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Turbidity curtains secure a contaminated environment Synthetic materials provide cost-effective sediment control while minimizing environmental disturbance in a lakeshore remediation project. By Murray Banting, R.E.T. and Ryan Wonnacott, B.Sc. ____________________________ Faced with the challenge of cleaning up and remediating an abandoned mine site, a Canadian mining company applied an innovative solution that utilized synthetic materials to replace conventional methods and materials. The mine was operational for over fifty years and was located near the bay of a large lake in British Columbia, Canada. While the mine was operational, barges docked and loaded ore to transport out of the bay. Site runoff from mining operations deposited common mining contam-inants such as lead, zinc, cadmium, copper and sulphates in the bay’s beach sediments and along the shoreline. The removal, replacement, and capping of the contaminated beach soils and sediments was determined to be a suitable re-mediation. This process would consist of the removal of the existing beach soils to a depth of 0.5 m (1.6 ft.) below the low water level of the lake and to cap sediments to a depth of 3 m (10 ft.) below the low water level.

Clean sands and gravels would be used to reconstruct the beach and to cap the deeper sediments under water. The beach area consisted of the entire end of the bay, measuring approx-imately 170 m (557.6 ft.) in length and 30 m (98.4 ft.) in width, including the section that was submerged beneath the lake waters. The bay has a south-ern aspect, with a 17 km (10.5 mi.)

fetch that creates large wave action within the bay. The challenge was to select a construction method that would be cost effective and have a minimal impact on the lake and surrounding areas, as the disturbance of the sediments during the construction phases would be very significant (Photo 1). Preventing these

suspended particles from entering the lake was essential, so disturbance of

the fine sediments would have to be confined to the work zone and be kept separate from the lake body. The options available to isolate the construction zone were: build a dam across the bay and

dewater the site, build a permeable rock dyke and

work in wet conditions, sheet pile the bay, or utilize a floating barrier system

(turbidity curtain). With 17 m (55.7 ft.) deep water in a 170 m (557.6 ft.) wide bay, none of the proposed methods would be easy. For various reasons, three of these options would simply not be appropriate. Building any sort of earth fill across the bay would be a very costly and potentially environment-ally damaging approach, since placing large amounts of fill into the water would create high turbidity levels, and fill removal would be potentially more disruptive. Using a rock dyke or

Photo 1: Turbidity curtains were chosen as part of a construction method that would have a minimal impact on the lake and surrounding areas, which had already been disturbed by reconstruction efforts.

Photo 2: In addition to the two floating barriers, a log boom was installed to trap debris. As shown on the left of this photo, the log boom also

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sheet piling offered similar challenges and risks. The floating turbidity barrier option proved to be the most attractive solution, as it offered minimal disturbance to the lake bottom, efficient deployment, and cost effectiveness. Turbidity Barrier Construction Plans called for two turbidity curtains, and Applied Geo-Environmental Solutions Inc. (AGES Inc.) of Calgary, Alberta was selected to supply and deploy them. The barriers consisted of a floating boom section, a top tension cable system, a permeable or impermeable skirt, and a ballast chain weight system. These compon-ents were selected to match the design requirements of the project. In this case the curtains were required to span the open bay for a distance of approximately 170 m (557.6 ft.), to a consistent depth of 6 m (19.7 ft.) and 17 m (55.7 ft.). With the 17 km (10.5 mi.) fetch, a heavy-duty system cap-able of withstanding large breaking waves was required. The skirt section suspended from the floating boom was weighted to the lake bottom by the ballast chain, and was required to be impermeable in order to confine the fine suspended particles.

The first barrier (closest to the work area) was labeled the primary curtain, as it confined the particles in the immediate work area. This 6 m (19.7ft.)-deep curtain was to be moved as required to encom-pass the equipment working in the water. It was also equipped with an oil absorbent

boom, in the case of an oil leak from an excavator or other piece of equipment.

