ارزيابي علت پديده لهيدگي در تونل‌ها بر اساس ريزساختارهاي مشاهده شده در سنگ نمك

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

سنگ نمك در مقايسه با سنگ‌هاي سيليكاته رفتار شكل‌پذير زيادي را حتي در شرايط پاييني از دما و فشار نشان مي‌دهد. بنابراين بررسي ريزساختارهاي سنگ نمك در درك مكانيزم‌هاي دگرشكلي كه تغييرشكل سنگ‌ها را كنترل مي‌كنند كمك كننده ‌است. در اين پژوهش ريزساختارهاي موجود در سنگ‌نمك طبيعي متعلق به گنبدنمكي دهكويه با استفاده از روش پراش الكترون‌هاي برگشتي (EBSD) به‌منظور يافتن علت پديدۀ لهيدگي در مقياس ميكروسكوپي بررسي شد. بررسي ريزساختاري نشان داد كه جريان شكل‌پذير نمك نتيجۀ عملكرد خزش نابه‌جايي و تبلور مجدد است. برخي از دانه‌ها تغييرشكل درون بلوري مهمي را نشان دادند. تحليل دگرشكلي دانه‌هاي تغيير شكل يافته نشان داد كه تغيير شكل درون دانه‌اي نتيجه لغزش بر روي سيستم لغزشي <110> (110) و <110> (111) است. لغزش روي چنين سطوحي مي‌تواند در همگرايي ديوارۀ تونل نيز مؤثر باشد. ريزساختارهاي مشاهده شده در اين پژوهش با ريزساختارهاي ارائه شده براي نمونه‌هاي شيستي در منطقۀ هيماليا قابل قياس است. از اين‌رو، اين پژوهش پيشنهاد مي‌كند كه وقتي سنگ‌هاي اطراف تونل دربردارندۀ برخي از ريزساختارهاي خاص باشند لهيدگي بوقوع مي‌پيوندد. به‌نظر مي‌رسد نوع سنگ و تاريخچۀ تكتونيكي مهم‌ترين عوامل در وقوع اين پديده هستند.

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 micro-tectonics, 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 Pre-Cambrian 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 Methods Deh 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´2 ´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.