Ömer Aydan

The squeezing potential of rocks around tunnels; Theory and prediction

SummaryThe deformational behaviour of tunnels, which underwent large deformations, socalled squeezing, have been recently receiving great attention in the field of rock mechanics and tunnelling. Contrary to rockbursting phenomenon in which the deformation of the medium takes place instantaneously, the deformation of the surrounding rock in squeezing phenomenon takes place slowly and gradually when the resulting stress state following the excavation exceeds the strength of the surrounding medium. Although there are some proposals for the definition of squeezing rocks and prediction of their squeezing potential and deformations of tunnels in literature, it is difficult to say that they are concise and appropriate.In the first half of this paper, the squeezing phenomenon of rock about tunnels and its mechanism and associated factors are clarified by studying carefully observed failures in-situ and laboratory model tests. Then, an extensive survey of tunnels in squeezing rocks in Japan is presented and the results of this survey are summarised. In the second half of the paper, a new method is proposed to predict the squeezing potential and deformations of tunnels in squeezing rock. Then, the method is applied to actual tunnelling projects, where squeezing problems have been encountered, to check its validity and applicability. As a concrete example, an application of the method to predict the squeezing potential and deformations of the rock along a 300 m long section of an actual tunnel was made.

The stability of slopes and underground openings against flexural toppling and their stabilisation

SummaryThe stability of slopes and underground openings during and after excavation is always of great concern in the field of rock engineering. Depending upon the geologic conditions and material properties of rock and discontinuities, and the geometry of excavations and topography, various kinds of instabilities are likely to be encountered. One of these is the flexural toppling failure which has become to be known recently. A stability analysis method for slopes and underground openings under various loading conditions against the flexural toppling failure is proposed. In addition, for the stabilisation of structures, a method is suggested to take into account the reinforcement effect of fully grouted rockbolts. The applicability and validity of the proposed method is checked through model tests carried out in laboratory under well controlled conditions.

The squeezing potential of rock around tunnels: Theory and prediction with examples taken from Japan

SummaryLarge deformations of surrounding media around tunnels are often encountered during excavations in rocks with squeezing characteristics. These deformations may sometimes continue for a long period of time. Predictions of deformations of tunnels in such grounds are urgently needed, not because of stability concerns, but also of their sevicability. In the present study, the squeezing phenomenon of rock around tunnels and its mechanism and associated factors are first clarified by carefully studying failures of tunnels, and a survey of tunnels in squeezing rocks in Japan is presented and its results are summarised. Then, a practical method is proposed to predic the squeezing potential and deformation of tunnels in squeezing rock and this method has beeen applied to actual tunnelling projects, where squeezing problems were encountered, to check its applicability and validity. Finally, an extension of this method to the time-dependent behaviour of squeezing rocks is given and an application of this method to an actual tunnel is presented.

Assessment of rock mass strength for underground excavations

Abstract When the stability of underground openings is assessed, one must concern with local and global failure modes. Local failures may take place as long as rock blocks are kinematically movable into openings while global failure modes are associated with the stress state and the strength of rock mass surrounding openings. This fact is always confused in underground rock engineering. First, the authors briefly describe several approaches how to assess the strength of rock masses together with some new proposals for underground excavations. Then, the estimations by these methods are compared with each other as well as with experimental data. Finally, the validity of these methods are discussed and some recommendations are made for practical applications.

A New Rock Mass Quality Rating System: Rock Mass Quality Rating (RMQR) and Its Application to the Estimation of Geomechanical Characteristics of Rock Masses

The qualitative description of rock masses by means of classification systems and subsequent correlation to establish engineering quantities or design parameters has become one of the most challenging topics in rock engineering. Many rock mass classification systems have been proposed for rock masses with the consideration of a particular rock structure and/or specific purposes. Therefore, direct utilization of these systems, in their original form, for the characterization of complex rock mass conditions is not always possible. This is probably one of the reasons why rock engineers continue to develop new systems or modify and extend current ones. The recent tendency is to obtain rock mass properties from the utilization of properties of intact rock and rock classification indexes, which have some drawbacks. In this study, it is aimed to propose a new rock mass quality rating system designated as Rock Mass Quality Rating (RMQR). This new rock mass rating system is used to estimate the geomechanical properties of rock masses. In the first part of this paper, the input parameters of RMQR and their ratings are given and discussed. In the second part, the unified formula proposed by the first author is adopted for the new rock mass rating system for estimating the rock mass properties and compared with the results of the in situ tests carried out in Japan and those estimated from some empirical relationships developed by other investigators, and the outcomes of these studies are presented and discussed.

ISRM suggested methods for determining the creep characteristics of rock

It is important to note that creep is only one aspect of the time-dependent behavior of rocks. In Fig. 1, three cases are illustrated with respect to the complete stress–strain curve: creep, i.e., increasing strain when the stress is held constant; stress relaxation, i.e., decreasing stress when the strain is held constant; and a combination of both, when the rock unloads along a chosen unloading path. This ISRM suggested method deals only with the case of creep, which is particularly relevant for cases where the applied load or stress is kept constant. Creep tests have also been carried out on soft rocks such as tuff, shale, lignite, and sandstone, medium-hard rocks such as marble, limestone, and rock salt, and hard rocks such as granite and andesite (i.e., Akagi 1976; Akai et al. 1979, 1984; Ito and Akagi 2001; Berest et al. 2005; Doktan 1983; Passaris 1979; Serata et al. 1968; Wawersik 1983; Okubo et al. 1991, 1993; Masuda et al. 1987, 1988; Ishizuka et al. 1993; Lockner and Byerlee 1977; Boukharov et al. 1995; Fabre and Pellet 2006; Aydan et al. 1995; Chan 1997; Cristescu and Hunsche 1998; Hunsche 1992; Hunsche and Hampel 1999; Ito et al. 1999; Mottahed and Szeki 1982; Perzyna 1966; Slizowski and Lankof 2003; Yang et al. 1999). These experiments were mostly carried out under compressive loading conditions. There are few studies on rocks using creep tests under a tensile loading regime (Ito and Sasajima 1980, 1987; Ito et al. 2008; Aydan et al. 2011). In particular, shallow underground openings may be subjected to a sustained tensile stress regime, which requires the creep behavior of rocks under such conditions. Please send any written comments on this ISRM suggested method to Prof. Resat Ulusay, President of the ISRM Commission on Testing Methods, Hacettepe University, Department of Geological Engineering, 06800 Beytepe, Ankara, Turkey.

