ENGINEERING BEHAVIOUR AND CLASSIFICATION OF WEATHERED ROCKS

Transcription

1 ENGINEERING BEHAVIOUR AND CLASSIFICATION OF WEATHERED ROCKS By ANAND SWAROOP GUPTA A thesis submitted in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Civil Engineering INDIAN INSTITUTE OF TECHNOLOGY, DELHI FEBRUARY 1997

2 I. I. T. DELHI. LIBRARY No 7 r

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4 CERTIFICATE This is to certify that the thesis entitled, "Engineering Behaviour and Classification of Weathered Rocks" being submitted by Mr. Anand Swaroop Gupta to the Indian Institute of Technology, Delhi for the award of the degree of DOCTOR OF PHILOSOPHY is a record of the bonafide research work carried out by him. Mr. Gupta has worked under my guidance for the submission of this thesis which to my knowledge has reached the requisite standard. The thesis or any part thereof has not been presented or submitted to any other University or Institute for any degree or diploma. Dr. K. Seshagiri Rao Assistant Professor Department of Civil Engineering Indian Institute of Technology, Delhi

5 ACKNOWLEDGEMENTS On the completion of this work, it is my great proud privilege to express my deep sense of gratitude and immense indebtedness towards my supervisor for his inspiring guidance, unfailing support, stimulating discussions and constant encouragement throughout the research programme. I am highly indebted for his special interest and care in the accomplishment of the thesis. I am extremely thankful to Prof. T. Ramamurthy for his sincere advice during the study. I thankfully acknowledge to Head of the Civil Engineering Department for the required financial support for ent research programme. Thanks are due to Mr. K. Ramachandra Rao (VRCE, Nagpur), Mr. K.K. Chatterjee (GSI, Nagpur) and Mr. Sindupe (HCL, Malanjkhand) for providing necessary help in the field study. The assistance rendered by Mr. D. Biswas and Mr. Omprakash and other staff of Engineering Geology, Geotechnical Engineering, XRD and SEM laboratories and Civil Engineering Workshop. Mr. N. K. Saini's (XRF Lab., WIHG, Dehra Dun) help is also thankfully acknowledged. I would like to thank some of my friends, Anupam and Mohanta (Jawaharlal Nehru University), Ms. Sangeeta and Mukta (Delhi University) and Harish (Univ. of Missouri), who extended their valuable help particularly in laboratory experiments. ii

6 Mr. Mahendra Singh (MNREC, Allahabad) and Mr. C.S. Gokhale (Goa Engg. College) have kindly read the draft of the thesis several times and made useful suggestions for its improvement. Their advice is sincerely acknowledged. Including both of them, I would like to thank other fellow research scholars particularly, Amit Jain, Ashutosh Shukla, M. Hashemi, S. Rao and Ashok Gupta for sharing the cherished and memorable moments of discussion. I also take this opportunity to thank my friends, especially Ravi Kumar, Salil, Sunil, Maconnen, Ajay, and Bhupendra for creating amusing and hilarious environment during the difficult time. Finally, no words are adequate to express my indebtedness to my family for all their pains and sufferings they have undergone to bring me up to this stage. To them, I bow in deepest reverence. Anand S. Gupta iii

