1EXPLICIT CHARACTERISATION AND

INTERACTIVE ANALYSIS

FOR ENGINEERING DESIGN OF ROCK CAVERNS

S C Bandis, J C Sharp, R A MacKean, E A Bacasis

GEO-DESIGN

United Kingdom and Greece

HKIE-HKIP Conference on Planning and Development of Underground Space

HONG KONG, 23-24 SEPTEMBER 2011

The principal elements for the designof caverns are :

Geological interpretation

Engineering characterisation of the rock mass

Rock mass model development

Design analyses

Engineering assessment

2The discontinuous nature of rock masses predicates the use of comprehensive, explicit methods for engineering characterisation and analysis

aiming to provide a realistic model of the intrinsic load bearingresponse and hence the degree of stabilisation required for safeand economic cavern development.

The Distinct Element Method (DEM) provides the most appropriate approach for analysis of discontinuous rock mass behaviour.

The DEM in rock engineering is implemented by the state-of-art software UDEC and 3DEC, developed by Dr Peter Cundall

R

v-

v+

n

t

Simulation of Rock Mass as a Discontinuum

3The rock mass is simulated as an assembly of relatively rigid (rock) blocks in contact across interfaces representing the geological discontinuities.

Residual soil

Weathered

granitic rock

Fresh granitic

rock

4The discontinuum approachis appropriate when the geological structure controls anisotropy and deformation modes of the rock mass.

IT IS APPLICABLE TO MORE THAN 80% OF ROCK MASSES.

The equivalent-continuum approach is appropriate where the density and orientation of jointing are such, that no preferential paths of stress strain responses are present.

IT IS APPLICABLE TO LESS THAN 20% OF ROCK MASSES.

Simulation as equivalent continuum Discontinuum analysis

18m cavern inbedded rock

Pseudo-Continuum vs. Discontinuum

5Comparison between the Ground Reaction Curves derived from theUDEC-BB, FLAC (M-C material behaviour) and FLAC-Ubiquitous Joint analyses.

Pseudo-Continuum vs. Discontinuum

EXPLICIT ROCK MASS MODEL DEVELOPMENT REQUIREMENTS

ROCK MASS

ROCK MATERIALS

ROCK DISCONTINUITIES

Lithological Variability

Structure Geometry

Weathering

(Sets, Orientation)

Types of Discontinuites

Index Properties

Stress-Strain Behaviour

Shear Strength Stiffness Behaviour

Engineering

Characterisation

6EXPLICIT ROCK MASS MODEL DEVELOPMENT

12

Weathering

Zone

Typical

Subhorizontal

Joint Type2

Associated Characteristics

Typical Constituent

Material Grade

Distribution1

Typical Core

Loss (per core

run)

Typical

RQD

range

D SH4 &5I,II & III >85%

IV & V

7ROCK MASS ASSESSMENT

SITE SPECIFIC ROCK MASS CLASSIFICATION

RM03 RM06

RM 07RM02 RM05

RM01 RM04

14

8ROCK MASS CLASS O4

Weathering zone BTypical RQD 90-100%

Typical core loss

9Q 1Q 10

10

Most common strength criteria for rock discontinuities

Barton - Bandis

BARTON BANDIS CONSTITUTIVE MODEL OF COUPLED BEHAVIOUR OF ROCK DISCONTINUITIES

The model enables stress-and scale-dependent coupling of shear stress with shear displacement and dilation, and of normal stress with closure.

The model input data comprise the following joint characterization indices:

JRC=Joint Roughness Coefficient

JCS=Joint Wall Compressive Strength

Lo, Ln=Block size or length of joint

Eo =Mechanical (true) joint aperture or opening

r=Angle of residual friction

11

The rock mass is simulated as an assembly of relatively rigid (rock) blocks in contact across interfaces representing the geological discontinuities.

vj

Kn

Ks

u

UDEC

B-B MODEL

+

=

UDEC - BB

12

Explicit (Discontinuum) Analyses in Cavern Engineering Practice

Design prediction Construction reality

Ideally, the requirements from design analyses are to establish if a rock mass will fail under applied loadings or calculate the displacements required for a system in order to attain a new equilibrium either at the intrinsic or the engineered states.

