By Devon Mothersille, Geoserve Global and convenor to TC 182/Working Group 3
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Cement-based grout is typically the medium through which grouted anchors transfer tensile forces from the tendon to the surrounding ground. However, it is generally acknowledged within the anchor industry that the nature of grouted anchor construction is such that the integrity of in situ placed grout cannot be guaranteed. For this reason acceptance testing is a mandatory requirement for all grouted anchors before they are judged suitable for service.
Across Europe, and indeed worldwide, different approaches to anchor testing have evolved albeit that they are generally based on the principle of applying a tensile load to the anchor installation and monitoring its behaviour against established acceptance criteria. Such criteria provide limits for load transfer, from the anchor head to the fixed anchor zone, and the anchor’s ability to sustain its load with acceptable creep or load loss.
For many years attempts have been made to develop a pan-European standard for anchor testing which would, at the very least, provide a harmonised approach. The objective of this technical note is to initially explain the approach to grouted anchor testing from a UK perspective and then to describe the key elements of the new testing standard and how this document accommodates traditional UK practice.
2.0 Relevant technical background
Historically, UK clients and engineers have maintained a general preference for anchor-time behaviour to be presented to them as load loss in kN, or as a percentage of the lock-off load. Such data allows non-anchors experts to evaluate any potential detriment to the anchored structure. It was for this reason that the BS8081 committee originally favoured monitoring of load change with time using load cells, compared to direct monitoring of creep in mm at constant load, albeit that both methods are described in BS8081:1989; a code of practice that is still specified in many regions outside Europe.
Since the early 1980s in the UK there has undoubtedly been a reduction in confidence in the reliability and accuracy of load cell monitoring of load change through both short and long-term periods. The application of load cells on construction sites subjected to large changes in ambient temperature, or in damp areas, may be the main contributory factor to unsatisfactory load cell performance.
In addition, some contractors did not necessarily apply the same level of corrosion protection to the load cells as that specified for the anchor head and this led to a deterioration in load cell performance.
The situation was compounded by reports from sites that loads measured indirectly from jack pressure gauges were not consistent with those recorded on the load cell. In such circumstances where the identical applied jack pressure is recorded simultaneously from two calibrated gauges, and jack ram force is constant then it is difficult to justify an error in the evaluated load applied and maintained by the ram.
As a consequence of the above challenges, demand for the use of load cells reduced and preference was given to pressure measurement from two large faced hydraulic gauges or a single gauge supplemented by a digitalised read-unit of pressure or direct load.
The fact that load cell technology has advanced does not appear to have altered the perception of inherent inaccuracies, and often load cells are regarded as more suitable for measuring trends in load change as opposed to the absolute load value.
Prior to 2013, anchor testing in the UK was generally governed by BS8081: 1989. Within this document two conditions were applied and the anchor had to satisfy both. Firstly, the anchor performance was judged acceptable if the proof load had not reduced, during a 15 minute period, by more than 5% after allowing for any temperature changes and movements of the anchored structure. Secondly, if after anchor lock-off, the initial residual load did not reduce by more than 1% per unit of time (five, 15 and 50 minutes extended to three, eight and 24 hours where appropriate) then the anchor was deemed acceptable.
In the absence of load cell usage tendon displacement under constant load was conveniently measured using a dial gauge or an appropriately graduated steel rule and the equivalent load change simply presented to UK clients and engineers by converting tendon displacement using the following:
Load change = (tendon displacement) x (tendon area) x (tendon elastic modulus)/(free tendon length)
The above was a pragmatic approach, established from work carried out in the 1960s, and importantly no failures have resulted from this methodology. However, it should also be noted that acceptance criteria based on the above may be seen to have a bias towards the acceptance of anchors with longer free lengths.
The use of acceptance criteria based on displacement is related to a percentage of elastic extensions which are proportional to tendon free lengths. Therefore, it is possible that adjacent anchor installations with different free tendon lengths will have a different acceptance criterion. For this reason, the UK has now amended its approach and resorted to adopting measurement of creep rate as an acceptance criterion in the new testing standard. By doing this. each anchor installation will be subjected to a single acceptance criterion for creep behaviour irrespective of the tendon free length.
3.0 UK grouted anchor testing guidance
Over recent years a number of documents have emerged in relation to grouted anchor technology and many practitioners have expressed a need for clarification as to how these have evolved into the current suite of documents available to the industry in the UK.
Given the requirement to test all grouted anchors, and in keeping within the Eurocode environment, there have been attempts to develop a pan-European testing standard for well over a decade. The objective was to produce a document that, where possible, embraced the different anchor testing practices across Europe whilst providing a common basis of approach.
