Test load on bonded anchors with insufficient anchorage base

[Translate to English:] Abbildung 1: Prüfaufbau schematisch

[Translate to English:] Abbildung 1: Prüfaufbau schematisch

Abbildung 2: Last- und Verformungskurven

Abbildung 2: Last- und Verformungskurven

In the shaft of the detour tunnel of the Malter Dam, a steel staircase anchored to the shaft wall was installed in spring 2019, leading approximately 23 meters into the depth.

The stair flights were suspended in the landings so that they could be easily dismantled. The landings had to be fastened individually to the shaft wall, as it was geometrically impossible to support them all the way into the slide gate basement. The landings were each placed on three brackets anchored in the shaft wall.

During construction (1913/1914), the rock excavation was leveled by a tamped concrete shell (t > 50 cm) with strengths between 3 N/mm² and 30 N/mm² and widespread gravel pockets.In 1996, the shaft wall was faced by a 10 cm thick pre-hung shotcrete shell to counteract seepage.The shotcrete shell is structurally unsuitable for fixing the brackets. The brackets therefore had to be placed on top of the stamped concrete and anchored in it.

Commonly used and suitable composite anchor systems have general building authority approvals by ETA or DIBT (abZ) for concrete strengths above 20 N/mm². Their use was therefore not permissible without further ado. It was not certain whether the expected failure mechanisms would still be covered by the verifications according to ETAG or ENSO.The verifications were nevertheless carried out and could no longer be performed if 3 N/mm² were used. Initially, therefore, it was planned to temper the old concrete by injection. However, because of the risk of impairing the performance of the drainage layer located between the tamped concrete and the shotcrete shell, it was decided to anchor in the untempered concrete. The adequate load-bearing capacity was to be ensured by trial loads.

Pure tensile tests were out of the question because, in particular, the failure of the hole soffit under shear force was also considered as a failure mechanism. The stress in later operation was to be mapped as accurately as possible in the test.

One of the two test setups is shown in simplified symbolic form in Figure 1. The load was applied by tightening the tensioning nut screwed onto the anchoring cable against the bracket. A dowel tester (HILTIDPG) was interposed between the tensioning nut and the bracket for force measurement. Deformation was measured directly on the bracket by means of a dial gauge. The test regime was specially developed on the basis of pile test loads according to DIN 14199, since no adequate procedures could be researched. The definition of limit and failure criteria proved to be particularly difficult.

Three criteria were formulated, with those described below proving particularly useful. According to these, the increase in deformation had to converge to zero over the load-holding time. In addition, the deformation had to remain constant in the last 120 seconds of the load holding time. Starting from a preload, the characteristic load was set at the first load level, and the design load at the second. Figure 2 (see page 3) shows typical load and deformation curves. The load drop at the beginning of the hardening phase of the initial load is typical and is due to the slip to be overcome in the test setup. Four of the 54 brackets to be tested did not meet the two-minute criterion at load level 2. As a result, it was decided to reduce the permissible live load of the staircase from 5 kN/m² (category T2 according to DIN 1991-1-1/NA) to 3 kN/m² (T1). This live load is covered by the first load level and meets the requirement for stairs in normal building construction without heavy equipment.

Dominik Fiedler – Dresden