rock mechanics and rock cavern design_ice hka

Presentation Title

Keith Kong FICE FIMMM MHKIE CEng RPE(G)1

Rock Mechanics and Rock Cavern Design29 November 2016

16 December 2016Black & Veatch1

2Black & Veatch (HK & SG) involved in Underground Space Development

List of Past 20 years Underground Space Projects Kau Shat Wan Underground Magazine, (1997)1.42 km long tunnel (including adits to caverns), Caverns size: 6.5 m span, 5.5 m high, 20m long and rectangular chamber of 13 m wide x 6.8 m highTai Po Treatment Works Raw and Treated Water Aqueducts, (2001)12 km, 3.8 m dia. of TBM tunnel and 2.5 km drill & blast tunnels with span 3.8 m to 14 m span, 6m heightWest Kowloon Drainage Improvement Tai Hang Tung Storage Scheme, (2004)136 m x 130 m and 9.5 m deep storage tank under the existing rugby pitch and football pitchTsim Sha Tsui East Station - Signal Hill Tunnel (pedestrian subway), (2005)120 m long, 12 m wide x 9.5 m high horse shoe shapedHong Kong West Drainage Tunnel (FS), (2005)10.5 km with tunnel size 6m and 8m, plus 7.9 km of adits with dia. 2.5 m & 3.5m dia.HEC Bowen Road to Kennedy Road Cable Tunnel, (2008)0.23 km tunnel, 2.5 m wide x 2.8 m high horseshoe shaped tunnel and two joint bay caverns 3.3 m wide 4.8m highUnderground Service Reservoir behind The University of Hong Kong Centennial Campus, (2009)Caverns size 15m span & 15m high; tunnel span 8mHappy Valley Underground Stormwater Storage Scheme (2015)The underground storage tank with capacity of 60,000 m under the existing rugby pitch, football pitch and race course.Sai Kung Sewage Treatment Works to Cavern (FS), (current)Process caverns 20m span, 13 15m high.Diamond Hill Service Reservoirs to Cavern (FS), (current)Proposed caverns size 18m x 15mUnderground Drainage and Reservoir System, Singapore (current)The storage volume for the UDRS is expected to be 100 Mm

4Kau Shat Wan Underground Explosives Magazine, 1997

Portals

5

Existing Western Fresh Water and Salt Water Pumping StationNew Salt Water Service Reservoirsin Caverns

New Pipe Gallery

New Fresh Water Service Reservoirs

Historic Building

Underground Service Reservoir behind HKU

6

17.6 m Span Excavation (Header)

7 m (approx.)Underground Service Reservoir behind HKU

7AgendaGround Investigation and Rock ParametersIn-situ Stress ConsiderationsJoint Orientations and EffectsIntact Rock and Rock MassRockmass ClassificationsRock Support / Rock Reinforcement DesignPillar Stability Analysis


8Rock Mechanics is the subject concerned with the response of rock to an applied disturbance, which is considered here as an engineering, i.e. a man-induced disturbance. For a natural disturbance, rock mechanics would apply to the deformation of rocks in a structural geology context, i.e. how the folds, faults, and fractures developed as stresses were applied to the rocks during orogenic and other geological processes.

Soil Mechanics / Geotechnical Engineering is concerned with the engineering behaviours of earth materials (i.e. soils, and weathered rock).Difference of Rock Mechanics and Geotechnical Engineering

9

Rock Mechanics/EngineeringStructural EnineeringStudy InterestsGeology

Ground Investigation and Rock Parameters10


11(a)SuitabilityTo assess the general suitability of the site(b)DesignTo enable an adequate and economic design.(c)ConstructionTo plan the best method of construction; To foresee and provide against difficulties and delays that may arise during construction; andTo explore sources of indigenous materials for use in construction.(d)Effect of ChangeTo determine the changes that may arise in the ground and environmental conditions.

Objective of Ground Investigation (GI)Risk Management

12Geotechnical Risks & Failures ofUnderground Projects

13Water Ingress

Water ingress ~2 to 4 liter/sec

14Ground SubsidenceCollapsed area: 100m by 130m; settlement up to 15m

Elura Mine, NSW, AustraliaSource: http://en.wikipedia.org/wiki/Image:Elura.png#file

15

Chimney Failure by Shear Rupture(Btournay 1995)

16Common Rock Wedge Failure

Wedge Failure

17Squeezing GroundSource: http://www.danieledebernardi.it/professional/my-research

18

High Insitu Stresses Induced Failure(Martin 1997)

Ground Condition Risks19Fookes (1997) study indicated:~50% (confidence) of the anticipated geological model from desk study.~65% (confidence) of the geology should be known if a walkover survey is added to the desk study.95% (confidence) if comprehensive GI works to be done.

