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 to get serious about film industry expansion
A new organisation will nurture local talent and entice producers to consider Guernsey's unique locations.
JT Group acquires Isle of Man's leading telecoms provider
JT Group and CVC DIF have signed an agreement to buy Manx Telecom Group in the Isle of Man.
First brick laid at Guernsey's upcoming Wildlife Hospital
The GSPCA has entered the next phase of its big build with the foundations complete and the cornerstone laid.
Farming in Herm dates back six millenia
Recent excavation finds evidence of a settlement thought to date back 6,500 years.
Guernsey's water storage increases by 1% in 24 hours
57mm of rainfall during Storm Benjamin begins to replenish supplies.
Guernsey primary school gets daily vegetable tasters
The Health Improvement Commission launched the Munchy Lunches initiative in Le Rondin last year.
Storm Benjamin dramatically ends Guernsey's dry spell
Winds were generally mid Channel but hit 80 mph overnight at St Helier harbour.
Tireless Guernsey fundraiser "re-energised" by awards evening
Jo Priaulx, walked the red carpet at the Pride of Britain Awards ceremony.
