Session 3: Geology and Rock Mechanics Fundamentals

Transcription

1 Session 3: Geology and Rock Mechanics Fundamentals Geotechnical Engineering Appreciation Course (Jointly organised by IES Academy and GeoSS) Dr Zhou Yingxin, Senior Principal Engineer, DSTA Adjuct Associate Professor, NTU 1

2 Recommended references: Practical rock engineering. E. Hoek, (free download) The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: " known as the blue book. ISRM members (through SRMEG) can download chapters FOC at members area. Rock Mechanics Fundamentals, Zhou Yingxin -2-

3 Outline Rock and rock mass Rock material properties Discontinuities and shear strengths Rock mass classification Rock mass properties Failure mode and support design Rock Mechanics Fundamentals, Zhou Yingxin -3-

4 Rock Mechanics Defined Rock mechanics is the study of the statics and dynamics of rocks and rock masses. Rock engineering involves engineering with rocks, especially the construction of structures on or in rock masses, and includes the design process. Engineering rock mechanics is the study of the statics and dynamics of rocks and rock masses in anticipation of the results being applied to engineering. Rock Mechanics Fundamentals, Zhou Yingxin -4-

5 Why Rock Mechanics? In rock mechanics we are primarily interested in predicting the future. What will happen if a tunnel or cavern of a specific size is constructed in this rock mass at this orientation? What would happen if the tunnel or cavern were constructed at a different orientation or a different depth? The answers to this type of question are required for rock engineering design. The rock engineer has to have a predictive capability: without it there is no basis for coherent design. Rock Mechanics Fundamentals, Zhou Yingxin -5- Hudson, 2008

6 Rock and Rock Mass 6

7 Primary Factors F 1 F 2 F 3 Boundary conditions Geology Intact rock Excavation F n Rock stress Water flow Intact rock Fractures Discontinuities Rock mass properties Water flow Engineering activities Modelling (not just numerical modelling!) Rock Mechanics Fundamentals, Zhou Yingxin -7- Hudson, 2008

8 The rock mechanics conceptual model Rock stress Intact rock R-structure Tunnel R-structure Fracture Borehole EDZ can we identify these features in rock masses? Rock Mechanics Fundamentals, Zhou Yingxin -8- Hudson, 2008

9 Intact Rock F 1 F 2 F 3 Boundary conditions Intact rock Excavation F n Water flow Fractures Rock Mechanics Fundamentals, Zhou Yingxin -9- Hudson, 2008

10 Intact Rock Rock Mechanics Fundamentals, Zhou Yingxin -10- Hudson, 2008

11 Rock Mass Rock Mechanics Fundamentals, Zhou Yingxin -11- Hudson, 2008

12 The Rock Mass Rock Mechanics Fundamentals, Zhou Yingxin -12- Hudson, 2008

13 Effects of Discontinuities Intact rock Rock with 1 joint Rock mass 160 MPa Rock Mechanics Fundamentals, Zhou Yingxin Will slide when angle is greater than friction angle without any load Mpa? Rock mass is made weaker by the joints and weathering effects to them

14 The Mandai Granite Rock Mechanics Fundamentals, Zhou Yingxin -14-

15 Rock Mass Rock mass is a matrix consisting of rock material and rock discontinuities. Properties of rock mass therefore are governed by the parameters of rock joints and rock material, as well as boundary conditions. The behaviour of rock changes from continuous elastic for intact rock materials to discontinuous running of highly fractured rock masses, depending mainly on the existence of rock joints. Rock material Rock Mass Rock mass = Rock material + discontinuities Rock Mechanics Fundamentals, Zhou Yingxin -15-

16 Important Rock Mass Characteristics Discontinuous (faults, folds, bedding planes, joints) Inhomogeneous (varying properties from place to place) Anisotropic (different properties in different directions) Non-linear Rock Mechanics Fundamentals, Zhou Yingxin 16

17 Important Concept Rock material or intact rock basic element Discontinuities omni-present Rock mass do not behave the same way as in lab test of rock material Rock Mechanics Fundamentals, Zhou Yingxin -17-

