The A. V. regards the two words as a strong expression of a single idea; but the idea is twofold: he dug (through the sand), and deepened down into the solid rock. So Rev., rightly, he digged and went deep.
The flood (πλημμύρας)
There is no article: a flood. The word occurs in Luke only, and only in this passage. As a medical term it is used of excess of fluids in the body: flooding.
Beat vehemently (προσέῤῥηξεν)
Rev., more literally, brake. Used by physicians of a rupture of the veins. It occurs only here and Luk 6:49. Matthew has προσέκοψαν, beat.