Project Description

Construction of a wharf extension South of quay 24-4 in the National City Marine Terminal included extending the shoreline from its previous location by installing a cellular sheet pile bulkhead and filling behind the bulkhead. A pile supported wharf was constructed on top of and outboard of the sheet pile bulkhead. Filling was required behind the bulkhead to develop the backland area. The new pile-supported wharf extends 1,025 feet and is 75 feet wide for the current phase of development.

The project is located within the "San Diego Embayment" which consists of a fault bounded, sedimentary basin. On a seismic standpoint, the site falls in the Rose Canyon Fault Zone, the nearest major active strand being the Silver Strand Fault laying approximately at 2.5 miles distance, east from the site.

Seismic Mitigation

Seismic analyses carried out on the basis of the preliminary soil investigations indicate that the majority of the existing fills and bay deposits have a high potential for liquefaction to depths of approximately 50 to 60 feet. Manifestations of soil liquefaction can include loss of bearing and lateral capacity for foundations, surface settlement and tilting in level ground. Soil liquefaction can also result in instabilities and lateral deformations in areas of sloping ground and lateral spreading on reclaimed areas.

Improvement of the fill characteristics inside the caissons and at the underwharf slope - using the vibro-compaction method - was therefore critical to provide stability of the system and insure good performance of the new wharf. Vibro-replacement method is a modification of vibro-compaction in which granular columns (stone columns) are used to backfill the hole created by the vibratory probe. This method is used on finer grained material as it provides reinforcement of the soil and drainage for excess pore water pressures.

The currently on-going soil densification works are performed after the cells are driven and the slope in front is cut. It necessitates a bottom feed system designed to operate under off-shore conditions (slope). Monitoring of the columns installation using crane on-board instrumentation is an important tool for quality control during the works.