Micropiles are used when deep, boulder laden soils and fills need to be penetrated to allow bearing on bedrock. The drilling capacities and the pipe serving as a casing make micropiles ideal for difficult sites. West Virginia has karst bedrock, which is known to form pinnacles. In such conditions, steel bearing piles and drilled shafts are not a good choice since they will deflect along the sides of the pinnacles. Micropiles, on the other hand, will not. Micropiles are also used to underpin existing footings, to repair drilled shafts, and in cases where space is limited.
Only gravity fill, Type A, micropiles should be used in near-vertical applications to avoid fracturing the rock. When used in tieback applications, pressure grouting should be used. The WVDOH requires that clean, full-strength grout circulate from the bottom of the micropile to the surface for all micropiles to verify good communication of grout within the bond zone. Design and field testing must be in accordance with the latest edition of the AASHTO LRFD Bridge Design Specifications.
Properly sized plates are to be embedded into the cap when uplift or punching shear is a concern. Nominally, 7-in and 9-in micropiles are used for approximately 200 and 400-kip structural capacities, respectively. Greater capacities can be obtained by adding more steel. The bond zone within the rock is sized based on the factored geotechnical resistance to maximize the use of the factored structural resistance. The micropile lateral load capacity at threaded casing joints can be evaluated and designed in accordance with Section 5.18.3 of the FHWA-NHI-05-039 specifications. Alternatively, the joint strength can be assumed to be 50% of the strength of the pipe material. The reduced joint strength must be compared to the maximum factored tensile stress from combined bending and uplift at the worst-case joint location.
Tension field testing is to be used to verify the bond resistance. The geotechnical engineer must be present during field testing to verify the procedures and to verify that the presumptive resistance is met or exceeded. The geotechnical engineer must provide a letter to the WVDOH indicating that the micropiles’ factored resistance meets or exceeds the factored demand. The letter must be signed and sealed by a professional engineer practicing geotechnical engineering in the state of West Virginia.