18 Rock Material Properties 18

19 Rock Material Properties Uniaxial compressive strength Brazilian tensile strength Point load index Young s modulus and Poisson s ratio Direct shear strength of rock joints Size matters - influence of specimen size Rock Mechanics Fundamentals, Zhou Yingxin -19-

20 Uniaxial Compressive Strength Uniaxial compressive strength is the ultimate stress a cylindrical rock specimen under axial load. It is the most important mechanical properties of rock material, used in design, analysis and modelling. Along with measurements of load, axial and lateral deformations of the specimen are also measured. Rock Mechanics Fundamentals, Zhou Yingxin -20-

21 A Compression Machine Rock Mechanics Fundamentals, Zhou Yingxin -21-

22 Young s Modulus P E = Stress Strain Rock Mechanics Fundamentals, Zhou Yingxin -22-

23 Poisson s Ratio Axial strain: Lateral strain: Poisson s ratio: = Lateral strain Axial strain Rock Mechanics Fundamentals, Zhou Yingxin -23-

24 Brazilian Tensile Strength Direct tensile test is difficult Rock material tensile strength can be obtained from several types of indirect tests. The most common tensile test is the Brazilian test. Rock Mechanics Fundamentals, Zhou Yingxin -24-

25 Brazilian Test (Indirect Tensile Test) Specimen diameter (D): => 54 mm Specimen thickness (t): about one radius Rock Mechanics Fundamentals, Zhou Yingxin -25-

26 Point Load Index Point load test is a simple index test for rock material. It gives the standard point load index, I s(50). I s = P/D e 2 Granite 5 15 Gabbro 6 15 Andesite Basalt 9 15 Sandstone 1 8 Mudstone Limestone 3 7 Gneiss 5 15 Schist 5 10 Slate 1 9 Marble 4 12 Quartzite 5 15 Rock Mechanics Fundamentals, Zhou Yingxin -26- All point loads with diameter not equal to 50 mm is converted to I s(50) by using a correction factor, F.

27 Point Load Index and Strengths Uniaxial Compressive Strength c 22 I s(50) Correlation factor can vary between 10 and 30. For Bukit Timah granite, the average is about Tensile strength t 1.25 I s(50) I s(50) should be used as an independent strength index. Rock Mechanics Fundamentals, Zhou Yingxin -27-

28 Typical Intact Rock Strengths Rock UC Strength (MPa) Tensile Strength (MPa) Granite Dolerite Gabbro Basalt Sandstone Shale Dolomite Limestone Gneiss Slate Marble Quartzite Rock Mechanics Fundamentals, Zhou Yingxin -28-

29 Rock Mechanics Fundamentals, Zhou Yingxin -30- Ref: Geology of Singapore (2 nd ed.), DSTA

30 Discontinuities and Shear Strengths of Rock Joints 31

31 Discontinuities Discontinuities Joints (random but typically appear in sets) Fractures (also from blasting) Bedding planes (singular and large scale) Shear zones (singular and large scale) Faults (singular and large scale) In engineering, joints and fractures are often used to mean same thing In shallow depths, discontinuities often control structural behaviour Rock Mechanics Fundamentals, Zhou Yingxin -32-

32 Description of Rock Joints Key parameters for joint description Number of sets Persistence Orientation Spacing, frequency, block size, and RQD Roughness Aperture and filling Refer to the ISRM Blue Book for details Rock Mechanics Fundamentals, Zhou Yingxin -33-

33 Joint Sets Joints are generally in sets, i.e., joints with similar orientation. The number of joint sets can be up to 5. Typically one joint set cuts the rock mass into plates, two perpendicular sets cut rock into columns and three into blocks, and more sets cut rocks into mixed shapes of blocks and wedges. More joint sets provide more possibilities of potential slide planes 1 set 3 sets Rock Mechanics Fundamentals, Zhou Yingxin -34-

