Obstructions can be difficult, because you often do not know they are there until you begin to install a deep foundation. Helical piles cannot install through obstructions; however, here we highlight the different options you have for dealing with obstructions while installing helical piles. One of the benefits of helical piles is that you do not have to abandon the pile. In most cases, it can be removed and reinstalled (unlike other deep foundation options).
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Helical piles are a displacement piling system that moves the soil away from the central axis of the shaft. This has many advantages that include no spoils to remove, but also comes with disadvantages as the shaft diameter increases. The helical piling industry categorizes pipe helical piles into low, medium, and high displacement. Each category has advantages and disadvantages that a specifying engineer must consider when selecting a pile that is applicable and economical for the project.
The largest volume pipe shaft helical pile used in the North American market are low displacement pipe piles. Low displacement piles are defined as piles with central shaft diameters up to 4.5 inches.
On many construction projects, soil borings are not completed due to the property owner wanting to reduce costs or, quite simply, being unaware of the need to obtain soil strength data for foundation design. During the installation of CHANCE® Helical piles, monitoring torque can provide real time data defining underlying soil strength and its load capacity. As a helical pile is installed (screwed) into increasingly denser/harder soil, the resistance to installation (called installation energy or torque) will increase. The higher the torque, the higher the axial capacity. In most projects, the installation torque increases with depth, and the capacity of CHANCE helical piles can be determined at the time of installation. Regardless of whether the pile is being installed in clay or sand soil, the torque to correlation factor (Kt) for each shaft size, is multiplied by the effective installation torque (T), resulting in the ultimate capacity for each pile. The standard equation for ultimate capacity is Kt * T. The torque correlation factors for CHANCE helical piles can be found in the CHANCE Technical Design Manual-4th Edition. The effective torque is the average torque taken over the last 3 feet of installed depth, measured in 1 foot increments.
The shaft type/size of a helical pile is critical to both the axial and lateral capacity – especially for compression in soft/loose overburden soils where lateral stability of the shaft must be considered. The following is a brief summary of the 4 different shaft types commonly used for helical piles and their relative advantages and disadvantages based on site conditions and application. It is very important to understand that helical pile installation must be considered in the design process.
There are several solutions engineers and contractors can choose from when a deep foundation is required. With a deep foundation the structure’s load is transmitted to soils that are deeper in the ground. A deep foundation is used when a shallow foundation is not possible, not practical, or will not carry the load. Examples are weak, unstable, or expansive surface soils. Two popular options for deep foundations are helical piles and drilled shafts, also known as drilled piers or caissons.
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The first question we often get is, “are helical piles a new technology?”. Though they are growing each year in use and popularity, many Geotechnical and Structural Engineers are often unfamiliar with the technology. Complicating the matter is that, in many cases, students who will be our future engineers know very little about helical piles and their history. The truth is that helical piles [a.k.a. screws, screw piles, helical piers, helicals] appeared on the scene in the early 1800s. Alexander Mitchell (1780-1868) applied the use of screw piles in 1836 for moorings and, in 1838, for a lighthouse foundation. To put things into perspective, screw piles preceded the advent of Portland Cement [1850’s] and first use of a gasoline-powered automobile . According to some historians, screw piles were the major foundation technology of the 19th century.
ATLAS Resistance Piers are a versatile underpinning solution for existing structures that are in a state of movement and distress. The system components and installation methods lend themselves to a very quick and relatively easy installation. It is also a very cost-effective means of transferring structural loads from the foundation level to a suitable soil or rock bearing material.