Underwater Piles

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A large number of structures in coastal regions or crossing waterways are supported on piles. These piles are commonly made of concrete or wood. Additionally, steel piles are often encountered in certain structures such as offshore oil platforms. The adverse environment introduced by seawater, high humidity/high temperature, and dry-wet cycles causes rapid deterioration of these structures. The most severe conditions are found in the splash zone area which encompasses the portion of the pile between the low and high tide water levels. The high concentration of chloride ion in seawater allows it to penetrate and reach the steel even in high-quality concrete. As a result, the passive layer that normally protects steel is destroyed making corrosion of reinforcing or prestressing steel inevitable.

Since corroded steel occupies a larger volume, it causes lateral forces on the surrounding concrete that far exceed the tensile strength of concrete. This results in cracking and spalling of the cover concrete. The deterioration of concrete and the loss of cross sectional area of the reinforcing steel results in reduction in capacity of the piles. Repairs often require creating a formwork around the pile and filling it with grout or concrete to enlarge the section, thereby increasing its load-carrying capacity. The wet marine environment combined with access difficulties are not suitable for use of formwork made with wood or steel. In the last twenty years, Glass Fiber Reinforced Polymer (GFRP) jackets have become a material of choice for formwork in marine construction. In many cases, these forms are used as stay-in-place forms.

GFRP jackets are lightweight and moisture tolerant and have been used extensively for underwater pile repair. A handful of manufacturers supply the majority of the products used in north America. These products are typically constructed of two half shells made out of fiberglass that are bolted or glued together or held together with external straps to form a jacket around the pile. In some cases, the connection may be a tongue and groove type that gets filled with epoxy in the field. There are also other forms where the two half shells are supplied as U-shapes where the sides are overlapped and bonded together in the field with epoxy or screws to create a solid rectangular or square shell. Some of the jackets commonly used to date are shown in Fig. 1.

All existing pile jackets require advance planning for the contractor to order the right size jacket for the project. Often times the jackets are available in standard sizes only. This leads to large annular spaces between the jacket and the pile which is unattractive and adds to the weight and cost of the grout or resin that is needed to fill the annular space. The jackets are also bulky, resulting in higher shipping and storage costs. From a structural engineering point of view, the vertical seam along the jacket whether glued or bolted in the field introduces a plane of weakness that minimizes the confining pressure offered by the jacket. To overcome these shortcomings, a new product has been developed recently as described below.

PileMedic™ Laminates

Repair and strengthening of structures by external bonding of FRP products was introduced by the author in the late 1980s. The technique, known as wet layup, includes saturating fabrics of carbon or glass with resin in the field and applying them to the surface of beams, columns, walls, slabs, pipes, etc. By the next day when the FRP is cured, it forms an impervious sheet that is 2-3 times stronger than steel. This technique has seen worldwide acceptance and growth in the last two decades for repair of buildings and bridges. While new advances have been made to develop resins that cure underwater, repair of piles with this technology is not very efficient. Among the major disadvantages is the fact that wet layup systems can be applied to smooth surfaces. For deteriorated underwater piles, the forming of the surfaces requires construction of coffer dams, adding significant expense to the project. It is therefore preferred to use FRP forms which create a shell around the pile, allowing the annular space to be filled with grout.

The author has recently developed a new form of FRP called PileMedic™ that offers major advantages for repair of piles. Sheets of glass or carbon fabric are impregnated with resin in the manufacturing plant and pressed together under high pressure and heat to produce very thin laminates. Laminates with thickness of ¼ inch or higher have been available for years and are fairly easy to manufacture. However, PileMedic™ laminates vary in thickness between 0.010 to 0.025 inches. The development of a process and specially-designed equipment to allow construction of such uniform thin sheets is not a trivial matter. Depending on the design requirements, the laminate may include multiple layers of uniaxial or biaxial carbon and glass fabrics. Typical laminate rolls are less than 0.025 inch thick X 4-ft wide X 300-ft long (Fig. 2).