34 Joint Persistence ISRM Suggested Description Surface Trace Length (m) Very low persistence < 1 Low persistence 1 3 Medium persistence 3 10 High persistence Very high persistence > 20 Rock Mechanics Fundamentals, Zhou Yingxin -35-

35 Joint Orientation Orientation is defined by dip angle (inclination) and dip direction (facing). Importance of Joint Orientation Orientation of joints controls the possibility of unstable conditions or excessive deformations. The mutual orientation of joints determines the shape of the rock blocks. Rock Mechanics Fundamentals, Zhou Yingxin 36

36 Joint Orientation The dip direction is measured clockwise from North Strike N Vertical plane Dip angle N Measured on vertical plane: 55 Horizontal plane Line of maximum dip Dip direction Measured clockwise on horizontal plane: 220 The dip angle for a horizontal joint is = 0 o Orientation: Dip direction / Dip angle 220/55 The dip angle for a vertical joint is = 90 o Rock Mechanics Fundamentals, Zhou Yingxin -37-

37 Joint Spacing Joint spacing is the perpendicular distance between joints. For a joint set, it is usually expressed as the mean spacing of that joint set. Often the apparent spacing is measured for practical purpose. Measurements of joint spacing are different on different measuring faces and directions. For example, in a rock mass with mainly vertical joints, measurements in vertical direction have far greater spacing than that in horizontal direction. Rock Mechanics Fundamentals, Zhou Yingxin 38

38 Measuring Joint Spacing Apparent spacing on the plane Apparent spacing in x, y and z directions True spacing Rock Mechanics Fundamentals, Zhou Yingxin 39

39 Classification of Joint Spacing ISRM Suggested Joint Spacing Classification Description Joint Spacing (m) Extremely close spacing < 0.02 Very close spacing Close spacing Moderate spacing Wide spacing Very wide spacing 2 6 Extremely wide spacing > 6 Rock Mechanics Fundamentals, Zhou Yingxin 40

40 Joint Surface Profile A joint is an interface of two contacting surfaces. The surfaces can be smooth or rough; they can be in good contact and matched, or they can be poorly contacted and mismatched. The condition of contact also governs the aperture of the interface. The interface can be filled with intrusive or weathered materials. Rock Mechanics Fundamentals, Zhou Yingxin 41

41 JRC - Joint Roughness Coefficient JRC number is obtained by directly comparing the actual joint surface profile with the typical profile in the chart. JRC 20 is the profile for 20 cm and JRC 100 for 100 cm. The value of JRC decreases with increasing size. Rock Mechanics Fundamentals, Zhou Yingxin 42

42 Joint Aperture and Filling In a natural joint, it is very seldom that the two surfaces are in complete contact. There usually exists an opening or a gap between the two surfaces. The perpendicular distance separating the adjacent rock walls is termed as aperture. Joint opening is either filled with air or water (open joint) Rock Mechanics Fundamentals, Zhou Yingxin 43

43 Classification of joint aperture Aperture Description < 0.1 mm Very tight 0.1 ~ 0.25 mm Tight 0.25 ~ 0.5 mm Partly open 0.5 ~ 2.5 mm Open 2.5 ~ 10 mm Widely open 1 ~ 10 cm Very widely open 10 ~ 100 cm Extremely widely open > 1 m Cavernous "Closed feature" "Gapped Feature" "Open feature" Rock Mechanics Fundamentals, Zhou Yingxin 44

44 Shear Strength of Rock Joints All (almost) rock masses contain discontinuities (joints) Sliding of rock blocks mainly controlled by shear behaviour of rock joints Rock Mechanics Fundamentals, Zhou Yingxin -45-

45 Mohr-Coulomb Criterion Mohr-Coulomb s shear strength is made up of two parts, a constant cohesion (c) and a normal stress ( n ) dependent frictional component, angle of internal friction ( ), p = c + n tan It is a straight line, with an intercept c on the -axis and an angle of with the n -axis. In the case of residual strength, r = n tan r Rock Mechanics Fundamentals, Zhou Yingxin -46- c n

46 Peak and Residual Shear Strength Shear stress Normal stress Rock Mechanics Fundamentals, Zhou Yingxin -47- Hoek, Practical Rock Engineering

