The 58-story
Millennium Tower in San Francisco is an example of where differential settlement across the pile foundation caused significant tilt in the tower and it has reduced the serviceability of the structure, and caused trouble for the residents of the tower. However, the City of San Francisco recently concluded that the tower is structurally safe and can withstand a magnitude 8.0 earthquake. While all of the details of the geotechnical engineering and the foundation design are not yet widely published, it appears that the tower foundations were designed to satisfy strength and extreme event loading, but service loading and displacement of the tower foundations may not have been considered in the foundation design. More on the geotechnical issues...
Displacements of groups of driven pile foundations are often considered to be negligible in the foundation design for a heavily loaded structure. In practice, the presumption has often been that driven piles fully mobilize their resistance when they are driven, therefore only minimal elastic displacements should be expected once the piles are loaded with the structure. This presumption is incorrect because cohesive soils tend to creep (i.e. deform slowly over time) and groups of piles can have a group loading effect that can extend significantly deeper below the tip of the piles in the group, and cause consolidation settlement of underlying soil layers. Piles are generally driven to a required driving resistance with the common presumption that once that resistance is reached, no settlement of the pile should be noticed that would affect the structure once it is loaded because it is typically driven to two to three times the unfactored design load for allowable stress design methodology. If the pile can support twice the design load, no settlement should be noticed once it is loaded to the design load, right? Unfortunately not, due to the reasons described above.
Serviceability and settlement of the structure should be considered in any foundation design, which requires expert local knowledge of the subsurface conditions, and requires the geotechnical engineer to be involved during foundation construction to confirm that the subsurface conditions encountered during pile installation are as expected, and that the foundations are installed in accordance with the engineer's recommendations.
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