A water company has pleaded guilty to health and safety charges after an explosion at one of its sites killed four workers, including a teenage apprentice.Luke Wheaton, 16, Ray White, 57, Brian Vickery, 63, and Mike James, 64, died in the blast at Wessex Water's treatment site in Avonmouth, Bristol, in December 2020.A police investigation into manslaughter was dropped in 2024 but the Health and Safety Executive (HSE) brought its own charges earlier this year.Judge Lynn Matthews referred the "complex and sensitive" case to Bristol Crown Court where sentencing was due to take place on 8 January.After Avon and Somerset Police's investigation into the explosion ended in 2024, the HSE continued its own investigation to determine if criminal offences had been committed under health and safety laws.At a hearing at Bristol Magistrates' Court earlier, Wessex Water pleaded guilty to three charges under the Health and Safety at Work Act.