Testing of rock anchors is conducted after grouts have well achieved the minimum required unconfined compressive strength before stressing.

  • Grout in-the-hole and under cover caps, minimum 3,500 psi
  • Grout for bearing pads, min. 5,000 psi
  • Grouts must have a bleed <= to 2% at 50 psi
  • Grout testing done by independent and certified laboratories
  • Grouts must be of minimal shrinkage

Performance Testing of Rock Anchor

Performance testing is done in 6 incremental cycles then held for 10 minutes to check for creep movement. The cycles for a performance test can be seen in the chart below (loading schedule) along with the results from an actual rock anchor test. Errors in testing can come from inaccurate gauges, poor hydraulic pressure regulation, or misreading of instruments.

It can be seen in the above chart that the design load is equal to 35 kips (1.0 x DL) and is attained at the end of the 4th cycle. The 5th cycle is tested to 1.2 x DL, and the 6th cycle is tested to 1.33 x DL. Creep is held at 1.33 x DL for 10 minutes. All the testing is done within the elastic range of the steel, and the grade of the steel must be selected so that testing load does not exceed 90% of the yield strength (preferably keeping the maximum test loads between 80 to 90% x fy). Therefore, the design load should not exceed 60% of the yield strength (fy). The total time of the test loading cycles and creep is approximately 37 minutes, using zero hold time in between each cycle.

During the testing, elongations are measured at the end of the bar and plotted along with the load increments and cycles as shown in the chart below.

Elongations should all be contained within the unbonded portion of the anchor. Therefore, elongations can be predicted and when they are above and below certain limits, they indicate a potential problem with the rock anchor system. The limits, based on the strength or grade of the steel, present an accurate way to determine the performance of the rock anchor system and have shown through experience to be a good gauge of structural integrity.