EMPRICAL TORQUE INSTALLATION METHOD TO PREDICT CAPACITY
An empirical method has been derived and used in the screw anchor industry for many years. Installation torque is used to calculate the ultimate capacity of the anchor. The average torque achieved during the last three (3) to five (5) feet of installation is directly proportional to the ultimate axial capacity of the pier.
A pullout test to failure is performed with the capacity achieved recorded as the ultimate capacity. Using the ultimate capacity at the given installation torque, an empirical torque factor can be calculated. (NOTE: A tension test is often performed instead of a compression test because they are quicker to set up and perform and the capacities are generally less than the compression tests – inherent factor of safety).
From the pullout test, an empirical torque factor, Kt can be calculated using the following:
Kt = Qt / T
T = Average Installation Torque (Ft. Lbs.)
Qt = Ultimate Pier Capacity (Lbs.) from load test
Kt = Empirical Torque Factor (1/ft.)
Typical values for Kt range from 20 to 2, with the majority of soils giving a Kt value of 7 to 10. Unless load tests are performed to provide a Kt value, a conservative Kt value should be selected when designing piles. It is important to note that the value for Kt is a combination of soil and anchor properties, primarily relating to friction during installation. As an example, Kt for dense, dry sand would normally be less than for a hard, wet clay.
The factor for 2-7/8” pipe anchors is recommended to be around 7 and is usually in the 7 to 10 range for most soils. Pile Power LLC requires all installers to use a K value of 7 for pier installation.
Appropriate safety factors should then be applied (minimum S.F. = 1.5 – 2.0).