Modeling and Analysis of a Small Diameter Pneumatic Boring Tool

This paper documents the development, solution, and application of a computational model for the dynamic response of a small diameter, pneumatic tool used for boring horizontal tunnels in the soil. The model consists of (i) the tool component kinematics and kinetics, (ii) mechanics of the tool and soil interactions, (iii) the compressible air dynamics, and (iv) the pressure control valve switching logic. The resulting model is represented by a set of coupled, sixth-order nonlinear differential equations. The boring tool design has several unique features, including dual pistonheads and a pilot-pressure actuated spool valve used to control the oscillatory piston. Implementation of these and other tool features in the computational model is discussed at length. The dynamic simulation and associated parametric studies establish the feasibility of the design for small diameter (25.4 mm) horizontal boring tools. Results for this design predict tunneling rates of about 60 meters/hr in a medium clay soil.