47 Direct Shear Test P N P S Rock Mechanics Fundamentals, Zhou Yingxin -48- Hoek, Practical Rock Engineering

48 Peak and Residual Shear Strength Peak Shear Strength: Residual Shear Strength: Apparent cohesion Rock Mechanics Fundamentals, Zhou Yingxin -49-

49 Rock Joint Properties (Mandai Granite) Joint conditions Friction Angle, Cohesion, C (o) (Kpa) Freshly fractured and dry Freshly fractured and saturated Freshly fractured and dry (weathered rock) Natural and dry Natural and saturated Mineral filled and dry Mineral filled and saturated Weathered and dry Weathered and saturated * Shear strengths strongly influenced by joint conditions Rock Mechanics Fundamentals, Zhou Yingxin -50-

50 Rock Stresses In situ stress existing in situ before any engineering activity Induced stress due to engineering activities such as tunnel excavation Rock stresses are important for stability analysis in design Some stresses can be favourable for tunnel stability while others can create stability problems Rock Mechanics Fundamentals, Zhou Yingxin -51-

51 In Situ Stresses Vertical Stress and Overburden Typically, vertical stress in rock is the overburden stress generated by weight of the overlying material. The average specific gravity of rocks is about 2.7. The vertical stress at depth can be estimated as v (MPa) z (m) H h v z Rock Mechanics Fundamentals, Zhou Yingxin 52

52 In situ Stress Horizontal Stresses: Horizontal stress h = K v K = horizontal stress ratio. Actual K for shallow depths is generally greater than 1. Singapore: K = 2-3 for Bukit Timah granite and Jurong formation Rock Mechanics Fundamentals, Zhou Yingxin 53

53 Horizontal Stress Ratio, K K, Upper Limit K, Lower Limit Rock Mechanics Fundamentals, Zhou Yingxin 54 Hoek, Practical Rock Engineering,

54 Ground Water Rock mass strength Reduction in effective stress and shear strength Swelling and squeezing (clay rich rock) Degradation of rock quality over time Solutioning effects (limestone) Migration of substance (e.g. pollutants) Construction No.1 enemy! Grouting Pumping Flooding (safety implications, too) Ground subsidence due to drawdown Delays and cost increases Operations (pumping cost) Rock Mechanics Fundamentals, Zhou Yingxin 55

55 Rock Mass Classification Rock mass very difficult to characterise and model Experience and empirical methods Engineers need numbers Means of communication 56

56 Rock Mass Rock mass is a matrix consisting of rock material and rock discontinuities. Properties of rock mass therefore are governed by the parameters of rock joints and rock material, as well as boundary conditions. The behaviour of rock changes from continuous elastic for intact rock materials to discontinuous running of highly fractured rock masses, depending mainly on the existence of rock joints. Rock material Rock Mass Rock mass = Rock material + discontinuities Rock Mechanics Fundamentals, Zhou Yingxin -57-

57 The Rock Mass Rock Mechanics Fundamentals, Zhou Yingxin -58-

58 Primary Parameters The primary factors governing the behaviour of a rock mass Joint Parameters Material Parameters Boundary Conditions Number of joint sets Orientation Spacing Aperture Surface roughness Weathering and alteration Compressive strength Modulus of elasticity Groundwater pressure and flow In situ stress The rock mass is divided into structural regions based on major structural features or change in rock type. Each region is classified separately Rock Mechanics Fundamentals, Zhou Yingxin -59-

59 Rock Quality Designation (RQD) RQD is defined as the percentage of intact core pieces longer than 100 mm (4 inches) in the total length of core. Rock Mechanics Fundamentals, Zhou Yingxin -60- RQD is a directionally dependent parameter and its value may change significantly, depending upon the borehole orientation. The use of the volumetric joint count can be quite useful in reducing this directional dependence.