7 ABSTRACT Rocks, at and near the earth surface, commonly occur in weathered state. The important geotechnical parameters, strength and deformational properties greatly deteriorate by weathering on mass as well as material scale. Therefore, to understand and predict the physico-mechanical behaviour of weathered rock and characterization of extent of weathering on material and rock mass is of great significance. Most of the available methods for characterization of weathered rocks involve subjective terminology and are not well established. Hence, they lack the ability to be used in any method of strength and deformation prediction. Keeping these facts in view, a detailed investigation has been carried out over three common Indian rocks, namely Delhi quartzite, Malanjkhand granite, and Nagpur basalt: (i) To study the intrinsic properties and characterize the rock materials in fresh and differently weathered conditions, geologically and geotechnically. (ii) To study the strength and deformational behaviour of weathered rocks in unconfined and confined compression state. (iii) To develop a quantitative classification and assessment of strength behaviour of weathered rock mass. The weathered rock masses have been studied at 13 vertical profiles, in the field. Rock materials have also been identified by simple field methods. Detailed material characterization was carried out by using advanced laboratory techniques, such as X-ray diffraction, optical microscopy, Scanning Electron Microscopy, and X-ray fluoroscopy. The physical and strength properties were determined by using ISRM and Indian Standards (IS) test procedures. Triaxiat tests were conducted at confining pressures, = 0.5, 1, 2, 5, 10, 25. and 50 MPa over the weathered and fresh rock specimens. iv

8 Detailed petrographic study reveals that Delhi quartzite, which is dominantly composed of quartz, weathers through increasing intensity of microfracture. Decomposition in quartzite is very obvious through high order of staining and dissolution of quartz grain. Weathering in Malanjkhand granite is conspicuous by density of microfracture and content of secondary minerals. Alteration features of felspars and shattering of quartz grains also mark the development of weathering in granite. The fine grained basic volcanic rock, Deccan basalt of Nagpur, has several distinction features at different weathering stages. Initial stage of weathering is identified by development of thin microfractures accompanied with altered micropores. At later stages, complete transformation of minerals into clays and well-developed macrofractures prevail. Geochemical study of the weathered and fresh rocks indicates that, in general, Na followed by Ca and K is the highest mobile element in the weathering of all three rocks: Almost all physical and strength properties are significantly influenced by extent of weathering. Similar to strength properties, deformational modulus, E, is also greatly affected by weathering. However, it is evident that initial modulus, E, reduces more drastically than the E, with progressive weathering stages. Close relationship between E, and a, is observed throughout the weathering stages. This indicates that modulus ratio (MR = Ei /o-c) does not change significantly even at highly weathered grade. This may be due to the fact that both ac and Et decrease systematically with progress of weathering. Good correlations have been observed among the index properties varying with weathering grade of a particular rock. However, the comparison of these relationships for different rock weathering sequences reveals that a common relationship among the index properties can not be valid for all the rock types. In general, the index properties of rock material alone can not represent the extent of weathering, unless compared with their values for fresh rocks. Hence, for characterization of weathering extent, indices other than index properties should be preferred. Study reveals that none of the existing chemical weathering indices is valid for engineering purposes. Loss on ignition may provide approximate estimation of altered

9 minerals (clays and hydroxides) in crystalline rocks. It has also shown good correlation with petrographic indices and other index properties. Among the petrographic indices, crack density (o ) and Unsound Constituent indices provide good correlation with engineering index properties. To quantify the extent of weathering in terms of strength degradation due to weathering in rock, an index is proposed as Strength Ratio (R,) which is the percentage of a, of weathered rock with respect to a, of fresh rock. Its usefulness is shown through the relationships with other indices for several rock types including sedimentary and metamorphic rocks. Furthermore, it can also be used in prediction of strength and deformation of weathered rocks in confined conditions, and classifications of weathered rock material. Triaxial test results and their analysis show that the shear strength parameters, c and vary with a3 and effect of weathering. The cohesion intercept, c increases non-linearly and angle of internal friction, decreases linearly with increasing o-3 for all the rocks. The stress-strain response under uniaxial compression is also observed to be changing with increasing weathering stage. The crystalline rocks, in fresh state, deform linear-elastically (and quasi-elastically). At higher weathering grades, the rocks behave more plastically with characteristic S-shaped stress-strain curves; with increasing confining pressure, the shape of stress-strain curves also changes. Observation on failure pattern under unconfined compression reveals that each weathering stage has a characteristic mode of failure. Generally, fresh rocks fail with tensile splitting and shear fractures. In many cases, formation of cones has been associated with failure. In moderately weathered rocks, fractures are more prominent than tensile fractures at failure. Highly and completely weathered materials deform due to multiple shear fractures propagated throughout the material. The strength prediction made based on IITD criterion has been compared with those from Hoek and Brown criterion, It shows good agreement with experimental data of vi

