While helical screw piles have their foundations in the construction industry since the 1800s, they have not been widely used in modern-day construction until recently. However, the engineering community is increasingly realizing the benefits of the screw pile system.
Screw piles, also known as helical piles or screw anchors, are structural, deep foundation elements used to provide stability against forces exerted by axial compression, tension, and/or lateral loading. They consist of one or more circular, helical plates affixed to a central shaft of smaller diameter. For screw piles with multiple helices, the helices may be of equal diameters or have diameters tapered towards the pile tip. The helices are generally attached to the shaft by welding, but may also be bolted to, riveted to, or monolithically made with the shaft.
Screw piles are embedded into the soil by applying a turning moment to the head of the central shaft, which causes the helix or helices to penetrate the ground in a “screwing” motion. A downward force may also be applied to the screw pile during installation to facilitate the helices in “biting” into the soil and advancing the downward movement of the pile. Powerful hydraulic motors are used to apply the large torques required to screw a screw pile into the ground. The motors are fastened onto handling machines that range in size from ½ T Bobcats, 20T Excavators, or up to 100T mast mounted crane rigs.
Screw piles are installed in segments of length corresponding to the height of the torque head above the ground surface. If more than one length is required, additional shaft lengths are simply welded or threaded onto the pile as installation progresses. Screw piles are typically installed to depths of less than 10 meters, and installation usually requires only two people on a crew and approximately 30 minutes per pile.
The diameter of the helical plates is governed by the pre-determined loading and soil condition. Helical anchors / piles derive their load carrying capacity through both end bearing on the helix plates and skin friction on the shaft. The overall number and spacing of the plates are designed to maximize the allowable load capacity of the soils. The size and spacing of the plates is determined at the design stage. The number and sizes of the helixes used is a function of the soil profile, with deep firm clay profiles requiring large plates and 2 to 3 helixes. This reduces the depth the pile would have to go, with conventional piles having to penetrate deeper into the profile. This “gearing” principle gives the screw pile considerable design flexibility. Shallower soil profiles over rock would require a small single helix plate.In general they are supplied in 2 metre sections and are bolted together as they are screwed into the ground by the use of a torque drive unit which can be attached to a modern excavator. The piles are screwed into the ground until design depth and torque is achieved. The torque is measured from a reading taken on the calibrated torque drive unit. The axial capacity of the pile is directly related to the torque achieved throughout the last 3 to 4 ft. of installation (i.e. three times the diameter of the largest helix). This torque vs. capacity relationship for low displacement piles (i.e. <= 3.50” shaft dia.) provides for an excellent on-site quality control method.
Screw piles are be fabricated in a wide variety of sizes and configurations, depending on the proposed application and the likely soil conditions to be encountered. At present, most helical pile shafts range in sizes from 1.25” to 2.00” square, and from 2.875” to 10.75” diameter hollow round. The sizes of helices typically range from 6” to 14” diameter. Maximum ultimate pile capacities (not including skin friction) range from 34 to 150 kips. To provide higher capacities, larger helical pile shafts and helices will be required.
Methods of connecting the pile to the structure depend on the type of structure to be supported. Connections can range from complex welded brackets to holes drilled into the top of the pile. The major consideration for this connection is to assure that there is a clean transfer of load from the structure to the pile.
Helical piles offer unique advantages over other foundation types. The main advantages associated with the use of screw piles are that they can be loaded to their full capacity immediately after installation, they may be in-stalled rapidly with very little noise or vibration, and may be installed using various sizes of lightweight equipment which makes them especially suited for use on soft or marshy terrain or on sites with restricted access, including inside of existing buildings. Screw piles can be particularly cost-effective in cases of high groundwater tables, as dewatering is not required, and may also be removed after installation and re-used, which can create significant economic and environmental advantages in the construction of temporary structures.
The installation equipment for screw pile foundations is generally smaller, lighter, and less specialized than that required for other types of foundations such as drilled piers, driven piles, and auger cast piles. Installation of a new foundation system consisting of 20 helical piles is conducted in typically less than a few hours. Installation can even be done with portable, hand-operated equipment in limited access areas such as inside crawl spaces of existing buildings.
With traditional forms of piled foundations, the load-carrying capacity is generated from a combination of “end bearing” at the pile tip, and “skin friction” of the vertical sides of the pile shaft with the surrounding soil. Screw piles generate their load capacity solely from the flat contact area of the projecting helical plates with the surrounding soil mass. As a result, screw piles have a higher bearing capacity than conventional piles of the same length. The necessary load-bearing capacity is achieved by bolting more sections of pile together and screwing them in.
