A large number of structures in coastal regions or crossing waterways are supported on timber or wood piles. In an unforeseen sequence of events, our successes in cleaning the nation’s waterways have made the comeback of many marine worms possible (Read Recent New York Times Article). These worms attack wooden piles and there has been a surge of infestation and deterioration of these piles in recent years. The figure below shows one of thousands of bridges in the Midwest with deteriorating timber piling. The problem is not limited to smaller structures; the Tappan Zee Bridge in New York, for example, is supported on 303 miles of timber piles!
Shortcomings of Current Pile Jacket Systems
Unfortunately, an engineer faced this problem will soon realize that the choice of systems for strengthening submerged piles is limited at best. Various rubber or glass FRP jackets are available on the market. While these jacket materials themselves are durable, they offer little in way of long term repair and strengthening.
Some of the jackets available in North America are depicted in the left figure abovet. The tensile strength of these jackets varies between 12 to 24 ksi which is significantly lower than the 60-155 ksi strength offered by PileMedic™ laminates. A critical review of these systems makes it clear why they have not been successful in providing a long-term solution for this problem.
These jackets are typically constructed of two half shells made out of fiberglass or rubber 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 bolts to create a rectangular or square shell.
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 an engineering point of view, the vertical seam along the jacket whether glued or bolted in the field is a major weakness that results in:
a) a plane of weakness that minimizes the confining pressure offered by the jacket, and
b) a pathway for moisture and oxygen to reach the pile and fuel the corrosion process.
Many DOTs, for example, simply build a new 4-6 inch thick reinforced concrete collar around the existing pile, similar to those shown in the photograph above. Obviously it will be only a matter of time before the new concrete will suffer the same fate as the original pile.
Repair in Shallow Waters without Coffer Dams
The procedure for repair of piles in shallow waters is shown on the right where a number of deteriorated concrete piles were repaired in Miami, FL. The procedure for repair of a timber pile will be very similar to the steps shown here. As the figure indicates, the laminate can be cut to the desired length. QuakeBond™ 220UR (underwater Resin) epoxy is used that cures in water; this is significant as it eliminates the costly construction of coffer dams. The sheet is coated with QuakeBond™ 220UR epoxy (Fig. (b)).
Workers walk into water to wrap the PileMedic™ laminate around the pile and create the shell. Ratchet straps can be used to temporarily hold the shell in desired shape (Fig. (c)). The bottom of the shell can be sealed in a variety of ways. In this particular project the shell rested on the soil at the bottom of the canal. In most timber piles used in short span bridges that would b.ve the case. A special underwater tremie mix is pumped into the annular space (Fig. (e)). As the tremie mixture rises to the top, it displaces the water in the annular space until the entire annular space is filled with grout.
Repair of Submerged Piles with Minimal Use of Divers
In repair of deeper piles such as those encountered in ports, larger bridges and off-shore structures, a significant cost of the repair is for divers that must be hired to install these jackets. The use of PileMedic™ laminates eliminates or significantly reduces the need for divers. As shown in the figure below, the crew can start construction of the shell directly on the deteriorated pile from a work platform on a barge. If needed, spacers can be attached to the pile to fix the size of the annular space, Fig. (a). The PileMedic™ laminate can be cut into narrower strips (about 12-24 inches wide) for ease of handling.
The PileMedic™ laminate is wound around the pile in a helical manner to create a shell (Figs. (b) and (c)). After a couple of turns, the shell has enough stiffness that it can be lowered into water (Fig. (d)) and the process continues until a shell of desired height is constructed. At that point the bottom of the shell is sealed (Fig. (e)) and a ratchet strap is used to temporarily secure the top end of the shell and prevent it from unraveling. The sealing of the bottom can be achieved with an embedded rubber bladder (similar to a bicycle tube) or oakum rope that expands when exposed to water. Once the annular space is filled, it can be sealed at the top and the grout or resin can be pressurized to penetrate the deteriorated pile.
Use of Grout vs. Epoxy Resin
A preferred method for repair of timber piles is to fill the annular space with QuakeBond™ 320LV which is a low-viscosity moisture-insensitive resin. The use of low viscosity resin is a unique feature offered by the "seamless" PileMedic™ system; all other commercially available jackets have a seam or bolted connection along the side that will allow the resin to escape from the annular space. Moreover, with the seamless shell provided by the PileMedic™ system, the pressurized epoxy can find its way into all cracks and voids in the timber pile, resulting in a stronger and longer-lasting repair.
The low viscosity QuakeBond™ 320LV resin that is injected into the annular space can penetrate all voids and crevices of the wood, producing a solid section. As shown in the figure to the right this procedure has been successfully documented. Because the strength of the epoxy used for injection is about twice that of the timber, the filling of the voids with QuakeBond™ 320LV epoxy results in a net gain in strength for the pole. In addition, the strength of the PileMedic™ laminate that is bonded to the exterior of the pole contributes to flexural resistance of the pile. If necessary, a few long grooves can be cut along the height of the pile and reinforcing bars can be embedded in the timber pile prior to wrapping.