ارزيابي روش AE و DRA در اندازه‌گيري تنش برجا و بررسي اثر زمان تاخير در اين دو روش

Assessment of AE and DRA Methods for In-situ Stress Measuring and Study of Delay Time in both Methods

تخمين تنش برجا به روش آزمايشگاهي به خاطر هزينه كم و سريع بودن، امروزه مورد توجه پژوهشگران مكانيك سنگ قرار گرفته است. از روش‌هاي آزمايشگاهي تخمين تنش برجا كه پژوهش روي آن افزايش يافته است، روش انتشار اكوستيك(Acoustic Emission) و آناليز نرخ تغيير شكل(Deformation Rate Analysis) است. دقت نتايج در اين روش ها برگرفته از بافت سنگ، نوع ترك‌ها و اثر زمان تاخير است. در اين پژوهش توانايي روش AE و DRA در تخمين تنش برجا و اثر زمان تاخير در مقدار تنش اندازه گيري شده و بررسي شده است. زمان تاخير فاصله زماني بين كرگيري از محل و زمان آزمايش است. براي اندازه گيري تنش برجا از تكيه گاه سد سيمره، 6 سري نمونه با زمان تاخيرهاي متفاوت (دو سري نمونه با زمان تاخير 5 سال و چهار سري نمونه با زمان تاخير 20 روز) تهيه و آزمايش روي آنها انجام شد. مقدار تنش تخمين زده شده از روش هاي AE و DRA محاسبه و با نتايج روش شكست هيدروليكي (Hydroulic Fracturing) مقايسه شد. نتايج حاكي از هم‌خواني خوب بين روش AE و DRA با روش شكست هيدروليكي است. تنها اختلاف در مقدار تنش قايم بوده كه علت اين امر اين است كه در روش HF فرض برابري تنش قايم اصلي معادل با وزن روباره است. نتايج نشان مي دهند كه زمان تاخير باعث پراكندگي نتايج در دو روش AE و DRA شده و براي حصول نتيجه بهتر در مقدار اندازه گيري شده زمان تاخير بايد كمينه ممكن باشد.

There are several direct and indirect methods to estimate in-situ stress. Generally in all methods rock breaking is required for in-situ stress determination in which broken rock response is due to displacement/strain or hydraulic pressure. Hydraulic fracture, Relief method, Jacking method, Strain Recovery method and borehole breakout method are known as direct methods. On other hand, the indirect methods estimation is based on changing of some physical or other properties of rock which represent from applying stress. Even though, these methods are also laboratory methods, however, indirect techniques are non destructive, low cost and simple for estimation of in-situ stress. This is the reason which has motivated rock mechanics researchers to use Acoustic Emission (AE) and Deformation Rate Analysis (DRA) methods. Results aaccuracy of the methods are affected by rock texture, cracks type and delay time. The delay time means that time between coring in field and test time for stress measuring. The capability of AE and DRA methods in stress estimation have been assessed in this research and the effect of delay time on estimated stress are worked out. AE and DRA tests were carried out on the samples for estimation of in-situ stress. Stress has a nature tensor with 9 components, in which six of them are independent components and the rest (3 components) are dependent components. Each component of tensor need to be determined with a total of six tensors which required six tests in six different orientations. Six series of samples with different delay times (4 series with 20 days as delay time and 2 series with 5 years as delay time) were obtained from abutments of Seyamreh dam (South-West of Iran). AE and DRA tests were carried out. The values obtain from these AE and DRA tests were compared with result of Hydraulic Fracturing (HF) method which carried out at the same locations. The comparison shows a good conformity between the result of AE and DRA methods with those of HF method for horizontal stresses where as there are variations noticed for vertical stress values. This may be due to this fact that in the HF method assumes the vertical stress is equal to overburden weight which is not correct assumption. The other result of this assumption is that the others two principal stresses are horizontal. It is obvious if one of principal stresses would be vertical the two other have to be horizontal. Furthermore, this investigation infers that delay time is very important parameter on the results obtained by AE & DRA methods. Thus better accuracy can be computed when the delay time is minimized