Holding ground on the waterfront

Seismic design for marine structures is an evolving science

"Necessity is the mother of invention." This old adage rings true in the area of seismic design and strengthening provisions for maritime or waterfront facilities. To the surprise of the uninitiated, there are currently no formal or prescriptive seismic design and strengthening provisions for marine structures in the United States.

Engineers and their clients are now faced with a set of challenges and conditions that often generates innovations, new applications of existing seismic technology and a collaborative approach with owners and regulatory officials.

The Pacific Northwest region is seismically active with several recognized sources of earthquakes: a shallow crustal event capable of producing up to a magnitude 6 to 6.5 earthquake; a deeper source that produced earthquakes of magnitudes 7.1 and 6.5 in 1949 and 1965, and is capable of producing an earthquake with a magnitude as great as 7.5; and the Cascadia Subduction Zone that could generate earthquakes of magnitude 8 or greater. Finally, the recently discovered Seattle Fault provides another source of seismic activity.

Generally there are three ways waterfront structures are vulnerable to earthquakes:

Earthquake ground motions and resulting vibrations resonate in amplitudes that exceed the structure's load or displacement capacity. Waterfront structures are typically massive because they must be designed to carry heavy service loads and resist large environmental loads and berthing loads from ships. This mass tends to increase the forced vibration demands during seismic design.

Because waterfront structures are located near shorelines they are commonly built on loosely consolidated river (deltaic) deposits, such as the Duwamish Head, or loose man-made hydraulic fills, such as Harbor Island and the Central Waterfront. These loose soils can lose their shear strength during earthquakes (liquefaction), and as a result cause slope failures, settlements, and increase loads on retaining structures. The slope movements can cause pile failures in piers and wharves; displacements of crane rails, and settlements can cause severe cracking in pavements and damage to buried pipelines.

At the Port of Seattle's Terminal 5, a 400-foot wharf extension incorporated a technique combining the use of timber pinch piles and enhanced pile cap connections.

Damage at a site depends on many factors such as the magnitude of the earthquake, distance to the epicenter, site soil conditions and type of structure. Seattle's waterfront is a complex mixture of geological and human-created elements that could potentially spell disaster for residents, businesses and structures in the event of an earthquake of a 7.5 magnitude or greater.

The Olympia-area based quake in 1949 (7.1 magnitude) and the 1965 quake here (6.5 magnitude) resulted in considerable damage to dock structures, piers, buildings and facilities. During the 1949 quake, structures in near the waterfront on filled ground areas suffered considerable damage. Major water spouts were observed from ground fissures.

Similar widespread damage occurred in 1965, when nearly every waterfront facility at the Port of Seattle was damaged to some extent. Structures shifted toward the water, bulkheads (and the fill behind them) settled dramatically.

Seattle's downtown waterfront can have a high liquification potential. This is because the existing shoreline is built on reclaimed tidal flats filled with sluicing material from adjacent upland areas, including the famed Denny Regrade project early in this century.

Finding solutions

Historically, designers have tried to incorporate the Uniform Building Code seismic design criteria into waterfront design. However, since the UBC is developed primarily for building structures where site soils do not liquefy and permanently displace significantly during earthquake ground motions, this extrapolation is marginal at best. A better approach is to adopt the intent of the UBC criteria which can be summarized as follows: resist minor levels of earthquake ground motion without damage; resist moderate levels of earthquake ground motion without structural damage, but possibly experience some nonstructural damage; resist major earthquake ground motion without collapse, but possibly with some structural as well as nonstructural damage.

By understanding the basic seismic design for waterfront structures and implementing the intent of the UBC, a number of design and engineering innovations have been spawned from the unique challenges set by waterfront structures.

At the Port of Seattle's Terminal 5, a 400-foot wharf extension uses timber pinch piles and enhanced pile cap connections. Also, in order to implement the intent of the UBC Code, a two-level, performance-based seismic design criteria was developed to maintain terminal serviceability following a moderate (6.5 magnitude) quake while maintaining life safety and structural stability during a major earthquake. At this project, the additional cost to meet the serviceability criteria came with a price tag slightly more than the minimum construction requirements for the major level earthquake, however the added service value was desired by the owner.

Located in the highly unstable region of the West Duwamish estuary, slope stabilization measures were required to secure the wharf under-dock slope, which was subject to failure due to earthquake induced liquefaction.

By recycling existing timber piles from a demolished adjacent pier, the stability of the underdock slope was enhanced to meet the seismic performance requirements. The timber piles were driven into the lower half of the slope in a 5 foot square grid pattern, both densifying the soil to reduce the potential for liquefaction and providing additional soil shear strength at failure planes. The use of recycled materials was extremely cost effective and reduced disposal costs.

At the Port of Long Beach, which is situated in Zone 4 (near-fault conditions), a batter pile lateral system was connected to the wharf. In recent earthquakes, batter pilings have had a higher failure rate than desirable during a moderate or higher level quake, primarily because they are too stiff to resist deformations.

Rather than throwing the high lateral strength batter piling out with the proverbial bath water, a system was developed using a yielding link between the batter piling and the wharf. During moderate level events, slope deformations are small and the link stays elastic; however, during major level earthquakes slope deformations are larger and incompatible with the batter piling. To avoid failure, the link is designed to yield inelastically and essentially un-couple the slope deformations from the batter piling and preclude failure.

Because of the lack of design code, establishing a collaborative dialogue with design review and regulatory officials in the pre-design process is essential. Since waterfront structures fall "somewhere between a building and a bridge," engineers are forced to make early decisions and judgements about the design criteria. It is important to have early discussions with local design review and permitting officials when setting these criteria in order to head off potential-and costly-problems down the road.

Close, strong working relationships must be forged between owners, scientists, engineers, city planners, government officials and other involved parties, so that the nature of geological hazards in marine and coastal environment is better understood and the resulting negative impacts on people and property are minimized. By clearly understanding the seismic design problems, communicating the performance goals to all parties and utilizing current technology, cost efficient seismic designs can be accomplished for waterfront structures.

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