20What is Comprehensive GI Works?

21US National Committee on Tunnelling Technology (1984) suggested:1.5 linear metre of borehole per route metre tunnel alignment, and

~3% of cost of tunnelling civil works for ground investigation.

22Ground Investigation Works(GI)


23

GI for Hard Rock OpeningsSource: AGS (HK)

DH(I)DH(V)

24Typical Tests Required to Interpret Design ParametersIn Situ Tests:

SPT, Water absorption test, Packer test, Lugeon tests, Impression packer/BH televiewer

Geophysical surveys: seismic, resistivity, micro-gravity, magnetic, cross-hole shear wave test

In situ modulus: High Pressure Dilatometer or Goodman Jack, etc

In situ stress tests (e.g. Hydraulic Fracturing Test, Flatjack, Overcoring Test, Pressuremeters, High pressure dilatometer)Laboratory Tests:

Index tests, Triaxial shear strength and Oedometer for overburden

Point load, UCS, Young's Modulus, Poisson's ratio, Rock shear test on joints, shear-box test for joint, saw cuts for rock, Modulus of rupture of rock, etc.

Testing for TBM/Machinery selection:-Thin section petrography, Punch test, Rock abrasively test, Brazilian test, Machine Excavation Performance test, Cuttability & Drillability Test

Field and Laboratory Testing

25Rock Tunnel/Cavern Design ParametersGeological model (desk study, GI)Groundwater level, permeability of soil/rock mass (GI, field testing)Insitu Stresses (field testing)Rock Mass Quality (e.g. RMR, Q, GSI) (field mapping, rock cores inspection)Joints orientations, shear strength (c & f), stiffness (field mapping, lab testing, empirical methods)Rock and Rock mass strength, modulus, shear strength (c & f), Poisson's Ratio (field and lab testing, empirical methods)

26Special Techniques ofGI Methods

27Ground Investigation on Remote Site

Use of scaffolding and platform

Air mobilisation

28Inclined Boreholes

29

Horizontal Directional Coring (HDC) Borehole

30HDC

3D-magnetometersand accelerometers to define magnetic and gravity tool face, azimuth and inclination of the borehole

31

Specification of HDC Technique

In-situ Stress Considerations32


33Insitu Stresses FieldRock at depth is subjected to stresses resulting from the weight of the overlying strata and from locked in stresses of tectonic origin. When an opening is excavated in this rock, the stress field is locally disrupted and a new set of stresses are induced in the rock surrounding the opening. (Hoek 2007)

34Insitu Stresses Field Vertical Stresssv = g zWhere: sv is the vertical stressg is the unit weight of the overlying rock andz is the depth below surface

(After Brown and Hoek 1978)

35Insitu Stresses Field Horizontal Stress

Normally, the ratio of the average horizontal stress (sh) to the vertical stress (sv) is denoted by the letter k such that:sh = k sv = k g zk = sh / sv(Hoek et al 2000)(k sv)

36Insitu Stresses Ratio vs Depth

(Brown & Hoek 1978)(k)Depth Below Surface

37Insitu Stresses Ratio vs Depth (Hong Kong)

(Kwong & Wong 2013)

38

Influence of Lithology on the Distribution of Insitu Stresses Field

39

The Influence of Topography on Initial Stresses

(NGI 2015)

40Effects of Insitu Stresses to Openings

Sigma-1 contourk = 3 k = 1

41Effects of Insitu Stresses to OpeningsSigma-3 contourk = 3 k = 1

42

Example of High Insitu Stresses Induced Failure(Martin 1997)

43

(Source: www.niwa.co.nz)

44Field Testing and Measurements of Insitu Stresses

45Field Testing and Measurements ofInsitu StressesMethod :Flat JackHydraulic Fracturing Test including hydraulic tests on pre-existing fracturesOvercoring Test

CSIR / CSIRO cellBorre probe cellUSBMSigra IST

46

Flatjack

pin

47

(PIN SEPARATION)Pin Separation (Deformation)vs Slot Excavation-Time and Flatjack Pressure

48

Application of Flatjack

49Suggested method for deformability determination using a large flat jack techniqueJ. Loureiro-Pinto

International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, Volume 23, Issue 2, April 1986, Pages 133-140