The secondary curtain was deployed in deeper water to act as a second entrapment area for particles that escaped when the primary curtain was moved as work progressed. The depth of this curtain was 17 m (55.7 ft.). In addition to the two floating barriers a log boom was installed by the contractor to act as a breakwater and debris trap to stop invasion by floating logs or other objects. The log booms also secured warning indicators (Photo 2). AGES Inc., with the assistance of Brockton Equipment/Spilldam of Boston, Massachusetts, recommended the type of materials and the hardware required for curtain fabrication. The materials selected were recommended to meet the specific requirements of the containment in this project, and to withstand the duration and severity of the site conditions. The challenge with this particular curtain was a com-bination of depth and the requirement of a heavy-duty curtain. The curtains were constructed in 15 m (49.2 ft.) sections that were then connected on site to span the entire width of the bay. Each 300 mm (11.8 in.) diameter float was approximately 3 m (10 ft.) in length and was sealed in a 22 oz. orange PVC waterproof cover. Each section was equipped with a top tensioning cable for connection to adjoining sections and shore anchors. The curtain utilized a heavy top

tensioning cable that was interlocked at each section with an eye and clevis connection. The ballast chain was interlocked to each adjoining curtain panel in the same manner. The sides of the curtain sections were adjoined along their entire depth by lacing the hemmed and grommeted sides with marine rope. The base of the curtain was weighed down to the lake bottom with the ballast chains held in sleeves sewn at the bottom of the curtain. Once the curtain sections were laced together on site, the skirt material was then “accordion” folded under the float and temporarily tied in place. Each curtain was then deployed into the lake from one shore, and pulled across the bay with a small tugboat. The tension cables were affixed to the shore on both sides of the bay and the curtain was then freed by removing the ties and letting it drop to the bottom of the lake. Sufficient slack was left in the curtain to withstand a lake water elevation increase of ap-proximately two meters. The entire process of connection and deployment took approximately 12 hours. The time required in comparison to sheet piling the bay or infilling with rock is of no comparison; weeks or months were saved with no construction impact on the bay.

Photo 3: After a lake-bottom mudslide traveled perpendicular to the curtains, half of the secondary curtain was still attached to the east side of the bay, intact and above water.

Diagram 1: Mudslide that caused the curtain to be dragged under the water surface.

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Expect the Unexpected During the reconstruction phase of the beach an unexpected event occurred that compromised the secondary curtain. It was concluded that due to the movement of the heavy equipment on the beach area during beach reconstruction, the lake bottom became unstable in localized areas, and a section of the lake bottom between the primary and secondary curtains produced a mudslide that traveled perpendicular to the curtains. The slide approached the secondary curtain on the lake bottom, where it buried the bottom of the curtain and proceeded to push the curtain into deeper water (Diagram 1). This slide occurred rapidly, as witnessed by the project engineer who was viewing the construction progress from near the west anchor point. This west tension cable anchor connection suddenly broke and the curtain proceeded out into the lake at a high rate of speed before submerging. Approximately half of the curtain was still attached to the east side of the bay, intact and above water (Photo 3). The primary curtain was not affected and it provided containment while the secondary curtain was repaired and replaced.

Extraction The extraction of the curtain was difficult since it took time to evaluate what had happened and then remove the debris that covered the curtain. Scuba divers were required to inspect the curtain, help dislodge it from the lake bottom and to cut away damaged sections. A large crawler excavator was used to pull the curtain out of the lake, from under tons of gravel and mud. The high strength hardware of the curtain permitted the use of heavy equipment to pull on the components and remove the trapped curtain sections. A tugboat was used to tow other sections of the curtain to shore and to pull it from under the mudslide material. The tugboat was fully taxed, pulling at full throttle while changing the direction of pull until it had dumped the debris from sections of the submerged curtain. Finally the boom rose to the surface with the skirt intact but in need of repairs. Although the curtain was not designed to be handled in this manner, there were no other options to retrieve the curtain. Beach reconstruction was stopped until this secondary curtain was redeployed. Working in the low water window and with contract deadlines, there was plenty of motivation for all

parties to cooperate and repair the secondary curtain. The tugboat oper-ator and the contractor’s equipment operators were instrumental in making the extraction of the curtain a success. It is important to note that when the curtain material for this project was selected, the project managers were correct in choosing a highly durable system. A less expensive, lighter-weight composition may have been a sufficient barrier for the turbidity issue, but would have been rendered useless in such severe circumstances.

