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Geomechanics Challenges and Its Future Direction – Food for Thought*

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Geomechanics Challenges and Its Future Direction – Food for Thought* Geomechanics Challenges and its Future Direction – Food for Thought* F. T. Suorineni Suorineni F. T. (2013), “Geomechanics Challenges and its Future Direction – Food for Thought”, Ghana Mining Journal, pp. 14 - 20. Abstract Geomechanics has proven to be the backbone of safe and cost effective mining practice. However, experience shows it still does not have the appreciation of most mine managements until there is a fatality or costly ore sterilization. The problem can be traced back to the training of mining engineers. The subject is also picked up later in life by other professionals with limited background in geology. The consequences of these limitations are lack of fundamental knowledge in the subject by some who practice it. This lack of understanding has affected the growth and maturity of the subject of rock mechanics, a key component of geomechanics. Today, we are beginning to understand that failure criteria that were inherited from soil mechanics for appli- cation to rock are wrong and misleading. Despite these problems, some major achievements have been made towards a better understanding of rock behavior under load. Many more challenges still lie ahead. This paper takes a look at the history of rock mechanics, and therefore geomechanics, its development, required training, present status and what lies ahead in the future. The goal is to provide educators, industry and practicing engineers what it takes to be a good rock engineer and the real benefits of the subject for the good of society. 1 Introduction was in 1963 Professor Bjerrum, from Norway, speaking on behalf of A. Casagrande (President of Even the definition of geomechanics is confusing. Is the International Conference on Soil Mechanics) there a difference among geotechnology, geotechni- brilliantly explained how rock mechanics should be cal engineering, and geomechanics? Furthermore, is integrated into soil mechanics as a mere chapter of a there a difference between engineering geology and wider, more advanced technical science. geological engineering, areas also closely related to geotechnology, geotechnical engineering and ge- The links between rock mechanics, engineering ge- omechanics? ology, geological engineering and classical geology are intricate and complex. It is not possible to think Historically, geotechnology and geotechnical engi- about rock mechanics without examining and dis- neering are courses or branches of civil engineering. cussing these links. Geotechnology often implies the study of soil me- chanics. Until the 18th century, no theoretical basis Geotechnical engineering is soil mechanics and for soil design had been developed, and the disci- foundation engineering (VanDine, 1987). The princi- pline was more of an art than a science, relying on ple of effective stress, bearing capacity theory, and past experience. Failure of important structural foun- the theory of consolidation are all attributed to Ter- dations such as the Leaning Tower of Pisa prompted zaghi. Roscoe et al. (1958) introduced the theory of the development of a more scientific approach to the critical state soil mechanics that is now the basis for use of soil mechanics. This initiative was led by many contemporary advanced constitutive models Gauthier (1717) with the development of earth pres- describing the behavior of soil. sure theories and soil classification based on unit Geomechanics is defined in Wikipedia as the geo- weight (Muni, 2007). This development was fol- logic study of the behavior of soil and rock. Note the lowed by the work of Coulomb (1773) and Mohr keywords in the definition as geology, soil and rock. (1882). The Mohr-Coulomb failure criterion is well While geotechnology and geotechnical engineering known in both soil and rock mechanics. Other con- focus on soils, geomechanics combines geology, soil tributors to the science of soil mechanics include mechanics and rock mechanics. Hence, it is danger- Darcy, Rankine, Atterberg and Reynolds. ous to be trained in geomechanics without a good Advances in soil mechanics and foundation engi- background in geology. This is the focus of this neering peaked in 1925 with the publication of Erd- paper. baumechanik by Karl Terzaghi (a civil engineer and The limitations in the knowledge of soil mechanics geologist) who is commonly referred to as the father practitioners in geology prompted the introduction of of soil mechanics and geotechnical engineering. Ter- the engineering geology profession. In 1951, one of zaghi was a mechanical engineer by training. Note the earliest definitions of "Engineering geologist" now the use of the geotechnical engineering term. It * Manuscript received February 10, 2012 Revised version accepted April 7, 2012 14 GMJ Vol. 14, June, 2013 was provided by the Executive Committee of the to understand the earth through his knowledge in Division on Engineering Geology of the Geological geology and to manage it through his understanding Society of America as a geologist trained in the dis- of engineering principles. cipline