بررسی تاثیر درجۀ سیمانتاسیون بر پارامترهای مقاومتی و سرعت موج برشی آبرفت‌های درشت‌دانه شرق تهران

توسعۀ شهری در کلان شهر تهران باعث شده که برای ساخت سازه های بلند، بزرگ راه ها، تونل ها و سایر تاسیسات شهری، حفاری هایی با عمق و حجم زیاد انجام گیرد. آبرفت های شرق تهران (سری a)، نهشته های درشت دانه، همگن و سیمانته هستند. در این پژوهش تاثیر درجۀ سیمانتاسیون بر پارامترهای مقاومتی و سرعت موج برشی خاک بر اساس پژوهش‌های انجام شده در بزرگراه شرق تهران انجام شده است. به‌منظور تعیین خصوصیات ژئوتکنیکی و سرعت موج برشی خاک، از آزمون های برجا همانند بارگذاری صفحه، برش برجا، پرسیومتری و لرزه ای درون‌گمانه ای، برداشت های صحرایی و آزمون های آزمایشگاهی استفاده شده است. هم‌چنین تجزیۀ شیمیایی برای شناسایی نوع کانی ها و جنس مواد سیمانته انجام شده است. نتایج آزمایش ها نشان داد که سیمانتاسیون این آبرفت ها متوسط تا زیاد است و جنس مواد سیمانی در این خاک ها عموما کربناتی و به‌ویژه کلسیتی است. جمع بندی نتایج نشان داد که با افزایش درجۀ سیمانتاسیون، مدول تغییر شکل حدود 25 درصد، چسبندگی حدود 55 درصد و سرعت موج برشی حدود 30 درصد افزایش می یابد. افزایش ها این پارامترها با عمق رابطۀ مستقیم دارد. این در حالی است که درجۀ سیمانتاسیون تاثیری قابل توجهی روی زاویۀ اصطکاک داخلی خاک ندارد.

The Study of Cementation Level Effect on the Strength Characteristics and Shear Wave Velocity of Coarse-Grained Alluvium in Eastern Areas of Tehran, Iran

IntroductionSeries A of coarsegrained alluvial deposits of Tehran are extended in eastern and northeastern areas of Tehran. Analyzing and studying of these alluvial deposits from a geological point of view as well as their creation time and general characteristics such as the deposits rsquo; mineral types, their source, and formation conditions, gives a better point of view to geotechnical engineers about exploring their characteristics as well as geotechnical aspects in underground structure design, excavations, and foundation design processes. On the other hand, in order to analyze stability, estimating the factor of safety and the seismic design of these structures, considering their location, which is in Tehran with a high seismic hazard area, the necessity of knowing the exact mechanical and dynamic properties of Tehranchr('39')s alluvium is felt more than ever.Material and methodsDue to the grain size of Tehran rsquo;s coarsegrained alluviums (series A) as well as high level of cementation of them, it is impossible (or maybe so difficult) to make undisturbed samples in order to do experiments. Such that it is excavated 23 boreholes with 30 to 140 meters depth as well as 17 test wells with 20 meters depth in an area which was extended in 10 kilometers in long which were located in Tehran rsquo;s No. 13 and No. 14 districts (as it can be seen in Figure 1). During the excavation of the entrance ramp and tunnel of eastern highway of Tehran, insitu tests have been done in different sequences. Since it was important to investigate real behavior of these alluviums, different insitu tests such as plate load test, insitu shear test, pressuremeter test, and downhole test have been done as well as many laboratory and field tests. Furthermore, (1) XRay Diffraction (XRD) and (2) Xray Fluorescence (XRF) as well as (3) Scanning Electron Microscopy (SEM) methods, have been used to explore the type of minerals and those used in cementation. (ب) Figure 1. a) Geological plan and the location of boreholes and test wells in the alignment of East Tehran FreewayResults and discussionBased on the results of XRD tests, it is quite clear that the largest weight percentages of tested samples are lime and silica.Calcium and magnesium levelsas the highpower cations in flocculation processin soil sample No. 1 (soil with high cementation level) are much more than soil sample No. 2 (soil with moderate cementation level).This is the cause of high cementation level of soil sample No. 1 comparing with soils sample No. 2. A rapid increase in stress level can be seen in insitu shear test results, in low shear displacements, up to reaching a maximum of tau;p (peak point) and afterwards reduction in shear stress with softening behavior. Cohesion and shear strength levels also increase by increasing the depth. According to the plate load tests results, an increase in soil modules changes can be seen in different depths by depth increasing.Large tendencies to increase in volume and dilation can be seen in under shear load cemented soils, after applying a primary compression on them. A brittle behavior with the occurrence of a certain peak can be seen in cemented samples. The significant increase in strength is directly related to the severe dilation rate, which can be seen in cemented samples results. The shear strength would be decreased, if this cement is broken during the particles rsquo; displacements.The results of downhole tests are shown in Figure 2. According to this figure, it has been explored that Vs,30 is about 600 m/s in moderate cemented soils while it is about 850 m/s in highly cemented soils. Because of the homogeneity and uniformity of sedimentary deposits, shear wave velocity is increasing due to the higher density of the layers and high level of cementation in both of the soil types. However, this increase is not significant at depths above 25 meters.ConclusionBased on the results, cementation level of the eastern coarsegrainalluvium of Tehran is moderate to high and minerals used in cementation of this type of soil are generally carbonated and especially calcite.Investigating the level of cementation of soil as well as the results of chemical analysis and insitu tests, it can be found that the strength and deformation parameters of the soil are directly related to the degree of its cementation.Based on the obtained results, the deformation modulus increases by about 25%, the cohesion by about 55% and the shear wave velocity by about 30% with increasing the degree of cementation (Table 1).Increases of these parameters are directly related to depth. However, the cementation level does not significantly affect the internal friction angle of the soil.Table 1. Average results of insitu shear testsDeformation Modulus (MPa)Peak Friction Angle (deg.)Cohesion(kPa)USCSDepth(m)Sample5060393035GWGM5Moderately Cemented Soil (M.C. Soil)7585415060SPSC108590415060GWGC1595105415060GWGC206070393540GWGM5Highly Cemented Soil (H.C. Soil)7585395060GWGC10110120426575GWGC1512514041110120GC20