ارزیابی علت پدیده لهیدگی در تونل‌ها بر اساس ریزساختارهای مشاهده شده در سنگ نمک

سنگ نمک در مقایسه با سنگ‌های سیلیکاته رفتار شکل‌پذیر زیادی را حتی در شرایط پایینی از دما و فشار نشان می‌دهد. بنابراین بررسی ریزساختارهای سنگ نمک در درک مکانیزم‌های دگرشکلی که تغییرشکل سنگ‌ها را کنترل می‌کنند کمک کننده ‌است. در این پژوهش ریزساختارهای موجود در سنگ‌نمک طبیعی متعلق به گنبدنمکی دهکویه با استفاده از روش پراش الکترون‌های برگشتی (ebsd) به‌منظور یافتن علت پدیدۀ لهیدگی در مقیاس میکروسکوپی بررسی شد. بررسی ریزساختاری نشان داد که جریان شکل‌پذیر نمک نتیجۀ عملکرد خزش نابه‌جایی و تبلور مجدد است. برخی از دانه‌ها تغییرشکل درون بلوری مهمی را نشان دادند. تحلیل دگرشکلی دانه‌های تغییر شکل یافته نشان داد که تغییر شکل درون دانه‌ای نتیجه لغزش بر روی سیستم لغزشی <110> (110) و <110> (111) است. لغزش روی چنین سطوحی می‌تواند در همگرایی دیوارۀ تونل نیز موثر باشد. ریزساختارهای مشاهده شده در این پژوهش با ریزساختارهای ارائه شده برای نمونه‌های شیستی در منطقۀ هیمالیا قابل قیاس است. از این‌رو، این پژوهش پیشنهاد می‌کند که وقتی سنگ‌های اطراف تونل دربردارندۀ برخی از ریزساختارهای خاص باشند لهیدگی بوقوع می‌پیوندد. به‌نظر می‌رسد نوع سنگ و تاریخچۀ تکتونیکی مهم‌ترین عوامل در وقوع این پدیده هستند.

Evaluation of the Cause of Squeezing Phenomenon in Tunnels on the Basis of Observed Microstructure in Salt Rock

Introduction Generally, in engineering geology physical and mechanical properties of rocks are investigated in macroscopic scale, and less attention is paid to investigate the texture and microstructure developing in rock during deformation. Salt rock, as a best example of ductile rocks, has attracted the attention of many researchers. Compared to silicate rocks, salt rock exhibits extensively ductile behavior at even low temperature and pressure. In microtectonics, salt is important, because of it is useful as an analogue material for understanding the microstructural processes and textural development in silicate rocks. Deformed salt rock can display microstructures developed in silicate rocks at high pressures and temperatures. Regarding the similarity between microstructures of salt rock and silicate rocks, investigation of microstructure and deformation mechanism in salt rock can be helpful in understanding the main cause of the squeezing phenomenon in tunnels.One of the effective factors on squeezing phenomenon is the structures and microstructures of rock. Rock mass classifications that contain rock mass structures are used in the predicting methods. But, so far, no attention has been paid to the role of rock microstructure in predicting the squeezing phenomenon.This study is aimed to identify deformation mechanisms occurring in microscopic scale in rocks and lead to tunnel convergent in large scale. To achieve this goal, the microstructures in a naturally deformed Late PreCambrian to Early Cambrian Hormuz salt rock from the active Deh Kuyeh salt fountain in Fars province were investigated using Electron Backscatter Diffraction (EBSD).Materials and MethodsDeh Kuyeh salt diapir was located at about 27 km NE of Lar city. Salt samples were taken from top of the east and west glaciers (S1 and S2) and from the middle part of diapiric stem (sample S3). Raw samples were first cut dry into slabs (approximately 3 acute;2 acute;1 cm). Thin sections were prepared following the procedure of Schleder and Urai (2005) and Urai et al. (1987).Halite crystallographic orientation data were collected using a Zeiss SIGMAVP FEGSEM. EBSD patterns were collected using an accelerating voltage of 30 kV, beam current of ~ 100 nA and a working distance of about 30 mm. Oxford instruments AZTEC software was used for data acquisition. EBSD large step size (50 mm) mapping was used to examine the overall microstructure in each sample. EBSD data were processed using HKL Channel 5 software.Results and DiscussionAll samples showed relatively similar microstructures. Samples comprise a small number of large grains in a matrix of smaller grains. Most grains were irregular in shape with lobate boundaries and internal distortion. Microstructural study revealed that the ductile flow of the salt was accommodated by dislocation creep and dynamic recrystallization. Salt grains show lattice distortion and a prevalence of lowangle boundaries that are evidence for dislocation creep and recovery processes. Misorientation analysis suggests that (110) <110> and (111) <110> slip systems are responsible for crystal plastic deformation of salt grains. Schmid factor analysis showed that stresses acting on inclined directions lead to the maximum activity of these slip systems.The observed microstructures in the salt are comparable with the microstructures presented for schist samples from Himalaya region. The rock along Himalaya main trusts also showed evidence of dislocation creep and development of crystallographic preferred orientation. Hence, this article suggests that the rock type and its microstructures are the most important factors in occurrence of tunnel convergent.ConclusionsThis article proposes that deformation mechanisms occurring in microscale control the rock behavior in large scale. All rocks can behave as a ductile material depending on the temperature and pressure. In intrinsically ductile rocks like salt rock, presence of many active slip systems facilitates rock deformation under lower pressures and temperatures than silicate rocks. High tectonic stresses in shear zones lead to development of a strong shape preferred orientation and crystal preferred orientation in rocks. These microstructures facilitate rock deformation under stresses exiting in tunnels. It can be said that rock type and tectonic history of the area play the most important role in occurrence of squeezing phenomenon. Other factors such as current stress system in the area control deformation speed in tunnel. It seems investigating microstructures of rocks from tunnel route before and after excavation can be effective in identifying places with high possibility of squeezing.