Providing pavements to support traffic from wheeled equipment with axle loads as high as 90 tons on loose dry soils is tough enough under the best of conditions. But try doing that by installing and compacting a six-foot thick base layer on top of a weak, saturated clay subgrade ?EUR??,,????'??+ where environmental concerns prevent the use of lime to establish a layer of stabilized soil underneath the base ?EUR??,,????'??+ and when time is running short. That can be a real challenge. It?EUR??,,????'???s one that design engineers and construction crews faced during construction of the container storage area for Berths 55/56 at the Port of Oakland, California, in the spring and summer of 2000. Lack of drainage discovered during excavation combined with unexpected wet weather threatened to delay the project. However, by employing an eight-inch thick layer of crushed concrete, held in place and strengthened by a cellular confinement system, they were able to build a stable, yet flexible subgrade to support construction of the base layer. This approach cut construction costs by reducing thickness of this reinforcing layer ?EUR??,,????'??+ which lowered excavating and spoil disposal costs ?EUR??,,????'??+ and by using recycled on-site material as fill. It will reduce future maintenance and repair expenses, compared to other alternatives. The work was part of the continuing $560 million project to expand the Port of Oakland by converting a former 531-acre U.S. Navy site to a maritime container terminal. The general contractor assigned to build the two berths was Port of Oakland Constructors. Traffic at the intermodal facility includes loads from tractor-trailer units to large cranes with axle loads up to 212,000 pounds. Supporting such loads requires a properly prepared subgrade for the pavement. The original plan was to excavate 61,300 cubic yards near the shoreline of San Francisco Bay to a depth of six feet, prior to bringing in and compacting engineered fill. That work was being done by subcontractor McGuire and Hester. However, about half way through this work, the excavating crews ran into a problem. Pothole tests revealed large areas of unstable subgrade. The deeper they went, the more water they found. In the past they would have repaired soft soils by overexcavating the area, covering it with a lightweight geofabric and backfilling with some type of engineered fill. But the success of that approach would be suspect with the heavy loads of the container handling area. A high-strength, stiffening layer was needed against which the fill could be compacted. Harza Engineering (Fugro West) was called in to solve the problem. ?EUR??,,????'??We needed a way to stabilize the subgrade within a short depth below grade to minimize the volume of material to be excavated,?EUR??,,????'?? explains Harza geotechnical engineer and project manager, Darius Abolhassani. ?EUR??,,????'??We also needed a very stiff, integrated layer which would allow us to compact the fill above it to 98 percent maximum density required to support the heavy traffic loads.?EUR??,,????'?? Regulations to protect the water quality of San Francisco Bay prevented use of a conventional calcium-based treatment, such as cement and lime, to strengthen the subgrade material. There were concerns that the lime could leach into a nearby estuary and threaten aquatic life. Meanwhile, time and labor expenses and disposal costs of excavated material also ruled out use of a flexible geogrid structure to build a suitable bottom layer for the fill. For these reasons, project engineers recommended the use of the fully engineered, perforated Geoweb??????oe system. This three-dimensional system features expandable, polyethylene honeycomb-like cells that improve the load-support performance of infill. The system produces a stiff, flexible base that stabilizes subgrade and is designed to help distribute any localized subgrade contact pressures over a larger area, controlling differential and total settlements, even on low-strength subgrades like those on this project. The textured surface and pattern of cell wall perforations optimizes frictional interlock with the infill material and improves lateral drainage, resulting in better performance in saturated soils. The product?EUR??,,????'???s ability to improve load distribution and reduce pavement deflection and rutting can pay off in intermodal yards and in other uses like road bases, railroad track ballast, parking areas, porous pavements, retaining wall spread footings and low level water crossings. The system is easy to install and eliminates the time and expense of overexcavating and placing large quantities of aggregate. ?EUR??,,????'??In load support applications the Geoweb system is normally installed in the base layer, which is located at the top of the fill materials,?EUR??,,????'?? says Samuel Randolph, Soil Stabilization Products Co., the material supplier. ?EUR??,,????'??However, as this project shows, the system can also be used to provide a stiff layer at the base of the fill where it contacts an extremely soft subgrade. The Geoweb system offers a way to compact the fill, which wouldn?EUR??,,????'???t be possible otherwise. In this case, the system made construction possible and now provides protection to the subgrade from the heavy and high frequency loads above.?EUR??,,????'?? Various full-scale cyclic loading tests have shown that the number of load applications required to produce a given permanent deflection of an unconfined aggregate base are increased by a factor of 10 to 15 when the aggregate is confined within the prefabricated system. Based on preliminary design information and a composite pavement analysis, Abolhassani drew up the final design that called for installing 213,000 square feet of the cellular confinement system. He recommended overexcavating the areas of weak soils to a depth of one-foot below the Geoweb system, and covering this area with a nonwoven filter fabric to separate the cellular system?EUR??,,????'???s infill material from the underlying mud. The design featured the use of recycled, crushed concrete to fill the cells. ?EUR??,,????'??The three-inch minus crushed concrete produced a gradation of interlocking material ideal in this application,?EUR??,,????'?? Abolhassani says. ?EUR??,,????'??It was pushed into the cells with a dozer and compacted by the traffic of large all-terrain dump trucks used to haul in the material.?EUR??,,????'?? Pad-foot and smooth-face vibratory rollers were utilized by McGuire & Hester crews to compact the infill materials within the sections. With the cellular system, mobilization of the system?EUR??,,????'???s load support capability is instantaneous once the infill is fully compacted. Installation of the system was completed in about three weeks.