Probability of Boulders

Ove Dalager Ditlevsen

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    To collect background information for formulating a description of the expected soil properties along the tunnel line, in 1987 Storebælt initiated a statistical investigation of the occurrence and size of boulders in the Great Belt area. The data for the boulder size distribution were obtained by use of aerial photographing of cliff beaches with subsequent stereo metric measurement on the photographs. To get information about the density of occurrence, a series of continuous seismic scannings were also made along parallel lines in a corridor with the tunnel line as about the central line. The data collection part of the investigation was made on the basis of geological expert advice (Gunnar Larsen, Århus) by the Danish Geotechnical Institute (DGI).The statistical data analysis combined with stochastic modeling based on geometry and sound wave diffraction theory gave a point estimate and a 95% confidence interval (say) for the expected number of boulders with largest dimension larger than 1m (say) per volume unit of till along the tunnel line. Using quantitatively prudent assessments of the uncertainties of the seismic measuring method in the calculations, the resulting point estimate varies linearly from about 35 to about 53 boulders of maximal size larger than 1 m per 100 000 m3 of till across the seismic scanning corridor from the south to the north boundaries. The assessed 95 % confidence interval for the number of boulders along the north boundary ranges from 22 to 112 boulders. Contractor and client representatives systematically recorded the boulders from the first 61 500 m3 of tunnel excavation in the in the upper till.The observed empirical distribution given in the range from about 0.5 m to about 1.2 m is well within the 95% confidence interval. For 1 m it is by interpolation obtained as about 45, convincingly close to the prediction of 44.
    Original languageEnglish
    Title of host publicationEast Tunnel
    Place of PublicationCopenhagen
    PublisherA/S Storebæltsforbindelsen
    Publication date1997
    Publication statusPublished - 1997

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