Is it permissible to combine the Dhuhr and Asr prayers, or to perform the obligatory prayer without the Sunnah, due to difficult work circumstances?

The default is to perform each prayer at its appointed time, based on Allah's saying: (Indeed, prayer has been decreed upon the believers a time-bound ordinance), and this was the guidance of the Prophet, peace and blessings be upon him. However, it is permissible to combine two prayers while resident for a dire need or hardship. It is more prudent for the combining to be suriyyan (figurative), by performing the first prayer at the very end of its time and the second prayer at the very beginning of its time.