50Hydraulic Fracturing (HF) and Hydraulic Testing of Pre-existing Fractures (HTPF)

51

Hydraulic Fracturing Test

(SINTEF 2005)

52

Straddle Packer and HF Instrument Impression Packer Flowmeter and pressure transducer

53

Flow Rate (litre per min.)Pressure (MPa)HF/HTPF (Time vs Pressure & Flow Rate)

54

Flow Rate (litre per min.)Pressure (MPa)Time (minute)HF/HTPF (Time vs Pressure & Flow Rate)

55International Journal of Rock Mechanics & Mining Sciences 40 (2003) 10111020

,

56Assumptions/Considerations of HF (or HTPF)sv = gravity body force of rock at depthPrincipal stresses orientated at true vertical and horizontalTest at shallow ground (i.e. < 30m) may give a questionable results

57 Overcoring Test

58InstrumentNo of active gaugesMeasuring depthsContinuous loggingBorehole requirementsCSIR Cell12Normally 1050 m; modified versions up to 1000mNo38mm pilot hole, usually 90mm drillhole. Modified versions accept waterCSIRO Cell9 / 12Normally up to 30mYes, via cable38mm pilot hole, usually 150mm drill hole. Problems in water filled holesBorre probe cell9Practiced to 620 m. Tested for 1000mYes, built in datalogger36mm pilot hole, 76mm drillhole.Accepts water-filled holesUSBMNormally 3; modified versions 4Normally 1050 m; modified versions up to 1000mNo38mm pilot hole, usually 90mm drillhole. Modified versions acceptwaterSigra IST3, in two or three levelsUsed to 700 m. Designed for1500mYes, built in datalogger25mm pilot hole, 76mm drillhole.Accepts water-filled holes

List of Overcoring Testing Cell

59

Borre Probe used in the Overcoring Method

(Sjberg et al 2003)


60Video of Overcoring Test (by Sigra IST)

61

International Journal of Rock Mechanics & Mining Sciences 40 (2003) 9991010

62Joints Orientation and Effects


63Joints Orientation vs Openings

UnfavourableFavourable

64Methods:Impression Packer TestBorehole Acoustic and Optical Televiewers Field MappingRock Joint Survey

65

Impression Packer Test

66

Acoustic Televiewer(for filled water borehole)

67

Optical Televiewer(for dry borehole)LED Light

68

Field Rock Joint Survey / MappingUse of geological compass

69 Rock Joint Analysis

70Hemispherical Projection Method (also called Stereo-graphic Projection), there are two projection methods:Use of Hemispherical Projection Method Equal Area ProjectionReducing areal distortion and improving visual estimates of clusters and variabilities.Equal Angel ProjectionWhen performing kinematic analysis, angular relationships and shapes are preserved.

71

Equal Area Projection & Net180270090

30 / 270 (great circle)Pole90 degree

72Equal Angle Projection & Net

30 / 270 (great circle)Pole90 degree

73

Jointing data ofLower Road Slopes

Jointing DataOf Upper RoadJointing data ofLower Road WorksLegend Drillholes Cut slopes Fill slopes Disturbed terrain

Jointing data ofLower Road SlopesRock Joint Analysis Example

74Rock Joint Analysis Example

Pole Plot with contour

75Rock Joint Analysis Example

Rosette Plot

(Tunnel Axis)Favourable orientation (Tunnel Axis) Unfavourable orientation

76Kinematic Identification of Unstable Blocks in Underground Openings

77

Example of Computer Modelling (e.g. UNWEDGE)

78Kinematic Identification of Unstable Blocks(Using Stereo Plot)

Stable Block (Husdon & Harrison 1997)

79Kinematic Identification of Unstable Blocks(Using Stereo Plot)Block Falling

(Husdon & Harrison 1997)

80Kinematic Identification of Unstable Blocks(Using Stereo Plot)Block Sliding

(Husdon & Harrison 1997)

81

Inclined Hemisphere Projections(Husdon & Harrison 1997)

Priest (1985), Hemispherical Projection Methods in Rock Mechanics

82Rock and rockmass


83

Jointed and Weathered RockmassJointed RockmassBlocky RockmassIntact Rockmass

Wedge Failure

84

Relationship of Discontinuities and Rockmasses for Engineered Openings

85 Intact Rock

86

Complete Stress-Strain CurveAxial StressAxial Strain


87Mohr-Coulomb Criterion

88Mohr-Coulomb Criterion

89Generalised Hoek-Brown Criterion

(for intact rock)

90

Hoek-Brown Empirical Failure CriterionFor highly fractured rock, it reduces in value of s (i.e. < 1) and tends towards zero as the strength is reduced from peak to residual.

