Timber Piles

As described in Section 3.7, there are many causes of deterioration of timber piles in the marine environment. There are also many methods for the repair and maintenance of timber piles. Repair and maintenance methods fall into four categories:

• Concrete encasement

• Partial replacement

• Complete replacement

5.9.1 Concrete Encasement

Concrete encasement can be used for timber piles that have not deteriorated to less than 50 percent of their cross-section area. The general procedures for repairing timber using concrete encasement are described in Table 5-21. They are essentially the same as for steel piles as described in Section 5.5.1. Planning and estimating data are included in Table 5-29. Some general information is provided below. Refer to the figures in Table 5-21 for an outline of the procedure. The repair procedure involves:

• Cleaning the pile

• Installing reinforcing, spacers, and forms about the pile

• Pouring concrete to fill the space between the pile and the form

Nonmetallic spacers are used to maintain a uniform concrete thickness and ensure a minimum (2 inches) distance between pile and reinforcing steel and reinforcing and form. Each form manufacturer provides detailed instructions relating to installing their specific designs.

Table 5-28

Planning and Estimating Data for Concrete Sheet Pile Repair Using Concrete Reconstruction

Description of Task: Repair a severely deteriorated 12-inch-thick concrete sheet pile wall by reconstructing the deteriorated portion with cast-in-place concrete. Unit area to be reconstructed is 10 ft2. Access to the wall is from both sides.

Size of Crew: Dive station, two laborers.

Special Training Requirements: Familiarity with procedures for removal of marine growth, concrete pump/truck operations, concrete cutting/chipping machine operation, concrete formwork fabrication and installation.

Equipment Requirements: High-pressure waterblaster, hydraulic grinder with Barnacle Buster attachment, high-pressure pump for waterblaster, hydraulic power unit, hydraulic drill with bits, hydraulic chipping hammer, hydraulic hammer drill with bits, oxygen arc cutting equipment, rigging equipment, float stage or work platform.

Productivity of Crew: 40 hours per 10-ft2 patch. Materials:

Concrete Form Material - The concrete form is required for casting the repair section. The formwork must exist on both the front and back of the repair section and should extend a minimum of 3 inches over the existing sound concrete. The frontal plus the rear area of the repair plus the form overlap is the required area of formwork.

Steel Reinforcing Bars - Steel reinforcing bars must be placed in the repair section to replace those bars that were removed. Estimates can be made by measuring the gaps between sections of sound reinforcing steel. Allow for sufficient lap of new bars with existing bars.

Concrete - Concrete quantity is determined by the volume of the area to be filled. A conservative estimate should include an allowance of 10 percent extra concrete over the theoretically calculated quantity for waste and overfill.

Potential Problems: There may be difficulty in matching the existing concrete. Care must be taken in removing the forms; removing them too early will result in the concrete sagging at the top and bulging at the bottom. Forms must be dewa-tered before pouring concrete, if possible, or underwater concrete placement techniques must be used.

underwater maintenance and repair procedures

Table 5-29

Planning and Estimating Data for Timber Pile Repair Using Concrete Encasement

Description of Task: Repair a deteriorated timber pile by installing a concrete encasement from 1 foot above the high waterline to 1 foot below the mudline. The total length of encasement is 20 to 30 feet.

Size of Crew: Dive station, two laborers.

Productivity of Crew: 8 hours per pile repair.

Special Training Requirements: Familiarity with manufacturers' instructions for the rigid or flexible form used, concrete pump operation, jetting or air lifting procedures, and removal of marine growth.

Equipment/Materials Requirements: High-pressure waterblaster, hydraulic grinder with Barnacle Buster attachment, hydraulic power unit, concrete pump with adequate hose, concrete mixer (if ready-mix concrete is not available), jetting pump and hose, rigging equipment, float stage, scaffolding.

Form Material - Either flexible or rigid forms may be used. Follow manufacturers' recommendations regarding lengths and diameter of forms, top and bottom closures, spacers, bands, straps and special fittings. Forms are ordered prefabricated in the required length and diameter. For flexible forms, allowance on the length must be made for extra fabric that may be required around blocking at the top and bottom of the jacket.

Spacers - A conservative estimate of the number of spacers must be made. Rough water and batter piles will require more spacers than calm water application.

Wire Mesh Reinforcing - Usually 6x6-10/10 welded wire fabric is adequate. Calculate the width of wire fabric based on its circumference, taking into consideration the thickness of the spacers. Allow a 9-inch overlap of the ends.

Concrete - Be conservative on the amount of concrete required. Usually an allowance of 10 percent extra for stretching jacket and waste is adequate.

Form Reinforcing Straps and Special Fittings - The spacing and number required depend on the type of form and the hydrostatic pressure of the concrete fill. Some types of reinforcing straps are reusable; allow for a loss of 20 percent of the straps each time they are used. In addition to reinforcing straps, closure forms, blocking hangers, pump inlet valves, and clamps will be required, depending on the forming system being used.