Example of excavation sequence Intrinsic performance

13

SV2 (major) joint

Extended rotational shearing of the bedded rock due to formation of a large destressed haunch wedge.

Block detachment under rotational shear at the haunch due to shear overstressing of the bedded rock.

Contiguous zones of tensile yielding are evident forming the outer boundaries of the collapsing crown block. The distributions of tensile yield points at the fixities and the centers of the rock layers indicate typical failure mechanism of a laminate composed of simply supported beams

Zone of tensile yield at the rock beam fixities

Example of collapse mechanism of an unsupported 18m span cavern

Material yield intension

INTRINSIC (UNSUPPORTED) CAVERN RESPONSE

EFFECT OF ROCK MASS QUALITY

(Fully Drained State)

GOOD QUALITY RM 01 POOR QUALITY RM 05

25 mm

55 mm

OVERALL STABLE CONDITIONS (EXCLUDING DETACHED WEDGES)

14

Example of groundwater modelling

The inferred patterns represent partial drainage with zero pressure at 1m from theexcavation periphery, gradually reverting to full hydrostatic pressure throughinferred intermediate regimes according to the permeability profile.

Zero groundwater pressure Under groundwater pressure

Extensive shearing along both the SH and SV discontinuities diminishes the flexural stiffness of the rock slabs COLLAPSE STATE

EFFECTS OF GROUNDWATER PRESSURE

POOR QUALITY RM 05

OVERALL STABLE STATE(EXCLUDING DETACHED WEDGES)

INTRINSIC (UNSUPPORTED) CAVERN RESPONSE

15

INTRINSIC (UNSUPPORTED) CAVERN RESPONSE

INFLUENCE OF HORIZONTAL STRESSES GOOD ROCK MASS QUALITY

Ko=4.0Ko=0.7

Enhanced normal stresses across the SV structure mobilize their dilatant shear strength, radial destressing is minimised, normal stresses/shear stiffness across theSH structure maintained de-lamination / slab deflections minimal

12 mm45 mm

Simulation of Excavation Support Sequence

Derivation of induced loadings on rock reinforcementand shotcrete lining support systems

16

Intrinsic vs. Supported excavation

Detailed engineering evaluation of the numerical analyses is an extremely important stage of developing comprehensive detailed designs.

Such an evaluation should also involve a realistic appreciation of the limitations inherent in all computational schemes.

Engineering Evaluation of Numerical Analyses Resultsin Cavern Engineering Practice

17

Temporary Support

Rock Reinforcement

Modelling details of the reinforcement units (notably the typeof bonding and allowable axial strain) are significant.

Typically Safety Factors of 1.5 against tensile failure of bar for temporary units and 2.0 for permanent units are usually considered.

Interaction Diagram

for Reinforced Concrete DesignAccording to BS 8110, Part 1:1997 & Part 3:1985

Input Data: TABLE BAL - Concept I - 20m - K=0.7

SHOTCRETE 350mm - PILLAR (30mm Relaxation)

h = 350 mm Intrados: cover 50 mmb = 1000 mm diameter 10 mm

fcu = 25 N/mm2

spacing 150 mm

fy = 460 N/mm2

As1 = 524 524 mm2

Es = 210000 kN/mm2

Extrados: cover 20 mmdiameter 10 mm

Partial material factors: spacing 150 mmmc = 1,5 concrete As2 = 524 524 mm

2

ms = 1,05 reinforcement

M N d1 = 55 mm169,770 2279 0,260 0,055 d2 = 25 mm144,520 2212 0,253 0,047 d = 295 mm119,290 2205 0,252 0,03975,000 2150 0,246 0,02440,420 2101 0,240 0,01314,390 2086 0,238 0,0050,310 2075 0,237 0,000

18,520 2056 0,235 0,0062,940 2039 0,233 0,0017,690 2002 0,229 0,003

47,280 2008 0,230 0,01560,000 1989 0,227 0,020

0,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,000

Interactive Diagram

-0,1

0

0,1

0,2

0,3

0,4

0,5

0,6

-0,02 -0,01 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09

= [M/fcu.b.h2]

=

[N

/fcu.b

.h]

