This document discusses excavation design in stratified and jointed rock masses. It describes the properties of stratified rock, including low tensile and shear strength along bedding planes. When an opening is excavated, the immediate roof will part from the rock mass. The document reviews various observational methods for assessing roof stability, including rock mass classification pioneered by Terzaghi. It notes the challenges in analyzing a stratified and jointed roof given the lack of a closed-form solution. The document also discusses design approaches such as analytical, empirical and numerical modeling and divides the area around an excavation into near and far fields.

1. Excavation Design InExcavation Design In
StratifiedStratified
AndAnd
Jointed RockMassJointed RockMass

2. IntroductionIntroduction
 A stratified host rock mass is a commonA stratified host rock mass is a common
feature in mining and civil engineeringfeature in mining and civil engineering
where excavation in sedimentary rock iswhere excavation in sedimentary rock is
attempted.attempted.
 Stratified rock (Fig. 2.1a) is defined asStratified rock (Fig. 2.1a) is defined as
composed of a succession of parallel layerscomposed of a succession of parallel layers
whose thickness is small compared withwhose thickness is small compared with
the span of the opening. (Obert and Duvall,the span of the opening. (Obert and Duvall,
1976).1976).
 There are two principal mechanicalThere are two principal mechanical
properties of bedding planes that areproperties of bedding planes that are
significant in the context of undergroundsignificant in the context of underground
projects.projects.

3.  1)1) low to zero tensile strength;low to zero tensile strength;
 2)2) low shear strength.low shear strength.
 If an opening is excavated in this type ofIf an opening is excavated in this type of
rock the roof of the excavation will part fromrock the roof of the excavation will part from
the rock mass due to low tensile strength ofthe rock mass due to low tensile strength of
bedding planes, thus forming the immediatebedding planes, thus forming the immediate
roof.roof.
 Investigation of immediate roof stabilityInvestigation of immediate roof stability
commenced more than a century ago whencommenced more than a century ago when
Fayol (1885) conducted experiments on aFayol (1885) conducted experiments on a
stack of wood beams spanning a simplestack of wood beams spanning a simple
support, simulating the bedded sequence ofsupport, simulating the bedded sequence of
roof span.roof span.

4.  By noting the deflection of the lowest beam asBy noting the deflection of the lowest beam as
successive beams where loaded onto the stack,successive beams where loaded onto the stack,
Fayol demonstrated that at a certain stage noneFayol demonstrated that at a certain stage none
of the added load of an upper beam was carriedof the added load of an upper beam was carried
by the lowest member.by the lowest member.
 The load of the upper beams was transmittedThe load of the upper beams was transmitted
laterally to the supports, rather than verticallylaterally to the supports, rather than vertically
as transverse loads to the lower members.as transverse loads to the lower members.

5.  For such a configuration beam theory can beFor such a configuration beam theory can be
employed to assess deflection, shear stresses,employed to assess deflection, shear stresses,
and maximum stresses in the immediate roofand maximum stresses in the immediate roof
as a function of elastic parameters, rockas a function of elastic parameters, rock
density, and beam geometry (Obert and Duvall,density, and beam geometry (Obert and Duvall,
1976).1976).
 Goodman (1989) incorporated inter-beddingGoodman (1989) incorporated inter-bedding
friction into the beam analysis, thus extendingfriction into the beam analysis, thus extending
the capabilities of this method.the capabilities of this method.
 These analyses however are limited toThese analyses however are limited to
continuous, clamped beams only.continuous, clamped beams only.

6. Figure 2.1. a) Horizontally laminated rock massFigure 2.1. a) Horizontally laminated rock mass
and immediate roof deflection; b) Horizontallyand immediate roof deflection; b) Horizontally
laminated rock mass with vertical joints (afterlaminated rock mass with vertical joints (after
Brown and Brady, 1993)Brown and Brady, 1993)

7.  In practice, stratified rock masses are in mostIn practice, stratified rock masses are in most
cases transected by numerous joints forming acases transected by numerous joints forming a
matrix of individual rock blocks.matrix of individual rock blocks.
 Horizontal stratification with vertical jointing oneHorizontal stratification with vertical jointing one
common case (Fig. 2.1b).common case (Fig. 2.1b).
 The analysis of a stratified and jointed roof isThe analysis of a stratified and jointed roof is
complicated by the fact that there is no closed-complicated by the fact that there is no closed-
form analytical solution for the interaction ofform analytical solution for the interaction of
these blocks.these blocks.
 In absence of a closed-form solution, theIn absence of a closed-form solution, the
practicing engineer/geologist should rely onpracticing engineer/geologist should rely on
other methods for assessing the stability of theother methods for assessing the stability of the
roof.roof.

