Load testing of ground anchorages

Clause 8.8 8.4.1. Acceptance tests Clause 8.1.2.3

Clause 8.4(10)P

These tests are performed on all working anchorages to confirm that each meets the design requirements. Clause 8.4(10)P, second sentence, requires the design resistance to be checked by acceptance tests, in accordance with EN 1537. EN 1537 states in its Annex E (informative) that the proof load, P_P, is the greater of 1.25P₀ and R_{a, d} when the anchor is loaded in incremental steps (‘test method 3’), or P_P= 1.25P₀when the anchor is loaded in cycles (‘test methods 1 and 2’) from a datum load to the test load; P₀is the calculated prestress force (lock-off) and R_{a, d}is the design value of the anchorage resistance. Applying the test load in incremental steps is the most common method. The acceptance criterion is expressed as a maximum deformation per logarithmic time unit.

As the design value of the resistance of a grouted anchorage R_{a, d}is close to its characteristic resistance R_{a, k}(recommended value of partial factorγa= 1.1, see Annex A, Table A.12) and to the failure load, the combination of the requirements of EN 1997-1 and EN 1537 leads to very high values of the proof load compared with the working load of the anchorage; on average, a proof load P_p= R_{a, d}(where R_{a, d}≥ Pd) would then become as much as 1.4–1.7 times the working load under service conditions, P_k, because the ratio between anchorage load in a ULS and in an SLS, P_d/P_k, usually ranges from 1.4 to 1.7. The acceptance criteria set out in EN 1537, when applied to a proof load equal to R_{a, d}(‘method 3’), may be severe. It should be remembered in this respect that, in EN 1537, a value of γa≥ 1.35 is applied (EN 1537, Annex D), whereas in EN 1997-1 the recommended value is 1.1. This guide therefore recommends a broad interpretation of the wording ‘shall comply with EN 1537’ and

‘meets the design requirement’; the consequences of running acceptance tests up to the design resistance, R_d, of the anchorage should be carefully evaluated. The acceptance test should be planned in such a way that the proof load remains far enough from pull-out failure to avoid adverse affects on the anchorage, but close enough to prove that the as-built anchorage will meet the design requirements. It may be preferable to apply a proof load equal to the more severe of 1.1–1.3 times the load under serviceability conditions (the ‘working load’) or 1.25 times the lock-off force, and to put an acceptance criterion on a measured deformation in a defined time interval.

Clause 8.7 8.4.2. Suitability tests Clause 8.1.2.4

Clause 8.4(10)P

These tests are performed on selected anchors to confirm that a particular anchor design is adequate or to determine the characteristic resistance of the anchorage. Suitability tests should preferably be run to failure in order to assess the characteristic pull-out resistance of the anchorage, R_{a, k}, from the measured pull-out resistance, R_a. The pull-out resistance R_ais defined as the load corresponding to the vertical asymptote of the creep slope curveα in millimetres per log cycle time (time in minutes) plotted against the applied load. If the asymptote cannot be defined, R_a is the load corresponding to α = 5 mm (see EN 1537, Annex E.4.4 (informative), Fig. E5).

Clause 8.5.2(3) The characteristic value of the pull–out resistance may be related to the results of the

suitability tests by applying aξ factor (see Section 8.2.3 above).

If a certain design resistance R_{a, d}≥ Pdhas to be proven, the proof load should be at least equal to the expected resistance of the anchorage, i.e. to

P_p≥ ξaR_{a, k}=ξaR_{a, d}γa≥ Pdγaξa (D8.3)

where P_dis the calculated design value of the anchorage load.

At this value of the proof load, P_P, the anchorage is adequate if it does not reach a state of failure between grout and ground, as defined above, e.g.α < 5 mm.

The establishment of test standards for anchorages is one of the tasks of CEN Technical Committee 341 on ground investigation and testing.

Clause 8.1.2.5 8.4.3. Investigation tests

Clause 8.4(8)P Investigation tests are necessary for new types of anchorage or for ground or load conditions

where no experience is available for the considered anchorage system. These tests are required to proceed to failure between the grout and the soil, or are performed as creep tests.

8.4.4. Proof load as an action to the structure

The proof load is an unfavourable (characteristic, monotonic) action applied to the structure.

