Load Capacity of Deep Foundations
3.5.1 Deep Foundations – What Are They?
This is a more difficult question than you might imagine. Back in the nineteenth and early twentieth century, most footings were shallow spread footings that supported building columns and walls. Often when soft soils were encountered in the upper 10-feet of the site, engineers and constructors just dug through the soft material to obtain footing support on harder soil. These footings were called deepened spread footings, and I suppose you could call them “deep foundations.” When side walls of the excavations in soft soils became unstable and started collapsing, they used timbers and wood sheeting to support the excavation walls. When the wood sheeting used to support the excavation sides started to push into the excavation, they added wood timbers across the excavations to brace the wood sheeting. If you have ever seen a wood- sheeted, timber-braced excavation it is like chaos in motion with cross members going everywhere. There are photographs of these sheeted/braced excavations in old engineering books, and they are still used in Third World nations to this day. Topics of retaining structures and laterally braced retaining structures are included in Sections 4.2 and 4.3.
Early bridge engineers trying to cross rivers couldn't normally use deep spread footings to support their bridges because they couldn't keep river water out of the required excavations. Some bridge engineers used timber piles which ...