10 rocks at all weathering grades. The relationships have been proposed to evaluate the material constants for complete weathering range. The strength prediction involves the 0 3 values of fresh and weathered rocks, and material constant values for fresh rock. The results of triaxial tests show that the variation of E, is more systematic than Ei with increasing a3 and degree of weathering. In normalized form, Et is related with a3 by power relationship. The determined constants show good correlation with index of weathering, R,. This allows to make the prediction of deformation modulus of rock at any confining pressure and stage of weathering. In common practice, the rock masses are studied and characterised by a classification system. Virtually, lack of well applicable and quantitative rock-weathering classification has guided to develop such a classification useful for common engineering purposes. This may also be applicable for the prediction of rock mass strength. The classification system has been developed by detailed field study of 13 weathered profiles. Important attributes of the classification system involve the recognition of weathered material, delineation of weathered zones at exposed profile, and study of rock mass parameters at profile. The profile and its zones are classified according to the final rating, R, which is the sum of the ratings of rock mass parameters. Besides grading, it has also been attempted to link the RW with failure criterion in order to predict the strength of weathered rock mass. For this, few relationships have been proposed employing the limited data available in literature. Based on the outcome of the study, final statement can be made that with the help of given guidelines, and visual description one can classify the weathered rock in both mass and material scale. The suggested indices and relationships will be useful in predicting the engineering behaviour of rocks occurring in weathered conditions. The overall classification system may be effectively functional at preliminary investigation stage of any rock engineering project where detailed and expensive field tests are not feasible. Vii

11 CONTENT PAG E Certificate Acknowledgements Abstract Content List of Tables List of Figures List of Notations ii iv viii xvi xxi xxxv CHAPTER 1 INTRODUCTION General Objectives Organisation of The Thesis 5 CHAPTER 2 LITERATURE REVIEW General Weathering Processes Physical Weathering Chemical Weathering Factors Influencing The Rock Weathering Topography Vegetation and Time Parent Material Climate Rate of Weathering Rates of Mineral Weathering Rates of Rock Weathering Products of Weathering 21 viii

12 2.3 Weathering Pattern Mobility of Elements Weathering of Minerals Weathering of Common Rock Material Granite Basalt Quartzite Weathering Pattern in Rock Mass Problems of Rock Weathering in Ground Engineering Effects of Weathering on Index Properties Point Load Strength Schmidt Hammer Value Tensile Strength Unconfined Compressive Strength Deformation Modulus Permeability Weathering Indices Chemical Weathering Indices Mineralogical and Petrographical Indices Engineering Indices Factors Influencing the Strength of Rocks Geological Factors Physical Properties Mechanical factors Environmental Factors Triaxial Strength of Weathered Rocks Deformational Behaviour of Weathered Rocks Failure Pattern in Weathered Rocks Strength Prediction Engineering Classification of Weathered Rocks Conclusions 93 ix

13 CHAPTER 3 FIELD AND LABORATORY STUDY Field Study: Data Collection Delhi Quartzite Malanjkhand Granite Nagpur Basalt Field Techniques Material Identification Study of Weathered Rock Mass Sampling Laboratory Studies Mineralogical and Petrographical Study X-ray Diffraction Method Transmitted Light Microscopy Scanning Electron Microscopy Geochemical Study: X-ray Fluoroscopy Geotechnical Study Specimen Preparation Physical Index Properties Strength Index Tests Schmidt Hammer Test Point Load Strength Test Brazilian Strength Test Line Load Test Unconfined Compressive Strength Triaxial Test Slake Durability Test 133 CHAPTER 4 MINERALOGICAL, TEXTURAL AND GEOCHEMICAL CHANGES Mineralogical and Textural Changes Granite 139

