IADC WOB(kN/mm)(bit dia)   RPM(r/min)   Applicable Formations  115    0.3--0.75  200--80  Very soft formations with low compressive strength and high drillability,such as clay,mudstone,chalk ,etc.  125    0.3--0.85  200--80  Soft formations with low compressive strength and  
high drillability,such as mudstone,gypsum ,salt,soft shale,soft limestone,etc.  135    0.3--0.95  180--80  Soft to medium soft formations with low compressive strength and high drillability.such as medium soft shale,
hard gypsum,medium soft limestone,medium soft sandstone,soft formation with harder interbeds,etc.  215    0.35--0.75  180--80  Medium formations with high compressive strength ,
such as medium soft shale,hard gypsum,medium soft
limestone,medium soft sandstone,soft formation with 
harder interbeds ,etc.