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Practical Advice for Foundation Design – Micropiles for Structural Support - page 17 / 25

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GSP 131 Contemporary Issues in Foundation Engineering

The connection between the top of the micropile and the reinforced concrete pile cap can vary depending on the required capacity of the connection, the type of pile reinforcement, and the details of the pile cap. Some connections need only work in compression, while others will be required to work in compression and tension.

For example, Figure 3 shows a composite reinforced pile connected to a new (or extended) footing. The footing tension and compression load is transferred to the pile through the top bearing plate. The stiffener plates provide bending strength to the plate, plus provide additional weld length for transferring the load from the bearing plate to the pile casing. The stiffener plates can be eliminated if the support of the top plate and additional weld length are not required. Additional considerations for this connection detail include the following:

  • The portion of the tension load carried by the reinforcing bar can be transferred to the top plate through the nut, reducing the plate-to-casing weld requirement.

  • The bond between the pile casing and the footing concrete can be utilized, reducing the load capacity required for the top plate and top plate to casing weld.

Figure 4 shows a composite reinforced pile connected to an existing footing. The pile is installed through an oversized hole cored through the existing footing or slab. After the pile is installed, the core hole is cleaned and filled with cement grout. Steel rings are welded to the top section of the casing prior to pile installation. The use of non-shrink grout may be appropriate to avoid shrinkage problems in large cored holes.

These rings transfer the pile load from the casing to the non-shrink grout. Adequate spacing must be used between the rings to avoid combining bearing stresses in the concrete and grout. The total capacity of the connection is controlled by the sum of the bearing strength of the rings, the capacity of the load transfer across the interface between the non-shrink grout and the existing concrete, and the shear capacity of the existing concrete.

Grooves may be chipped into sides of the core hole (typical dimension = 20 mm deep and 32 mm wide) to increase the load carrying capacity of the grout to existing concrete. Also, vertical reinforcing bars may be drilled and epoxied into the existing concrete around the exterior of the connection to increase the punching shear capacity.

For thick existing footings, the shear rings and grooves in Figure 4 may be eliminated. Load tests on the connections are appropriate to verify the casing to grout bond and grout to existing concrete bond for the proposed materials and methods.

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