Shield method and method of modifying agent

PROBLEM TO BE SOLVED : To provide a processing method capable of making effective use of excavated earth and sand produced from a muddy water type shield construction method and a modifier. SOLUTION : In this method for processing earth and sand derived from a muddy water type shield construction method, excavated earth and sand derived from the muddy water type shield construction method is classified into primary processed earth 11 and secondary processed earth 12 having a smaller average grain size than the primary processed earth 11, after which a modifier 13a containing gypsum and the secondary processed earth 12 are admixed to form a mixture (modified secondary processed earth 12a); the modified secondary processed earth 12a and the primary processed earth 11 or modified primary processed earth having the modifier 13a added thereto are admixed to obtain reformed earth and sand 20. The modifier 13a is added to the excavated earth and sand derived from the muddy water shield construction method and contains gypsum.