60 Rock Quality Designation (RQD) Rock Mechanics Fundamentals, Zhou Yingxin 61

61 Rock Quality Designation (RQD) RQD = (L1 + L2 + + Ln) / L x 100% (joint frequency) = number of joints / length = n / L Measuring RQD along a scanline Rock Mechanics Fundamentals, Zhou Yingxin 62

62 Rock Quality Designation (RQD) RQD represents the degree of fracturing of the rock mass. It partially reflecting the rock mass quality. RQD Rock Mass Quality < 25 Very poor Poor Fair Good Excellent Rock Mechanics Fundamentals, Zhou Yingxin -63-

63 Rock Mass Rating RMR (Bieniawski) Parameters: 1. Strength of intact rock material: uniaxial compressive strength or point load index; 2. Drill core RQD; 3. Spacing of discontinuities: average spacing of all rock discontinuities; 4. Condition of discontinuities: joint aperture, roughness, joint surface weathering and alteration, infilling; 5. Groundwater conditions: inflow or water pressure. 6. Orientation of discontinuities (when a tunnel is put in the rock) Rock Mechanics Fundamentals, Zhou Yingxin -64-

64 RMR Basic Rating Rock Mechanics Fundamentals, Zhou Yingxin -65- Hoek, Practical Rock Engineering

65 RMR Effects of Joint Orientation Drive direction Drive with dip Drive direction Drive against dip Rock Mechanics Fundamentals, Zhou Yingxin 66

66 Meaning of RMR RMR Ratings < 20 Rock mass class A B C D E Description very good rock good rock fair rock poor rock very poor rock Average stand-up time 20 years for 15 m span 1 year for 10 m span 1 week for 5 m span 10 hours for 2.5 m span 30 minutes for 1 m span Rock mass cohesion (KPa) Rock mass friction angle > < 100 > < 15 Rock Mechanics Fundamentals, Zhou Yingxin -67- Bieniawski, 1989

67 Stand-up Time Based on RMR Rock Mechanics Fundamentals, Zhou Yingxin -68-

68 Guidelines for Rock Support Note that these guidelines have been published for a 10 m span horseshoe shaped tunnel, constructed using drill and blast methods, in a rock mass subjected to a vertical stress < 25 MPa equivalent to a depth below surface of <900 m). Rock Mechanics Fundamentals, Zhou Yingxin -69- Hoek, Practical Rock Engineering,

69 Rock Tunnel Quality Index, Q RQD - Rock Quality Designation. J n - joint set number. J r - joint roughness number. J a - joint alteration number indicating the degree of weathering, alteration and filling. J w = joint water reduction factor. SRF = stress reduction factor. Rock Mechanics Fundamentals, Zhou Yingxin 70

70 Physical Meaning of Q Parameters Block size (RQD/Jn) 2. Inter-block shear strength (Jr/ Ja) 3. Active stress (Jw/SRF) Rock Mechanics Fundamentals, Zhou Yingxin 71

71 Rock Quality Designation, RQD Rock Mechanics Fundamentals, Zhou Yingxin 72

72 Joint set number, Jn Rock Mechanics Fundamentals, Zhou Yingxin 73

73 Joint roughness number, Jr Rock Mechanics Fundamentals, Zhou Yingxin 74

74 Joint Alteration Number, Ja Rock Mechanics Fundamentals, Zhou Yingxin 75

75 Joint water reduction factor, Jw Rock Mechanics Fundamentals, Zhou Yingxin 76

76 Rock Mechanics Fundamentals, Zhou Yingxin 77

77 Q Value and Rock Mass Quality Class Q-value Class Rock mass quality 400 ~ 1000 A Exceptionally Good 100 ~ 400 A Extremely Good 40 ~ 100 A Very Good 10 ~ 40 B Good 4 ~ 10 C Fair 1 ~ 4 D Poor 0.1 ~ 1 E Very Poor 0.01 ~ 0.1 F Extremely Poor ~ 0.01 G Exceptionally Poor Rock Mechanics Fundamentals, Zhou Yingxin 78

