Hoek-Brown Görgül Yenilme Ölçütüne İlişkin Değişiklik Önerileri ve Bunların Uygulanabilirliği

Year 2000, Volume: 24 Issue: 1, 1 - 22, 15.12.2000

Reşat Ulusay , Harun Sönmez

Abstract

Sık aralıklı süreksizlikler içeren kaya kütlelerinin dayanımlarının laboratuvarda tayininde kullanılacak örneklerin temsil edici olabilmesi amacıyla, klasik örnek boyutlarının dışına çıkılarak, çok büyük örneklere gereksinim duyulmaktadır. Ancak, bu tür kaya kütlelerinde metre boyutunda örnek alımı mümkün olmayıp, bu boyuttaki örneklerin yerleştirileceği çok büyük deney hücrelerinin geliştirilmesi de güçtür. Bu güçlük, 80’li yıllardan bu yana kullanılan Hoek-Brown görgül yenilme ölçütüyle aşılmaya çalışılmaktadır. Zayıf kaya kütlelerine uygulanmasında ortaya çıkan bazı sınırlamalardan dolayı, önerildiği 1980’den günümüze değin ölçüt, yaratıcıları tarafından pek çok kez değişikliğe uğramıştır. 1994’e kadar Jeomekanik Sınıflama Sistemi’nden belirlenen RMR değerini esas alan ölçüte, 1994’ten sonra doğrudan RMR’ın kullanılması yerine, yine RMR değerinden belirlenen Jeolojik Dayanım İndeksi (Geological Strength Index-GSI) dahil edilmiştir. Ölçütün 1997 yılındaki son versiyonunda; RMR’dan bağımsız ve herhangi bir kaya kütlesi parametresi için sayısal değer esas alınmaksızın, sadece kaya kütlesinin görsel olarak tanımlanmasına dayalı bir GSI Sınıflama Sistemi’nin kullanımına geçilmiştir. Güncel GSI Sınıflama Sistemi’nde, GSI değerleri kaya kütlesinin görünümüne ve tanımlanmasına göre belirlenmekte ve bu uygulama genel olarak sübjektif bir değerlendirmeye yol açmaktadır. GSI sistemi ile ilgili tartışmaya açık diğer bir husus ise, yenilme ölçütünün son versiyonunda örselenmiş ve örselenmemiş kaya kütlelerinin mühendislik parametrelerinin belirlenmesine yönelik bir kılavuzun mevcut olmayışıdır. Ölçütün son versiyonunun desteklendiği herhangi bir araştırmanın bulunmayışı da, güncel GSI sisteminin güvenilirliği ve/veya performansı hakkında yorum yapılmasını olanaksız kılmaktadır. Bu yazıda, GSI sistemine ait yukarıda değinilen belirsizlikler dikkate alınarak, öncelikle GSI değerlerinin daha gerçekçi şekilde ve kolaylıkla tayin edilebilen kaya kütlesi parametrelerine bağlı olarak hesaplanması amacıyla yazarlar tarafından önerilen değişiklikler ve bunlarla ilgili puanlama parametreleri sunulmuştur. Ayrıca, kaya kütlesi parametreleri üzerinde kazı yöntemine bağlı olarak ortaya çıkan örselenmenin etkisini dikkate alan bir yöntem önerilmiştir. GIS’ın yazarlar tarafından modifiye edilmiş bu son hali, Türkiye'den seçilmiş sık eklemli kaya kütlelerinde açılmış şevler ile pasa yığınlarında meydana gelmiş duraysızlıkların geriye dönük analizlerinde kullanılarak, önerilen sistemin ve yöntemin performansı sınanmıştır. Analizler, önerilere göre hesaplanmış GSI değerlerinin kullanılması ve örselenme etkisinin yenilme ölçü¬ tüne dahil edilmesi halinde gerçekçi sonuçların elde edilebileceğini göstermiştir.

