Resat Ulusay

Prediction of engineering properties of a selected litharenite sandstone from its petrographic characteristics using correlation and multivariate statistical techniques

Abstract The determination of rock strength and elastic parameters in laboratory tests requires high quality core samples which are not always available from fractured and/or weathered strata. The purpose of this study is to apply a correlation analysis in conjunction with multivariate statistical techniques to petrographical and laboratory geomechanical data for the investigation of correlations that exist between these properties of a selected sandstone, and to suggest inexpensive predictive models in a preliminary investigation. Sandstone samples from a borehole drilled for a preliminary investigation of a shaft location at a hard coal region (Black Sea, Turkey) were tested to determine uniaxial compressive strength, elastic parameters, unit weight, porosity, point load index and quality index. Secondly, fifteen selected samples were subjected to petrographic examination which included fifteen parameters and modal analysis. Based on the statistical analysis results, polynomial prediction equations were developed for estimating mechanical properties from petrographic characteristics and index properties. The study revealed that the influence of textural characteristics appear to be more important than mineralogy for predicting engineering properties. It is also noted that type of contacts, grain shape and size, rock fragments with packing density and proximity are the petrographic characteristics which have significant influence upon engineering properties of the sandstones. Results of this study were presented in the form of correlation coefficient models and predictive models.

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.

A simple test and predictive models for assessing swell potential of Ankara (Turkey) Clay

Abstract During the past three decades, damage due to swelling action of Ankara Clay has been observed more clearly in some parts of Ankara where rapid expansion of the city led to the construction of various kinds of structures. In this study, a comprehensive research program has been conducted (i) to investigate the effect of remoulding and desiccation on the swelling behavior of Ankara Clay and its swelling anisotropy, (ii) to estimate depth of active zone, (iii) to develop a simple technique in determining the magnitude of swelling based on water content of the soaked specimen after 24 and 72 h ( w max24,72 ), and (iv) to produce predictive models which could be used to estimate the swelling potential of Ankara Clay from its mineralogical and simply measured engineering characteristics. A laboratory testing program was carried out using both undisturbed, and remoulded and desiccated samples selected from 20 different locations. Montmorillonite was identified as being the main clay mineral present. Based on the moisture content variation with depth, the active expansive zone was considered to be about 2 m. The test results suggest that swelling pressure of the clay considerably decreases and/or dies out when the water content is greater than 30%. It is also noted that the measured lateral swelling is significantly in excess of the vertical equivalent indicating an anisotropy with respect to swelling. Statistical assessments indicate that a new parameter ( w max24,72 ) from a simple test, suggested in this study, is a very strong parameter for predicting swelling parameters of Ankara Clay. Based on 60 empirical predictive equations with coefficients of correlation between 0.96 and 0.66 from multiple regression analyses, w max24,72 , methylene blue value, liquid limit, dry unit weight and smectite content are the most important index and mineralogical properties to predict the swelling parameters of Ankara clay with small deviations from the measured values.

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.

Draft isrm suggested method for determining block punch strength index (bpi)

1.1. Rock strength, particularly the uniaxial compressive strength (UCS) is an important parameter in rock mass classification methods and in various rock engineering design approaches. Measurement of rock strength requires testing which must be undertaken on test specimens of particular sizes in order to fulfill testing standards. However, there are some shortcomings associatedwith these conventional tests. When rock cores are only divided into small discs, due to the presence of thin bedding or schistosity planes, the core length may be too short to allow preparation of the specimens long enough even for the point loadstrength index test. 1.2. To overcome the above-mentionedd ifficulty, the possibility of using relatively short samples for a rock strength or index test has always been attractive. The block punch strength index (BPI) test apparatus, which was similar to that usedfor the measurement of d irect shear strength of a thin plate of rock [1–2], has been developed in Delft University, The Netherlands, as an index test in directly assessing UCS by Schrier [3]. However, in the previous studies, rock-disc specimens of about 40 mm in diameter and 10 mm in thickness were tested, and the size effect of the test specimens and the use of the BPI test in rock engineering have not been considered. The studies by Ulusay and Gokceoglu [4–6] indicated that size correction was indispensable in the BPI test and the use of a generalizedsize correction factor established from the experimental data should be used. A considerably important correlation foundbetween UCS andBPI indicates that BPI tests lead to insignificant errors in determining UCS when compared to those obtained from point loadtesting, particularly for laminatedweak rocks [4–7]. It was also suggestedthat BPI be usedas an alternative input parameter for intact rock strength in rock mass classification andas a measure of anisotropy using orientedd isc samples [4–7]. 1.3. The BPI test describedin this suggestedmethodis appliedto the rock-d isc specimens, andinvolves the use of size correction, andd etermination of the strength in the strongest direction where only core samples from boreholes drilled at any angle to the weakness planes are available. 1.4. In this suggestedmethod , the apparatus and operating procedure are described together with data evaluation. There is an explanation for the presentation of the results. The empirical relationships to predict some strength parameters from BPI are also presented in the last chapter.

