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Stump rock anchors

Suitable for use even in solid rock.

Stranded rock anchors are the best example of progress and evidence of the advances in modern anchoring techniques for solid rock faces. These anchors are generally used only where the entire length of the force transmission area comprises solid rock. However, Stump rock anchors may be used in other circumstances with the approval of a geotechnical expert. 

The use of specialised grouting techniques enables reliable, heavy-duty bolt anchors to be used with even the most difficult soil types and rock formations

Developmental milestones.

Works on the Eder dam wall represent a significant milestone in the development of the Stump rock anchor system. Buoyancy control for the Eder dam wall was provided by 104 stranded permanent anchors of type 6-34 with a load resistance of 4,500kN. Test loads applied up to 12,500kN of force to anchor lengths of between 68m and 73m. In this project, it was essential to drill holes into the dam wall with great care and precision across the entire area. Drilling discrepancies of 0.5% represented a figure well below the 1% variation allowed.

Holes with a diameter of 146mm were coredrilled prior to the fabrication of the rock anchors. These holes were then expanded to a diameter of 276mm. Cement-based grout injections into the mountainside and walls enabled us to apply targeted tempering to the area, particularly in the force transmission area around each anchor. This was verified using WD tests (lugeon tests). A mobile crane and special deflection construction were used to install the anchors, to avoid damage to the anchor encapsulation during installation.

To tension each anchor, we used a prestressing jack developed specifically for this purpose. 10 out of 104 built-in anchors were equipped with force measuring boxes and fibre-optic cable sensors for permanent ongoing monitoring of the anchors. All the anchor heads are designed so that the pressure on the anchor can be measured at any time, and the anchor tensioned if necessary.

A large fissure in the Kammereck rock face, between St Goar and Oberwesel, was secured with the installation of 23 permanent rock anchors with a load resistance of 2,000kN each. The rock massif looms directly above the heavily-trafficked national highway B9, which passes through a Deutsche Bahn railway tunnel beneath the rock. Each of the up to 40m-long permanent anchors is composed of 23 individual strands arranged along two reinforced concrete cross bars. The service load was increased in two stages, first to 1,350kN and then to 2,000kN. We are constantly monitoring the deformation of the cliff face via multiple extensometers.

Approval (PDF): Felsanker (Z-34.11-223) (only available in German)