78 Rock Mechanics Fundamentals, Zhou Yingxin 79

79 Excavation Support Ratio in Q-system Excavation Support Ratio (ESR) Excavation Category ESR A Temporary mine openings. 3 5 B C D E Permanent mine openings, water tunnels for hydro-electric projects, pilot tunnels, drifts and headings for large excavations. Storage rooms, water treatment plants, minor road and railway tunnels, surge chambers and access tunnels in hydroelectric project. Underground power station caverns, major road and railway tunnels, civil defense chamber, tunnel portals and intersections. Underground nuclear power stations, railway stations, sports and public facilities, underground factories Rock Mechanics Fundamentals, Zhou Yingxin -80-

80 Comments on Rock Mass Classification Most widely used methods are the RMR and Q- system in empirical design. All classification methods have strengths and weaknesses Better to use more than one method for cross checking and confirmation Always check to make sure you are using the most updated one Check for accuracy from quoted sources Rock Mechanics Fundamentals, Zhou Yingxin -81-

81 Rock Mass Properties 82

82 Rock Mass Properties Hoek-Brown empirical strength criteria Rock mass deformation modulus Global strength Rock Mechanics Fundamentals, Zhou Yingxin -83-

83 Hoek-Brown Empirical Criterion As the classic strength theories have been found not to apply to rock materials over a wide range of applied compressive stress conditions, a number of empirical strength criteria have been introduced for practical use. One of the most widely used criteria is the Hoek- Brown criterion for isotropic rock materials and rock masses. Rock Mechanics Fundamentals, Zhou Yingxin 84

84 Hoek-Brown Strength Criterion The generalised Hoek-Brown criterion is: Where 1 and 3 are the max and min principal effective stresses at failure M b, s, and, a are related to the rock material and rock mass properties ci is the uniaxial compressive strength of the intact rock Rock Mechanics Fundamentals, Zhou Yingxin 85 Source: Practical Rock Engineering, Hoek, 2007.

85 Hoek-Brown Criterion Parameters In order to use the H-B criterion for estimating rock mass strength and deformability, the following three properties have to be estimated: Uniaxial compressive strength, ci Value of the H-B constant for intact rock, m i Value of the Geological Strength Index (GSI) for the rock mass Rock Mechanics Fundamentals, Zhou Yingxin 86

86 Hoek-Brown Criterion for Intact Rock The Hoek-Brown criterion for intact rock, as originally proposed: 1 c Where t 3 m i = material constant for intact rock, and ci is the rock material uniaxial strength. They can be calculated from triaxial tests using statistical regression analysis. Rock Mechanics Fundamentals, Zhou Yingxin 87 Source: Practical Rock Engineering, Hoek, 2007.

87 Estimates of Hoek-Brown Parameters Hoek-Brown Failure Criterion 1 / c = 3 / c + (m b 3 / c + s) 0.5 Carbonate rocks - dolomite, limestone, marble Argillaceous rocks - mudstone, siltstone, shale, slate Arenaceous rocks - sandstone, quartzite Fine grained igneous - andesite, dolerite, basalt, rhyolite Coarse metamorphic & igneous - gabbro, gneiss, granite Intact rock material RMR = 100,Q = 500 m i = 7.0 s = 1.0 m i = 10.0 s = 1.0 m i = 15.0 s = 1.0 m i = 17.0 s = 1.0 m i = 25.0 s = 1.0 Very good quality rock mass RMR = 85, Q = 100 m b = 3.5 s = 0.1 m b = 5.0 s = 0.1 m b = 7.5 s = 0.1 m b = 8.5 s = 0.1 m b = 12.5 s = 0.1 Good quality rock mass RMR = 65, Q = 10 m b = 0.7 s = m b = 1.0 s = m b = 1.5 s = m b = 1.7 s = m b = 2.5 s = Fair quality rock mass RMR = 44, Q = 1.0 m b = 0.14 s = m b = 0.20 s = m b = 0.30 s = m b = 0.34 s = m b = 0.50 s = Poor quality rock mass RMR = 23, Q = 0.1 m b = 0.04 s = m b = 0.05 s = m b = 0.08 s = m b = 0.09 s = m b = 0.13 s = Very poor quality rock mass RMR = 3, Q = 0.01 m b = s = 0 m b = 0.01 s = 0 m b = s = 0 m b = s = 0 m b = s = 0 Rock Mechanics Fundamentals, Zhou Yingxin 88

