Abstract:
This presentation will present and compare the results of a full-scale testing program that was performed to evaluate threaded micropile casing connection (threaded joint). The casing selected for testing has a nominal outside side diameter (O.D.) of 9.625-inch, a nominal casing wall thickness of 0.545-inch, and a minimum yield strength, fy of 80 ksi.
The goals of this investigation were to assess the bending capacity of the casing at the threaded joints, to identify the type of failure of the joint, and to assess whether typical design methods can correctly predict the bending capacity of the joint. In general, design guidelines specify that the bending and tensile capacity of a micropile threaded joint is approximately one-half that of the full casing. There is, however, insufficient published data on threaded micropile casing joints that permits to fully support common design assumptions. Also, because manufacturers tend to produce their own thread styles, standard design procedures may under- or over-estimate the structural capacity of threaded joints and, therefore, it is good practice to require test data for each type of threaded casing joints that are to be subject to bending or tension.
In addition, this investigation examined the influence of grout and of a central bar on the bending capacity of the casing and threaded joints. Common structural design practice negates considering the influence of grout and/or that of a central reinforcing bar when calculating the bending resistance or bending stiffness of a micropile. However, this is often questioned during project work when a threaded joint will occur at a point of elevated bending moment.