Hi @daisy4,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Treated pine sleepers have an H4 treatment rating which makes them suitable for use in direct contact with the earth.
You might like to check out How to choose the right timber for the job for some information on the different types of timber and their applications.
Simply put, yes you can put sleepers partly below the ground to stop your dogs digging under your fence.
Let me know if you have any further questions.
Jacob
