Stop the machine close to the stack and parallel to it.
Lower the stabilising jacks, (if fitted), making sure that they are on firm, level ground. Use spreader plates if in doubt.
Disclaimer: The legislative information contained on this web site is my interpretation of the law based on many years in the health and safety business. A definitive interpretation can only be given by the courts. I will therefore not be held responsible for any accident/incident/prosecution arising as a consequence of anyone using any information obtained from this web site.