91

Relationships between major and minor principal stresses for Hoek-Brown and equivalent Mohr-Coulomb criteriaHBMC(Hoek, 2002)

92Rockmass Properties

93If the discontinuity is parallel or perpendicular to the applied loading, it will have no effect on the sample strength. If the discontinuity orientated at some angles, it will significantly reduce the strength of the sample.Strength of Rock with Single Joint

Intact RockMC Model

94

Mohr's Circle - Possible modes of failure for rock containing a single plane of weakness.Circle A represents the case when the failure locus for the discontinuity is just reached, i.e. for a discontinuity at the angle 2bw = 90 + fw. Circle B For a case when failure can occur along the discontinuity for a range of angles, as indicated in the figure.Circle C For the case where the circle touches the intact rock failure locus, i.e. where failure will occur in the intact rock if it has not already done so along the discontinuity.

95Strength of Jointed Rock

(Hoek & Brown 1980)

96

Strength of Jointed RockEach discontinuity would weaken the sample (as discussed in previous slide), but the angular position of the strength minima would not coincide. As a result the rock is weakened in several different directions simultaneously. Hence, heavy jointed material tends to become isotropic in strength, like a granular soil (Hudson & Harrison 1997).

97

Isotropic medium Anisotropic mediumStrength of Jointed RockmassIn most of numerical model, the geomaterials (soil/rock) are considered to be Continuous, Homogeneous, Isotropic and Liner-Elastic (CHILE). However, in reality the geomaterials are Discontinuous, Inhomogeneous, Anisotropic and Non-Elastic (DIANE).[e.g. GSI=30 (or Q=0.1); RQD=25]

98

Deformation Modulus of Rockmass(Different Estimation method)(Hoek & Diederichs 2006)

99Rock Joint Properties

100

Shear Testing of Discontinuitiestp = c + sn tan ftr = c + sn tan fr

101Shear Strength of Rough Surfaces

fb is the basic friction angle of the surface andi is the angle of the saw-tooth face.

102Barton (1990) equations:Where:JRC= joint wall roughness coefficient JCS= joint wall compression strengthsn= normal stress of the blockfb= basic friction angle of rock jointBartons Estimate of Rock Joint Shear Strength

103Joint Wall Roughness, JRC

JRC joint wall roughness, estimation from joint surface profile matching (Barton et. al., 1977)Slickensided or smooth planarRough stepped

104Joint Wall Compressive Strength, JCS

Estimate of joint wall compressive strength (JCS) from Schmidt hardness(after Barton et. al., 1977 and 1985)Bandis et al (1983) suggested: F to SW: (sc / JCS) ~< 1.2MW: 1.2 < (sc / JCS) ~< 2W: (sc / JCS) > 2

105Joint Wall Stiffness (Barton 1972)For a single joint set with an average spacing L, oriented perpendicularly to the direction of loading, the joint normal stffness (kn) is:where Em = rock mass modulus; Ei = intact rock modulus, Gm = rock mass shear modulus; Gi = intact rock shear modulus, L = mean joint spacing.Joint shear stiffness (ks) is:

106 Rockmass Permeability

Water Ingress Assessment for Underground OpeningsReference:

Kong, W.K. 2011. Water Ingress Assessment for Rock Tunnels: A Tool for Risk Planning. Rock Mechanics and Rock Engineering, Volume 44, Number 6, pp. 755-765.

Open access to download: http://link.springer.com/article/10.1007/s00603-011-0163-4?view=classic

107Rockmass classification

108Rockmass ClassificationTerzaghi's rockmass classification (Terzaghi, 1946) Geomechanics Classification or the Rock Mass Rating (RMR) system (Bieniawski, 1976)Rock Tunnelling Quality Index, Q (Barton et al, 1974)Geological strength Index (GSI) (Hoek ,1994)

