9.6 Suitability test
Prior to the execution of suitability tests full consideration of the benefits of investigation tests should be made and the results from any such tests should be assessed.
The objectives of the suitability test are as follows:
- a) where investigation tests have been carried out suitability tests confirm either the acceptable creep or load loss characteristics at proof and lock-off load levels for future acceptance tests, or a critical creep load;
- b) where no investigation tests have been carried out and if no results of investigation tests on similar anchors in similar ground conditions are available, suitability tests demonstrate the characteristics as in a) above, and provide acceptance creep or load loss criteria at proof load level of acceptance tests, or provide the critical creep load;
- c) to determine the apparent tendon free length.
At least three suitability tests shall be carried out on anchors constructed under identical conditions to working anchors.
Where investigation tests have not been carried out the suitability test anchors may contain a tendon with a capacity higher than that of a working anchor.
9.7 Acceptance test
Each working anchor shall be subjected to an acceptance test. The objectives of the acceptance test are as follows:
- a) to demonstrate that a proof load, which will depend on the test method, can be sustained by the anchor;
- b) to determine the apparent tendon free length;
- c) to ensure that the lock-off load is at the designed load level, excluding friction;
- d) the creep or load loss characteristics at the serviceability limit state, when necessary.
9.8 Maximum lock-off load
If a creep or load loss limit is not exceeded, the maximum lock-off load (P0) shall be limited to 0,60 Ptk.
If, in the case of either a suitability test or an acceptance test, the creep or load loss limit is exceeded, the lock-off load shall be reduced to a level where the creep or load loss criterion is satisfied.
9.9 Evaluation of the apparent tendon free length
The apparent tendon free length, Lapp, is calculated from the measurement of the tendon extension As from the point of fixing the tendon to the jack or from a reference point coupled to the tendon. This measurement defines the location of a fictitious fixed anchor which is compared with the end of the tendon free length and the start of the tendon bond length.
NOTE The following equation is generally used to calculate the apparent tendon free length:
Lapp = (AtEtΔs)/ΔP
Limits within which Lapp shall fall are:
— upper limit Lapp ≤ Ltf + Le +0,5Ltb;
Lapp≤1,10Ltf + Le;
whichever is the larger;
— lower limit Lapp ≤ 0,80Ltf+ Le.
Where there is significant friction in the free length the method shown in Figure 3 may be used, by considering the hysteresis loop between a load and an unload curve, to estimate the magnitude of apparent elastic stiffness of the free length (ΔP/Δs).
NOTE If friction exceeds 5 % Pp then this may be taken into account in determining the minimum proof load or lock-off load. If necessary the proof load may be reduced.
Where the apparent tendon free length lies outside the limits the anchor may be subjected to repeated load cycles to Pp. If the anchor demonstrates repeatability of load/extension behaviour the anchor may be accepted by the designer.
- 1 Anchor load (P)
- 2 Friction
- 3 Slope of load vs displacement curve ΔP/Δs
- 4 Friction
- 5 Displacement (s)
9.10 Supervision of construction and testing
The installation and testing of all anchors shall be supervised and records shall be made at the site (see clause 10).
If inspection reveals uncertainties with respect to the quality of installed anchors, additional investigations shall be carried out to determine the as-built conditions of the anchors.
Ground anchors can be installed with a monitoring facility. Where a structure is sensitive to changes in load or ground movement use can be made of this facility to monitor the behaviour throughout its design life.
The number of anchors to be monitored and the intervals between measurements shall be specified.
NOTE In certain cases due to structural movement, it may be necessary to restress the anchors periodically to keep the residual anchor force above the minimum required level.
The corrosion protection of the accessible parts of the anchor heads shall be inspected periodically and renewed, if necessary.