Piers have many uses, but the most basic is to provide access from land into a body of water. Many government and public entities require this access to water but probably the most common are the United States Navy and the United States Coast Guard. The Navy and Coast Guard use piers for the mooring of ships as a homeport and for the loading and unloading of personnel and materials. In addition, piers are commonly used to support mobile and gantry cranes for ship repair and material handling operations.
In our experience, the most common type of pier is an open timber pier. Timber piers are often used for residential water access and light industrial applications. The structure of timber piers is relatively simple and typically consists of pile bents, stringers and decking. A pile bent consists of a line of timber piles that can vary in size, with a typical minimum diameter of 12 inches for industrial piers. As part of each pile bent there are typically two large beams – one on each side of the piles – at the top of the pile bents called pile clamps or headers. Pile bents are typically spaced at 10 to 15 feet on center along the length of a pier, allowing water and marine organisms to pass under the structure, which is why it is referred to as an open structure. Bearing on top of the headers are closely spaced heavy rectangular timber stringers that span between the pile bents. Laid flat on top of the stringers are more heavy rectangular timbers called decking. The lateral resistance for timber piers typically consists of batter piles and/or timber braces within or beside the pile bents. These batter piles are driven at an angle to resist horizontal loads from ship berthing and wave action and to provide the load path for the transfer of those loads to the ground.
The second most common type of pier consists of concrete slabs on precast concrete or steel piles. These types of piers can typically handle heavier loadings than timber piers; consequently, the piles are driven significantly deeper than the timber piles of timber piers. The structures of concrete piers are relatively similar to the timber piers described above, with pile bents at variable spacing and decks spanning between these bents. Most commonly, the piles consist of square or hollow cylindrical precast concrete or steel pipe piles. The decks can consist of monolithic cast – in – place concrete slabs or concrete slabs on various concrete beam elements. Frequently, precast concrete slabs are used as stay – in – place forms that act compositely with a cast – in – place concrete topping. In addition, the use of precast concrete for beams and pile caps is also relatively common.
A less common type of pier consists of concrete slabs and beams on fill material and timber piles surrounded by a series of large steel sheet pile cells. These types of piers do not allow for water to pass below and are thus called closed piers.
The primary enemy of timber structures is marine borers that attack the timber structures in the splash zone and at the mudline. As the name implies, these organisms bore holes into the structural elements. Some borers attack the outer surface, while some borers destroy the cores of the elements. When the borers attack the outer surfaces, the weak material left at the surface is easily removed by waves and other forces, eventually causing an hourglass – like shape of the piles or brace members. Most commonly, timber elements are protected from these organisms by pressure – treating with preservatives. These preservatives are very similar to those used in residential applications – applied in much heavier doses. Some states prohibit the use of pressure – treated lumber and other materials susceptible to leaching. Alternative, non – polluting materials include plastic, greenheart or other untreated wood, polymer coated pressure – treated wood, concrete or other inert products.
Deterioration of concrete occurs most often when the reinforcing steel inside the concrete corrodes, which is caused by moisture and chlorides penetrating the concrete and reaching the steel. When the steel corrodes, it expands and causes the concrete to spall off. Concrete can be protected from this type of deterioration with admixtures, increased cement content and various types of cement and cement substitutes (like silica fume) that help reduce the penetration of moisture and chlorides. Alternative or additional protection can be provided with coatings on the steel reinforcing, such as epoxy paints or hot – dip galvanizing. Adequate cover over the steel reinforcing, an adequate amount and spacing of reinforcing and proper curing to control cracking are the most cost – effective ways of protecting the concrete from deterioration.
Steel elements like pipe and sheet piles deteriorate due to corrosion caused by moisture and chlorides. In the case of the sheet pile cells, corrosion holes in the steel sheet piles lead to loss of fill within the cells that can lead to additional problems, including instability or loss of strength of the overall structural system. Steel elements can be protected from corrosion with coatings such as paints and hot – dip galvanizing. Alternative or additional protection can be provided by using thicker sections than required by analysis and allowing portions of the thickness to corrode and to be sacrificed. In addition, cathodic protection with sacrificial anode material can be used to significantly slow the rates of corrosion. However, the initial investment costs associated with these systems can be significant.
Additional deterioration can be caused by overstressing the structural elements, leading to yield or rupture of the material. This type of damage can be prevented by designing the structural elements with adequate strength and providing fender systems that absorb ship impact energy.
Timber structures can deteriorate quickly with significant loss of section in less than 10 years or, depending on the local conditions and activity of marine borers, they can last 20 years or more without significant maintenance. Compared to timber, steel and concrete structures are significantly more durable and typically have much longer lifespans before maintenance is required.
Reasons for rehabilitation
While there are many reasons to rehabilitate a pier, the most obvious and most important is to maintain a safe work environment for the users. Occasionally, workers on the pier may become uncomfortable due to excessive movement of the pier or due to fear of structural failure of elements that have a poor physical appearance. This can lead to significant down time or work stoppages. Another significant reason to rehabilitate a pier is to reduce maintenance costs and potentially reduce insurance costs for replacement or liability. Occasionally, piers need strengthening to adapt to new uses, including new cranes, new equipment or the mooring of larger vessels. A more obscure reason for rehabilitation is to maintain the pier’s footprint over the water to facilitate the acquisition of future environmental permits for future construction.
