Determining the relative density of deep sand in overburden layer based on standard penetration test
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Abstract:
The relative density of sand characterizes its compactness in the soil layer and is an important indicator of its physical property. It has a significant impact on its mechanical properties and is a basic control condition for conducting mechanical property tests. Due to the unstable particle structure of the deep sand layer, the interference of conventional sampling methods may cause significant changes in macroscopic properties (including relative density) of sand. Meanwhile, special sampling methods, such as frozen sampling, are complex to operate and costly. For general engineering projects, the undisturbed sampling of deep-buried sand is difficult in practice. Therefore, a simple, efficient, and reasonable method for determining the relative density of deep-buried sand is a necessity. Internationally, the standard penetration test is a commonly used field test, and its test results can directly or indirectly represent various mechanical properties of the soil. It has been demonstrated that there is a correlation between the standard penetration blow count and the relative density of sand under certain overburden stress conditions. Based on relevant research results internationally, standard penetration test results of 5 typical sand were summarized by Skempton, and the correlation among standard penetration test blow count, relative density and effective overburden stress was formulated. Based on the recommended standard penetration test energy transfer rate (60%) proposed by Seed et al., the standard penetration test blow count was standardized as standard penetration test blow count with the same energy transfer rate by Skempton. Therefore, the above formula was standardized. Based on these relationships, a relative density estimate formula and the corresponding method for deep-buried sand in the overburden layer based on the results of standard penetration tests were established. Based on the moisture content and gradation of 5 typical sand summarized by Skempton, a sand relationship selection method was proposed for selecting the characteristic parameters of the empirical relationship between relative density, standard penetration blow count, and effective overburden stress. The effectiveness and stability of the established method for estimating the relative density of sand were demonstrated by the analysis of various engineering examples. A method was suggested to replace uncertain overburden sand density with estimated sand density, and the influence of this method on the results was analyzed through an example calculation. The analysis results indicate that replacing uncertain overburden sand density with this method has little impact on calculation results. It is recommended that an iterative calculation method may be used when high accuracy is required. Additionally, indoor tests should be conducted simulating field calibrations on the analyzed sand for projects with strict safety and accuracy requirements.
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