PURPOSE: To prevent the degradation of the ground by a method in which a vertical pit is excavated by a striking hammer while turning an excavating hammer set in a casing with rotation of the casing, and only the excavating hammer is turned and the vertical pit is further excavated by means of the striking hammer.

CONSTITUTION: A casing 1 is rotationally penetrated into the usual ground 3 by means of a casing rotary penetrator 2. A grab bucket is hung in the casing 1 to discharge soil in the casing 1, and a vertical pit 6 is excavated to a hard ground 7. The bucket is removed, an auxiliary casing 8 is connected to the upside of the casing 1, and a striking hammer 9 is inserted into both the casings 1 and 8. The casing 8 is penetrated while being turned, a hammer 9b is driven while penetrating the casing 1 and turning the hammer 9 to excavate the ground to form a vertical pit. Penetrating the casing 1 is stopped, the casing 8 is turned, and the hammer 9b is driven while turning only the hammer 9 to excavate the ground 7 to more deepen the vertical pit 6.