Keywords

Geriye Dönük Analiz, Hoek-Brown Yenilme Ölçütü. Jeolojik Dayanım İndeksi, Kaya Kütlesi, Pasa Yığını, Şev Duraylılığı

Suggested Modifications to The Empirical Hoek-Brown Failure Criterion and Their Applicability

Abstract

The strength of a rock material is determined in the laboratory on representative standard samples. In the ca se of a closely jointed rock mass it is not possible to obtain a sample with suitable dimensions to represent the whole rock mass. Since /980, the empirical Hoek-Brown failure criterion (Hoek and Brown, 1980) began to be used to overcome the difficulties in laboratory shear strength determination of jointed rock masses. The empirical failure criterion was used in conjunction with the Geomechanics Classification System-RMR ( Bieniawski,/ 989) until 1994. However, due to the limitations in the RMR classification scheme particularly for very poor qualify rock masses and for unrealistic rating adjustments for discontinuity orientation in slopes, the failure criterion has been modified over the years. Recently, the originators of the criterion introduced a new index, called Geological Strength Index (GSI), into the criterion. The GSI is based upon the visual impression on the rock mass structure and consists of twenty codes to identify each rock mass category and to estimate the GSI value (Hoek and Brown, 1997). Because rock mass classification requires time consuming procedures and has some limitations, the existing GSI system seems a more practical parameter for the determination of the strength of jointed rock masses from field observations. However, the system is lack of measurable and more representative parameters, and related interval limits or ratings for describing the structure and surface conditions of discontinuities. This situation results in subjective assessments on the determination of the GSI value. In other words, it is possible to estimate different GSI values for the same rock mass by different persons. The other important problem of the criterion is the use of undisturbed and disturbed rock mass categories for determining the parameters in the criterion, for which clear guidelines are lacking. It is also noted that the data supporting of the revisions have not been published. These uncertainties make it difficult to judge their validity and performance. In order to provide a more quantitative basis for evaluating GSI values, the authors of this paper suggested some modifications by introducing easily measurable rock mass parameters with ratings and/or intervals. For the purpose, two terms, namely Structure Rating (SR) based on volumetric joint count and Siu face Condition Rating (SCR) from the input parameters are introduced into the GSI system, and the modified GSI chart is established. In the latest version of the failure criterion (Hoek and Brown, 1997; Hoek et al., 1998) average undisturbed in-situ conditions are considered to estimate the GSI without application of any adjustment due to any disturbance effect, such as blasting. Method of excavation, major planes of weakness or change in stress are considered as local features influencing the rock mass at a particular location. Therefore, the influence of such factors should be compensated, and necessary adjustments should be taken into consideration. In this study, a method was proposed to assess the influence of disturbance on rock constants due to method of excavation as discussed in detail by Sönmez, and Ulusay (1999). The modifications and the method suggested have been applied to well studied five slope instabilities from Turkey to check the validity and performance of the modifications and the methodology of parameter estimation. Four cases were selected from the slopes excavated in heavily jointed rock masses and one from spoil piles in a strip coal mine. The application of the suggested modifications and the method examined by the back analysis of the failures indicated that the use of GSI value determined from the suggested modified chart and consideration of disturbance effect confirmed the limit equilibrium condition for the investigated failed slopes. The back analysis results from a spoil pile instability revealed that spoil pile materials consisting of blocky and angular rock pieces with small amount of fines could be categorized as a disintegrated rock mass in the GSI system and the criterion seemed to be applied to spoil materials. However, future applications of the suggested modifications onto failure case studies both from surface and underground excavations may provide a better tool for more precise guidelines and to check the performance of the equations of the criterion.

Keywords

Back Analysis, Hoek-Brown Failure Criterion, Geological Strength Index, Rock Mass, Spoil Pile, Slope Stability.