of engineering geology. Emphasis is the per- son being a geologist first and then trained by prac- Finance Geomechanics. Water tice in engineering. This background in geology pro- Resources Geophysics vides the engineering geologist with an understand- Civil Engineering- ing of how the earth works, which is crucial in miti- Geotech. Mining Tunnelling engineer Engineering gating for earth related hazards. This training also Ground Control enabled the geologists to effectively communicate Exploration Geological Engineer Geology Mine Technical with engineers. The need for geologist on engineer- Engineering Superintendent Management – ing works gained worldwide attention in 1928 with Director Univ. Risk Analysis the failure of the St. Francis dam in California and President Careers Teaching- the loss of 426 lives. Commenting on the disaster Environmental University Prof. Ransome (1928) wrote “so far as can be ascertained, Science/Engineering Computer no geological examination was made of the dam-site Science/En Research before construction began. The plain lesson of the gineering Mineral Scientist/Engineer Processing disaster is that engineers, no matter how extensive Processing their experience in building of dams cannot safely meneralogist dispense with the knowledge of the character and structure of the adjacent rocks, such as only an ex- Fig. 1 Versatility of the Geological Engineering pert and thorough geological examination can pro- Profession vide.” (Ransome, 1928). Turning geologists into engineers solved some prob- lems. The limitations of this approach originate from 3 Soil Mechanics Principles and Theo- the fact that the so called engineering geologists had ries and how they have misled Rock difficulties in integrating design principles into their Mechanics evaluations to appreciate and understand the de- mands and frustration and the burden of the engineer The Mohr-Coulomb failure criterion is given by as being the safe keeper of society. Price (2009) Equation (1). states: (1) c n tan “Engineering geologists” are essentially geologists who deliver basic geological data to engineers, with- where t is the shear strength of the soil, c is the co- out interpretation.” hesion of the soil. σn is the normal stress acting on Few geologists had sufficient engineering knowl- the shear plane, and f is the frictional resistance of edge to understand the requirements of the engineer, the soil. and few engineers had more than the most superfi- Anyone who has sat in both a soil mechanics class cial knowledge of geology. It was then thought that and a rock mechanics class would have seen this to resolve the issue, it is best to train someone simul- equation at least twice, once in soil mechanics and a taneously with engineering and geological principles second time in the rock mechanics class. at the same time. The geological engineering pro- fession was born. Equation (1) implies that the shear strength of the material is due to the simultaneous mobilization of Mathews (1967) describes the geological engineer cohesion and frictional resistance or strength. best, when he stated that the geological engineer is one soundly trained in both geology and engineering What is wrong in applying this criterion, which has its origin in soil mechanics, to rocks? fundamentals, and that that is the man, he believes, is best qualified to work closely with the civil engi- First, what is the definition of a soil? Second, what neer responsible for the execution of engineering is the definition of rock? works. Craig (2004) defines soil as any uncemented or weakly cemented accumulation of mineral particles 2 The Geological Engineer formed by the weathering of rocks, the void space The geological engineering profession is a versatile between the particles containing water and/or air. one (see Fig. 1). The following figure illustrates how The key words in the definition of soil are versatile the geological engineering profession is. It “uncemented” and “accumulation of mineral parti- is my belief that to be able to control or manage the cles”. earth, one must understand the earth. By the nature By the definition of soil, its shear strength can be of the training of the geological engineer, he is able mobilized by simultaneous activation of its cohesive 15 GMJ Vol. 14, June, 2013 , ' and frictional strength. This is because of the fact ' ' m 3 (2) that the particles are uncemented or weakly ce- 1 3 ci s ci mented, and therefore Equation (1) applies. On the other hand, by definition a rock is a naturally occur- ring and coherent aggregate of one or more minerals. ci (3) The key words here are “coherent” and “aggregate”. 1 3 3 Friction occurs when two or more surfaces rub where σ and σ are the induced major and minor against each other as will occur in soils because of 1 3 principal stresses, and σci is the uniaxial compressive their nature. However, in rocks free surfaces rarely strength of the intact rock. exist until fracture occurs through breaking the cohe- sive bonding in the aggregate of minerals! Thus, the Equation (3) implies that for massive strong brittle strength of rocks is NOT a simultaneous mobiliza- rocks friction plays little role in their failure.
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