H. Sonmez

Modifications to the geological strength index (GSI) and their applicability to stability of slopes

Abstract Determination of the strength of closely jointed rock masses is difficult since the size of representative specimens is too large for laboratory testing. This difficulty can be overcome by using the Hoek–Brown failure criterion. Since its introduction in 1980, the criterion has been refined and expanded over the years, particularly due to some limitations in its application to poor quality rock masses. In the latest version, the geological strength index (GSI) was introduced into the criterion by its originators. However, the GSI classification scheme, in its existing form, leads to rough estimates of the GSI values. Another particular issue is the use of undisturbed and disturbed rock mass categories for determining the parameters in the criterion, for which clear guidelines are lacking. Furthermore, the data supporting some of these revisions, particularly the latest one, have not been published, making it difficult to judge their validity. In this study, in order to provide a more quantitative basis for evaluating GSI values, some modifications are suggested by introducing easily measurable parameters with their ratings and/or intervals which define the blockiness and surface condition of discontinuities. In addition, a method is proposed to assess the influence of disturbance on rock mass constants due to the method of excavation. The modifications to the GSI and the suggested method have been applied to slope instability case histories selected from Turkey by performing back analysis, to discuss the validity of the criterion and the methodology of parameter estimation. It was shown that the failure conditions in each case were confirmed, i.e. the analysed failure surfaces satisfied factors of safety of unity, when the suggested modifications and disturbed rock mass condition are considered. On the basis of the results, a chart to assess the effect of disturbance in terms of method of excavation was also suggested. The back analysis of a spoil instability indicated that spoil pile materials consisting of blocky and angular rock pieces could be categorized as a disintegrated rock mass in the GSI classification and the criterion seemed to be applied to such materials. The method suggested herein must, however, be verified by additional data from slope failures before more precise guidelines can be formulated.

Factors affecting the durability of selected weak and clay-bearing rocks from Turkey, with particular emphasis on the influence of the number of drying and wetting cycles

Weathering can induce a rapid change of rock material from initial rock-like properties to soil-like properties. The resistance of a rock to short-term weathering is described through a durability parameter called the slake durability index. As durability is an important engineering parameter, particularly for weak and clay-bearing rocks, it was assessed by a number of tests. The main purpose of this study is to assess the influence of the number of drying and wetting cycles and controls of mineralogical composition and strength on durability. For this purpose, 141 samples of different types of weak and clay-bearing rocks were selected from different parts of Turkey, and relationships between the above-mentioned rock characteristics were statistically investigated. The samples were subjected to multiple-cycle slake durability testing, X-ray diffraction (XRD) analysis and uniaxial compression testing. In addition, to assess the influence of mineralogical composition on durability, the mineral contents of the original material and the material passing from the drum of the slake durability apparatus after each cycle were also determined by XRD. The results indicate that the type and amount of clay minerals are the main factors influencing the variations of the slake durability index in all samples. The durability of the clay-bearing rocks studied correlates best with the amount of expandable clay minerals. A strong relationship between the uniaxial compressive strength and the fourth-cycle slake durability index is found only for the marls among the rock types studied. Assessment of gradation results of the spoil pile materials consisting of clay-bearing rocks also reveals that the increase in percentage of fines in old piles is indicative of material degradation, as is evident by multiple-cycle slaking. It is emphasized that two-cycle conventional slake durability testing did not appear to offer an acceptable indication of the durability of weak and clay-bearing rocks when compared with multiple-cyclic wetting and drying. Comments on the performance of the test are made that aim to make the testing process and interpretation of the results more reliable.

Discontinuity controlled probabilistic slope failure risk maps of the Altindag (settlement) region in Turkey

The evaluation of potential rock slope problems using stereographic projection techniques known as kinematic analysis is one of the most important parts of a slope stability investigation to be carried out in jointed rock media. In conventional stereoprojection techniques for the assessment of possible rock slope failures, the peak orientations of joints together with the slope geometry and the friction angle of the weakness planes are used. Other possible joint orientations which may be encountered in the rock media are ignored, although they belong to the group of joint peak orientations. In this study, nearly vertical jointed andesites cropped out at the Altindag settlement region in Ankara were studied in order to evaluate the relevance of this ignored discontinuity orientation data on slope stability. As a result, probabilistic risk maps for planar, toppling and wedge failures were produced using the kinematic rules and digital elevation model of the study area. The comparison of the distribution of the actual failures in the area and the probabilistic risk maps prepared for the study area revealed that all of the identified failures are found to be located in the higher risk zones on the probabilistic risk maps.