Thus, the structure (the wall, waling, etc.) has to be checked for this unfavourable action according to the principles of clause 2.4.7.

Example 8.1: assessment of proof load for suitability and acceptance tests

The lock-off force and design values of the anchorage load per metre of wall in an SLS and a ULS are obtained from the SLS and ULS calculations given in Chapter 9 (see Example 9.2: Table 9.6, for DA-1, DA-2 and DA-3 using the ‘limit equilibrium model’ (LEM);

Table 9.7, for DA-2 using a spring model; and Table 9.9 for SLSs). The anchorages are spaced at 2.4 m intervals, and inclined at 10° to the horizontal. The design value of the action to be sustained by the anchorage, P_d, in the ULS, and the SLS is obtained by multiplying the design value of the horizontal component of the anchor force F_hby the distance between the anchors (2.4 m) and correcting for the anchorage inclination by a factor 1/cos 10° = 1.015, as summarized in Table 8.1.

In this example, the proof load to be applied to the anchorages in suitability and acceptance tests will be assessed. At least three suitability tests will be performed.

The design value of the anchorage load in ULSs is greater than 1.35 times the anchorage load in SLSs; thus, the ULS design of the anchorage will start from the ULS anchorage loads.

Table 8.2 indicates:

• the required characteristic anchor resistances, R_{a, k}= R_{a, d}γa≥ Pdγa

• the required resistances to be proved by the suitability tests, Ra, measured, min≥ Ra, kξa, 2

for aξa, 2value equal to 1.05, or Ra, measured, mean≥ Ra, kξa, 1forξa, 1= 1.10 (three tests). The values ofξa, 1andξa, 2are chosen for the purposes of this example. As the partial factors of DA-3 cannot be used to check the suitability of anchorages from the results of a load test, the partial factors of DA-2 are used to meet the requirements for the proof loads in the suitability tests. Of course, the value P_dfrom the wall calculations of DA-3 are used; the required proof loads for the acceptance test on each anchor are determined according to

GUIDETOEN1997-1

Table 8.1. Design value of anchorage load P_dobtained from SLS and ULS design of the retaining wall using DA-1, DA-2 and DA-3

Lock-off force (kN/m)

ULS and SLS design value of horizontal anchorage load F_hfrom wall design (kN/m)

Lock-off force (kN)

ULS and SLS design value of anchorage load P_d(kN/anchorage)

SLS

DA-1 LEM

DA-2(L) LEM

DA-2(S) Spring model

DA-3

LEM SLS

DA-1 LEM

DA-2(L) LEM

DA-2(S) spring model

DA-3 LEM

100 112 172 228 157 172 244 273 419 557 383 419

Table 8.2. Overview of required characteristic anchor resistance and proof loads for suitability and acceptance tests (all values are axial forces in the anchorage, in kN)

Lock-off force, P₀

Anchor load in SLSs, P_SLS

Required characteristic anchor resistance, R_{a, k}(γa= 1.1)

Required proof load for suitability tests

(ξa= 1.1 and 1.05) Required proof load P_pfor acceptance tests

DA-1 DA- 2(L) DA-2 (S) DA-3^a DA-1 DA-2 (L) DA-2 (S) DA-3^a DA-1 DA-2 (L) DA-2 (S) DA-3

244 273 461 611 421 461 Mean 484 642 442 484 EN 1537 419 557 383 419

Minimum 507 672 463 507 Realistic 313 313 313 313

aSuitability to be checked for P_dcalculated in DA-3 by applying the partial factors of DA-2

the requirement of EN 1537, i.e. the greater of 1.25P₀and R_{a, d}(see the first row of proof loads for acceptance tests).

Table 8.2 shows that, in all Design Approaches, the proof load for acceptance tests is very high compared with the lock-off force and to the SLS anchorage load. These high proof loads, P_p, for acceptance tests can lead to creep of the anchors during testing because there is only a very small margin between the proof load and the anchor resistance (compare the values indicated for the proof load for suitability tests, keeping in mind that proof loads for the suitability tests are failure loads). Table 8.2 proposes a more realistic proof load (second row, in bold) obtained from

P_P= max{1.25P₀, 1.15R_SLS}