The SQL server Log folder is expanding because of the SQL Dump files (what to do with it ?) The "Log" folder SIZE in the SQL server root directory (X:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Log) becomes too big , ~80 GB.

When I checked it, I see that there are a lot of SQLDumpxxxx.mdmp /SQLDumpxxxx.txt files in that folder.


what to do with them ?
can I delete them and if it is good thing to do ?


10X
First of all, everything that Shanky said in his answer is 100% correct.  I would like to add:


The age of the files - If your log folder has several dumps from two years ago, then no dumps for several months, then a few recent dumps, you can safely delete the two year old dumps.
Duplicate dumps - When dumps occur there are three files created: .txt, .log &amp; .mdmp.  Open the .txt file for several of the dumps.  If they are different, keep the dumps.  If they are the same, delete the old ones.
Your available disk space - If the disk where your dumps are accumulating is also the same disk where your database files or transaction log files also reside, and you are going to run out of disk space if you don't do something, and you have no where to move the dump files to, then by all means, delete them.
Types of dumps - If the .log file indicates "Non-yielding Scheduler",  "Non-yielding IOCP Listener" or "Non-yielding Resource Monitor", these are performance related specific to CPU.  You can troubleshoot those following the steps in this blog:  https://blogs.msdn.microsoft.com/karthick_pk/2012/08/21/non-yielding-iocp-listener-non-yielding-scheduler-and-non-yielding-resource-monitor-known-issues-and-fixes/


If the .log file indicates "Deadlocked Schedulers", this is also performance related specific to CPU.  You can troubleshoot those following the steps in this blog:  https://blogs.msdn.microsoft.com/karthick_pk/2010/06/22/how-to-analyze-deadlocked-schedulers-dumps/ 


Corruption Dumps - Just as the first answer states, run DBCC CHECKDB.  If the output from DBCC CHECKDB indicates corruption in your indexes, rebuild them.  If it indicates that "repair allow dataloss is the minimum" necessary to repair the database, then restore from last known good backup.
AV Dumps - You can attempt to tackle this yourself:  https://blogs.msdn.microsoft.com/sqlcat/2009/09/11/looking-deeper-into-sql-server-using-minidumps/
If you run into difficulty, open a case with Microsoft, but just as Shanky said, ensure you are on a supported version and you have the latest updates.