DEEP FOUNDATIONS VERSUS SHALLOW FOUNDATIONS

As the names would seem to imply, the primary difference between a shallow and deep foundation is the depth that the foundation extends into the ground.  Shallow foundations spread structural load to soils that are near the ground surface.  Shallow foundations are commonly implemented in residential construction or for any structure with generally light loads (weight of the overlying structure).  Deep foundations, as you might have guessed, extend structural loads to soils that are not near the ground surface.  These deeper materials generally have a much greater load bearing capacity than material near the ground surface.  Deep foundations are commonly implemented when dealing with very large design loads, such as those associated with large buildings or bridges.  Some deep foundations are driven into the ground, such as concrete piles, while others are put in place by excavation and drilling.

Aside from difference in depth, another significant difference between shallow and deep foundations is the way that the design loads are transferred to the soil.  Shallow foundations primarily transfer loads to the soil by bearing pressure (loads are supported by the soils directly below the foundation).  Deep foundations also transfer loads to soil through bearing pressure (albeit to much deeper soils with generally greater load bearing capacity), but additionally transfer loads by friction with the soils along the length of the foundation; making deep foundations significantly more stable than their shallow counterparts.

The need for bridges was clear even in ancient times.  As early roads began to connect villages together, the necessity of traversing natural obstacles encountered along the way became increasingly important.  While a person carrying limited supplies could likely cross a river or stream by swimming; the same was not true for the carts and wagons needed to transfer large amounts materials and resources between destinations.  Thus, the need for permanent sturdy bridges was born.  This is a need that has only grown over time as human society entered the modern era.
Some modern bridges traverse huge distances, such as the Danyang-Kunshan Grand Bridge in China which runs approximately 102.4 miles in length; while others are much smaller, sometimes simply extending across a creek or slough.  However, a common requirement in the construction of all modern bridges is the need for stability.  As is the case with all construction (bridge, house, skyscraper or otherwise) stability begins with the foundation.  The foundations of a bridge are of critical importance as they must support the entire weight of the bridge and the traffic loads that it will carry.

The most common type of deep foundation for modern bridge construction is piling.  Piles used for bridge construction may be concrete, steel, or timber; the most common of which is pre-cast concrete piles.  In short, the piles are put in place by a large crane and driven into the ground to competent subsurface material by means of a large (usually diesel powered) hammer which is hoisted above the pile by crane.   The lengths of the piles vary greatly depending upon the depth to competent material in the area where the bridge is being constructed.

As the piles are hammered into the ground, soils are displaced and friction builds up along the “skin” of the pile.  This friction factor significantly supplements the bearing pressure of the pile.  When the desired depth is achieved and the piles are sitting atop competent bearing material, the piles are capped and tied together; allowing the bridge to be constructed upon the solid foundation provided by the piles.

As we look to the future, the scale and complexity of bridge design continues to become ever grander.  The aesthetics of a finished bridge is also increasing in importance in the modern era; as bridges are viewed not only as functional constructs to traverse natural obstacles, but as works of art.  However, one must always remember that a structure is only as strong as its foundation.

Gascoigne, Bamber. HistoryWorld. From 2001, ongoing. http://www.historyworld.net/wrldhis/plaintexthistories.asp?historyid=ab97

Katharine Gammon. LiveScience. February 28, 2013.  http://www.livescience.com/34448-worlds-longest-bridge.html

A&SW Consultants, Inc. Construction Training and Qualification Program: Pile Driving Inspector. Version 1.1-6/10. Prepared for the Florida Department of Transportation