References

• Balmer, G., 1952. A general analytical solution for Mohr’s envelope. American Society of Testing Materials, 52, 1269 - 1271.
• Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications. John Wiley and Sons, 237 p.
• Franklin, J.A., Mearz, N.H. and Bennett, C.P., 1988. Rock mass characterization using photo analysis. International Journal of Mining and Geological Engineering, 6, 97-112.
• Hoek, E., 1983. Strength of jointed rock masses, 1983 Rankine Lecture. Géotechnique, 33(3), 187 -223. Hoek, E., 1994. Strength of rock and rock masses. ISRM News Journal, 2(2) 4-16.
• Hoek, E., 1998. Reliability of the Hoek-Brown estimates of rock mass properties and their impact on design. International Journal of Rock Mechanics and Mining Sciences, 35, 63-68
• Hoek, E. and Brown, E.T., 1980. Underground Excavations in Rock. Institution of Mining and Metallurgy. Stephen Austin and Sons, London, 527 p.
• Hoek E. and Brown, E.T., 1988.The Hoek-Brown failure criterion: a 1988 update. Proc. 15th Canadian Rock Mechanics Symposium: Rock Engineering for Underground Excavations, J.C. Jurran (ed.), University of Toronto, 31 -38
• Hoek, E. and Brown, E.T., 1997. Practical estimates of rock mass strength. International Journal of Rock Mechanics and Mining Sciences, 34(8), 1165 - 1186.
• Hoek, E., Wood, D. and Shah, S., 1992. A modified Hoek-Brown criterion for jointed rock masses. ISRM Symposium: Eurock’92 -Rock Characterization, J.A. Hudson (ed.), Thomas Telford, 209 - 213.
• Hoek, E., Kaiser, P.K. and Bawden, W.F., 1995. Support of Underground Excavations in Hard Rock. A.A. Balkema, Rotterdam, 214 p.
• Hoek, E., Marinos, P. and Benissi, M., 1998. Appli cability of the geological strength index (GSI) classification for very weak and sheared rock masses: the case of the Athens schist formation. Bulletin of Engineering Geology and Environment, 57, 151 - 160.
• ISRM (International Society for Rock Mechanics), 1981. ISRM Suggested Methods: Rock Characterization, Testing and Monitoring.E.T. Brown (ed.), Pergamon Press, London, 211 p.
• Kendorski, F.S., Cumming, R.A., Bieniawski, Z.T. and Skinner, E.H., 1983. Rock mass classification for block caving mine drift support. Proc. 5th International Congree on Mechanics, ISRM, Melbourne, B51 - B63.
• Laubscher, D.H., 1990. A geomechanics classification system for the rating of rock mass in mine design. Journal of South African Institute of Minerals and Metallurgy, 90(10), 257 - 273.
• Romana, M.A., 1993. Geomechanical classification for slopes: Slope Mass Rating. J.A. Hudson (ed.), Comprehensive Rock Engineering, 3., Pergamon Press, London, 575 - 599.
• Singh, A., Scoblc, M., Lizotte, Y. and Crowther, G.,1991. Characterization of underground rock fragmentation. Geotechnical and Geological Engineering, 9, 93 - 107.
• Sönmez, H., Ulusay, R. and Gökçeoğlu, C., 1998. A practical procedure for back analysis of slope failures in closely jointed rock masses. International Journal of Rock Mechanics and Mining Sciences, 35(2), 219 - 233.
• Sönmez, H. and Ulusay, R., 1999. Modifications to the Geological Strength Index (GSI) and their applicability to stability of slopes. International Journal of Rock Mechanics and Mining Sciences, 36(6), 743 - 760.
• Ulusay. R., 1991. Geotechnical evaluations and deterministic design considerations for pit wall slopes at Eskihisar (Yatagan-Muğla) strip coal mine. PhD dissertation, Middle East Technical University, Ankara, Turkey, 340 p (unpublished).
• Ulusay, R. ve Yücel, Z., 1989. Zayıf kayaçlarda açılan şevlerin duraylılığına bir örnek: Başkoyak barit işletmesi. Yerbilimleri, 15, 15-27.
• Ulusay, R., Arıkan, F., Yoleri, M.F. and Çağlan, D., 1995a. Engineering geological characterization of coal mine waste material and an evaluation in the context of back analysis of spoil pile instabilities in a strip mine, SW Turkey. Engineering Geology, 40, 77 - 101.
• Ulusay, R., Yoleri, M.F., Çağlan, D. and Arıkan, E, 1995b. Design evaluations for spoil piles at a strip coal mine considering safety of the haul road. International Journal of Surface Mining, Reclamation and Environment, 9., 133 - 140.
• Ulusay, R., Çağlan, D., Arıkan, F. and Yoleri, M.F., 1996. Characteristics of biplanar wedge spoil pile instabilities and methods to improve stability. Canadian Geotechnical Journal, 33(1), 58 -79.
• Ulusay, R., Ekmekçi, M., Gökçeoğlu, C., Sönmez, H., Tuncay, E. ve Erdoğan, E., 1998. Himmetoğlu Linyit Açık İşletmesi şev stabilite etüdü: 1. Aşama- A panosu şevlerinin duraylılığı. Hacettepe Üniversitesi, Proje No: 97- 0058, 245 s.