Foundation design on a site under highly susceptible liquefaction hazard
Case Study
In many countries, particularly those composed of numerous islands or bordered extensively by coastal areas, the proximity to large bodies of water significantly influences the geotechnical characteristics of the soil. These locations often have soil conditions that are vulnerable to changes in moisture content and seismic activity. One critical issue that arises in such environments is soil liquefaction, a phenomenon where saturated soil substantially loses strength and stiffness in response to applied stress typically due to earthquake shaking causing it to behave like a liquid.
Liquefiable soils tend to have reduced bearing capacity, making them unsuitable for conventional shallow foundations. Furthermore, they are prone to significant settlement, which can lead to uneven support for structures. Settlement refers to the downward movement of the ground surface caused by the compression of underlying soil layers. When liquefaction occurs, this settlement can be rapid and substantial, posing a serious threat to the stability of infrastructures.
A geotechnical investigation was carried out on the project site, which confirmed that the area is classified as having a high liquefaction susceptibility. Based on this assessment, the recommended foundation system involves the use of deep foundations, specifically reinforced concrete bored piles. These piles are designed to transfer structural loads through the weak, liquefiable layers and into the more competent soil strata beneath. According to the findings, the piles must penetrate to a minimum depth of 32.5 meters below ground level to ensure they are anchored within a stable, non-liquefiable layer.
Innovative Design Approach
Given the project constraints and site-specific geotechnical conditions, a more efficient foundation strategy was adopted. Instead of assigning an individual pile to support each piece of equipment, a consolidated approach was implemented. This involved the use of a pile cap system, wherein multiple equipment units are grouped together and supported collectively by a single pile cap anchored by multiple piles as seen in Figure 1. This approach allows for a more streamlined load distribution while optimizing the number of piles used.
The piles implemented in this design are each 33 meters in length, deliberately exceeding the minimum required penetration depth. This ensures complete traversal of the liquefiable layers and engagement with deeper, more competent soil strata capable of safely bearing the imposed loads. The additional depth also provides greater resistance to potential ground displacement and improves the overall seismic performance of the structure.
Benefits of the Consolidated Pile Cap System
This method offers several key benefits:
- Material Optimization: Reduces the total number of piles required by combining structural loads.
- Labor Efficiency: Minimizes installation time and workforce requirements.
- Construction Time Savings: Simplifies the installation process, leading to faster project delivery.
- Improved Structural Integrity: Enhances load-sharing and redundancy, particularly important in seismic events.
- Future Proof: Ensuring the structure would last longer than intended life span of structure.
At PSS, we continuously strive to implement innovative, efficient, and safe engineering solutions tailored to the unique challenges of each project. Our design practices are not only aligned with local and international codes and standards but are also grounded in rigorous technical analysis and sustainable construction methodologies. Through this approach, we deliver high-performance foundations that prioritize safety, resilience, and cost-effectiveness.