Why is drilling fluid added when pulling the drill string back through the hole with a fluid-assisted boring machine?

Drilling fluid is introduced when pulling the drill string back through the hole primarily to aid the back-reamer and reduce drag. When utilizing a fluid-assisted boring machine, the back-reamer, which is attached to the drill string, works to widen the borehole or remove any obstructions as it is pulled back. The presence of drilling fluid helps lubricate the drill string and the borehole, effectively minimizing friction. This lubrication reduces the resistance experienced by the drill string, allowing for a smoother and more efficient operation.

Additionally, the fluid can help carry away cuttings and debris that may be generated during the boring process, keeping the borehole cleaner and facilitating easier back-reaming. This means that adding drilling fluid is vital for both the mechanical performance of the drilling equipment and the overall success of the operation.

While cooling the equipment is a benefit of using drilling fluids, it is not the primary reason for their addition during the retracting process. The fluid does not serve to increase the weight of the drill string significantly or primarily facilitate visual inspection of the hole, as these aspects are not the main functions of drilling fluid in this context.