May 9, 2001: Water Online

GEOTECHNICAL WORK HELPS ANCHOR NEW BEACH RESORT

A new world-class resort will soon be a major tourist destination in Southern California.

Formerly a private mobile home park on bluff-top land, the anchor will be a 274-room luxury hotel that will also feature a number of quality retail shops and restaurants. Fourteen residential condominiums and 14 custom residential estate lots within the complex will have full access to the resort facilities and services. The condominiums and estates will be architecturally integrated with the design of the hotel. The project is slated to be completed in late 2002.

As part of the resort development, a 7.25-acre bluff-top park and publicly accessible resort gardens are being planned. Public beach access will be facilitated by two existing beach access ramps, an existing stairway, and construction of a new handicapped accessible boardwalk and stairway leading to the public sand beach.

Not surprisingly, a project of this magnitude required extensive geotechnical consultation. Law/Crandall was retained by the project developer, The Athens Group, to conduct a geotechnical investigation at the site and to provide cost effective solutions in designing the foundations. The firm also provided technical and regulatory consultation to the California Coastal Commission to satisfy the requirements for geologic review for stability and preservation of the seaside bluffs.

While the investigation outlined the static physical characteristics of the soils at the site, recommendations were provided for foundation design, floor slab support, excavation and earthwork for the project, which has one to five levels, including up to two levels below grade. The hotel will have up to about 490,000 square feet of floor area.

The natural soils at the site consist of marine and non-marine terrace deposits, which are composed of medium dense silty sand, sand, and sandy clay and medium stiff to stiff silty clay. Miocene-age San Onofre Breccia underlies the natural soils. The foundation recommendations comprised spread footings that could be used for the structures. Spread footings are square or continuous pads of concrete placed on the ground near the ground surface, upon which columns and walls are supported. Spread footings are also the least costly type of foundation for a building structure; their use for the structures saved considerable money, potentially millions of dollars, compared to deep foundations, such as drilled piles, which were being considered before Law/Crandall was retained.

Investigation services included:

• Subsurface explorations (borings) to determine the nature and stratigraphy of the subsurface soils, and to obtain undisturbed and bulk samples for laboratory observation and testing.
• Laboratory tests of soil samples to evaluate the static physical soil properties.
• Liquefaction study to determine the liquefaction potential underlying the site.
• Engineering evaluation of the geotechnical data to develop recommendations for the design of the project.

The excavation for the subterranean levels of the project presented the opportunity to use soil-nail walls instead of the standard shored excavation. Soil-nailing consists of excavating a vertical wall in the natural soil, and as the excavation process progresses, drilling small diameter holes nearly horizontally to reinforce the soil mass.

As a measure of protection against erosion and surficial sluffing, a layer is applied against the vertical soil surface. Soil-nailing is a relatively new technology for excavation, and has not been used significantly in Southern California for commercial developments because it does not have a long track record in the region (although it has been implemented widely elsewhere).

Soil-nailing was utilized for the beach resort because it was more cost-effective than conventional shoring. The concept behind a conventional shoring wall is to physically hold back the soil with a temporary steel and wood wall, which would generally require a row of steel soldier piles be placed at the excavation line in addition to tie-back anchors (which are similar to the soil nails). Therefore, a soil-nail wall does not require as much structural materials or labor to accomplish the same excavation.

Geotechnical analysis for the existing bluffs at the site comprised studying sea cliff erosion rates, setback requirements, short- and long-term stability, and methodologies for reconstructing portions of the bluff in which new storm-water pipelines were installed. The natural soil was reused as compacted fill in small portions of the slope. To increase the stability of rebuilt portions of the slope, geosynthetic fabric was used within the layers of fill.