Repair The barrier sections were brought to shore and laid out for damage inspection. Damage ranged from minor tears and holes of 300-600 mm (11.8 in. to 23.6 in.) in size, to large gaping rips, and the total tearing away of the ballast chain and sleeve. The majority of the damaged occurred to the lower sections of the curtains, and to the grommeted and reinforced side connection areas (Photo 4). As part of the supply and install contract, AGES Inc. offered a 24 hour callout service. A technician was required to travel 900 km (559.2 mi.) to provide emergency repairs, and was on site with repair equipment within 19 hours of the call. The repairs included the use of new connection hardware, chain links, cable connectors, bolts and on-site sewing. All of the curtain materials were recovered, and thus the repair consisted mainly of re-attaching ballast chains and sleeves, re-sewing side grommetted sections and patching torn areas. New ballast sleeves were sewn in, chains re-inserted, and new side connection sleeves created. The majority of these repairs were done with a portable, hand-held sewing machine equipped with high strength nylon tread. None of the floating boom sections were damaged or ruptured. As repairs were completed the sections were again accordion folded under the floating boom, tied, and readied for re-deployment. The repair took ten hours on-site, and the curtain was re-deployed the next morning using the same procedure as the first deployment. Performance It was required on several occasions to add curtain sections and to supplement the original sections when gravels or other bottom materials moved onto the base of the curtain. The process of excavation and fill placement, coupled with the steep lake bottom, con-tributed to gravel and mud sliding onto the base of the curtain. As the base was buried, the curtain was

Photo 4: An example of the damage the mudslide caused to the curtain's grommeted and reinforced side connections.

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pulled down, and this was the cause of several short-term breaches of the primary curtain (due to its proximity to the work area). Morrow Environmental Ltd. mon-itored turbidity levels and results showed turbidity levels throughout the project duration were maintained within required levels. Three zones were monitored: the immediate work area up to the primary curtain, the zone between the primary and sec-ondary curtains, and the lake past the secondary curtain. The turbidity levels in the containment zones ranged from approximately 100 NTU to 700 NTU, while measurements taken immed-iately outside the secondary curtain in the lake averaged 4 NTU. This dramatic difference is clearly visible in the Photo 5. Removal The curtains were left in place at the completion of the beach reconstruc-tion work for approximately three weeks. This allowed the lake to reach it's maximum elevation and rinse residual fines from the coarse beach rock. Monitoring of turbidity continued at lower frequency during this period to confirm that values were consistently less than the regulatory criterion and consistent with back-ground values. Once the lake had crested, approval was sought from the provincial regulator to remove the barriers. The curtains were removed in a similar manner to which they were deployed. The sole difference being that they were pulled to the opposite shore of the bay to take advantage of vehicular access (the dock on the other side of the bay could no longer be accessed). Each panel was discon-nected and folded up as it came to shore. It was not necessary to wash or rinse the curtains as they had "self-cleaned" during the three-week post-construction period. The recovered curtain panels were sold by the mining company for re-use at a different site.

Design Modifications When mass movement of lake bottom debris is a potential occurrence, the floating barriers can be modified to address this issue. Had this been a foresight on this project, measures could have been implemented to mitigate the problem. A potential solution to this issue would have been to design the curtain with an appropriate anchorage and float system that would allow moving debris on the lake bottom to pass underneath the curtain. Using addit-ional buoy anchor points and having a relief mechanism for the curtain base could accommodate this modification. Summary The floating barrier system was the correct selection for this project. The project engineers, general contractor, floating barrier specialists, owner, and regulators all contributed to the suc-cess, and participated in the learning opportunities the project presented. The turbidity curtains succeeded in containing the turbid waters during construction, and secured the lake from potential contaminants. The

largest obstacle to the proper func-tioning of the curtains was the movement of gravels and mud on the lake bottom. Design modifications of the curtain can be implemented to avoid this issue on future projects. The barriers were sufficiently durable to withstand severe and unforeseen treatment and were capable of being repaired and re-deployed in a short period of time causing little down time for the contractor. The salvage value of the floating barrier system added to the success of the selection and application of a floating barrier system for this reclamation project. Being one of the first Canadian applications of turbidity curtains for a project under such rigorous condit-ions, Canadian regulators and mining officials across the country can be more confident when utilizing this procedure in the future. _______________________________ Murray Banting is a Registered En-gineering Technologist and President of the Calgary, Alberta-based Applied Geo-Environmental Solutions Inc. (AGES). Ryan Wonnacott, B.Sc., is in the Technical Department with AGES.

Photo 5: The water behind the curtains (gray areas) showed turbidity levels of 100 to 700 NTU, while water immediately outside the secondary curtain (blue area) averaged 4 NTU.