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 teenager launches coffee business
Shortly after finishing her GCSEs, Harriette Guille became her own boss, with her own coffee van.
Stowaway squirrel spotted in Guernsey
The GSPCA has received multiple reports over the last week of a grey squirrel in the St Sampson area.
Guernsey hotel group encourages guests to skip a room clean
By saying 'no thanks' to a daily room clean, guests at the Sarnia Hotels Group can help island charities.
Protestors to meet deputies ahead of Guernsey's GST debate
Islanders against GST will make their presence felt outside the Royal Court for the tax reform debate.
Venue change for World Cup semi-final screening in Guernsey
The England Fan Zone, organised by Owl Events, has been moved indoors.
Discarded glass blamed for Little Sark fire
Glass that may date back to the Occupation is seen as the likely cause of the major wildfire on Little Sark.
Appointment of Guernsey's first female Bailiff
Jessica Roland will become Guernsey's first ever female Bailiff, following the approval of her appointment by His Majesty the King.
Sensitive demolition at Guernsey's largest building site to begin soon
The remaining buildings at Leale's Yard off the Bridge will be demolished carefully over the coming winter.
