برآورد خصوصيات فيزيكي و خميري خاك‌هاي رسي مارني تبريز بر اساس عدد SPT

Estimation of Physical and Engineering Properties of Clayey Soils According to Standard Penetration Test: A Case Study of Tabriz Clayey Marl

آزمون نفوذ استاندارد (SPT) يكي از متداول‌ترين‌ترين آزمايش‌ها در تحقيقات ژئوتكنيك است. در اين تحقيق با استفاده از نتايج آزمايش نفوذ استاندارد، برآورد خصوصيات خميري و فيزيكي خاك‌هاي رسي باروش­هاي آماري بررسي شده است. محدودۀ بررسي شده مربوط به انواع لايه­ هاي رسي و مارني در شهر تبريز واقع در شمال‌غرب ايران است. دو گمانه ماشيني حفاري شده و آزمايش نفوذ استاندارد به‌همراه ساير آزمون‌هاي آزماشگاهي براي تعيين همۀ خصوصيات خميري و فيزيكي روي نمونه‌هاي تهيه شده انجام شد. با استفاده از نتايج پژوهش‌هاي پروژه‌هاي مهم موجود در منطقه، بانك اطلاعاتي مشتمل بر 107 سري از خصوصيات لايه‌هاي منطقه تهيه شد. ضريب هم‌بستگي اسپيرمن بين خصوصيات فيزيكي و خميري خاك شامل درصد رطوبت طبيعي، درصد ريزدانه و درصد ذرات رسي، حد رواني، حد خميري، دامنۀ خميري، شاخص استحكام، فعاليت، تخلخل اوليه، وزن مخصوص خشك و تنش مؤثر قائم با عدد نفوذ استاندارد تصحيح شده تعيين شد. 8 رابطۀ تجربي موجود براي برآورد نفوذ استاندارد تصحيح شده با داده‌هاي اين تحقيق صحت‌سنجي شد. براي اين روابط عموماً نتيجه ضعيف از نظر ضرايب آماري شامل ضريب تبيين (R2)، خطاي ريشه ميانگين مربعات (RMSE) و شيب خط رگرسيون(CR) به‌دست مي‌آيد. با استفاده از رگرسيون غيرخطي و بر اساس داده‌هاي تحقيق حاضر، هشت رابطۀ تجربي جديد بررسي و پيشنهاد شد. ضريب تبيين، خطاي ريشه ميانگين مربعات و شيب خط رگرسيون براي روابط پيشنهادي نسبت به روابط موجود بهبود يافته است. ضرايب آماري براي مناسب‌ترين رابطه به‌منظور برآورد شاخص استحكام خاك با استفاده از نفوذ استاندارد تصحيح شده حدود 0.7 با خطاي 129/0 به‌دست آمد. هم‌چنين دو رابطه براي هم‌بستگي تنش مؤثر قائم و عمق نمونه با نفوذ استاندارد تصحيح شده پيشنهاد شد.

The standard penetration test (SPT) is one of the most common tests in geotechnical investigations. The results of this test are known as a simple, inexpensive, and tangible criterion in geological and geotechnical engineering. Many computational methods and engineering judgments depend on the results of this test. In this research, estimation of physical and engineering properties of clay soils was carried out using statistical methods based on standard penetration test results. The scope of this case study is related to a variety of clayey soils in Tabriz (the northwest of Iran). The existing relationships were confirmed based on database of this study. After statistical analysis of the database, eight relationships including single and two-variable associations have been proposed to estimate the physical and engineering properties with better performance using nonlinear regression. Material and Methods Different types of clayey silt and marl layers spread in Tabriz were included for the purpose of this study. The geological age of these layers dates back to the Miocene and Pliocene era. This research was conducted in two sections of the field and analysis. Two machine boreholes were drilled, and, based on ASTM, a standard penetration test with other laboratory tests were performed on the soil specimens in order to determine the physical and plasticity properties. According to the results of this study and the existing data, a total of 107 series were prepared. Based on the soil properties, 11 variables were selected including the fine grain percentage (FGP), liquid limit (LL), plastic limit (PL), percentage of clay particles (C), plastic index (PI), consistency index (CI), activity (A), dry unit weight (γd), natural moisture content (wn), initial void ratio (eo), and effective vertical stress (σ'v). The standard penetration tests were run for each meter in drilled boreholes. The results of this test were corrected according to NCEER method. The correlation between the variables and corrected standardized penetration test results (N60) were studied by Spearman ranking coefficient. Verifications of the existing eight experimental relationships between standard penetration and other soil properties, proposed by Kayabasi (2015) and Hoshmand et al., (2012), were checked out using the findings and data of the present study. The linear, exponential, logarithmic, and exponential regressions between each variable and N60 were investigated using SPSS software, version 16. The best regression with the highest R2 for each variable was selected. Eight new relationships were proposed. Performance of the suggested relationships was compared with the existing relationships. Results and Discussion The findings of the current study could be summarized as: 1. The clay soils of the studied area in Tabriz were classified into four categories including CH, MH, CL, and ML according to USCS classification. The range of changes in plastic index and liquid limits of the samples were 9.19 ~ 45% and 29 ~ 77%, respectively. The corrected standard penetration test results (N60) changed from 9 to 28 showing that soil compression was low to high. 2. The highest positive and negative Spearman correlation coefficients were related to the consistency index (+0.772) and moisture content (-0.759), respectively. 3. The existing empirical relationships, based on the database of this study, were found to have better statistical coefficients in terms of consistency index, activity, moisture percentage, and fine grained percentage. In term of sample depth, the experimental relationship, showed the lowest statistical coefficient. 4. Four single-variable and two-variable relationships were proposed by nonlinear regression analysis. Using these relationships, clay soil properties including activity, moisture content, fine grain percentage, and consistency index were estimated based on N60. In addition, two relations were proposed between sample depth (D) and vertical effective stress (σ'v) with N60. The statistical coefficients of the suggested relationships were better than the existing empirical relationships. The proposed relationship of estimating the consistency index with coefficient (R2) of 0.673 and regression line slope of about 1 had the best performance. Conclusion In general, the main objective of this study was to investigate the correlation between physical and plasticity properties of clay soils and N60 on Tabriz clayey soils. Clay soils of the present study included various silty and marl layers. Sufficient correlation was observed between the physical and engineering properties of clay soils and N60. The validation of the existing experimental relationships based on A, wn, FGP, and D resulted in weak statistical coefficients (R2 <5) employing the database of the current study. Six new experimental relationships were proposed to estimate A, FGP, wn, and CI as well as two correlations of N60 with effective stress and sample depth. Generally, the results have been revealed that the statistical coefficients of the proposed relationships were improved compared with the existing relationships. The most suitable relationship was the estimation of soil consistency index (R2~70) and root mean square error (RMSE=129). Finally, due to the novelty of this research topic, verification and development of the proposed relationships for the soils has been recommended in other areas.