Active and passive anchors

Active anchors refers to pre-stressed anchors, in which the tensile force is established by tensioning a tendon free length against the anchor head. The anchoring force is established without further deformations in the ground. Passive anchorage means that a tendon or bolt which is tension-free at the time of installation is activated when the ground alongside the anchor is deformed.

Eurocode 7 - Anchors

The dimensioning standard, Eurocode 7, lays down rules for dimensioning of active anchors in Chap 8, "Anchors". Passive anchoring is rather cursorily treated in the same standard, with a short comment that it may dimensioned as for piles in Chap. 7.

Temporary and permanent anchors

A distinction is made temporary and permanent anchors. The distinction is a dimensioning lifetime of less than or more than two years. The implementation standard NS-EN 1537 for active anchors lays down requirements for corrosion protection of permanent anchors.

Tendon

The tendon between the anchor head and the anchoring body is usually steel, but fibreglass, carbon or kevlar are also used. Steel tendons may be cables or laces of drawn steel wire or else whole-rolled steel rods.

Enkel fjellbolt

Anchor head

The anchor head usually consists of a bearing plate and a nut for rod anchors or plate and wedges for fixing cables or laces. Steel rod tendons require a spherical bearing to equalise angular deviations. Cables or laces must be so soft that they can absorb the angular deviation without special measures.

Anchoring body

The anchoring body is usually a length of tendons in which the tensile force is transferred to the ground through shear (adhesive) stresses from the tendon and to the surrounding grout and from there to the ground alongside the drill hole. The anchoring body is usually in tension, but may be in compression (removable anchors).

Bolts

Bolts used for supporting the bedrock in cuttings or rock caverns is usually a passive anchor. If the bolt is installed at the time of blasting of the cutting or close to the working face in a rock cavern, it will be significantly pre-stressed by the deformation of the bedrock during further blasting.

Self-drilling anchors

Self-drilling anchors are a type which combines the drilling equipment, crown and drilling rod with the anchoring body and tendon. The advantage is that the drilling fluid, the grout, stabilises the borehole while at the same time the mortar anchors the tendon to the ground.

Interaction between the construction and the ground

An anchor is a construction which requires an understanding of the interaction between the construction and the ground. Most anchors in Norway are made in rock. It is rare for anchors to terminate in soil. One exception is "deadman anchors". Anchorages in firm soil and loose rock are common in other countries.