Failure of the Ground-Grout Bond of Self-drilling anchors

In general, the ground-grout bond is mobilized progressively in uniform soil or rock as the stress is transferred along the bond length.

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Ground anchors mobilize skin friction between the anchor bond zone and the ground. In general, this bond is dependent on the normal stress acting on the bond zone grout and the adhesion and friction mobilized between the ground and the grout. Anchors which are underreamed may also develop the base resistance of the increased annular area. In general, the ground-grout bond is mobilized progressively in uniform soil or rock as the stress is transferred along the bond length. Initially, as the anchor is stressed, the portion of the bond length nearest the load application elongates and transfers load to the ground.

As the resistance in this portion of the bond length is mobilized, stress is transferred farther down. During this process, the anchor continues to elongate to mobilize deeper bond zones. Once the stress is transferred to the end of the bond zone and the ultimate ground-grout bond is exceeded, anchor failure by pullout occurs.

Self-drilling Anchors which have been improperly grouted such that a column of grout exists between the bearing plate or wall and the top of the bond zone will show no load transfer into the bond length when the load is increased. Factors influencing stress transfer for small diameter ground anchors with bond lengths in a uniform soil are summarized in table 3. Experience has shown that increasing the bond length for typical soil anchors beyond 9 to 12 m does not result in significant increases in resistance.

A possible reason for this observation is that after the load has been transferred that distance down the bond zone, sufficient movement at the ground-grout interface has occurred in the upper bond length to decrease the upper ground-grout interface resistance to residual strength levels.

Bond lengths greater than 12 m may be used effectively provided special procedures are used to bond the tendon to the grout such that capacity can be mobilized along the longer length.

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

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