The new testing standard, together with EN 1997-1:2004+A1:2013 and EN 1537:2013, form the trinity which;
- EN 1997-1:2004+A1:2013 (EC7) defines the design requirements for grouted anchors, including the limits of proof load and limiting criteria by testing of grouted anchors, which may be specified in the national annex (for EN 1997-1) or a similar national application document for ISO countries;
- EN 1537:2013 defines the execution of grouted anchors
Currently, UK practice for anchor design and construction is guided by the recommendations of BS8081:2015+A2:2018 (formerly BS8081:2015 and before that BS8081: 1989). It is noteworthy that BS8081: 2015 and its revised version BS8081:2015+A2: 2018 were drafted to provide non-contradictory, complimentary information (NCCI), so that the recommendations provided do not conflict with the existing anchor standards; EN 1537:2013 (execution of ground anchors) and EN 1997-1:2004+A1:2013 (design of anchors). Previously, testing aspects were placed in Annex G of BS8081:2015 but these have since been removed in BS8081:2015+A2:2018 so that all reference to grouted anchor testing will be in accordance with the new EN ISO 22477-5:2018 (publication imminent).
4.0 Key features of EN ISO 22477-5: 2018
In broad terms the standard is complicated by the fact that it describes three test types and associated with these are three test methods (denoted test methods 1, 2 and 3). The working group assigned to technical committee TC182 (WG3) unanimously agreed that the three established test types should remain unchanged, namely investigation tests, suitability tests and acceptance tests in order to maintain full compliance with definitions in EC7.
The investigation test is a load test to establish the geotechnical ultimate resistance of an anchor and to determine the characteristics of the anchor in the working range. The suitability test is a load test to confirm that a particular anchor design will be adequate in particular ground conditions and the acceptance test is a load test to confirm that an individual anchor conforms with its acceptance criteria. Of the three types of test, only the acceptance test is required for all grouted anchors, the other are specified depending on the particular circumstances presented on the project.
In relation to UK practice it is relevant that in previous drafts of EN ISO 22477-5 test method 2 was originally split into two parts, giving the user the option to use creep displacement of the anchor or load loss as the means of assessing behaviour. However, this was judged to be confusing by European counterparts and the new standard was amended so that test method 2 represents load loss only, and test method 1 represents measurement of the creep rate of the anchor under load. So even though there is an option, it is envisaged that UK engineers will resort to using the new test method 1 since it most closely relates to existing UK practice.
The principle in all grouted anchor testing is that the anchor is subjected to tensile loading and its load vs extension or load loss vs time behaviour is monitored and assessed against established acceptance criteria. In test method 1 the anchor is loaded stepwise by one or more load cycles increasing from the datum load to the proof load (see figure 1). At each load step the displacement of the tendon end is measured during a fixed time period. In test method 2 the anchor is loaded stepwise by load cycles increasing from a datum load to the proof load. At each load step the load loss in the anchor is measured during a fixed time period (see figure 2). In test method 3 the anchor is loaded in incremental steps from a datum load to a maximum load. The displacement of the tendon end is measured under maintained load at each loading step (see Figure 3).
Note that in figures 1,2 and 3, Pp and t represent “proof load” and “time” respectively, and Po is the lock-off load in figure 1.
5.0 Points pertinent to UK anchor testing practice
Within UK practice anchors are routinely subjected to an initial cycle of loading up to the proof load, prior to testing, since this has proved useful in bedding the system in and reducing free tendon length friction. Such practices are not part of general European practice (eg in Germany and Denmark) but has nevertheless been included as an option within the new standard at the UK’s request.
The determination of creep rate could be considered a new approach but this concept was present in EN 1537: 1999. This was an earlier version of the execution standard that included anchor testing and despite the existence of BS8081:1989 was still adopted by some UK specialist anchor contractors in the past. Within the current execution standard the testing methodologies have been removed since these aspects are covered in the new testing standard.
Reference to the determination of creep rate (and critical creep load) are also included in Appendix M.10 of BS8081:1989 as an example of European practice. Importantly, the acceptance criteria (ie creep rates defining acceptance and indeed failure) associated with the different types of test are to be covered in a revised UK National Annex to EC7.
The testing of grouted anchors (referred to as anchors in EC7 and ground anchors in EN1537: 2013), is a mandatory requirement. The development of a suitable standard that harmonises the traditional approaches adopted across Europe was not an easy task and the process has taken nearly 15 years since it was first conceived.
The main challenge was gaining consensus amongst the various member countries on a document that specifies three test types and utilizes three different test methods within each type of test. This was ultimately achieved by making it clear that in developing a document of this type, member countries will not necessarily get everything they want but that as a collective, we could produce a document that provides a high degree of harmonization that is useable and it is judged that EN ISO 22477-5: 2018 has satisfied this objective.
From a UK perspective, contractors should be reassured that the process of grouted anchor testing has not radically changed and in fact the harmonisation with Europe has resulted in reductions in testing time for routine acceptance testing.