109Terzaghi's rockmass classification (1/2)Rock classType of RocksDefinitionIHard and intactThe rock is unweathered. It contains neither joints nor hair cracks. If fractured, it breaks across intact rock. After excavation, the rock may have some popping and spalling failures from roof. At high stresses spontaneous and violent spalling of rock slabs may occur from the side or the roof. The unconfined compressive strength is equal to or more than 100 MPa.IIHard stratified and schistoseThe rock is hard and layered. The layers are usually widely separated. The rock may or may not have planes of weakness. In such rocks, spalling is quite common.IIIMassive, moderately jointedA jointed rock, the joints are widely spaced. The joints may or may not be cemented. It may also contain hair cracks but the huge blocks between the joints are intimately interlocked so that vertical walls do not require lateral support. Spalling may occur.IVModerately blocky and seamyJoints are less spaced. Blocks are about 1m in size. The rock may or may not be hard. The joints may or may not be healed but the interlocking is so intimate that no side pressure is exerted or expected.VVery blocky and seamyClosely spaced joints. Block size is less than 1 m. It consists of almost chemically intact rock fragments which are entirely separated from each other and imperfectly interlocked. Some side pressure of low magnitude is expected. Vertical walls may require supports.

110Terzaghi's rockmass classification (2/2)Rock classType of RocksDefinitionVICompletely crushed but chemically intactComprises chemically intact rock having the character of a crusher-run aggregate. There is no interlocking. Considerable side pressure is expected on tunnel supports. The block size could be few centimeters to 30 cm.VIISqueezing rock moderate depthSqueezing is a mechanical process in which the rock advances into the tunnel opening without perceptible increase in volume. Moderate depth is a relative term and could be from 150 to 1000 m.VIIISqueezing rock great depthThe depth may be more than 150 m. The maximum recommended tunnel depth is 1000 m.IXSwelling rockSwelling is associated with volume change and is due to chemical change of the rock, usually in presence of moisture or water. Some shales absorb moisture from air and swell. Rocks containing swelling minerals such as montmorillonite, illite, kaolinite and others can swell and exert heavy pressure on rock supports.

Terzaghi Rock Load16 March 2011111

Support pressure (pv) = g Hp

where g is unit weight of rock

To change Layout: Right click slide thumbnail, select Layout and choose appropriately. Or Select Layout from the Home tab.111

Terzaghi Rock Load112Comments on Terzaghi Rock Load Terzaghis method provides reasonable support pressure for small tunnels (B < 6 m).It provides over-safe estimates for large tunnels and caverns (Diam. 6 to 14 m) andThe estimated support pressure values fall in a very large range for squeezing and swelling ground conditions for a meaningful application.


Terzaghi Rock Load113

(Singh and Goel 2006)


114 Geomechanics Classification(RMR System)

115RMR System [Bieniawski (1973, 1974, 1989)]

Six parameters are used to classify a rock mass using the RMR system:Uniaxial compressive strength of rock materialRock Quality Designation (RQD)Spacing of discontinuitiesCondition of discontinuitiesGroundwater conditionsOrientation of discontinuities.RMR Rating = (1) + (2) + (3) + (4) + (5) + (6)

116

RMR System


117

RMR System Para. (1) : UCS

118Determination of RQD (RMR System)

38 + 17 + 20 + 35

119RMR System Para. (2) : RQD

120RMR System Para. (3) : Joints Spacing

121Chart for correlation between RQD and Joint Spacing

122

RMR System the 6th parameter

123Joints Orientation vs Openings

Unfavourable (RMR rating -12)Favourable (RMR Rating -2)

124

RMR System - Guidelines for excavation and support of 10 m span rock tunnels (After Bieniawski 1989)

RMR Support Pressure125Unal (1983), particularly applicable for flat roof coal mine with span < 10m.

Goel and Jethwa (1991) short-term support pressure for underground openings, but not for rock burst condition.where H = overburden or tunnel depth in meters (50600 m)


126

RMR System (Bieniawski 1989) - Stand-up time vs roof span

127 Q System(Barton et. al. 1974)

128Q-System

where RQD is the Rock Quality DesignationJn is the joint set numberJr is the joint roughness numberJa is the joint alteration numberJw is the joint water reduction factorSRF is the stress reduction factor

129The first quotient (RQD / Jn), representing the structure of the rock mass, is a crude measure of the block size, with the two extreme values (100/0.5 and 10/20).The second quotient (Jr / Ja) represents the roughness and frictional characteristics of the joint walls or filling materials.The third quotient (Jw / SRF) represents the active stress in rock. Jw is a measure of water pressure with the effect on the shear strength of joints due to a reduction in effective normal stress. SRF is a measure of:loosening load in the case of an excavation through shear zones and clay bearing rock,rock stress in competent rock, andsqueezing loads in plastic incompetent rocks.Q-System

130Q value can be ranging from 0.001 to 1000 corresponding to extremely poor to excellent rock conditionsQ-System

131Barton et al (1974) defined an additional parameter which they called the Equivalent Dimension, De:Q-System

Excavation Support Ratio (ESR) determined by:

132Estimate of Q Support Pressure Barton et al. 1974 recommended:(kPa), if no. of joint sets 3(kPa), if no. of joint sets 3Bolt length (m). B is span or height of opening whichever is larger.