Because of the significant cost of replacement and limited funding, owners often choose to rehabilitate existing piers. However, there are significant challenges to rehabilitation that may have been mitigated by complete replacement. Typically, a rehabilitation project begins with an underwater and above water inspection of the entire pier. These inspections can be costly and difficult, especially with timber construction, since there is often a forest of piles and braces below the deck that limit access. In addition, many piers have been repaired in the past, which may have added more piles and more bracing, which further limits accessibility. Furthermore, while underwater inspection is essential to verifying the condition of a structure, it can be challenging because of limited visibility due to water clarity and marine growth.
The forest of piles can also cause significant difficulty for design and construction. The locations of all new piles must be coordinated as much as possible with the location of the existing piles in order to avoid obstructions. This is further complicated with batter piles, since their depth is not always known and they cover much more lateral area.
Another significant challenge for pier inspectors can be confined spaces within the piers. This is especially true for sheet pile cells and utility tunnels within piers due to limited access points below the deck. These confined spaces require verification of adequate ventilation and verification that there are no other potential dangers like energized utilities.
Oftentimes, drawings of the existing structure are not available. Especially with timber piers, this lack of information can cause significant difficulties in determining load paths for lateral forces and in determining whether members are connected adequately to transfer the lateral loads to the subgrade. This can be further complicated by batter piles offset from pile bents and by previous modifications to the structure.
Another challenge encountered on many piers is the existence of utilities running along the pier. These utilities can be shore power, sanitary sewer, steam, potable water, and many others. These utilities can run above or below deck on one or both sides of the pier for a partial length or the full length of the pier. These utilities can be major obstructions that may have to be temporarily relocated until repair work is complete. Many times portions of the pier and portions of the utilities have to stay in service during construction, adding complication to the design and construction process. Further complicating matters, abandoned utilities are often encountered. These utilities have to be identified and investigated in order to determine whether they can be removed or if they have to stay in place.
Environmental permitting can frequently add constraints to the design and construction of pier rehabilitations. This can include limiting the footprint of new construction to within the footprint of the existing pier or slightly outside the existing pier. In addition, hazardous materials may be present in the structure or in the sediment below the pier that may require remediation or abatement.
The least expensive alternative is downgrading the allowable capacity of the pier. This is essentially the do – nothing alternative, but it still requires an inspection, assessment and structural analysis of the existing pier structure to determine an allowable live load.
If downgrading the load capacity for the pier structure is not an acceptable alternative, there are many ways to repair the structure or supplement it to add capacity. The most common method of repairing a pier consists of posting or encapsulating the piles. Posting of timber piles consists of cutting away the deteriorated sections, replacing those sections with new timber material and providing fish plates on either side of the new material to hold it in place. A slightly more expensive and more effective method of repairing piles consists of encapsulating the piles. This method can be used on timber, concrete and steel piles and consists of cleaning and wrapping the deteriorated area of the pile with a special form made of fiber reinforced plastic (FRP) or other material, placing reinforcing inside the form and grouting inside the form to fill all voids. This repair is typically done from above the splash zone down to slightly below the mudline. Deteriorated framing and decking members are typically repaired by selective replacement. Pile posting and encapsulation can be difficult and complicated work since it is typically completed underwater with construction divers and may require excavation. Encapsulation can return the piles to their original strength but posting typically does not.
An alternative to pile repair that may not involve underwater construction is driving new piles adjacent to deteriorated pile bents and creating new bents adjacent to the existing. This can be especially effective if the stringers or beams bearing on the existing bents are continuous over more than one span. Continuous deck framing can allow for a one – for – one replacement of bents, whereas simple span deck framing would require a new bent on either side of each existing bent, which is obviously more costly. Smaller supplemental bents can also be used to support crane runway beams or other specific concentrated loads. The advantages of these supplemental bents are that no or very little underwater construction is required and significant strength and lateral capacity can be restored depending on the configuration of the new bents. In addition, if the deck framing is in adequate condition, it can be left in place except where new piles have to be driven.
“The key to a serviceable, durable waterfront structure is a quality initial design and a maintenance program that is based on a solid understanding of the causes”
An alternative to supplemental pile bents, where vertical live load capacity is not as much of a concern as lateral mooring and berthing loads, is constructing new mooring or breasting dolphins within the footprint of the existing pier. These dolphins can vary in size and location along the pier, depending on the loadings and may consist of plumb and batter piles with a deck at the same elevation as the existing pier. These mooring dolphins could be connected to the existing pier or be independent, depending on the situation and condition of the existing pier. As with the supplemental pile bents, no underwater construction is required, but selective demolition of larger areas of the existing pier deck and piles below may be required. Another advantage to this system is that if the pier is eventually replaced, these dolphin structures can be incorporated into a new pier structure and provide significant lateral strength for the new pier.
All the alternatives above can be used on their own or in combination with each other. If the owner does not want to repair the existing pier and it has some remaining capacity, the pier structure may be used as soffit formwork for a new pier constructed directly on top of an existing pier. This method requires selective demolition of the existing pier deck at the specific locations of the new piles.
Factors affecting choice of proper solution
Many factors affect the choice of which repair solution to use. The primary factor is almost always the owner’s budget. Permitting requirements can also play a significant role in selection along with general live load requirements. Current and future use of the pier, including anticipated ships and lifting equipment on the pier, are also major considerations, depending on the loading requirements and necessary future dredge depths.
The key to a serviceable, durable waterfront structure is a quality initial design and a maintenance program that is based on a solid understanding of the causes of deterioration.
Jeffrey A. Fisher, P.E., S.E., is an associate at Clark Nexsen in Norfolk, Va. Contact him at jfisher@ClarkNexsen.com.