14 Fresh Granite Slightly Weathered Granite Moderately Weathered Granite Completely Weathered Granite Granitic Residual Soil Basalt Fresh Basalt Slightly Weathered Basalt Highly Weathered Basalt Completely Weathered Basalt Basaltic Residual Soil Quartzite Fresh Quartzite Slightly Weathered Quartzite Moderately Weathered Quartzite Highly Weathered Quartzite Completely Weathered Quartzite Quartzitic Residual Soil Mineralogical Variations Geochemical Variations Relative Mobility of Cations Conclusions 197 CHAPTER 5 INDEX PROPERTIES Physical Properties Specific Gravity and Density Porosity and Other Related Properties Ultrasonic Velocity and Electrical Resistivity Strength Index Properties Schmidt Hammer Value Point Load Strength 206 xi

15 5.2.3 Indirect Tensile Strength Unconfined Compressive Strength Deformational Modulus Poisson's Ratio Interrelationships Among Index Properties Porosity and Other Index Properties Ultrasonic Velocity and Strength Index Properties Schmidt Hammer Value and Other Strength Index Properties Tensile Strength (Indirect) and Compressive Strength Modulus Value and Strength Index Properties Conclusions 229 CHAPTER 6 WEATHERING INDICES Chemical Weathering Indices Petrographical Indices Microfracture Index Crack Density Micropetrographic Index Engineering Indices Strength Index Properties as Weathering Indices Quick Absorption Index Coefficient of Weathering Slake Durability Proposed New Index Conclusions 266 CHAPTER 7 STRENGTH AND DEFORMATIONAL BEHAVIOUR Strength Behaviour in Triaxial Condition Effect of Confinement on Shear Strength Parameters 268 xii

16 7.3 Deformational Behaviour Stress-Strain Behaviour in Unconfined Compression Stress-Strain Behaviour under Confined State Effect of Confinement on Failure Strain Failure Pattern in Weathered Rocks Prediction of Strength and Deformation Prediction of Strength Relations among Material Constants and Weathering Index Prediction of Deformational Modulus Conclusions 360 CHAPTER 8 ENGINEERING CLASSIFICATION OF WEATHERED ROCKS General Proposed Material Classification Visual Identification and Grading Discolouration and. Staining Texture and Fabric Disintegration Decomposition Relative Strength Quantitative Approach for the Classification Applicability of Classification Advantages Disadvantages 376

17 8.3 Rock Mass Classification Proposed Rock Mass Classification Identification of Rock Material Delineation of the Weathering Zones Rock Mass Parameters Influenced by Weathering Joint Wall Strength Joint Roughness Width of Joint and Infills Joint Frequency Other Parameters Assignment of Rating Link to Rock Mass Classification (RMR) Assessment of Strength and Deformational Properties Applicability and Limitations Some Typical Field Examples Delhi Quartzite Malanjkhand Granite Nagpur Basalt Some Other Examples (Barcelona Granite, Leona Rhyolite and Panguna Andesite Conclusions 413 CHAPTER 9 SUMMARY AND CONCLUSIONS General Field and Laboratory Studies Mineralogical and Textural Changes 417 xiv

18 9.3.1 Delhi Quartzite Malanjkhand Granite Nagpur Basalt Geochemical Variations Index Properties Weathering Indices Chemical Indices Petrographical Indices Engineering Indices Strength and Deformational Behaviour in Confined State Shear Strength Stress-Strain Response Failure Pattern Prediction of Strength in Confined State Prediction on Deformation Modulus in Confined State Engineering Classification of Weathered Rocks 426 REFERENCES 431 APPENDICES Appendix - A Results of the Chemical Analysis for the Trace Elements 451 Appendix - B Mohr Envelopes 453 Appendix - C Determination of Material Constants 459 Appendix - D Classification of Weathered Profiles 461 xv