Design of the Unbonded Length

Design of the Unbonded Length

The minimum unbonded length for rock and soil ground anchors is 4.5 m for strand tendons and 3 m for bar tendons. These minimum values are intended to prevent significant reductions in load resulting from seating losses during transfer of load to the structure following anchor load testing.

Longer unbonded lengths may be required to:

(1) locate the bond length a minimum distance behind the critical potential failure surface;

(2) locate the anchor bond zone in appropriate ground for anchoring;

(3) ensure overall stability of the anchored system; 

(4) accommodate long term movements.

In general, the unbonded length is extended a minimum distance of H/5 or 1.5 m behind the critical potential failure surface (see section 5.3.2) to accommodate minor load transfer to the grout column above the top of the anchor bond zone.

As a general rule, the anchor bond zone and unbonded zone should be grouted in one stage to maintain hole stability and to create a continuous grout cover for corrosion protection.

However, for large diameter anchors in which the unbonded length of the anchor extends just behind the critical potential failure surface, significant strains at the top of the anchor bond zone may cause load transfer into the grout column above the anchor bond zone.

Large diameter anchors have been grouted in two stages (two stage grouting). With two stage grouting, the anchor bond length is grouted (Stage 1) and the anchor is tested. The unbonded length portion of the drill hole is then grouted (Stage 2) after the anchor is tested.

The two-stage procedure is not recommended since local collapse of the ground can occur which will compromise the corrosion protection provided by the grout.

From:https://www.fhwa.dot.gov/engineering/geotech/pubs/if99015.pdf