation confirmed that Filter Units were achieving the scour prevention design objectives of the project owner, the Akashi Kaikyo Bridge Authority, and remain in service at this location to this day.

Kyowa’s Filter Unit—designed to protect rivers, seashores, bridges, and mono piles from scour damage—is designed of mesh net and rocks to do the job of preventing flood and bridge scour while also enabling small fish and plants to live in the spaces inside the Filter Unit. That addresses an increasing concern of protecting the aquatic environment in the use of stabilization techniques. The units “have been found to support revegetation on river bank projects,” notes O’Callaghan.

For the Akashi Bridge project, the Filter Units were made of nylon dope-dyed fiber, but “around 15 to 20 years ago, Kyowa switched to recycled polyester fiber—also dope-dyed—for Filter Units,” says O’Callaghan.

The mesh is designed to not rust or release toxic pollutants, and it is resistant to corrosion from saltwater or in highly acidic or alkaline soils. It is treated with a dye, so it also withstands UV exposure for up to 30 years. The total unit is designed to conform to the shape of the underlying soil.

The mesh bags are draped into a production box, and rock is poured into them with a loader, with the bags holding a volume of 1.25 cubic meters. Binding rope is closed on the bag.

Each Filter Unit comes with a cast iron ring, which connects to six fastening points of the net lifting rope, designed to ensure safety and accuracy in lifting and placing the bag. Multiple bags can be linked together with a rope. The hanging rope is pulled upward to allow the bag to be picked up and placed by a backhoe.

O’Callaghan notes that the Filter Units are designed to be efficient and cost-effective to fill and install.

“For river bank projects, Filter Units 2T and 4T can be filled and installed by two to three workers using a backhoe for filling and crane for placement,” he says, adding that on small jobs, only a backhoe may be necessary.

With such resources, in general, Filter Unit 2T can be filled and installed at a rate of 80 units a day and Filter Unit 4T can be filled and installed at a rate of 60 units a day, says O’Callaghan.

Site leveling and preparation is not needed for Filter Units, he adds. The units also can be filled offsite and transported to the project site, and can be installed at or below the water line. With a single lifting point, Filter Units are designed to be easily deployed by crane, says O’Callaghan.

Seabrook Harbor
Sedimentation and erosion at Seabrook Harbor in New Hampshire was starting to threaten fishing and farming operations. A sheet piling solution from Crane Materials International (CMI) has restored economic activities.

Erosion had posed a threat to property, increasing the flooding risk. Blackwater River was continually encroaching into the harbor, causing increased tidal flows and elevating the rate of silt building up in the harbor, making it a difficult environment in which to maneuver boats.

The US Army Corps of Engineers (USACE) had been dredging the Blackwater River annually in an attempt to mitigate the issue, but to no avail. After conducting studies on how to most efficiently address the problem, the USACE chose to repair the breach between the shore and the sand flats.

In so doing, engineers from the USACE New England District decided the river would need to be rerouted away from the town by constructing two permanent cofferdam-type structures on the harbor’s north side while dredging sand from the shoaled areas of the river to encourage the natural flow of water away from the town.

The solution also entailed using the dredged sand to fill between the cofferdams to restore the sand flats.

The objective of the project was to replace the lost intertidal sands, reduce sand migration into the harbor, and prevent shoreline erosion.

As the result of constructing and comparing hydrodynamic models to theoretical solutions and calculations, engineers specified 17-foot and 27-foot sheets of CMI UltraComposite UC 30, driving the sheets 15 to 20 feet into the ground to protect against potential scour.

Design parameters encompassed a 50-year low tide, 50% drainage infill, 12-foot depth to mudline, 2 tons horizontal load per linear foot, tieback 6-foot spacing, and a 200-pound-per-square-foot surcharge.

In a project that lasted from October 2004 to April 2005, Reed & Reed Inc. used an ICE 216 vibratory hammer hanging from a crane to drive the UltraComposite UC 30 sheets, installing 160 sheets, or 240 feet of bulkhead, each day.

CMI’s composite sheeting is a fiber-reinforced polymer sheeting manufactured in an ISO-certified facility. Initially, project construction had been delayed when a non-ISO-certified sheet piling product failed to meet USACE quality standards to endure installation rigors and be designed for consistent strength and durability.

Dredge spoils were used as fill between the double-walled structure. The walls were anchored by using a tieback system incorporating galvanized steel 18-foot-by-2.25-inch rods with connecting turnbuckles on 6-foot centers and two 10-inch channels.

Geogrids were used to protect the toe of the sheet piling. Monitoring sensors were installed to look for any movements in the wall over time.

The project was one of 13 funded by the National Shoreline Erosion Control Development and Demonstration Program.

Following years of service and monitoring, the project has successfully achieved its goals by reclaiming the clam flats and diverting the river to its previous course. The use of the walls, designed to be completely submerged under water during high tide, has ended the shoreline erosion and periodic flooding of the town, with fishing, clamming, and recreational activities thriving again.

Additionally, the local community and state are saving money by having to dredge the harbor only once every five years instead of yearly.

Stabilizing a Channel in Alabama
When stabilizing channel banks or constructing gabion retaining walls, the roll-stock approach is sometimes used to save gabion material and up to 25% in installation time and labor costs, notes George Ragazzo, general manager and erosion and flood control specialist for Modular Gabion Systems.

Roll-stock involves pre-engineered rolls of mesh connected with preformed spiral binders, which are standard for welded wire mesh gabions. The technique is used where a more monolithic, aesthetic, and economical gabion structure is preferred, says Ragazzo.

He adds that the only time roll stock is not recommended is when a project is so small that there will not be savings in material costs.

The use of roll stock “eliminates the joints, which are always the weak points, and the labor that is required to join the gabions together before filling them in,” says Ragazzo.

The approach is demonstrating long-term success in a channel stabilization project in Foley, AL, where a continuous system of gabions and gabion mattresses up to 300 feet long and without joints was used.

In Foley, southeast of Mobile near the Gulf Coast, a small channel was experiencing erosion in the late 1990s, caused by runoff from a culvert under Alabama State Route 59 and one from the north slope, says Ragazzo. The runoff was creating a lot of turbulence and scour.

For the project, base mesh was rolled flat over a geotextile fabric at the prepared site, with the edges meeting to form seams. The mesh for the vertical sides was rolled out atop the base mesh with one edge aligned with the seam of the two segments of the base mesh below. Spiral binders were then fed through the mesh to connect the three sections of mesh, base, and sides. The spiral binder is designed to speed construction and create strong and consistent joint strength.

Diaphragms were cut from the mesh and placed over the base mesh perpendicular to the sides. These panels divided the basket into uniformly sized cells to strengthen the structure and limit fill movement, with all connections made from spiral binders.

Ragazzo points out that the material is PVC-coated and galvanized.

It’s an approach that lends itself well to the area’s mostly sandy soils, which are easily erodible, he notes. EC_bug_web

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