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Assessment of site variability from analysis of cone penetration test data.
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Assessment of site variability from analysis of cone penetration test data.
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    Soil property values for use in geotechnical design are often estimated from a limited number of in situ or laboratory tests. The

    uncertainty involved in estimating soil properties from a limited number of tests can be addressed by quantifying the variability within

    individual soundings and of the collection of soundings at a site. It has been proposed that factors of safety or resistance factors used in

    design be linked to site variability. Site variability can be assessed by studying the correlation structure of in situ test data. The cone

    penetration test (CPT), which is a reliable and widely‐accepted in situ test, can be used for this purpose. Soil behavior type (SBT) charts

    are often used to obtain the subsurface soil profile from CPT parameters such as the cone resistance and the sleeve friction. A soil

    profile generation algorithm was developed in this research to generate a soil profile from an individual CPT sounding using two

    modified SBT charts. Soils are variable in both the vertical and horizontal directions. A vertical variability index (VVI) was defined to

    quantify variability in a CPT sounding. The average of the VVIs for all CPT soundings performed at a site is the site VVI. A site horizontal

    variability index (site HVI) was also developed, based on cross‐correlation between cone resistances, the cone resistance trend

    differences and the spacing between every pair of CPTs considered, to quantify the soil variability of a site in the horizontal direction. A

    site variability rating (SVR) system, integrating the vertical and horizontal site variability, was developed to assess the overall site

    variability. Depending on the SBT chart selected, the soil profile generated using the soil profile generation algorithm may be slightly

    different; however, the SBT chart effect on the variability indices that compose the SVR index is small. Close agreement was found

    between the SVRs obtained using the two SBT charts selected for this research. In order to illustrate the use of the algorithms

    for VVI and HVI calculations and SVR of sites, CPTs from across the state of Indiana were analyzed. CPT data were obtained from

    Purdue's own database, INDOT’s data repository and the U.S. Geological Survey (USGS) website. Site variability is calculated for specific

    depths of interest. For example, that depth of interest will be shallower for shallow foundations than for deep foundations. Site

    variability rating maps (SVR maps) for various depths of interest were constructed for the state of Indiana, illustrating the potential use

    of the site variability assessment methodology. An optimal sounding spacing calculation methodology was also developed to make the

    site investigation process more efficient, cost‐effective and reliable.

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