Predicting the deformation moduli of rock masses

Abstract Predictive empirical models for the mechanical properties of rock masses have been used in rock engineering because direct measurement of the properties is difficult due to the presence of discontinuities. Such empirical models are open to improvement because they are based on collected data. The purposes of the present study are to assess the existing empirical equations and to develop a new empirical approach. For this reason, in the first stage of the study, the prediction performance of the existing models proposed for predicting the deformation modulus of rock masses were evaluated statistically by using a database including 115 data values obtained from in situ plate loading and dilatometer tests. A new empirical approach with higher prediction capacity than the existing empirical models was developed in the subsequent stage of the study. The new empirical model considers the modulus ratio of intact rock ( E i /UCS), rock quality designation (RQD) and weathering degree (WD). Although, data obtained from very weak and weak rock masses were included in the development of the new empirical equation, the type of rocks employed in the study were limited. Therefore, a crosscheck between the new empirical equation and previous empirical approaches should be performed in the design stage.

An application of fuzzy sets to the Geological Strength Index (GSI) system used in rock engineering

Abstract Characterization of rock masses is one of the fundamental aspects of rock engineering. Particularly, as a rock mass characteristic, determination of the strength of closely jointed rock masses is difficult since the size of representative specimens including discontinuities is too large for laboratory testing. This difficulty can be overcome by using the Hoek–Brown empirical failure criterion in conjunction with the Geological Strength Index (GSI) Classification System. However, characterization of rock masses and determination of their strength may involve some uncertainties due to their complex nature. The fuzzy set theory is one of the tools to handle such uncertainties. This paper describes the application of fuzzy set theory to the GSI System by incorporating judgement and experience of practising engineers. For the purpose, the original GSI System and its modified form were defined by fuzzy sets, and Mamdani fuzzy algorithm was constructed using 22 “if–then” rules for evaluating discontinuity parameters and their ratings considered in the GSI System. In addition, slope instabilities in heavily jointed rock masses selected from two open pit mines in Turkey were back analysed and the results were evaluated to demonstrate and to check the performance of this approach.

A practical procedure for the back analysis of slope failures in closely jointed rock masses

Abstract Where closely jointed rock masses are encountered in slopes, failure can occur both through the rock mass, as a result of combination of macro and micro jointing, and through the rock substance. Determination of the strength of this category of rock mass is extraordinarily difficult since the size of representative specimens is too large for laboratory testing. This difficulty can be overcome by using a non-linear rock mass failure criterion or by back analysis of such slopes to estimate the rock mass strength. In this paper, a practical procedure and a computer program are presented for the back determination of shear strength parameters mobilized in slopes cut in closely jointed rock masses which obey a non-linear failure criterion rather than a linear one. The procedure shows that the constants to derive normal stress dependent shear strength parameters of the failed rock masses can be determined by utilizing a main cross-section and without a pre-determined value of rock mass rating (RMR). Trials are made for different RMR m and RMR s values corresponding to various possible combinations of the constant m and s , which are used in the Hoek–Brown failure criterion, satisfying the limit equilibrium condition. It is also noted that the procedure provides a quick check for the rock mass rating obtained from the site investigations. The method is used in conjunction with the Bishops method of analysis based on circular slip surfaces. The procedure outlined in this paper has also been satisfactorily applied to documented slope failure case histories in three open pit mines in Turkey.

Estimating the uniaxial compressive strength of some clay-bearing rocks selected from Turkey by nonlinear multivariable regression and rule-based fuzzy models

: Although the use of predictive models in rock engineering and engineering geology is an important issue, some simple and multivariate regression techniques traditionally employed in these areas have recently been challenged by the use of fuzzy inference systems and artificial neural networks. The purpose of this study was to construct some predictive models to estimate the uniaxial compressive strength of some clay-bearing rocks, depending on examination of their slake durability indices and clay contents. For this purpose, the simple and nonlinear multivariable regression techniques and the Mamdani fuzzy algorithm are compared in terms of their accuracy. To increase the accuracy of the Mamdani fuzzy inference system, the weighted if–then rules are extracted. To compare the predictive performances of the models, the statistical performance indices (root mean square error and variance account for) are calculated and the results are discussed. The indices reveal that the fuzzy inference system has a slightly higher prediction capacity than the regression models. The basic reason for the higher performance of the fuzzy inference system is the flexibility of the fuzzy approach.

