Basis of design

Guidance on the design of anchorages is currently split between EN 1997-1 and EN 15373 (the execution standard for ground anchors - see Chapter 15). Unfortunately some of this guidance is contradictory. Furthermore, EN 1537's testing requirements for ground anchors overlap the planned scope of ISO EN 22477-54 (the geotechnical investigation and testing standard for anchorages - see Chapter 4). The relevant CEN Technical Committees are considering moving design-related material from EN 1537 into EN 1997-1 and testing-related material into ISO EN 22477-5, leaving EN 1537 to deal solely with execution matters. However, UK practitioners argue5 that separating testing from execution is not a good idea and prefer the integration of content employed in BS 8081.6

In view of the above, the following discussion provides our tentative understanding of the way ground anchorages should be designed to Eurocode 7, highlighting areas where contradictions exist and need to be resolved in future editions of the codes. This book does not attempt to provide complete guidance on the design of anchorages, for which the reader should refer to any well-established text on the subject.7

The design pull-out resistance of an anchorage Ra,d must satisfy the following inequality: P < R

1 d ~ lxa ,d where Pd is the design load in the anchorage, i.e. the larger of the anchor force derived from an ultimate limit state verification of the retained structure (PULS) and that derived from a serviceability limit state verification

There is some debate as to whether the anchor force that is derived from an ultimate limit state (ULS) verification of an anchored retaining wall (i.e. PULS) is the largest force the anchor will have to withstand. At the ULS, earth pressures on the back of the wall approach limiting active (Ka) values and those on the front limiting passive (K^) values.

In some circumstances, it is possible for the serviceability limit state (SLS) force PSLS to be of similar magnitude to - or even larger than - PULS. At the SLS, earth pressures on the back of the wall may remain close to their in situ (i.e. Kq) values, particularly for inflexible walls in stiff soils. Since values of Kg can be considerably larger than Ka, it is possible for PULS and PSLS to be of similar magnitude.

The value of PULS obtained from ultimate limit state calculations of wall stability implicitly includes a load factor yg, which is equal to 1.35 in Design Approaches 11 and 2 and 1.0 in Design Approach 3. Because of this, anchorage designs based on Design Approach 3 may not be sufficiently reliable, particularly if the ratio PULS/PSLS is 1.1-1.2 (which can often be the case when a soil-structure interaction model is used). Frank et al.8 recommend multiplying PSLS by a model factor YRd = YG = 1.35 to rectify this. The design load in the anchorage Pd is then:

Eurocode 7 Part 1 discusses two ways of determining the design pull-out resistance Ra,d — from the results of tests (as discussed in Section 14.5 below) and by calculation (discussed in Section 14.6).

Eurocode 7 Part 1 has specific requirements regarding anchorage tests. Unless their performance and durability can be demonstrated by (documented) successful comparable experience, anchorage systems must be verified by investigation tests (see Section 14.4.1); the characteristic pull-out resistance Ra,k of grouted and screw anchorages must be determined from suitability tests (Section 14.4.2); and all grouted anchorages must undergo acceptance tests (Section 14.4.3). [en 1997-1 §8.4(8)P, (10)P, and 8.8(1)P]

Suitability tests are not intended to determine the characteristic pull-out resistance. Their purpose is to prove that the anchorages are suitable for the conditions on site. The important distinction is that the loading in a suitability test does not exceed the proof load. Usually the anchorage is subjected to a more rigorous number of load cycles. The test is not designed

+Yg = 1.35 in Combination 1 (which governs) and 1.0 in Combination 2.

to ascertain information on the ultimate or failure condition when establishing pull-out resistance.

Factors that must be considered in the design of anchorage systems include: tolerances on angular deviations of the anchor force and accommodation for other deformations; compatibility between the deformation performance of different materials used in the anchorage; and adverse effects of tensile stresses1 transmitted to ground beyond the anchorage.

Factors that must be considered in the construction of anchorage systems include: applying proof and lock-off loads to pre-stressed tendons; allowing pre-stressed anchorages to be de-stressed and re-stressed, as required; and avoiding adverse effects caused by the retained ground or existing foundations being too close to the ground that provides the resisting force.

Corrosion protection must comply with the requirements of EN 1537.9

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