According to the US and European civil aviation authorities (FAA & EASA) there is no regulatory minimum distance between the ground and an aircraft’s engines; which some may find surprising.
In fact, what matters, is that the plane complies with the certification "specifications". The design of an aircraft should allow it to meet the regulatory safety requirements set for commercial transport aircraft.
An example is the ability to function normally on a waterlogged runway. The water ingestion trials are there to ensure that water does not enter an aircrafts’ engines or the auxiliary power unit (APU).
Even though there is no specific regulation on the matter, an aircraft’s engines must still allow sufficient ground clearance with the aim of minimising the risk of any foreign object ingestion thrown up from the runway or taxiway, when the aircraft is taxiing, taking-off or landing. From observation, we can see that, generally, engines are provided with a minimum ground clearance of 20 inches (50 centimetres).
