The torque-to-capacity relationship for helical anchors and piles is an empirical method originally developed by the A. B. CHANCE Company beginning in the late 1950s. Today, the relationship is well recognized in the deep foundation industry. It is commonly referred to as torque correlation.
Predicting the holding capacity of an anchor can be difficult due to soil variations at different depths and different structure sites. It is critical to understand anchor capacity to ensure the integrity of your guyed structures.
The reliability of our overhead power and communication lines is as important today as ever before. The integrity of overhead construction often relies on guy anchor systems that can be trusted when put to the test during high wind loading situations and when tree branches come into contact with the lines. If an anchor pulls even the slightest amount, it causes the top of the pole to lean. This can cause improper line sag resulting in clearance issues.
It is well-documented that the best method for predicting the capacity of helical piles and anchors is to measure the installation torque. Accurate torque measurements are crucial to calculate the load capacity or holding capacity.
- Once all safety concerns have been addressed, attach the Kelly bar adapter and installing tool assembly to the Kelly bar on the installing truck.
- Insert the upper end of the anchors’ lead section into the installing tool. Position the anchor at the desired guy location and at a near vertical position; screw the first helix into the ground.
- When the first helix is buried, begin to make the angular adjustment for the desired guying angle.
- Remember, final angular adjustments should be made before the second helix penetrates the ground.
- When the installing tool becomes 12”-18” from the ground, disconnect it from the section in the ground and reconnect it to the next extension.
As with most mechanical devices, CHANCE® anchor-installing tools periodically require maintenance checks to ensure peak performance.
In the case of the shear-pin torque limiter, (see drawing below to the right) you should be able to rotate the tool shear halves independently from one another using a smooth-turning action. If rotation cannot be done by hand or if movement is challenging, disassemble the torque indicator to check the thrust bearing, washers and/or pin for wear.
- Choose anchor site carefully. Rock anchors will only perform effectively in solid competent rock (Class 0 Soil).
- Drill the hole into the competent rock a minimum depth of 12 inches along the drill steel. Be sure to drill so that the anchor rod will be in line with the guy.
- Holes should be drilled so the diameter is optimally an 1/8th inch larger than the diameter of the unexpanded anchor. For example, the hole drilled for a R315 is 1-7/8 inch diameter. However, drill bits are usually available in 1/4 inch increments. In practice, a 2 inch diameter hole is drilled for the R3_ series anchors and a 2-1/2 inch dia. hole is drilled for the R1_series anchors. Be sure to thoroughly clean the drilled hole of rock dust and debris.
"Proper alignment" and "down pressure" are simple phrases to summarize proper anchor installation technique. The Power Installed Screw Anchor (PISA®) wrench transmits torque from the digger’s Kelly bar to the anchor hub. Note: The anchor rod only has to be of sufficient diameter to support the guy load.
It is essential to use installing tools and anchors that are properly rated for trucks during the installation of power-installed screw anchors (PISA®). The high torque energy used in installing high-strength anchors requires the control of hazardous torque, which can be safely completed by proper tools.
The CHANCE® grouted rock anchor is designed to be used in situations where the soil is too rocky to use screw anchors, and the rock is fractured preventing the use of wedge style rock anchors. The forged knob on the end of the anchor along with any extension couplings provides the interference fit with the grout. The holding capacity of the anchor is dependent on the bond stress between the rock and the grout. The grouted rock anchor is designed to be used in line with the guy. Failure to install within 5° of alignment with the guy load will significantly lower strength.