A sonic-powered method for horizontal directional drilling, comprising: positioning a drilling apparatus proximate to an entry site at one end of a desired path for a generally horizontal bore to be formed, the drilling apparatus including a sonic drill head movably mounted on a drill mast and a sonic oscillator operatively engaged with the sonic drill head; attaching a sonic drill bit and a sonic drill rod to the sonic drill head; advancing the sonic drill bit and the sonic drill rod into and through the ground along the desired path by advancing the sonic drill head to move along the drill mast, wherein the sonic oscillator applies sonic energy defined by high frequency vibrations to the sonic drill rod and the sonic drill bit to cause the sonic drill bit to penetrate through an underground formation (56, 58, 60) in front of the sonic drill bit along the desired path; attaching further sonic drill rods to the sonic drill head and to previously-advanced sonic drill rod(s) and repeating the advancing of the sonic drill head to continue forming a generally horizontal bore by horizontal directional drilling along the desired path until the sonic drill bit emerges at an exit site at an opposite end of the desired path, wherein the generally horizontal bore formed by the advancing along the desired path defines a pilot bore for installing a line or conduit along the desired path; attaching a sonic drill casing to the sonic drill head, the sonic drill casing being larger in diameter than the sonic drill bit and sonic drill rods; advancing the sonic drill casing into and through the ground so that the sonic drill casing surrounds the sonic drill rods and the pilot bore as the sonic drill casing is installed, wherein the sonic oscillator applies sonic energy defined by high frequency vibrations to the sonic drill casing to cause the sonic drill casing to penetrate through underground formation(s) in front of the sonic drill casing; and attaching further sonic drill casings to the sonic drill head and to previously-advanced sonic drill casing(s) and repeating the advancing of the sonic drill head to continue casing the pilot bore, thereby stabilizing the walls of the pilot bore to prevent collapse of the pilot bore and/or to prevent escape of any drilling fluids used in the pilot bore, characterised in that the sonic drill head alternates between advancing one sonic drill rod and then one sonic drill casing such that the majority of the pilot bore is stabilized and cased behind the sonic drill bit during the horizontal directional drilling, and wherein the sonic drill head independently and separately applies the sonic energy to the sonic drill rods or to the sonic drill casings. positioning a drilling apparatus proximate to an entry site at one end of a desired path for a generally horizontal bore to be formed, the drilling apparatus including a sonic drill head movably mounted on a drill mast and a sonic oscillator operatively engaged with the sonic drill head; attaching a sonic drill bit and a sonic drill rod to the sonic drill head; advancing the sonic drill bit and the sonic drill rod into and through the ground along the desired path by advancing the sonic drill head to move along the drill mast, wherein the sonic oscillator applies sonic energy defined by high frequency vibrations to the sonic drill rod and the sonic drill bit to cause the sonic drill bit to penetrate through an underground formation (56, 58, 60) in front of the sonic drill bit along the desired path; attaching further sonic drill rods to the sonic drill head and to previously-advanced sonic drill rod(s) and repeating the advancing of the sonic drill head to continue forming a generally horizontal bore by horizontal directional drilling along the desired path until the sonic drill bit emerges at an exit site at an opposite end of the desired path, wherein the generally horizontal bore formed by the advancing along the desired path defines a pilot bore for installing a line or conduit along the desired path; attaching a sonic drill casing to the sonic drill head, the sonic drill casing being larger in diameter than the sonic drill bit and sonic drill rods; advancing the sonic drill casing into and through the ground so that the sonic drill casing surrounds the sonic drill rods and the pilot bore as the sonic drill casing is installed, wherein the sonic oscillator applies sonic energy defined by high frequency vibrations to the sonic drill casing to cause the sonic drill casing to penetrate through underground formation(s) in front of the sonic drill casing; and attaching further sonic drill casings to the sonic drill head and to previously-advanced sonic drill casing(s) and repeating the advancing of the sonic drill head to continue casing the pilot bore, thereby stabilizing the walls of the pilot bore to prevent collapse of the pilot bore and/or to prevent escape of any drilling fluids used in the pilot bore, characterised in that the sonic drill head alternates between advancing one sonic drill rod and then one sonic drill casing such that the majority of the pilot bore is stabilized and cased behind the sonic drill bit during the horizontal directional drilling, and wherein the sonic drill head independently and separately applies the sonic energy to the sonic drill rods or to the sonic drill casings.