Geomechanical Evaluation of Derinkuyu Antique Underground City and its Implications in Geoengineering

Derinkuyu Underground City, located in the Cappadocia Region of Turkey, is an important structure not only for its antique and archaeological characteristics, but also as a structure in terms of the long-term stability of underground rock structures excavated by mankind. The authors carried out some observational, experimental and theoretical rock mechanics studies in the region from 1996 in the context of a research project supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan for the assessment of the long-term behaviour of Derinkuyu Underground City, and these studies are still continuing. In addition to the monitoring of the environmental conditions such as temperature, moisture and air pressure, they also installed acoustic emission (AE) and electrical potential (EP) measurement systems to monitor the behaviour and response of the surrounding rock at the fifth and seventh floors of the underground city. In this article, the geology, seismicity and state of stress of the Cappadocia Region, climatic conditions in the underground city and its vicinity, short- and long-term behaviours of the surrounding rock, its index and mechanical properties, and effects of water content and freezing–thawing processes were investigated. The stability of Derinkuyu Underground City was also evaluated using theoretical and numerical methods, and the results were presented. Furthermore, its implications in modern geoengineering are also discussed.

The Inference of Geo-Mechanical Properties of Soft Rocks and their Degradation from Needle Penetration Tests

Abstract Needle penetration tests (NPTs) are used for inferring the uniaxial compressive strength of soft rocks, particularly in tunneling through squeezing rocks and foundations on weathered rocks in Japan. The device measures the applied load and the penetration depth of its needle. The ratio of applied load to penetration depth was originally called the needle penetration index (NPI). In this study, this device has been used to infer the geo-mechanical properties of soft rocks from Japan, Turkey, Indonesia, and Egypt. Various equations are presented to infer the geo-mechanical properties in terms of the NPI and compared with the experimental results. The possibility of evaluating the anisotropy of geo-mechanical properties is shown. Furthermore, the characterization of geo-mechanical properties of fault/fracture and slip (shear) surfaces is explained. Some additional equations are given to consider the degradation of geo-mechanical properties as a function of water content, weathering state, and number of cycles of freezing–thawing. Furthermore, the possibility of evaluating the time-dependency characteristics of soft rocks by needle penetration testing is discussed through experiments. It is shown that the effects of water content, weathering state, and number of cycles of freezing–thawing and time-dependency can be evaluated using the NPT technique.

A review of numerical analysis of tunnels in discontinuous rock masses

Rock masses in nature contain numerous discontinuities in the form of cracks, joints, faults, bedding planes, etc. Therefore, various continuum equivalent models of discontinuum models have been proposed and used to assess the stability of rock tunnels since the beginning of 1970. In this article, a brief but comprehensive review of numerical analysis of tunnels in discontinuous rock masses is presented. Firstly, various discrete models are briefly described and their fundamental features are summarised. Then, equivalent continuum models are described and their fundamental assumptions are presented. And then a brief outline of hybrid models proposed in literature is given. Finally, some recommendations as to how to use these models in practice are made. Copyright

ISRM Suggested Method for the Needle Penetration Test

Estimation of mechanical properties of intact rock is usually required for assessment of the stability of rock structures. They are also important elements of the rock classifications used in empirical assessment of rock masses. Measurement of these properties requires laboratory testing, which must be performed on samples of certain dimensions to fulfill testing standards and/or suggested methods. Laboratory tests are also time-consuming due to sample preparation, as well as experimental procedures often require high-capacity loading devices. High-quality core samples recommended by standards and/or suggested methods for the laboratory tests cannot always be obtained, particularly from weak and clay-bearing rocks. For these reasons, some simple and inexpensive index test methods have been developed to indirectly estimate the mechanical properties of intact rock (ISRM 2007). However, even preparation of smaller samples from weak and clay-bearing rocks for some index tests is still troublesome. In addition, geo-engineering and/or restoration studies on natural and man-made historical rock structures and monuments or buildings built with masonry construction techniques may require the determination of mechanical properties of intact rock. Sampling from such ancient sites is not allowed due to preservation, and environmental and other concerns resulting in the lack of mechanical data for those studies. To overcome the above-mentioned difficulties, a portable, lightweight and non-destructive testing device, called needle penetrometer, was developed in Japan and released as a suggested method by the Rock Mechanics Committee of the Japan Society of Civil Engineers (JSCE-RMC 1980). Similarly, Public Works Research Institute (PWRI 1987) published a draft manual of the test for weak rock mass Please send any written comments on this ISRM Suggested Method to Prof. Resat Ulusay, President of the ISRM Commission on Testing Methods, Hacettepe University, Department of Geological Engineering, 06800 Beytepe, Ankara, Turkey.