service tax on earth work

Not receiving notice has no meaning. The case will be decided ex parte, after serving the notice as per the procedure described by law.

It is better to receive notice and try to find ways to contest that. If no way is there, pay the tax n interest. Interest liability will keep on growing, the more u delay it.

Whether the non-apyment has been detected or not by the Department, it is always adviceable to assess the service tax on price-cum-duty basis  as you mentioned service  tax has not been collected and pay along with interest.

By doing this penalty imposable under Section 76 or 78 may be possible to avoided.