The higher quality control of the manufacturing process allows production of laminates with tensile strength higher than 150,000 psi. By mixing and orienting the fibers in different directions, laminates with infinite number of strength and stiffness characteristics can be produced. In the field, the laminates are cut to desired size and wrapped around the pile to create a multilayered shell of any desired size. The annular space between the shell and the pile is filled with grout or resin. A moisture-insensitive resin that cures in water is applied to the laminate as it is wrapped onto itself to create this free-standing shell.

Case Study

The first application of PileMedic™ for repair of underwater piles was recently completed at Guilford House Condominiums, Bay Harbor Islands, Florida (Fig. 3). The project is located on the environmentally sensitive South Florida intra-coastal waterway. Approval from the Florida Department of Environmental Protection DEP was required prior to restoration.

The 14-inch x 14-inch piles (Fig. 4a) had substantial steel corrosion and spalled concrete. There was no original design or repair methodology. The contractor, CSI, recommended using the new system for the piles that supported an overlook on the pool deck. Use of conventional pile jackets was contemplated. However, the contractor chose the PileMedic™ system for ease of installation and the added structural enhancement.

The laminates were packaged in 4-ft wide x 300-ft long rolls. The manufacturer’s specification requires a minimum double-layer wrap plus an 8-inch overlap beyond the starting point. This reduces the inter-laminar shear stresses between the two plies of PileMedic™ and ensures that the pile is confined by two plies of laminates all around, i.e. 360⁰. Unlike the jackets shown in Fig. 1, this system leaves no plane of weakness along the height of the jacket.

For this application, it was decided to encase the piles in 21-inch diameter cylindrical shells; this was a tightly fitting circle that would leave a small gap (about ½ inch) between the jacket and the pile at the four corners of the piles. The circumference of such shell is 66 inches. Thus, the laminates were cut into 66+66+8= 140 inch long pieces. This allowed for creating a two-ply cylindrical shell with a diameter of approximately 21 inches plus an 8-inch overlap at the end.

The two-component epoxy supplied by the manufacturer is environmentally safe and complies with the highly stringent requirements for lining potable water pipes. The epoxy is also moisture-insensitive and it cures under water, eliminating the need for any coffer dams. The epoxy was mixed using a jiffy mixer and it was applied with a thickness of 40 mil to the 66+8=74 inch length of the laminate; the first 66 inch length of the laminate that will be placed next to the pile does not need to be coated with epoxy (Fig. 4b).

Since the water was relatively shallow, the workers could pick up the laminate and walk it in water. At this time the laminate is wrapped around the pile to create the 21-inch diameter shell (Fig. 4c). At this stage, the uncured epoxy coating serves as a lubricant allowing the laminate to slide easily as the crew makes final adjustments to the diameter of the shell. Ratchet straps were used to fix the diameter of the shell and prevent it from unraveling before the epoxy cures (Fig. 4d). The cylindrical shell can be moved vertically along the height of the pile to its final position. In this case, it was pushed down into the mud to create a seal at the base.

An underwater grout was mixed and pumped with a hose into the annular space between the jacket and the pile (Fig. 4e). As the tremie mix rose to the top, it displaced the water in the annular space until the entire annular space was filled with grout. At this stage, the hydrostatic pressure of the grout pushes the two layers of the PileMedic™ jacket tightly against one another while the ratchet straps prevent the jacket from opening up. At the same time, the heat generated from the hydration process of the grout helps with the curing of the epoxy. Depending on the ambient temperature, the epoxy will cure in several hours at which time the ratchet straps can be removed. The repaired piles at the conclusion of the project are shown in Fig. 4f.

In repair of underwater piles, it is common to schedule the work around low tide hours and working continuous 8-hour shifts may not be efficient. Nevertheless, on this project, the repair of fourteen piles was completed in 4 working days using a 3 man crew.

Summary and Conclusions

Following his introduction of FRP products to the construction industry some twenty years ago, the author presents a new form of FRP laminate sheets. This product is a major advancement that offers unique solutions to a number of repair and retrofit problems that are not possible with conventional laminates or the wet layup systems that have served the construction industry for the last twenty years. This paper focuses on the application of these laminates to repair of underwater piles.

The high-quality plant-manufactured laminates reduce field construction time and cost significantly, while improving the quality of the repair.