If you want to penetrate and break rock quickly, lacking time to drill and plant explosives, then anti tank rounds are your best bet.
HE will just explode on the surface and do little damage, its a shame that all the footage I have seen just shows the Engineer firing the round and not the effect of the impact.
Fortunately, I understand that most of the Sichuan Geology is relatively soft Sedimentary Rock, has it been harder Igneous or Metamorphic strata, then they would have had no option but the slow and arduous process of drilling. Sedimentary Rock would probably break apart very satisfyingly!