The Failure of Anchorage B160 at Devonport Royal Dockyard, Plymouth; Reasons for Failure and Lessons Learnt

In 2004 as part of the routine maintenance of ground anchorages within 15 Dock, part of the Nuclear Facilities within Devonport Dockyard an anchorage known as B160 failed. The maintenance comprised of check lifts and visual inspections, in total a further 70 anchorages underwent check lifts without failure.

Due to the nature of the facility a significant investigation was undertaken in an attempt to identify the cause and evaluate implications for anchorages throughout Devonport Dockyard. The investigation comprised extraction of the anchorage enabling visual inspection of the fixed and free length, integrity of couplers and double corrosion protection, CCTV survey, coring of the base of the hole and laboratory analysis of grout.

Due to anchorage B160 being extracted with the couplers intact and with no break in the bar or failure of the encapsulation (factory) grout, the possible remaining factors contributing to the failure of the anchor are:

1. Grout to rock bond failure

2. Outer encapsulation to grout bond failure

3. Contamination of Grout

4. Incomplete Grouting

Due to the information obtained from the exhumation and subsequent laboratory testing, it is considered most likely that the failure occurred at the encapsulation / primary grout interface.

• Background

• Details of Failure

• Extraction Process

• Examination of Exhumed Anchorage

• Discussion of Factors Leading to Failure

• Conclusions

• Suggestions for Future Rock Anchorage Installation Contracts