Perhaps you should try some spelling variations:
"ground penetration rate" » "grout penetration rate", "cloud penetration rate", "rod penetration rate", "ground penetration a", "ground penetration chapter", "ground penetration tester"
"ground penetrating nevus" » "ground penetrating neck"

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• If you are trying to use Boolean operators, they must be ALL CAPS: (("ground penetration rate") OR ("ground penetrating nevus")).
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