A hole has appeared behind the end of the German wall at the eastern side of L'Ancresse, near the slipway.
The States of Guernsey has fenced the area off, and work will start next week to fill it.
Under the latest States resolution, Environment and Infrastructure are to keep repairing the crumbling World War Two defence work for a number of years.
A £150,000 repair plan was agreed in April.
Members of the public are being asked to avoid the area and not pass the barriers.
Guernsey Waste improves the quality of its free garden compost
The 'windrow' method ensures the temperature of garden waste is constantly monitored to provide optimum conditions.
Guernsey cat lover to ride the length of Britain in nine days
Former Commonwealth and Island Games cyclist Ann Bowditch expects the solo ride to be tough.
Unclaimed CI Co-op cash funds community projects
Money that was sitting unclaimed in dormant Co-op share accounts is being given to community projects in the Channel Islands.
Work to commence on 70 affordable flats and homes in Guernsey
Construction will begin this summer on the affordable housing project at the former CI Tyres site, now known as Court Falaise.
A new tax, more 'proportionate' to wealth, to be put to Guernsey's States
Deputy David Goy initially wants public feedback on a new tax system that would look at alternative sources of revenue, rather than GST.
Guernsey gives £75k for Islands Unlimited inter-islands sailings
Guernsey taxpayers have given inter-islands ferry service Islands Unlimited a £75,000 grant
Guernsey tax package to raise more States income but protect low earners
GST at 3% combined with benefits and States savings will bring in 59M a year.
Changes to make Guernsey's buses more reliable
Bus operator Stagecoach and Guernsey's States have tweaked the timetable to offer a more effective service.