88 The H-B Criterion Applies only to isotropic rock behaviour Rock Mechanics Fundamentals, Zhou Yingxin 89

89 Rock Mass Deformation Modulus Rock mass deformation modulus, E m, cannot be easily obtained from field testing. Empirical equations between Em and rock mass classification based on field testing E m = 25 log10q, for Q > 1 E m = 10 (Q ci /100) 1/3 E m = 2 RMR 100, for RMR > 50 E m = 10 (RMR 10)/40 for 20 < RMR < 85 E m = 10 (15 logq+40)/40 Rock Mechanics Fundamentals, Zhou Yingxin -90-

90 Rock Mass Deformation Modulus Rock Mechanics Fundamentals, Zhou Yingxin -91-

91 Rock Mass Strength Rock Mechanics Fundamentals, Zhou Yingxin -92- Hoek, 2004

92 Failure Mode and Rock Support 93

93 Basic Failure Modes in Tunnels Structurally controlled failure Stress-induced failure Two basic types of failure control the stability of tunnels Rock Mechanics Fundamentals, Zhou Yingxin -94-

94 Structurally controlled failure Rock Mechanics Fundamentals, Zhou Yingxin -95- Hoek, Practical Rock Engineering

95 Rock bolting to support unstable wedges Rock Mechanics Fundamentals, Zhou Yingxin -96- Hoek, Practical Rock Engineering

96 Deformations Around an Advancing Tunnel The deformation and load on the support will be a function of the rock mass strength, the support pressure, and timing of the internal support Rock Mechanics Fundamentals, Zhou Yingxin Hoek, Practical Rock Engineering

97 Interaction Rock Support and Deformation Simplified closed-form analysis of the formation of a plastic zone around a tunnel. In situ stress Assumptions: - Circular tunnel - Hydrostatic pressure Rock Mechanics Fundamentals, Zhou Yingxin Hoek, Practical Rock Engineering

98 Tunnel Closure and Strain Strain = tunnel closure / tunnel diameter Tunnel closure Tunnel diameter Rock Mechanics Fundamentals, Zhou Yingxin

99 Support interaction analysis for tunnels Critical support pressure (before plastic failure): Elastic deformation of tunnel wall (when there is no failure): Rock Mechanics Fundamentals, Zhou Yingxin Hoek, Practical Rock Engineering

100 Use of Yieldable Support Delay in activation of passive support by use of sliding joint. Yieldable support makes this approach more practicable Rock Mechanics Fundamentals, Zhou Yingxin Hoek, Practical Rock Engineering

101 Rock Mechanics Fundamentals, Zhou Yingxin 102 Hoek, Practical Rock Engineering

102 Influence of support pressure on tunnel deformation Rock Mechanics Fundamentals, Zhou Yingxin 103 Hoek, Practical Rock Engineering

103 Geotechnical issues and support requirements for increasing strain levels in tunnels in weak rock Rock Mechanics Fundamentals, Zhou Yingxin 104 Hoek, Practical Rock Engineering

104 Rock Support Rock Mechanics Fundamentals, Zhou Yingxin

105 Rock Support Rock bolts and shotcrete still very much empirical (usually for hard rock) Concrete lining more structurally based with assumed load (usually for soft rock) Water proofing or drainage a major consideration Spiling and fore poling for week grounds Concrete lining only for very week rock or special applications Rock Mechanics Fundamentals, Zhou Yingxin -106-

106 Empirical Design Process Site investigation and Rock mass classification Preliminary design based on dimensions and rock mass quality Tunnel mapping by engineering geologist after excavation Final support design prescribed based on mapped conditions and actual rock mass quality Supported by numerical modelling (if necessary) and instrumentation Rock Mechanics Fundamentals, Zhou Yingxin -107-