2+

133Range of QWallRoof FactoredQwallTemporaryQt wallTemporaryQt roofQ > 105.0 x Q5 x 5 x Q5.0 x Q0.1 < Q < 102.5 x Q5 x 2.5 x Q5.0 x QQ < 0.11.0 x Q5 x 1.0 x Q5.0 x Q

Q-value Adjustment for Tunnel Wall and Temporary Conditions of OpeningFor temporary case (< 1 year), ESRtemp = ESR x 1.5

134

Q Design Chart (NGI, 2015)

Bolt spacing based on f20mm dia. but design working load ???100kN?

135 Geological Strength Index (GSI) (Hoek 1994)

136Geological Strength Index (GSI)Heterogeneous Rock Mass

137GSI for Jointed Blocky Rock Mass

(Hoek et al 2013)

138Determination of RQD

38 + 17 + 20 + 35

JCond89 [RMR (Bieniawski, 1989)]

139Estimation of Deformation Modulus of Rockmass by GSI

(Hoek & Diederichs 2006)

modulus ratioDisturbancefactor

140

Guidelines for the selection of modulus ratio (MR) values in Eq. based on Deere (1968) and Palmstrom & Singh (2004)

141

Guidelines for estimating disturbance factor D (Hoek et al 2002)

142Correlation between SystemsRMR = 9 In Q + 44 (Bieniawski, 1989)RMR = 15 log Q + 50 (Barton 1995)GSI = RMR89 5 (Hoek & Brown 1997)Warning: The Q-system and the RMR system include different parameters and therefore cannot be strictly correlated. Palmstrm & Stille (2010), the relationship has an inaccuracy of 50% or more.

143Rock Support and Rock Reinforcement Design

144 Roof Arch Depth

145Leontovich (1959) gives solution for arches with raise-to-span ratio (r/Span) ranging from 0 to 0.6 for which the recommended assumptions for loading of such arches are believed to be safe:

For low rise arches (r/Span) = 0.2 or less, a uniform load may be assumed.

For higher rise arches (r/Span) > 0.2, a dead load consisting of uniform plus complementary parabolic loading (similar to Terzaghis rock load) may be assumed..Principle of Roof Arch Depth

Generic Structural Arch Beam Formula146

Symmetrical Three-Hinged Arches of any Depth(Milkhelson 2004)


147

Two-Hinged Parabolic Arches(Milkhelson 2004)Generic Structural Arch Beam Formula


148 Estimate Rock Support Force forRock Tunnels and Caverns

Estimation of Design Rock Load149Terzaghi Rock Load (Terzaghi 1946, Deere et al. 1970; Singh et al. 1995)RMR Support Pressure (Bieniawski 1984, Unal 1983, Goel and Jethwa 1991)Q Support Pressure (Barton et al. 1974, 1975, Grimstad and Barton 1993)Elastic Solutions (e.g. Kirsch Equations) for circular openingNumerical Modelling (e.g. FLAC, UDEC, PHASE2)



Support pressure (pv) = g Hp

where g is unit weight of rock



(Singh and Goel 2006)


Terzaghi Rock Load152Comments on Terzaghi Rock Load Terzaghis method provides reasonable support pressure for small tunnels (B < 6 m).It provides over-safe estimates for large tunnels and caverns (Diam. 6 to 14 m) andThe estimated support pressure values fall in a very large range for squeezing and swelling ground conditions for a meaningful application.


Comments on Terzaghi Rock Load (cont)153Barton et al. (1974) and Verman (1993) suggested that the support pressure is independent of opening width in rock tunnels. Goel et al. (1996) also found that there is a negligible effect of tunnel size on support pressure in non-squeezing ground conditions, but the tunnel size could have considerable influence on the support pressure in squeezing ground condition.


