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![Page 1: Case History SSoils Stonehenge › wp-content › themes › ...Location: Structural Soils, Stonehenge, England As part of ground investigations for a 3km tunnel 18 boreholes were](https://reader033.vdocuments.us/reader033/viewer/2022042402/5f141a2f5c59930fe0475379/html5/thumbnails/1.jpg)
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.