There are several types of pier foundation, defined by the pier material:
Concrete piers Concrete can be poured into tubular forms, or CMUs can be stacked and core-filled to create columns. The ground must be excavated to allow for the placement of the pier below the depth of frost, and the piers are typically backfilled with sand or gravel to provide drainage.
Helical piers Hollow galvanized steel tubes with a screw plate on the tip are wound into the ground with a hydraulic device until a predetermined torque reading is reached, indicating that the intended bearing capacity of the pier has been reached. Metal plates on the top of the pier are used to connect to the beam, slab, or floor framing. No excavation is typically required. They cannot be used on solid rock or in soils with large rocks. Pin piles Four steel pins are driven into the soil through a pre-cast concrete pile head. Each pair of pins acts in unison and bears on the wedge of soil immediately below. The four pins create a double-bearing wedge that is very stable and resists compressive and uplift loads. The pre-cast pier blocks and the pin diameters and lengths are engineered to meet site soil and loading conditions. Pin foundations do not require any site excavation. They cannot be used on solid rock or in soils with large rocks. CRiTERiA CONSiDERATiONS
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Wooden piers Wooden members of an appropriate diameter are used to support a floor deck above grade. A hole is drilled or excavated to a depth below the frost line and a gravel base and/or concrete footing are placed to receive the pier, which is made from a rot-resistant wood or treated to prevent decay. The excavation around the wooden pier is then backfilled with a well-draining sand/gravel mixture, and tamped in lifts to locate and secure the pier. Attachments between pier and beam can range from traditional timber-framing joinery to notch-and-bolt systems, to welded metal plates.
Foundation walls are built in the same manner as above-grade wood-framed walls (see page 122). Vertical studs are placed at regular intervals (commonly 12, 16, or 24 inches) between a single horizontal sill plate and a doubled horizontal top plate. The frame is sheathed with a structural grade of plywood on the exterior side. The wood materials are treated to prevent rot and allow the foundation to last a reasonable length of time in subgrade conditions.
There are numerous chemical-based treatments for lumber. Alkaline copper quaternary (ACQ), borates, and copper azole (CA) are all commonly available and used in treated wood foundations. The toxicity of each of these chemical treatments should be investigated to ensure that you are comfortable with using them.
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