Thanks Misha for your reply..
<Resource name="UserDatabase" auth="Container"
type="org.apache.catalina.UserDatabase"
description="User database that can be updated and saved"
factory="org.apache.catalina.users.MemoryUserDatabaseFactory"
pathname="conf/tomcat-users.xml"
readonly="false"
/>
is alredy present in server.xml..
Any other hint that I need to try?