8. Observational MethodsObservational Methods
 Standard engineering design both inStandard engineering design both in
continuous and structurallycontinuous and structurally
discontinuous rock is largely based ondiscontinuous rock is largely based on
observational methods known as rockobservational methods known as rock
mass classification methods.mass classification methods.
 Terzaghi (1946) formulated the firstTerzaghi (1946) formulated the first
rational method of classification byrational method of classification by
evaluating the rock loads appropriate toevaluating the rock loads appropriate to
the design of steel sets, based on rockthe design of steel sets, based on rock
mass description. Terzaghi’smass description. Terzaghi’s
descriptions are:descriptions are:

9.  Intact rockIntact rock contains neither joints nor haircontains neither joints nor hair
cracks. Hence, if it breaks, it breaks acrosscracks. Hence, if it breaks, it breaks across
sound rock.sound rock.
 Stratified rockStratified rock consists of individualconsists of individual
strata with little or no resistance againststrata with little or no resistance against
separation along the boundaries betweenseparation along the boundaries between
thebe weakened by transverse joints. Inthebe weakened by transverse joints. In
such rock the spalling condition is quitesuch rock the spalling condition is quite
common. strata. The strata may or may notcommon. strata. The strata may or may not

10.  Moderately jointed rockModerately jointed rock contains jointscontains joints
and hair cracks, but the blocks betweenand hair cracks, but the blocks between
joints are locally grown together or sojoints are locally grown together or so
intimately interlocked that vertical walls dointimately interlocked that vertical walls do
not require lateral support. In rocks of thisnot require lateral support. In rocks of this
type, both spalling and popping conditionstype, both spalling and popping conditions
may be encountered.may be encountered.
 Blocky and seamy rockBlocky and seamy rock consists ofconsists of
chemically intact or almost intact rockchemically intact or almost intact rock
fragments, which are entirely separatedfragments, which are entirely separated
from each other and imperfectlyfrom each other and imperfectly
interlocked. In such rock, vertical walls mayinterlocked. In such rock, vertical walls may
require lateral supportrequire lateral support

11.  CrushedCrushed but chemically intact rock has thebut chemically intact rock has the
character of crusher run. If most or all ofcharacter of crusher run. If most or all of
the fragments are as small as fine sandthe fragments are as small as fine sand
grains and no recementation has takengrains and no recementation has taken
place, crushed rock below the water tableplace, crushed rock below the water table
exhibits the properties of a water-bearingexhibits the properties of a water-bearing
sand.sand.
 Squeezing rockSqueezing rock slowly advances into theslowly advances into the
tunnel without perceptible volume increase.tunnel without perceptible volume increase.
 Swelling rockSwelling rock advances into the tunneladvances into the tunnel
chiefly on account of expansionchiefly on account of expansion

12.  According to Terzaghi’sAccording to Terzaghi’s classification for tunnelsclassification for tunnels
excavated in stratified rock the maximum expectedexcavated in stratified rock the maximum expected
overbreak, if no support is installed, is rangingoverbreak, if no support is installed, is ranging
from 0.25B to 0.5B, where B is the tunnel span. Thefrom 0.25B to 0.5B, where B is the tunnel span. The
lower estimate is assigned to vertically stratifiedlower estimate is assigned to vertically stratified
rock (Fig. 2.2b) while the higher is assigned torock (Fig. 2.2b) while the higher is assigned to
horizontally stratified rock (Fig 2.2a). For tunnelshorizontally stratified rock (Fig 2.2a). For tunnels
excavated in moderately massive jointed rock theexcavated in moderately massive jointed rock the
maximum expected over break is 0.25B. Formaximum expected over break is 0.25B. For
tunnels excavated in blocky rock mass thetunnels excavated in blocky rock mass the
expected over break ranges from 0.25B toexpected over break ranges from 0.25B to
1.1(B+Ht), where Ht is the height of tunnel, pending1.1(B+Ht), where Ht is the height of tunnel, pending
on the degree of jointing. This estimate is valid foron the degree of jointing. This estimate is valid for
tunnels at depth of up to 1.5(B+Ht), for deepertunnels at depth of up to 1.5(B+Ht), for deeper
tunnels the expected over break is constant attunnels the expected over break is constant at
1.15(B+Ht). Chapter 2 – Stability anal1.15(B+Ht). Chapter 2 – Stability anal

