بررسي خصوصيات مهندسي خاك هاي شهر كرمان و عوامل مؤثر بر ميزان رمبندگي آن‌ها

Investigating the Engineering Properties of Kerman Soils and Factors Controlling their Collapsibility

خاك‌هاي رمبن ده به‌طور گسترده ­اي در نواحي خشك و نيمه‌خشك در سراسر دنيا توزيع شده و حدود 10% از مساحت كل خشكي­ هاي زمين را تشكيل مي­دهند. اين خاك­ها به‌طور معمول ساختار لانه زنبوري نيمه‌پايدار دارند كه مستعد كاهش حجم زياد يا رمبش، به محض مرطوب شدن است. در اين پژوهش براي تعيين خصوصيات مهندسي نهشته­ هاي شهر كرمان علاوه بر گردآوري اطلاعات ژئوتكنيكي موجود، از 50 نقطه در سطح شهر نمونه‌گيري انجام شده است. پراكنش نقاط به‌گونه­اي طراحي شده كه پوشش قابل قبولي در سطح شهر داشته باشند. براي تعيين نوع كاني­ها و هم‌چنين بررسي ساختار خاك و نحوۀ آريش دانه­­ها به‌ترتيب از روش XRD و تصاوير ميكروسكوپ الكتروني استفاده شد. در بررسي تصاوير ميكروسكوپ الكتروني نمونه­هايي كه ميزان رمبندگي زيادي نشان مي­دهند، ساختار باز و منافذ بين‌دانه­اي دارند. در بخش­هاي مركزي شهر دانه­هاي خاك عموماً داراي اتصال گوشه به گوشه است و در ساختار آن‌ها نظم مشخصي ديده نمي­شود و آرايش ذرات تصادفي و بي‌نظم است و رمبن دگي كمي نشان مي­دهند. علاوه بر نحوۀ آرايش دانه ­ها در برخي نمونه­ ها وجود مواد انحلال‌پذير از جمله بلورهاي ژيپس و نمك نقش مهمي در ميزان رمبندگي خاك ايفا مي­كنند. براي ارزيابي پتانسيل رمبن دگي از معيارهاي دنيسف، هولتز و هيلف، آيين‌نامۀ ساختماني روسيه و آيين­نامه ASTM كه متداول­ترند، استفاده شده است. بر اساس نتايج به‌دست آمده خاك‌هاي شهر كرمان عمدتاً در محدودۀ خاك‌هاي با رمبن دگي متوسط قرار مي‌گيرند.

The problematic collapsible soils are deposits with wind origin that constitute about 10% of the total area of ​​the earth. Several countries, including China, Russia, the United States, France, Germany, New Zealand, and Argentina have vast areas of collapsible soils. These deposits usually form a semi-stable honeycomb structure and are highly susceptible to sudden changes in the volume reduction due to becoming humid. Collapsibility and other related issues such as different subsidences, land cracks and landfalls seriously damage the infrastructures constructed on these soils. By the growing rate of urbanization in different parts of the world, the probability of construction on these soils and consequently water availability for these soils will increase; as a result, humidity increases and the collapse of these soils may occur. Therefore, studying the behavior of these types of soils is very important. Over the past six decades, many researchers have studied the collapse mechanism of collapsible soils due to becoming humid. Discussions on this subject are summarized in three categories: traditional methods, soil structure studies, and soil mechanics-based methods. In the present work, collapsibility and its controlling factors in the soils of Kerman city are investigated. Material and methods To determine engineering properties of Kerman deposits in this research, the geotechnical information was gathered and 50 core samples were extracted from different parts of the city. The sampling points were selected such that they could have a high overlap. X-ray diffraction (XRD) was applied to determine the mineral type and soil structures while scanning electron microscopy (SEM) was used to study grain arrangement. Results and discussion Geotechnical characteristics of the samples collected from Kerman plain deposits include their physical and mechanical properties. Based on the obtained results, this fine-grained sediment generally includes two CL and CL-ML groups. The mineralogy studies of Kerman city soils show that the minerals in these deposits are mainly illite, chlorite, illite-smectite, calcite, quartz, and gypsum. In order to study the collapsibility level of the soils in Kerman through the field studies, samples were taken from different parts of the city and the tests were carried out to determine the physical properties, collapsibility index, and structural studies. Through the SEM analyses, samples related to Haft Bagh area, Motahhari Town, and Pedar Town revealed an open structure and intergranular pores and thus a high level of collapsibility. On the other hand, in the majority of samples taken from the central part of the city, such as Esteghlal Street, Azadi Square, Bahmaniyar Street, and Hafez Street, the soil aggregates generally have corner-to-corner connectivity, with no specific order in their structure, and the arrangement of the particles is random and irregular. The orientation of the particles mostly shows no sharp pattern. In addition to soil particles, they have shown random and disorientated cavities with small sizes, suggesting the density and compactness of the soil indicating a low to moderate collapsibility. In some areas (e.g., Pedar Township and Motahhari Township), crystalline salt and gypsum crystals are clearly seen. It is expected that by increasing the amount of water, these salts dissolve and their effects can be observed as dissolution cavities.