9 Testing, supervision and monitoring
ENV 1997-1 recognizes 2 classes of anchor testing, namely assessment tests and acceptance tests. In this standard three classifications of on-site load test will be considered separately.
- investigation test;
- suitability test;
- acceptance test.
The first two classes can be considered subdivisions of the general category of Assessment tests. Investigation tests establish in advance of the installation of the working anchors:
- a) the resistance Ra of the anchor at the grout/ground interface;
- b) the critical creep load of the anchor system; or
- c) the creep characteristics of the anchor system at loads up to failure; or
- d) the load loss characteristics of the anchor system at the serviceability limit state P0;
- e) an apparent tendon free length Lapp.
Suitability tests confirm, for a particular design situation:
- a) the ability to sustain a proof load Pp;
- b) the creep or load loss characteristics of the anchor system up to proof load;
- c) an apparent tendon free length Lapp.
Acceptance tests confirm, for each individual anchor:
- a) the ability of the anchor to sustain a proof load;
- b) the creep or load loss characteristics at the serviceability limit state, when necessary;
- c) the apparent tendon free length Lapp.
The supervision and assessment of all anchor tests shall be undertaken by a competent person experienced in anchor technology. The test methods prescribed for each test class shall apply to both temporary and permanent anchors.
On each project where encapsulations are grouted in the borehole a test involving the complete filling of the encapsulation with a quality grout in accordance with subclause 6.7 shall be undertaken by simulated operations under similar geometrical conditions prior to production anchor grouting. The test is carried out at the start of the works. Other system tests exist to verify the competence of the corrosion protection provided by the anchor system. These tests are described in clause 6.
9.2 Measurement accuracy
During hold periods where creep is measured in any test, the accuracy of displacement measurement shall be to 0,05 mm. Where no creep is to be measured, the accuracy of displacement measurement required is to 0,5 mm. The measuring equipment shall be able to resolve displacement of 0,01 mm when creep is being measured.
The measurement of loads in anchors shall be by hydraulic, electrical or mechanical load measuring device able to measure to an accuracy of better than 2 % of the maximum applied load during each test. Any load measuring device used for load loss tests shall be able to resolve loads to 0,5 % of the proof load.
9.3 Datum Load
The datum load Pa adopted from which measurement commences, is normally about 10 % of the proof load.
Higher datum loads are permitted in cyclic loading tests after load cycles where unusually high tendon extensions occur, see Figure 2.
|a) With load cycling||b) Without load cycling|
- 1 Anchor load
- 2 Optional higher datum load
- 3 Datum load Pa
- 4 Anchor displacement
9.4 Test methods
The Client's Technical Representative shall approve the test method and the associated interpretation system which shall be used in each test class. For each test class the ground anchor shall be loaded in stages in accordance with any procedure required for that test class.
Three examples of test methods applicable to each test class are given in annex E - these are:
- a) Test Method 1: The anchor is loaded incrementally in one or more cycles from a datum load to a proof load. Displacement of the anchor head is measured over a time period at the maximum load in each cycle;
- b) Test Method 2: The anchor is loaded incrementally in cycles from a datum load to a proof load or to failure. The loss of load at the anchor head is measured over a period of time at the maximum load in each cycle;
- c) Test Method 3: The anchor is loaded in incremental steps from a datum load to a maximum load. The displacement of the anchor head is measured under maintained load at each loading step.
During all testing the load shall be applied and released smoothly so that the anchor is not subjected to any shock or dynamic loads.
9.5 Investigation test
Investigation tests may be required to establish for the designer, in advance of the installation of the working ground anchors, the ultimate load resistance in relation to the ground conditions and materials used, to prove the competence of the contractor and/or to prove a new type of ground anchor by inducing a failure at the grout/ground interface.
Investigation tests should be carried out where anchors are used in ground conditions not yet tested by previous investigation tests or with higher working loads than those already adopted in similar ground conditions.
Anchors used for investigation tests are loaded more rigorously than anchors in acceptance tests so it may be necessary to increase the size of the tendon to accommodate this. Anchors subjected to investigation tests shall not be used in the permanent works if they have been loaded to failure.
The diameter of the borehole and dimensions of other components, apart from the tendon, should be kept the same as the working anchor.
Where an increase in capacity of the tendon is not possible then a shorter fixed length may be tested in order to induce a grout/ground failure.
Where failure of a test anchor with a reduced fixed anchor length is attained an increase in load resistance directly proportional to the increase in fixed length should not be expected for anchors of longer fixed lengths.
If the diameter of the borehole is increased the behaviour of an anchor in an investigation test may not be compared directly to the behaviour of working anchors.
The anchor shall be loaded to failure (Ra) or to a proof load (Pp) which shall be limited to 0,80Ptk or 0,95Pt0,1k whichever is the lower.