Geocells Transform Weak Soil at Long Beach Port and Beyond

The Port of Long Beach, a critical gateway for trade in Southern California, constantly undergoes expansion to manage the immense volume of goods it handles—approximately half a trillion dollars annually, in conjunction with the Port of Los Angeles. During an expansion project in the early 2000s at the Pier T Marine Terminal, engineers faced a significant challenge: converting an old, disused dry dock, much lower than the surrounding area, into a container storage yard. This required substantial fill material, but the available local soil was soft, water-logged silt with poor structural integrity, unsuitable for supporting heavy container traffic and machinery.

Traditionally, such a problem would necessitate excavating and removing the poor soil, then importing vast quantities of expensive backfill, a process costing millions and taking years. Instead, engineers opted for an innovative solution: geocells. These 3D plastic networks transformed the mushy dredged spoils into a high-capacity platform capable of supporting 100-ton machines and heavy container stacks.

Understanding Geotechnical Engineering and Soil Failure

Geotechnical engineers specialize in working with natural materials like soil and rock, which often present unique challenges compared to manufactured materials like steel or concrete. Unlike materials with strict specifications, soil and rock properties vary widely, and their behavior under load is crucial for the stability of built environments.

Soil failure typically occurs not by crushing or bending, but by shearing. When subjected to excessive pressure, the friction between soil particles becomes insufficient, causing them to slide past each other. This results in a downward, outward, and upward movement of the soil, leading to settlement or tipping of structures above. This phenomenon, known as bearing capacity failure, is a primary concern in the design of foundations for buildings, dams, retaining walls, and roadways.

Traditional Solutions for Weak Soil

Several traditional methods address weak soil conditions:

• Spreading the Load: Distributing weight over a larger area reduces pressure on the soil. This is commonly achieved with wide concrete footings, but concrete can be expensive, making it impractical for extensive applications like roadways.
• Subgrade Replacement: This involves excavating unsuitable soil and replacing it with "select-fill," a high-quality material like angular crushed stone, often referred to as "road base." Road base requires a mix of large chunks and finer particles for interlocking strength. The thickness of this layer is critical, as the stress from a load dissipates with depth. Weaker subgrades require thicker base layers, leading to extensive digging, hauling, placing, and compacting.

Geosynthetics: Reinforcing Soil

When traditional methods are impractical, geosynthetics offer a solution by reinforcing existing soil. These human-made materials enhance the ground's performance.

1. Geotextiles: These industrial-strength fabrics act as physical barriers, preventing gravel from sinking into mud and sometimes functioning as filters to allow water flow without soil erosion. However, they lack significant strength and can move independently of the soil, potentially leading to rutting and sagging under heavy loads.
2. Geogrids: Stiff plastic meshes that reinforce soil layers, similar to steel reinforcement in concrete. Geogrids are stronger than geotextiles and, crucially, interlock with the soil above and below, providing better reinforcement. While effective, geogrids are inherently 2D, meaning their reinforcing effect diminishes with distance from the grid. Multiple layers are often needed for thicker soil reinforcement.
3. Geocells: These are 3D structures formed from plastic strips welded into a honeycomb-like pattern. Unlike geogrids, geocells encapsulate the backfill material. This confinement prevents the soil from shearing and shifting under load, effectively transforming weak soil into a high-capacity platform.

Advantages and Applications of Geocells

The primary benefit of geocells lies in their ability to confine soil. When soil is confined, it cannot easily shear or shift, significantly increasing its bearing capacity. This allows for:

• Reduced Base Layer Thickness: Geocells can substantially decrease the required thickness of road base, as the plastic structure contributes significantly to load distribution.
• Use of Cheaper Fill Materials: The cellular confinement system is less reliant on the perfect interlocking properties of angular crushed rock, allowing for the use of more readily available and cheaper local materials, or even existing on-site soil. This reduces quarrying, crushing, sorting, and long-distance hauling costs.

Geocell technology was pioneered by the US Army Corps of Engineers in the 1970s for rapid construction of landing strips and roads on soft soils during the Vietnam War. NASA has also explored similar concepts for lunar infrastructure, considering geocells (or "lunarcells") for building on the moon due to their light weight and compact transportability.

Geocells and Washboarding

Geocells also offer a solution to washboarding, a common problem on unpaved roads where rhythmic corrugations form, causing vehicles to jostle and potentially lose traction. A demonstration using a model track showed that while unreinforced sand quickly developed washboards at a certain speed, the track with geocells remained remarkably stable, even at higher speeds. The geocells confined the sand, preventing the skip and push of particles that lead to washboard formation. This highlights how a small amount of plastic can dramatically alter the behavior of the system.

Environmental Considerations

While some lightweight fills like geofoam (styrofoam) raise environmental concerns, geocells, typically made from HDPE (high-density polyethylene), are relatively inert, especially when not exposed to UV light. In fact, geocells can offer environmental benefits:

• Reduced Excavation: Less excavation is needed, minimizing site disturbance.
• Local Material Use: They expand options for road base materials, reducing the need for long-distance transport of specialized fills.
• Reduced Pavement: In some cases, geocells can enable the construction of strong, low-maintenance unpaved roads, potentially eliminating the need for asphalt.
• Improved Permeability: Using cleaner fill with fewer fines can make surfaces more permeable, reducing impervious cover and mitigating flooding.
• Less Waste: Utilizing native materials reduces the generation of waste muck that needs to be transported off-site.

Geocells have diverse applications, including roadways, retaining walls, erosion control on steep slopes, and shipping terminals. While not a universal solution, they represent a clever engineering innovation that balances cost, maintenance, material availability, and traffic volume, offering an effective alternative to traditional, more resource-intensive methods.