Definition - What does Fouling mean?
Fouling is the buildup of undesirable material on hard or compacted surfaces, such as drill bits or the inner surface of bores degrades function. Fouling materials can be biological, such as grass, insects or other living matter – called biofouling – or a non-living substance, such as dirt, mud or rock chips, called inorganic fouling. Fouling is differentiated from other surface contamination because it inhibits useful function. Corrective action requires the removal of the material from the surface.
Trenchlesspedia explains Fouling
If you bore through mud that's thick enough to cling to the bit -- to the point the drill bit can no longer function -- the drill bit is fouled. If you make a bore hole that's not large enough for the pipe you wish to install the bore is fouled. If the bore is too wet or slushy to be compacted by the boring process, the bore may become fouled by the material through which you drilled.
If the bit or any other drilling component is fouled, you must take it out of service and clean the fouling from it. If a bore is fouled you may have to re-drill it. Both processes are expensive because of the lost time involved.