RMR Support Pressure154Unal (1983) for flat roof coal mine

Goel and Jethwa (1991) short-term support pressure for underground openings, but not for rock burst conditionwhere H = overburden or tunnel depth in meters (50600 m)


Q Support Pressure155Barton et al. 1974 recommended:(kPa), if no. of joint sets 3(kPa), if no. of joint sets 3


Elastic Solutions (e.g. Kirsch Equations for circular opening)156


157

Numerical Modelling Computer ProgrammesUDEC, 3DECPHASE2, RS3FLAC, FLAC3D

158 Q-system ApproachRock Support Design

159Q-system Design Approach: Determine an opening size (height, span)Determine Excavation Support Ratio (ESR)Tunnel/Cavern Support Design in Rock

160Calculate roof and wall supporting stress for different Q-valuesQ-system Design Approach (cont): (kPa), if no. of joint sets 3(kPa), if no. of joint sets 3Range of QWallRoof FactoredQwallTemporaryQt wallTemporaryQt roofQ > 105.0 x Q5 x 5 x Q5.0 x Q0.1 < Q < 102.5 x Q5 x 2.5 x Q5.0 x QQ < 0.11.0 x Q5 x 1.0 x Q5.0 x Q

1614) Determine bolt length, bolt force and bolt spacingQ-system Design Approach (cont): Bolt length (m). B is span or height of opening whichever is larger. For temporary case (< 1yr), ESRtemp = ESR x 1.5Design Working Load of Bolt 0.5 x characteristic yield strength of bolt (e.g. BS 8081)Bolt Spacing = (Support pressure / Working Load of Bolt)0.5

2+

162Q-system Design Approach (cont):

163 Rock Reinforcement Design(extracted from Kong & Garshol 2015)

The concept is based on improving the strength of the rockmass at the tunnel walls by application of confining pressure via the bolts.What is Rock Reinforcement for Underground Opening in Hard Rock164

(Bischoff and Smart, 1975)


How does it work of Rock Reinforcement?165

Photoelastic stress pattern of bolting Langs (1961) findings:


166

How does it work of Rock Reinforcement?(excerpted from Hoek, 2007)


How does it work of Rock Reinforcement?167Theoretical zone of compression by bolting (Hoek, 2007)

where, L 2 to 3 S; and S < 3a


Concept of Reinforcement of Rock Arch168

Where: c is the unconfined compressive strength of rockmass t is the tensile strength of rockmass (by consideration of MC criterion) Fb is provided bolt force

(in half span tunnel, kN)(Bischoff and Smart, 1975)


SSR The shotcrete liner is designed as a structural liner to support a failure wedge occurred between bolts. Detailed study on the SSR and structural shotcrete liner design has been carried out by number of researchers, they are:Fernandez-Delgado et al (1981)Holmgren (1987)Vandewalle (1992)Barrett & McCreath (1995)Morton et al (2009)Uotinen (2011) compatible with EurocodesKong & Garshol (2015)

Shotcrete-Rock-Reinforcement (SRR)169


Shotcrete-Rock-Reinforcement (SRR)170

Potential Unstable WedgeShotcrete LinerRockbolt Baseplate


Failure modes of SSR171

Adhesion FailureShear FailurePunching FailureFlexure Failure[modified from Barrett and McCreath (1995)]

unstable wedge between bolts, 45 projected from the base plate of the rock bolt (for critical case)


172Determining Adhesive Failure of SSRBased on Barrett and McCreath (1995), and Uotinen (2011)

Barrett and McCreath (1995) carried out back-calculations to reveal that the required adhesion strength for high grade shotcrete in hard rock was typically 0.5 MPa, and a minimum of 30 mm conservative bond width may be used in the design. If the adhesion strength is unknown, 0.4 MPa may be used for a conservative case. The Rad should be designed strong enough to retain a potential rock wedge forming in between rock bolts.Where:fak is the adhesion strength (bond strength in MPa) S is the perimeter of the load to be supported (i.e. bolt spacing in metres) b is width (in metres) of the adhesion area (if unknown, 30 mm may be used) c is the partial safety factor for concrete (BS EN 1992-1-1:2004 s. 2.4.2.4)

173Bending Capacity of Shotcrete Liner

Where:flex is the pure bending tensile strength [see Eq. (3.23) of BS EN 1992-1-1:2004] t is the shotcrete thickness (m)Designed Moment of Shotcrete LinerWhere:w is the contributed load of failure wedge (kN) S is the bolt spacing (m) c is width of the faceplates (m)Cflex > MoDetermining Flexure Failure of SSRBased on Barrett and McCreath (1995), and Uotinen (2011)