107 Q-chart for Support Design Rock Mechanics Fundamentals, Zhou Yingxin -108-

108 Excavation Support Ratio (Q-system) Excavation Category ESR A Temporary mine openings. 3 5 B C D E Permanent mine openings, water tunnels for hydro-electric projects, pilot tunnels, drifts and headings for large excavations. Storage rooms, water treatment plants, minor road and railway tunnels, surge chambers and access tunnels in hydroelectric project. Underground power station caverns, major road and railway tunnels, civil defense chamber, tunnel portals and intersections. Underground nuclear power stations, railway stations, sports and public facilities, underground factories Rock Mechanics Fundamentals, Zhou Yingxin -109-

109 Wall Support Using Q-System a. Select Q w = 5.0Q when Q>10 b. Select Q w = 2.5Q when Q <10 c. Select Q w = 1.0Q when Q <0.1 Rock Mechanics Fundamentals, Zhou Yingxin Barton & Grimstad, 1994

110 Rock Support using Q-System Example: highway tunnel of 20 m span, 10 m high A sandstone rock mass, fractured by 2 joint sets plus random fractures, average RQD is 70%, average joint spacing is 0.11 m, joint surfaces are slightly rough, highly weathered with stains and weathered surface but no clay found on surface, joints are generally in contact with apertures generally less than 1 mm, average rock material uniaxial compressive strength is 85 MPa, the tunnel is to be excavated at 80 m below ground level and the groundwater table is 10 m below the ground surface. Rock Mechanics Fundamentals, Zhou Yingxin

111 Rock Support using Q-System RQD 70% RQD 70 Joint set number 2 sets plus random J n 6 Joint roughness number slightly rough ( rough planar) J r 1.5 Joint alteration number Joint water factor highly weathered only stain, (altered non-softening mineral coating) 70 m water head = 7 kg/cm 2 = 7 bars J a 2 J w 0.5 Stress reduction factor c / 1 = 85/( ) = 39.3 SRF 1 Q (70/6) (1.5/2) (0.5/1) 4.4 Rock Mechanics Fundamentals, Zhou Yingxin

112 Design of Rock Support Ht l =10m 1 Q wall =2.5Q Q=4.4, ESR=1.0, Span=20m, De=20m 3 Rock Mechanics Fundamentals, Zhou Yingxin

113 Rock Support using Q-System Example (b): highway tunnel of 20 m span, 10 m high, in sandstone, with Q=4.4. Roof support requirement from the Q-chart: Bolt spacing at 2.1 m Bolt length of 5 m SFR shotcrete of 7-8 cm Side wall support requirement: Bolting at 2.4 m spacing Bolt length of 3 m Thin shotcrete to cover or no shotcrete Rock Mechanics Fundamentals, Zhou Yingxin

114 Guidelines on Tunnel Geometry Soil Bed rock Rock cover (>1-1.5 span) Arch height (0.2 times span) Span Wall height Rock Mechanics Fundamentals, Zhou Yingxin

115 Guide on Tunnel Geometry Rock cover : should be 1.5 times the span as a rule of thumb, and at least 1 time the span. With favourable horizontal stress, this can be optimised based on numerical calculations Arch height : should be roughly 0.2 times the span for optimum rock stability. Main objective is to ensure roof is in compression, much like an arch bridge Tunnel shape may be designed to optimise stress distribution but may be constrained by blasting operations and usage requirements Rock Mechanics Fundamentals, Zhou Yingxin

116 Tunnel Geometry Inspiration from Ancient Bridge Design The Zhaozhou Bridge in China Built more than 1400 years ago (the Sui Dynasty) Span of 37 m Limestone slabs joined by iron dovetails. Arch height/span ratio = 0.197! Rock Mechanics Fundamentals, Zhou Yingxin Survived at least eight wars, ten major floods and numerous earthquakes

117 What We ve Covered Rock material and rock mass Rock material properties and laboratory testing Joints and shear strengths Rock mass classification Rock mass properties Failure modes and rock support Rock Mechanics Fundamentals, Zhou Yingxin 118

118 Thank you! 119