大地工程之設計，普遍使用安全係數的概念。然而，未知的參數變異性會在計算安全係數時造成影響，若土壤參數相當均勻、變異性小，使用安全係數進行分析就能得到較可靠之結果。 本研究將透過自相關函數與關聯性長度的概念，對標準貫入試驗N值之水平變異性進行探討。自相關函數描述兩位置之參數，如何與相對距離產生關聯性，透過自相關函數，可求得關聯性長度，其意義為兩地之參數具有關聯性之臨界距離。本研究從163個鑽孔資料中求取其自相關函數與關聯性長度，該場址大多以粉質砂土為主。為了瞭解減少鑽孔數對自相關函數之影響，吾人自163鑽孔中分別提取10至80孔後再分析，以求得空間之變異性。 主要成果如下：(1) 以163鑽孔進行分析，求取自相關函數與關聯性長度，擬合程度最高的為Single exponential。(2) 隨著提取鑽孔之增加，關聯性長度之平均值與標準差亦隨之增加。(3) 細粒料含量之關聯性長度遠長於以SPT-N值求得之結果，表示其水平空間變異性較不顯著。 In terms of design in geotechnical engineering, the most common concept is to employ the notion of “Factor of Safety”. However, using a mere factor of safety also means the ignorance of variability of the parameters that may affect the results of analysis, If the geotechnical parameters are pretty uniform and do not vary much, then the employment of factor of safety may yield acceptable (and reliable) results. In this study, the horizontal variability of standard penetration test N values (SPT-N) is discussed through the concept of autocorrelation function and correlation length (or scale of fluctuation, SOF). The autocorrelation function describes how the parameters in two locations are related in terms of their relative distance. In addition, the derivation of correlation length can be obtained from autocorrelation function, and this means that beyond this correlation length, the parameters in two locations separated by this distance are most unlikely related. The main purpose of this study is to obtain the autocorrelation function and correlation length based on 163 boring logs from a site with mostly sandy-silty type of material. Afterwards, in order to know how the lack of partial boring logs can affect the autocorrelation function and correlation length, 10 to 80 boring logs were extracted from the 163 soil borings and remaining borings logs were analyzed again to obtain the spatial function. The main findings are as following: (1) Total of 163 soil boring logs are paired and grouped to obtain the autocorrelation function and corresponding correlation length. It was found that the best fit of function is single exponential function. (2) When less soil borings are extracted from existing 163 boring logs, it was found that as the number of boring logs are less, the longer the average and standard deviation of correlation length.(3) It was found that the stabilized correlation length of the SPT-N value is much less than that of the fine contents. This means that the horizontal spatial variability is less significant across the construction site.