Both flexible and rigid forms are available and manufacturers are listed in Appendix A. Forms may also be fabricated from materials on site. Selecting the form depends jjrimarily on availability and choice of the designer or construction crew.

Flexible forms fabricated from a porous fabric are usually left in place. Both open and closed top models are available. Open top fabric forms are usually suspended from the piling using nails or hooks. After filling, the concrete is topped with an epoxy cap or nonshrinking grout at a 45-degree angle as shown in Figure 5-17.

Rigid forms are generally open top and bottom and are removed and reused after the concrete has completely cured. Rigid forms can be fabricated from material found at the construction site. Rigid forms require a bottom seal and base plate to hold the form in place. A temporary friction clamp and base plate conforming to the pile contour can be fabricated and attached to the pile at the bottom of the repair area (Figure 3 of Table 5-21). The rigid form rests on the base plate to form the bottom seal.

The piles must be cleaned of marine growth and rotten wood. Failure to do so would result in poor bond between the concrete and pile. It is also important to ensure that the concrete be poured within 72 hours (or sooner in wanner waters) following cleaning to ensure that new marine growth will not have formed on the pile surface.

Concrete should be pumped using 15-psi minimum pressure through a suitable pipe or hose that extends down to the lowest point in the form. It is important to start filling with the hose at the bottom and to keep the discharge submerged in concrete during the pouring operation. This procedure will prevent voids from forming. Care must be taken when using the hose-tremie method as there is a tendency for the hose to become snagged on the wire mesh reinforcing. Some forms have built-in filling and overflow ports where the lower port is used for filling. Filling from a bottom port is preferred over extending the hose to the bottom of the form.

5.9.2 Partial Replacement

The repair of timber pile structures may involve the replacement of sections of the pile members. Sections are typically replaced using one of several posting techniques or by fish plating techniques.

A major drawback to these timber replacement techniques is that the replacement section is susceptible to marine borer attack, particularly at exposed cut ends. This can be minimized by covering the ends with a suitable preservative PVC wrap or encasing in concrete. Posting Techniques. Posting a damaged timber pile with a new pile butt post is described in Table 5-30 and illustrated in Figures 1 and 2. A water jet or air lift is used to excavate around the base of the pile. The damaged pile is cut below the mudline through sound wood. A new pile butt is cut to length and the top of the pile is cut to fit the pile cap. Slighdy undersized holes for drift pins are bored into the new pile butt section, after which the pile butt is maneuvered into place. Galvanized steel drift pins are driven into the piece of pile remaining in the seabed. Shims are placed between the pile and pile cap and held in place with 1-inch-diameter galvanized steel bolts. The area around the base of the pile is then backfilled.

Planning and estimating data for repairing timber piles by posting to the mudline are provided in Table 5-31 and by fish plating to below the low waterline in Table 531.

Table 5-30 Posting of Damaged Timber Piles

Problem: Severe damage to pile where greater than 50 percent of the pile cross section is damaged.

Description of Repairs: Many different techniques are available for partial restoration of severely damaged timber piles. In the majority of cases these repairs are considered temporary and should only be used as an interim solution until new piles can be installed.

• Cut out the damaged section of the pile to provide upper and lower sections with sound timber.

• Cut a replacement stub to fill the gap and shape end to fit pile and stub cap. A replacement section with the same diameter or cross section is recommended. If similar repair section is not available the bundled (strapped together piles) or larger piles may be used.

• Treat all wood, cut ends, and bolt holes with a suitable preservative.

• Install new pile section and attach to pile butt and pile cap shimming to pile cap for tight fit:

DOWEL METHOD (Figure 1) - Drive 1-inch-diameter steel dowel into the old pile smb. Bore hole for dowel in replacement section. Lower and maneuver replacement section in place.

DRIFT PINS (Figure 1) - Bore slightly undersized holes (see detail) for the drift pins in new pile section and maneuver into place. Drive galvanized drift pins through holes into old pipe stub.

PIPE SLEEVE (Figure 2) - Drill holes in pipe sleeve for 3/8-inch-diameter galvanized spikes and slide pipe sleeve over old pile butt. Maneuver new section into place and slide sleeve up so 1/2 length covers replacement section. Drive spikes through holes into upper or lower section to hold pipe sleeve in place.

FISH PLATES (Figure 3) - For joints below water, build two 3/4-inch-thick steel plates 36 inches long and a minimum of 3 inches wider than pile diameter.

TIMBER SCABS (Figure 4) - 2-4X12 treated timber scabs (4 if replaced section is longer than 20 feet). Through bolt to pile using 1-inch-diameter galvanized bolts, with nuts and 2 OGEE washers.

Note: These methods are typically for temporary repair. Extend useful life by using concrete encasement. Even with X-bracing, weak joints will exist and all splices and holes below water may be subjected to accelerated marine borer attack. PVC pile wrap may be used to prevent borer attack.

Existing deck stringer Existing pile cap

New treated timber pile post

MLW Backfill above splice joint

Existing pile cut below mudline

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