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Application: Geotechnical Technology: Chalkcharacterisation Location: StructuralSoils,Stonehenge,England
Aspartofgroundinvestigationsfora3kmtunnel18boreholesweredrilledrangingfrom146mmODboresto335mmODwells.Depthsvariedfrom35mto70m.Theclientwasparticularlyinterestedina>20mthicklayerofphosphaticchalkanditsstructuralimpactontheproject.Thekeyphysicalcharacteristicsofchalkarehighporosityandpermeabilitywithsoftfriableandsolublenature–whichcanbechallengingforgroundstabilityandfoundationdesign.Thischaracterallowsdissolutioninchalkbedsthatcancreateirregular,unpredictablesubsurfacevoidsandcavitiesleadingtoreducedrockstrengthandvulnerabilitytoongoingdissolution.Thesevoidsareextremelydifficulttodetectfromthesurface.Theobjectivewastodeterminethelocationandthicknessofthesephosphaticdeposits.
ToachievethisRobertsonGeoranits3-ArmCaliperprobewhichcontainsasodiumiodidenaturalgammacrystal.Thenaturalradiationemittedinaboreholesformationisdetectedbyscintillation:theproductionofatinyflashoflightwhengammaraysstrikethecrystal.Thelightisconvertedintoanelectricalpulsebyaphotomultipliertube.Thepulsesarethencountedbytheprobe’smicroprocessor.TheprobeiscalibratedinaccordancewiththeindustrystandardAPIunit.
Endvaluetoclient:
Naturalgammavaluesinchalkarenominallyverylow(<20API)duetothesparsityofpotassiumandheavierelements.Wherephosphaticchalkispresentthenaturalgammalevelcanrisesignificantly,upto140APIormore.Theaccurategammarayreadingsprovidedenabledtheclienttopreciselydeterminethissharpincreasealongwiththeexactdepthandthickness.
TheNaturalGammareadingillustratinganincreaseincountsata
changeinchalkbedding.