Micropiles are small diameter, reinforced load transfer elements capable of supporting design loads in excess of 200 tons. Micropiles can be installed through almost any type of subsurface condition from sands to clay to bouldery soils to rock. Micropiling is accomplished using low-vibratory drilling techniques, a critical consideration when working around sensitive structures. The micropiles can readily be installed in confined space and low headroom situations. Often installation can be accomplished without disrupting normal facility operations.
Vertical and/or inclined micropiles can also be installed to provide the additional foundation support needed to withstand increased structural loading, underpin existing shallow foundations during adjacent new construction, counteract uplift forces, stabilize slopes, and for seismic retrofit.
Hollow Bar Micropiles
Hollow bar micropiles are particularly suited to soils that are not conducive to open-hole drilling (collapsible soils). The pile is formed from the top down in one operation by the high-pressure injection of cementitious grout through the hollow bar, which also serves as the drill string. The grout exits the bar through ports at the end of the sacrificial drill bit. Continuous staged drilling and grouting causes the grout to permeate into the looser adjacent soils and into any voids in addition to forming a competent reinforced grout column.
A helical pile, sometimes called a screw pile, consists of a spiral-shaped head and galvanized steel pipe extension sections (flights). The pile is rotated into a stable bearing stratum, using a high-torque hydraulic motor, until the necessary load capacity has been achieved as verified by means of a pressure gauge. The pile is attached via a bracket to the base of the foundation wall to transfer the structural load to the pile. Helical pile installation creates no spoil and can be performed in limited access or low headroom situations. Pile systems can be used for new construction or retrofit of existing buildings.
Continuous Flight Auger (CFA) Piles
Under the right conditions, Continuous Flight Auger (CFA) piles, also commonly called Auger cast-in-place (ACIP) piles, offer a number of scheduling and economic advantages over other deep foundation options. Installation is rapid, vibration-free, and can be accomplished in low headroom situations if required. Since each pile is constructed in a continuous operation, no casing is required.
CFA piles are constructed by rotating a continuous flight hollow stem auger into the ground to design depth and pumping high strength concrete through the hollow stem as the auger is slowly withdrawn. If required, reinforcing can be added while the grout is still fluid. Typical pile diameters range from 12 to 18 inches, although piles with diameters up to 36 inches have been successfully constructed. Loads range from 10 to 50 tons for 12-inch diameter piles, and up to 100 to 150 tons for 18-inch piles. CFA piles should be considered where speed of installation is important, a large number of piles are required, and the soil conditions and project requirements are straightforward.
Drilled Shafts (Caissons)
Drilled shafts, also known as caissons, drilled piers or bored piles, are often the deep foundations of choice for buildings with high or concentrated column loads. Candidates for a drilled shaft foundation include multi-story buildings, parking decks, bridges and overpasses, and structures such as transmission towers that are subject to uplift loads.
Drilled shafts can be designed as end-bearing or friction shafts. Construction can be by the dry, wet or cased method, depending on the subsurface conditions and groundwater table elevation. Dry shaft construction, as the name implies, relies on the soil/rock to be self-supporting during excavation, reinforcement placement and concreting. Wet construction involves the use of a stabilizing agent only, either water or slurry, to maintain hole stability during excavation and completion of the shaft element. Temporary casing is often used where the soils are dry, but their characteristics are such that caving or sloughing is anticipated during excavation, or where clean granular soils are present below the water table and the shaft must be seated into an impermeable layer to prevent water flow into the borehole. For shafts installed through water, e.g. for bridge construction, the casing can also function as a form, becoming a permanent part of the foundation.
In dense urban environments, the selection of a deep foundation system for buildings and other structures with high or concentrated column loads must take into consideration the presence of underground structures such as subway systems, sewer lines or utility banks within the building footprint; restricted or congested site access; and noise or vibration limitations imposed on the installation process. Under such circumstances, mini-caissons offer a viable alternative to drilled shafts.
Mini-caissons are essentially large-diameter (12- to 24-inch casing), drilled micropiles, either rock socketed or bearing on rock, and are capable of carrying more substantial loads than standard micropiles. Like micropiles, mini-caissons can be installed through almost any type of subsurface condition from sands to clay to bouldery soils and accomplished in confined space and low headroom situations. Low-vibratory drilling techniques ensure minimal disruption to the surrounding area and load transfer is achieved without disturbing underlying structures.