Hi, There are not any specific criteria for the spacing of sample taken, and the decision is purely based upon site civil engineers and various criteria of soils and on observations. if you found soil which is less compacted or even is created then take samples for observation and check the bulk density of soil and moisture content. Please refer to the following article for details about core cutter method and its applications: Link: https://theconstructor.org/geotechnical/dry-density-of-soil-by-core-cutter-method/2680/18

Ricardo Kuhn
Construction Foreman