174Shear Capacity of Shotcrete LinerDesigned Shear Failure of Shotcrete Liner

Where: fctm is the shear (or tensile) strength (in MPa) of shotcrete grade S is the perimeter of the load to be supported (i.e. bolt spacing in metre) t is the thickness of the shotcrete layer (m) c is the partial safety factor for concrete (BS EN 1992-1-1:2004 s. 2.4.2.4)Where: w is the contributed load of failure wedge (kN) S is the bolt spacing (m)Rvd > RsdDetermining Shear Failure of SSRBased on Barrett and McCreath (1995), and Uotinen (2011)

175Punching Shear Resistance of Unreinforced ShotcreteDesigned Punching Shear acting on Shotcrete LinerIt may follow the requirements as stipulated in Section 6.4.4(1) of BS EN 1992-1-1:2004, to determine punching shear resistance, VRd,cWhere: w is the contributed load of failure wedge (kN) S is the bolt spacing (m)c is width of the faceplates (m)V =w (S c)VRd,c > VDetermining Punching Shear Failure of SSRBased on Barrett and McCreath (1995), and Uotinen (2011)

Suggested Wedge Height176

Swedge height


177

Rockbolt Spacing vs Shotcrete Thickness(after Kong & Garsholo, 2015)Workable thickness to be 25 mmSuggested min. thk.

178GSI = 40 (~Q=0.4), Joint Sets = 3.5 nos, UCS = 70 MPaExcavation Span = 24m, Rockbolt spacing = 1.0m SRR Approach Based on Q-systemSRR Vs Q-system

X

179Whats wrong of the models?

After rockbolts were installed, no joint networks should be added in the Zone of Compression to assess structural response of shotcrete lining. This zone is treated to become a continuous media and isotropic in strength.

Whatever it is PHASE2 or UDEC model, the failure wedge size is not true (Kong et al 2016), and hence the shear force given by the model is a reference value only (i.e. not a true value).

180Comparison with Q-system and Rock Reinforcement

RR(Lang, 1961; Bischoff & Smart, 1975)Q-SystemBolt lengthDepends on:rock strength,block size of rockmass,bolt spacingBolt force

Depends on:Q valueOpening sizeExcavation Support Ration(ESR)

L = 2 + (0.15B/ESR)Stablisation of individual failure wedgeNot consideredNot consideredRockmass strength < 25 MPaApplication QuestionableApplicable QuestionableShotcrete linerStructural liner against failure wedge occurred between boltsPrescribed thickness based on past experience.

181Pillar Stability Analysis

182

Surface Crown PillarPillarCrown Pillar

183Failure modes of pillar

Spalling FailureBearing FailureBuckling FailureFailure along JointsLateral bulgingShear Failure(Brady & Brown, 1985)

184Pillar Strength EstimationPillar Strength (Salamon and Munro, 1967):where sp (MPa) is the pillar strength, K (MPa) is the strength of a unit volumeof rockW and H are the pillar width and heightin metresFor square pillarwhere We = effective pillar width (m) Ap = cross-section area of pillar (m)Rp = pillar circuference (m)Wagner (1980) and Stacey & Page (1986) proposed:For rectangular pillar

185

(Maybee, 2000)List of Pillar Strength Estimation Methods

186Pillar Stress Determination Pillar Stress:Where:a and b is the pillar dimensionc is extraction widthH is the total depth of rock above pillarg is the unit weight of rock

187Pillar StabilityNumerical modelling (e.g. PHASE2) is able to give a FoS of Pillar Stability.Where: sp is pillar strength ss is pillar stressSuggested FOS for Pillar Stability:1.6Salamon and Munro (1967) (coal mines pillar study)>1.5Hoek & Brown (1980), after Salamon and Munro (1967)1.4Lunder and Pakalnis (1997), and width to height ratios of up to 1.51.4 Martin & Maybee (2000)Pillar Stability:

188 Crown Pillar

189

Flexure Failure

Punching Shear FailureDirect Shear FailureStress Induced FailureJoints FailureCrown Pillar Failure Modes

190Crown Pillar Safe Span EstimationConsiders as Fixed End Roof Beams (Adler and Sun, 1968):Where: S Safe Roof Span (m)R Modulus of Rupture of Rock (MPa)(rock testing refers to ASTM C99/C99M-2015)d Thickness of Roof BeamF Factor of Safety (from 4 to 8)Numerical modelling to verify overall stability of caverns including crown pillar is required under different load cases and the influences of insitu stresses.

191Design Chart of Safe Span vs Roof Beam Thickness (Adler and Sun, 1968)

R=6R=4R=8R=10R=2(e.g. Granite)

16 December 2016192Black & Veatch

www.bv.com

193Thank You

Question ?

rock mechanics and rock cavern design_ice hka

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