13. Figure 2.2. Maximum expected overbrake for unsupported
tunnels: a) horizontally stratified rock (top); b) vertically
stratified rock (bottom). From Terzaghi (1946).

14. DESIGN OF OPENINGSDESIGN OF OPENINGS

18. Division of area aroundDivision of area around
anan
excavationexcavation
 Near FieldNear Field
 –– Adjacent to the excavationsAdjacent to the excavations
 –– Area of interest to the designer•Area of interest to the designer•
 Far Field–Far Field–
 -Remote from excavation-Remote from excavation
 -The response of the rock is-The response of the rock is
essentially elasticessentially elastic

19. Requirements forRequirements for
StressStress
Analysis??Analysis??
 Rock Mass PropertyRock Mass Property
 –– Geological DiscontinuitiesGeological Discontinuities
 •• Fracture patterns, Fault zones,Fracture patterns, Fault zones,
Joints, Bedding planes etcJoints, Bedding planes etc
 –– Excavation GeometryExcavation Geometry

20. Design ApproachDesign Approach
 •• Analytical solutionAnalytical solution
 •• Empirical SolutionEmpirical Solution
 •• Physical ModelingPhysical Modeling
 •• Numerical ModelingNumerical Modeling

22. Circular OpeningCircular Opening
 Prediction of the stresses andPrediction of the stresses and
displacements around a circular opening indisplacements around a circular opening in
the rock mass at great depth is anthe rock mass at great depth is an
important problem in geotechnical,important problem in geotechnical,
petroleum and mining engineering such aspetroleum and mining engineering such as
the design of tunnels, boreholes and minethe design of tunnels, boreholes and mine
shafts.shafts.

24. Importance of ElasticImportance of Elastic
analysisanalysis
 Maximum and minimum stresses on theMaximum and minimum stresses on the
boundary of the opening.boundary of the opening.
 Boundary displacement induces by theBoundary displacement induces by the
excavation.excavation.
 Extent of zone of influence.Extent of zone of influence.
 The extent of the overstresses region.The extent of the overstresses region.
 The increase in strain energy, and theThe increase in strain energy, and the
dynamic energy released, when andynamic energy released, when an
excavation is generated.excavation is generated.

27. Design methods forDesign methods for
singlesingle
openingopening
 Any opening will be stable if the maximumAny opening will be stable if the maximum
stress occurring around the opening is lessstress occurring around the opening is less
than the failure strength of the rocksthan the failure strength of the rocks
defined in a failure criterion.defined in a failure criterion.
 Simplest methodSimplest method of designing is toof designing is to
determine what type of opening in whatdetermine what type of opening in what
geometry produces the maximum stressgeometry produces the maximum stress
and compare it withthe failure strength ofand compare it withthe failure strength of
the rocks.the rocks.

30. ConclusionConclusion
 For the EXCAVATION DESIGN OFFor the EXCAVATION DESIGN OF
UNDERGROUND OPENINGS, the knowledge ofUNDERGROUND OPENINGS, the knowledge of
stresses, strength and failure mechanism arestresses, strength and failure mechanism are
important. The idea of the stress concentrationsimportant. The idea of the stress concentrations
and their effects on the surroundings of theand their effects on the surroundings of the
openings helps the design engineers to plan aopenings helps the design engineers to plan a
suitable method of support system. However thesuitable method of support system. However the
knowledge of rockmass properties are still to beknowledge of rockmass properties are still to be
acquired and rockmass classification systems isacquired and rockmass classification systems is
an attempt towards the purposean attempt towards the purpose