Photos: courtesy of GeoSolv Design/Build
Pile foundations have existed as load carrying and load transferring methods for many years, to put it mildly. From the early days of civilization for both defense and strategic advantage, most settlements were located close to water sources, such as rivers and lakes. However, as most know, a high water table equals soft soils that are difficult to build on. These soft, low-weight-bearing soils had to be strengthened somehow, and so the most primitive form of piling was born. In those days, timber piles were the only form of pile available and were typically driven into the ground by hand, making it an effective, but very slow and sometimes dangerous and time-consuming task.1
What is a pile foundation?
In plain terms, a pile is a long column composed of a robust material, like iron or concrete. Piles are forcefully pushed into the ground to act as a stable support for structures built on top of them. The piles support the structure by remaining firmly placed in the rock and very hard soil and effectively transfer the load of the structure they support to hard ground layers – rocks or very hard soil – with high bearing capacity. A pile foundation is a distinct succession of columns constructed or inserted into the ground to transmit a structure’s heavy loads to a lower level of subsoil or rock.2
Shallow foundations vs. deep foundations
Shallow and deep foundations indicate the relative depth of the soil on which structures are built. When the absolute depth of a foundation is less than the actual width of the footing and is fewer than 10 feet deep, it’s considered a shallow foundation. Shallow foundations are usually employed when the surface soils are robust enough to support the forced loads easily. However, when the depth of a foundation is more than the width of the structural foundation, it’s considered a deep foundation.3
The age of the pile driver
Luckily, the pile driver came along to help ease some of the human burden. However, the source is greatly debated, and several persons are credited with coming up with one form or another of the device. A mechanically sound drawing of a pile driver appeared as early as 1475 in Francesco di Giorgio Martini’s treatise Trattato di Architectura drawing. Also, several other prominent inventors have been credited with inventing the piling device, including James Nasmyth (son of Alexander Nasmyth) who devised a steam-powered pile driver in 1845, watchmaker James Valoué, Count Giovan Battista Gazzola and even Leonardo da Vinci. In 1801, John Rennie came up with a steam pile driver in Britain. Otis Tufts is credited with inventing the steam pile driver in the U.S. That said, there is quite a bit of evidence to support that a type of piling device was used in construction as early as 5,000 years ago.4
Driven Ductile Iron Piles (DIPs)
One of the most popular types of piles out there, DIPs are a simple, fast and highly effective low-vibration driven pile system that uses high-strength ductile iron. Modular pile sections are connected by a proprietary Plug & Drive system, eliminating the need for welding and splicing, while providing a section at the connection, which is more than twice that of the pipe pile section, resulting in superior dimensional and driving stability and performance. With the use of a relatively small excavator fitted with a high-frequency hydraulic hammer, piles are installed by driving the pile sections in quick succession for fast and easy installation with minimal vibrations. DIPs can be designed and installed for end-bearing and friction resistance.
End-bearing
In an end-bearing installation, an end cap or driving point is fitted on the end of the DIP lead section and successive sections are added as the DIP is installed to transfer foundation loads through compressible soils or to fill down to soil that is more suitable for load-bearing or to bedrock. The pile can be further filled with grout or concrete, and additional central bars can be added as needed for additional capacity.
Friction resistance
DIPs for friction resistance are installed with an oversized conical grout point at the pile base. As the pile is driven, sand-cement grout is pumped through the pipe sections and exits through grout ports within the conical grouting point to fill the void space created by driving the oversized cap. This process encapsulates the pile in grout and forms the basis for the grouted bond zone within the surrounding load-bearing soil to achieve the design capacity. The system excels in supporting high-capacity foundations in constrained sites or urban settings where tight access and low vibration requirements influence foundation selection. The DIP results in zero waste onsite, as each cut-off is able to be used as a starter section for the next pile.
Ductile Iron Pile Applications
DIPs have numerous applications for foundation support, including:
- Constrained sites
- Adjacent structures
- Tanks
- Tension anchors
- Wind and solar
- Industrial and floor slabs
DIP Installation Methodology
End-bearing DIP elements are installed using five-metre bell-spigot sections and a special driving hammer with a dry driving shank and a pointed or flat tip. The result is a high-capacity end-bearing pile system. A concrete/grout central bar can be added to the inside of the pile to increase compressive capacity as needed. With many different sizes and combinations, end-bearing DIP elements can be installed in many soil types to practically any depth with full displacement and impact energy; improving the surrounding soils and reaching end-bearing, while driving the tip into the end-bearing soils to maximize both structural and geotechnical capacity. The end-bearing DIP is superior to helical piles, particularly in situations that involve quick transitions from very soft to very hard soils.
Friction resistance DIP elements are also installed using five-metre bell-spigot sections and a special driving hammer with a wet driving shank where grout is pumped continuously through the pile to a tremi-grout head at the base. A high-capacity friction system results from the installation, and a central bar can be added to increase tensile or compressive capacity as needed. Friction resistance DIP elements can be installed in many soil types with various diameters and to practically any depth with full displacement and impact energy, improving the surrounding soils for high frictional capacity, with more versatility than helical piles, and less cost than micropiles.
References
- http://onlinepubs.trb.org/Onlinepubs/hrr/1970/333/333-002.pdf.
- https://www.schoolofpe.com/blog/2017/06/basic-principles-and-classifications-of-pile-foundations.html.
- https://www.schoolofpe.com/blog/2017/06/basic-principles-and-classifications-of-pile-foundations.html.
- https://en.wikipedia.org/wiki/Pile_driver.
Mark Tigchelaar is president and founder of GeoSolv Design/Build, Inc., an Ontario-based geotechnical contracting firm that specializes in providing a range of innovative foundation solutions for challenging soil sites and provides clients with the options and advice necessary for them to make informed decisions about their projects. For inquiries about this article or ground improvement, reach Tigchelaar at mark@geosolv.ca.