Concrete Capacity

Including tension reinforcement

Capacity: tension +compression reinforcementSection Forces

Interaction Diagram

for Reinforced Concrete DesignAccording to BS 8110, Part 1:1997 & Part 3:1985

Input Data: TABLE BAL - DESIGN MODEL 20m - K=0.7

SHOTCRETE 350mm + LAG D2 (Ixx=773,45cm4, H=160mm, 2No32mm + 1No25mm) @ 1000mm

HALF TOP HEADING (at Relaxation 30mm)

h = 350 mm Intrados: cover 95 95 mmb = 1000 mm diameter 25 0 mm

fcu = 25 N/mm2

spacing 500 100 mm

fy = 460 N/mm2

As1 = 982 0 982 mm2

Es = 210000 kN/mm2

Extrados: cover 95 95 mmdiameter 32 0 mm

Partial material factors: spacing 1000 100 mmmc = 1,5 concrete As2 = 1608 0 1608 mm

2

ms = 1,05 reinforcement

M N d1 = 95 mm0,000 0,000 d2 = 95 mm0,000 0,000 d = 255 mm0,000 0,0000,000 0,000

66,000 2554 0,292 0,02271,810 2667 0,305 0,02353,880 2343 0,268 0,01830,400 2128 0,243 0,01013,040 2173 0,248 0,00413,530 2088 0,239 0,0045,670 1925 0,220 0,002

12,230 1818 0,208 0,00425,970 1680 0,192 0,0085,680 1588 0,181 0,002

20,300 1507 0,172 0,00746,740 1426 0,163 0,01575,650 1364 0,156 0,02596,970 1233 0,141 0,032

116,630 1176 0,134 0,038136,700 1126 0,129 0,045136,010 952 0,109 0,04498,860 1132 0,129 0,03299,340 1085 0,124 0,032

0,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,0000,000 0,000

Interactive Diagram

-0,2

-0,1

0

0,1

0,2

0,3

0,4

0,5

0,6

-0,02 -0,01 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09

= [M/fcu.b.h2]

=

[N

/fcu.b

.h]

Concrete Capacity

Including tension reinforcement

Capacity: tension +compression reinforcementSection Forces

N-M interaction diagrams for shotcrete line based on UDEC derived data

Temporary Support

Shotcrete Capacity

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Degree of Rock Mass Stabilization

Cavern profile deformation Discontinuity shear overstress

mob

peak

SSR = mobilised shear strength / peak shear strength

dV

S

dV = S /250

for temporary support

dV = S /500

for permanent support

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Ground loading on shotcrete / concrete linings

Rock load estimates are usually based on empirical approaches (andrelated classification schemes) and analytically derived (equivalentcontinuum) ground reaction curves and.

Numerical modelling enables derivations of ground loadings in aninteractive fashion, incorporating the effects of actual deformationsassociated with the prevailing stability mechanisms.

FIGURE 2- K=0,7 : WIL - FINAL LINING 600mm (GWP 2) - DISTRIB. LOADS

UNIVERSITY CAVERN

-100,0

0,0

100,0

200,0

300,0

400,0

30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220

Nodes

kP

a

NORMALSHEARWATER PRESSURE

ROCK LOADING Of CONCRETE LININGS

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CONCLUSIONS

Using comprehensive explicit geological and rock mass characterisation and design analysis methods the following is achievable:

Safely engineered, low risk, design for a range of rock mass conditions and cavern geometries

Optimised engineering design in terms of excavation sequencing, type and quantum of support.

Inappropriate design methods for discontinuous rock massesIncluding those in Hong Kong (re equivalent continuum methods), whilst attractive in terms of simplicity may lead to over- or under- dimensioned (and unsafe) designs.

Due to inevitable uncertainties in geological prediction, a planned

design implementation process, that includes designer

involvement and appropriate expertise, should be treated as an

essential component of a design methodology.

The key elements of the process should include:

Specialist evaluation of geological conditions as excavated.

In-line performance monitoring of key indicators (deformation, stress change, groundwater etc) in order to validate design

predictions.

Design model assessment and review (confirmation) for each construction stage.

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Particular rock engineering challenges facing Hong Kongcavern developments involve the excavation of cavernsbeneath existing high rise structures and the increasinglycomplex architecture of three dimensional space to producemore acceptable underground layouts and environments.

Thank you