Evaluation of the block punch index test with particular reference to the size effect, failure mechanism and its effectiveness in predicting rock strength

Measurement of rock strength requires testing which must be carried out on test specimens of particular sizes in order to fulfill testing standards. Often, the coring process breaks up the weaker core pieces, and they are too small to be used in either conventional strength tests or index tests such as point load test. The block punch index (BPI) test, which requires flat disc specimens without special treatment has been developed during the last decade. This paper presents the results of a study of BPI test device to provide new contributions to previous works on size effect in BPI test using a wide range of rock types, in addition, to assess the effectiveness of the test in predicting rock strength by an experimental way. About 2600 disc specimens were tested, and the results were combined with those from previous works and analyzed using statistical and graphical methods. Finite element modeling was also conducted to evaluate the stress distribution created within the rock specimen by punching action of the test device. The test results suggested that size correction in the BPI test was indispensable. Using a very large test database, the size-correction factors suggested by previous workers were modified in the present study. The finite element modeling indicated that the failure surface initiated at the top of the specimen and propagated into the specimen. This demonstrated that failure was not produced by a true shearing. It was also noted that cohesion values predicted from BPI tests were greater than those obtained from conventional tests. Both of these results suggested that the BPI test was not an accurate device for directly determining shear strength of the rock specimen and should only be used as a strength index. The experimental results showed that the corrected BPI values led to lower errors in determining uniaxial compressive and tensile strength when compared to those from point load testing. Possible uses of the BPI in rock mass classification were also briefly reviewed and a strength classification based on BPI was suggested.

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

Abstract Coal mine waste material, which usually consists of a mixture of coarse-grained particles to rock fragments grading to fine-grained particles, causes geotechnical and environmental problems on disposal. A study of the geotechnical aspects of the problem is, therefore, important in rational planning for the disposal, reclamation, treatment and use of such material. Spoil pile instability has been a continuing problem in the Eskihisar strip coal mine of southwestern Turkey. Particularly shallow-seated circular failures appearing along the haul road and instabilities covering large agricultural areas outside the dumping yard increase the importance of spoil pile stability. The fact that the height of spoil piles dumped by trucks on both sides of the haul road will increase much more in the future has focused the attention of the miners to this problem. This study outlines the geotechnical characteristics of waste material from the Tertiary coal-bearing rocks derived from field and laboratory investigations, and describes the causes and mechanism of spoil instabilities which threaten the safety of the haul road and the agricultural areas in the vicinity of the pit. Gradation results suggest that fines and coarser material were approximately equally represented in the spoil. The percentage of fines is indicative of material degradation as evidenced by slaking tests. Site observations and numerous back analysis of investigated failures reveal that the failures do not penetrate the foundation material and only occur along circular surfaces through the spoil material. It is also noted that surface water infiltration, reducing the effective shear strength of the spoil material, contributes to failures. Shear test results indicate that the spoil is a nearly cohesionless material with a residual internal friction angle of about 33°, which confirms the values derived from standard penetration tests. It is also noted from back analyses that the spoil material exhibits a shear strength at or approaching the residual value at the time of failure. The use of shear strength parameters defined by the linear Mohr-Coulomb envelope yields results that are comparable with those obtained when the non-linear failure envelope is used, provided that such strength parameters were defined at the operating normal stress levels on the failure surfaces.

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.

Empirical Correlations for Predicting Strength Properties of Rocks from P-Wave Velocity Under Different Degrees of Saturation

Determination of P-wave velocity (Vp), which is closely related to intact rock properties both in laboratory and in situ conditions, is a non-destructive, easy and less complicated procedure. Due to these advantages, there is an increasing trend to predict the physico-mechanical properties of rocks from Vp. By considering that no attempt on the estimation of mechanical properties of rocks from Vp under different degrees of saturation has been made, in this study, it was aimed to correlate strength properties (uniaxial compressive and tensile strengths) with Vp of various rock types under different degrees of saturation. For this purpose, fourteen different rock types were collected from several parts of Turkey and a comprehensive laboratory testing program was conducted. Experimental results indicated that strength and deformability properties of the rocks decreased with increasing degree of saturation, while Vp showed increasing and decreasing trends depending on degree of saturation. Simple regression analysis results indicated that although prediction of the strength properties of rocks directly from Vp at different degrees of saturation was possible, the equations developed would yield some under- or over-predictions. In the second stage of statistical analyses, a series of different prediction relationships were developed by using independent variables such as Vp, degree of saturation and effective clay content (ECC). The statistical tests suggested that the resultant multivariate equations had very high prediction performances and were very useful tools to estimate the strength properties from Vp determined at any degree of saturation. In addition, the comparisons between the theoretical Gassmann–Biot velocities, which were calculated at different degrees of saturation, and the experimental results suggest that the theoretical Gassmann–Biot velocities show inconsistencies with the experimental results obtained from the investigated rock types with high ECC. Therefore, it was concluded that the use of theoretical velocities is not suitable for rock types with high ECC.