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.
£8,000 still needed to refit Guernsey's only marine ambulance
Work on the 30-year refit of the Flying Christine III is progressing well.
Deputies submit alternatives to the funding of Guernsey's private colleges
Education's plans to phase out the £2.8M annual subsidy to reinvest in State education face opposition.
Public support for stricter anti smoking measures in Guernsey
A survey by Guernsey's Health Improvement Commission shows islanders want smoking banned outside places like pubs and sports venues.
Fresh calls for update to 90 year old housing standards
Guernsey's President of Environment and Infrastructure has issued fresh calls for introducing housing standards in the island.
Guernsey FC get ready to play last game at Footes Lane
After 11 years, and 269 games, GFC will bid farewell to Footes Lane - where the club was formed.
German Youth Orchestra to join Guernsey's Liberation Concert
Young musicians from Biberach will join Guernsey residents in concert this May.
Islander creates Guernsey AI platform
An enterprising islander has created a new AI chatbot that deals with everything Guernsey.
Guernsey CWGSY emails targeted in phishing scam
The Office of the Data Protection Authority says a series of fake telecoms bills are being sent to islanders.
