Can I use contact lenses instead of my very thick spectacles?

Maybe. If your eyes are healthy and your cornea is not irregular, then you can probably wear contact lenses. See your local eye doctor for a contact lens evaluation and fitting.
Maybe. It's best to check with an eye care provider to see if contact lenses are a good idea and come in the power needed. Depending on astigmatism, a toric contact may be needed. Also the corneal curvature may affect the ability to wear contacts as well as the tear film / dry eye, etc.