Begin with the ground, not the height
An 800-metre tower does not automatically require piles of a particular length. The foundation responds to the load distribution and the ground profile: strong rock near the surface presents a different problem from deep layers of compressible soil. Investigation has to establish groundwater conditions as well as strength. A borehole is a sample of the site, not a complete three-dimensional description. Engineers build a ground model from multiple observations and test how sensitive the design is to uncertainty. FHWA’s foundation resources connect investigation, selection, construction and acceptance, although their formal application is transportation infrastructure. [1]
Capacity and settlement answer different questions
Capacity asks whether the foundation can resist the applied actions. Settlement asks how far it will move while doing so. A foundation can have adequate resistance and still settle enough to damage partitions, alter drainage falls or load adjacent connections. Differential settlement is particularly consequential: a uniformly descending floor and a tilted floor create different demands. The structural model and geotechnical model therefore need to exchange more than one total tower weight. Core loads, perimeter-column loads and their evolution during construction matter. [2]
Jeddah: a foundation for an unusually large overturning demand
Thornton Tomasetti describes Jeddah Tower’s foundation as a five-metre-thick raft supported by 270 bored piles, 1.8 metres in diameter and reaching as deep as 105 metres. These are project-specific dimensions, not a recipe for other towers. The building’s three tapered wings distribute its structural system across a broad base. Read the dimensions alongside that geometry: the foundation has to receive the reactions generated by both gravity and lateral loading. The proposed height alone tells us little about how those reactions are distributed. [3]
Excavation is a neighbouring-building problem
Before the permanent foundation carries the tower, an excavation changes the support available to surrounding ground. Arup’s Merdeka 118 account describes a circular cofferdam using hoop action, avoiding internal struts and ground anchors. Monitoring tracked movement near important neighbouring landmarks. The broader lesson is that basement construction has its own load path and programme. A permanent structure that works perfectly in its final state does not establish that every temporary excavation stage is equally acceptable. [4]
What an engineer looks for in the evidence
A convincing foundation account should connect predictions to observations: ground investigation, trial construction, load testing, installation records and settlement monitoring. FHWA’s acceptance guidance is useful context for understanding why these are distinct forms of evidence. A load test examines response under an applied load; an integrity investigation asks a different question about the installed element. Neither should be described simply as proof that the entire tower is safe. For a reader comparing skyscrapers, the best question is not “whose piles are longest?” but “how was the assumed ground behaviour checked?” [1]
Sources & references
Sources reviewed 9 September 2026. Undated guidance and product pages can change. Dated research and announcements retain their original context; technical principles do not substitute for locally adopted project standards.
- Federal Highway Administration. Structural foundations: design, construction and acceptance.Undated; reviewed 9 September 2026.
- Federal Highway Administration. Drilled shafts: construction procedures and design methods.2010.
- Thornton Tomasetti. Jeddah Tower.Undated; reviewed 9 September 2026.
- Arup. Merdeka 118.Undated; reviewed 9 September 2026.