Security measures are being reviewed after two people gained access to the Aberdeen bypass site and scaled a crane.
Transport Scotland said "strict security measures" that the contractor had in place had been breached.
A spokesman said: "We are aware that two individuals gained access to the project site, outside of working hours."
Transport Scotland said the contractor was reviewing its security measures.
The spokesman added: "We strongly encourage all individuals not to enter the project site for their own safety and that of our workers."