Screw piles hold several distinct advantages over conventional piles for applications in soil conditions which permit their installation. Helical screw piles can be installed in almost all soils except for competent bedrock. Screw piles provide effective in-ground performance in a range of soils, including earthquake zones with liquefaction potential. Helical pile installation is unaffected by caving soils and groundwater. One of the unique features of the helical pile is its resistance to frost heave and expansive soils. The slender central shaft limits the upward stresses due to soil heave, while the helical bearing plates resist uplift. Entire subdivisions with hundreds of homes and decks have been founded on helical piles in areas of frost-susceptible or expansive soils. Screw piles are not particularly well-suited for use in very hard or gravelly soils, and may sustain damage to the helical plates during installation under such conditions.
There is virtually no vibration created when installing helical screw piles, which is important if pile locations are near existing structures or sensitive utilities. Each helical plate on any single pile will be of the exact same pitch as one another. This minimizes soil disturbance during installation compared with conventional piling techniques, such as CFA or rotary bored.
Eliminates the Use of Concrete
Like traditional piles, helical piles can be incorporated into a concrete pile-cap or slab. This can either be cast-in-situ or dependent on size, pre-cast, again reducing the delay in curing time onsite. A preferred method of ‘capping’ the piles is the use of a steel grillage which is bolted directly on to the piles. This eliminates the use of any concrete and provides the client with an instant foundation on which to build. Using helical piles and anchors can reduce the amount of concrete required and result in cost savings especially in remote sites.
Helical pile material is typically hot-dipped galvanized to extend the life of the pile in aggressive environments. Where special circumstances require a higher level of protection, special coatings can be utilized to improve corrosion resistance.
Helical pile foundations are an environmentally conscientious and sustainable construction practice. The construction of helical piles requires on the order of 65 percent less raw materials by weight to construct compared to driven steel piles and 95 percent less raw material by weight compared to drilled shafts or augercast piles. It takes fewer truck trips, to and from a construction site, to install a helical pile foundation system compared to other deep foundation systems. Fewer truck trips mean less traffic, less pollution, and less wear-and-tear on roads, streets, and highways.
Noise during installation is also significantly lower than driven piles, there is very little or no ground vibration, no concrete is used, and there is no spoil to remove. Due to their ability to be removed and reused elsewhere, they are sustainable and produce no muck-away saving considerable costs particularly where land is contaminated. Installation of helical screw piles does not create spoils that have to be disposed.
Helical piling has been used for a wide variety of applications throughout the construction industry. Geotechnical engineers have become increasingly aware of helical screw pile foundations and their applications.
Uses for helical piles include foundations for houses, commercial buildings, light poles, pedestrian bridges, and sound walls to name a few. Helical piles also are used as underpinning elements for repair of failed foundations or to augment existing foundations for support of new loads. Helical piles can be installed horizontally or at any angle and can support tensile in addition to compressive loads. As a tensile member, they are used for retaining wall systems, utility guy anchors, membrane roof systems, pipeline buoyancy control, transmission towers, and many other structures. In the electrical utility market, helical piles are used as guy wire anchors and foundations for transmission towers.
In residential construction, helical piles are used for new foundations, additions, decks, and gazebos in addition to repair of existing foundations. Small and maneuverable installation equipment and low mobilization cost make helical piles ideal for sites with limited access, such as narrow lots and backyards.
The ability to install helical piles without vibration in low-headroom areas within existing buildings has resulted in their use inside many commercial buildings where new loads are planned. Another example of how helical piles have been used inside existing buildings is to support mezzanines or additional floors. Helical piles have been used to support staircase and elevator additions for satisfying new commercial building egress requirements for a change of use. Helical piles also have been used to support heavy manufacturing equipment within commercial buildings
Another application of helical piles is for underground structures and excavation shoring. Helical anchors are often used as tie-backs in a variety of other shoring systems, including sheet piling and soldier piling.
Helical piling for deep foundations has made great strides in recent years. Advances in materials and equipment have resulted in helical piling with a wider range of capacities and applications. Larger hydraulic equipment enables the owners and contractors to specify helical piling on projects where higher foundation loads are required. Equipment output torque capacities have also increased which means higher helical piling design loads are achievable. Helical piles are practical, versatile, innovative, and economical deep foundations and are an excellent addition to the variety of deep foundation alternatives available to the practitioner.