A Mamdani Model to Predict the Weighted Joint Density

Estimating the block size is a major task for the quarry economy. Two approaches such as volumetric joint count and weighted joint density exist in the literature to assess the block size. However, due to the complex nature of discontinuities in the rock masses, this parameter could not be predicted easily everytime. Especially, when working in the rock masses having a wide discontinuity spacing, it is too difficult to perform a scanline survey. In this study, to overcome this difficulty, the photoanalysis method was considered to obtain the data required to construct a predictive model for weighted joint density. Considering the obtained data, a Mamdani fuzzy inference system was construct and its performance was assessed. As a result, a model proposed for predicting the weighted joint density in the present study.

Influence of seismic acceleration on landslide susceptibility maps: a case study from NE Turkey (the Kelkit Valley)

Particularly in the last decade, landslide susceptibility and hazard maps have been used for urban planning and site selection of infrastructures. Most of the procedures for preparing of landslide susceptibility maps need high-quality landslide inventory map. Although the rainfall and seismic activities are accepted as triggering factor for landslides, designation of the triggering factor for each landslide in the inventory is almost impossible when well-documented records are unavailable. Therefore, during preparation of landslide susceptibility map, whole landslide records in the inventory map are used together without classifying based on the triggering factors. Although seismic activity is accepted as a triggering factor, possible effect of the use of seismic activity on production of landslide susceptibility map was investigated in this study, and the subject is open to discussion. For this purpose, a series of stability analyses based on circular failure and infinite slope model were performed considering different pseudostatic conditions. The results of analyses show that gentle slopes have higher susceptibility to failure than steeper ones, even if their stability conditions (susceptibilities) are similar for static condition. The seismic forces acting on failure surfaces may not be sufficiently taken into consideration in the conventionally prepared landslide susceptibility maps. Employing the general decreasing trend in stability condition based on slope face angle and the seismic acceleration, a new procedure was introduced for preparing of the landslide susceptibility map for a scenario earthquake. The prediction performance of occurring landslides increased after the procedure was applied to the conventionally prepared landslide susceptibility map. According to the threshold independent spatial performance analyses of the proposed methodology and the produced landslide susceptibility maps, the area under ROC curve values were calculated as 0.801, 0.933, and 0.947 for the maps prepared by considering conventional method and scenario earthquakes having Mw values of 5.5 and 7.5, respectively.

Application of Chebyshev theorem to data preparation in landslide susceptibility mapping studies: an example from Yenice (Karabük, Turkey) region

Landslide database construction is one of the most crucial stages of the landslide susceptibility mapping studies. Although there are many techniques for preparing landslide database in the literature, representative data selection from huge data sets is a challenging, and, to some extent, a subjective task. Thus, in order to produce reliable landslide susceptibility maps, data-driven, objective and representative database construction is a very important stage for these maps. This study mainly focuses on a landslide database construction task. In this study, it was aimed at building a representative landslide database extraction approach by using Chebyshev theorem to evaluate landslide susceptibility in a landslide prone area in the Western Black Sea region of Turkey. The study area was divided into two different parts such as training (Basin 1) and testing areas (Basin 2). A total of nine parameters such as topographical elevation, slope, aspect, planar and profile curvatures, stream power index, distance to drainage, normalized difference vegetation index and topographical wetness index were used in the study. Next, frequency distributions of the considered parameters in both landslide and nonlandslide areas were extracted using different sampling strategies, and a total of nine different landslide databases were obtained. Of these, eight databases were gathered by the methodology proposed by this study based on different standard deviations and algebraic multiplication of raster parameter maps. To evaluate landslide susceptibility, Artificial Neural Network method was used in the study area considering the different landslide and nonlandslide data. Finally, to assess the performances of the so-produced landslide susceptibility maps based on nine data sets, Area Under Curve (AUC) approach was implemented both in Basin 1 and Basin 2. The best performances (the greatest AUC values) were gathered by the landslide susceptibility map produced by two standard deviation database extracted by the Chebyshev theorem, as 0.873 and 0.761, respectively. Results revealed that the methodology proposed by this study is a powerful and objective approach in landslide susceptibility mapping.