Publication: Correlations between the spt-N value and undrained shear strength for alluvium soil
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Date
2022-09-01
Authors
Tan, Jia Jun
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Abstract
Standard Penetration Test (SPT) is one of the most popular and common in-situ dynamic soil penetration testing methods used to determine the shear strength properties of soils. There are several empirical correlations between the SPT blow count value (SPT-N) and undrained shear strength (Su) of fine-grained soils. However, no available empirical correlations are established for the alluvial soils. Only a limited number of correlations were previously developed for certain soil groups (e.g., residual soil) at specific locations, with uncertain prediction accuracy for different soil groups and ground conditions, particularly in the alluvium formation. With that, this study aimed to evaluate the applicability of SPT in estimating the Su of fine-grained alluvial soils. A total of 234 pairs of SPT-N and Su were collected from previous soil investigation works in Penang Island, Malaysia. After the data screening process, 98 pairs of SPT-N and Su were employed to derive new predictive equations using simple linear regression. New predictive equations were developed according to type, plasticity, and geological origin of the studied soil data. The simple linear regression results revealed the significance of type and geological origin of the studied soil data in influencing the correlation between SPT-N and Su. This study proposed two newly developed equations, and comparisons were made with the studied soil data and correlation equations from previous studies. This study evaluated the prediction capability of the proposed equations and previous studies using graphical analysis (scatter diagram) and statistical analysis like absolute average relative error (AARE) and standard deviation (SD). The proposed equations were found to be reliable in estimating the Su of the fine-grained soils (AARE= 0.29 to xix 0.31, SD= 0.31 to 0.36). For previous studies, the results showed a more noticeable deviations between the soil data and predicted values (AARE= 0.60 to 0.65, SD= 0.31 to 0.41), signifying that the correlation equations may have underestimated or overestimated the shear strength values. The two newly proposed equations were also compared and validated with soil data of outside Penang Island, and these equations showed good reliability. In addition, this study investigated the effect of SPT corrections on the relationship between SPT-N and Su. When the corrections were applied to the SPT-N, for the previous published correlations, the AARE and SD increased by up to 46.5% and 70.73%, respectively. This research also provided new insights into the influence of each correction factor (borehole diameter, sampling method, and SPT hammer energy ratio) as well as the performance of existing correlations. The results revealed that the SPT hammer energy ratio correction was the most significant factor in influencing the relationship between SPT-N and Su. The study’s findings on the prediction capability of the empirical shear strength correlations for the purpose of geotechnical designs were expected to benefit the construction industry.