The affix kyap is used after the root khana 'to dig' and long ī is substituted for the final of the root.,
Thus खन् + क्यप् = ख + ई + य = खेयम् 'to be dug'. The long ई is used in the sutra for the sake of euphony. The same purpose would have been served by using short इ, thus 'इ च खनः'. But the long ई indicates that the rule 6.4.43 does not apply here. By that rule the अ of खन् would have been lengthened into आ before the affix क्यप्.,