Excavation Design In Excavation Design In Stratified Stratified And And Jointed RockMassJointed RockMass

Introduction Introduction A stratified host rock mass is a common A stratified host rock mass is a common

feature in mining and civil engineering where feature in mining and civil engineering where excavation in sedimentary rock is attempted.excavation in sedimentary rock is attempted.

Stratified rock (Fig. 2.1a) is defined as Stratified rock (Fig. 2.1a) is defined as composed of a succession of parallel layers composed of a succession of parallel layers whose thickness is small compared with the whose thickness is small compared with the span of the opening. (Obert and Duvall, 1976).span of the opening. (Obert and Duvall, 1976).

There are two principal mechanical There are two principal mechanical properties of bedding planes that are properties of bedding planes that are significant in the context of underground significant in the context of underground projects.projects.

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 of If an opening is excavated in this type of

rock the roof of the excavation will part from rock the roof of the excavation will part from the rock mass due to low tensile strength of the rock mass due to low tensile strength of bedding planes, thus forming the immediate bedding planes, thus forming the immediate roof.roof.

Investigation of immediate roof stability Investigation of immediate roof stability commenced more than a century ago when commenced more than a century ago when Fayol (1885) conducted experiments on a Fayol (1885) conducted experiments on a stack of wood beams spanning a simple stack of wood beams spanning a simple support, simulating the bedded sequence of support, simulating the bedded sequence of roof span.roof span.

By noting the deflection of the lowest beam as By 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 none Fayol demonstrated that at a certain stage none of the added load of an upper beam was carried of the added load of an upper beam was carried by the lowest member. by the lowest member.

The load of the upper beams was transmitted The load of the upper beams was transmitted laterally to the supports, rather than vertically laterally to the supports, rather than vertically as transverse loads to the lower members.as transverse loads to the lower members.

For such a configuration beam theory can be For 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 roof and maximum stresses in the immediate roof as a function of elastic parameters, rock as 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-bedding Goodman (1989) incorporated inter-bedding friction into the beam analysis, thus extending friction into the beam analysis, thus extending the capabilities of this method. the capabilities of this method.

These analyses however are limited to These analyses however are limited to continuous, clamped beams only. continuous, clamped beams only.

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

In practice, stratified rock masses are in most In practice, stratified rock masses are in most cases transected by numerous joints forming a cases transected by numerous joints forming a matrix of individual rock blocks. matrix of individual rock blocks.

Horizontal stratification with vertical jointing one Horizontal stratification with vertical jointing one common case (Fig. 2.1b). common case (Fig. 2.1b).

The analysis of a stratified and jointed roof is The 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 of form analytical solution for the interaction of these blocks. these blocks.

In absence of a closed-form solution, the In absence of a closed-form solution, the practicing engineer/geologist should rely on practicing engineer/geologist should rely on other methods for assessing the stability of the other methods for assessing the stability of the roof.roof.

Observational MethodsObservational Methods Standard engineering design both in Standard engineering design both in

continuous and structurally discontinuous continuous and structurally discontinuous rock is largely based on observational rock is largely based on observational methods known as rock mass classification methods known as rock mass classification methods.methods.

Terzaghi (1946) formulated the first rational Terzaghi (1946) formulated the first rational method of classification by evaluating the method of classification by evaluating the rock loads appropriate to the design of steel rock loads appropriate to the design of steel sets, based on rock mass description. sets, based on rock mass description. Terzaghi’s descriptions are: Terzaghi’s descriptions are:

Intact rock Intact rock contains neither joints nor hair contains neither joints nor hair cracks. Hence, if it breaks, it breaks across cracks. Hence, if it breaks, it breaks across sound rock.sound rock.

Stratified rock Stratified rock consists of individual consists of individual strata with little or no resistance against strata with little or no resistance against separation along the boundaries between separation along the boundaries between thebe weakened by transverse joints. In thebe weakened by transverse joints. In such rock the spalling condition is quite such rock the spalling condition is quite common. strata. The strata may or may not common. strata. The strata may or may not

Moderately jointed rock Moderately jointed rock contains joints and hair contains joints and hair cracks, but the blocks between joints are locally cracks, but the blocks between joints are locally grown together or so intimately interlocked that grown together or so intimately interlocked that vertical walls do not require lateral support. In rocks vertical walls do not require lateral support. In rocks of this type, both spalling and popping conditions of this type, both spalling and popping conditions may be encountered. may be encountered.

Blocky and seamy rock Blocky and seamy rock consists of chemically consists of chemically intact or almost intact rock fragments, which are intact or almost intact rock fragments, which are entirely separated from each other and imperfectly entirely separated from each other and imperfectly interlocked. In such rock, vertical walls may require interlocked. In such rock, vertical walls may require lateral supportlateral support

CrushedCrushed but chemically intact rock has the character but chemically intact rock has the character of crusher run. If most or all of the fragments are as of crusher run. If most or all of the fragments are as small as fine sand grains and no recementation has small as fine sand grains and no recementation has taken place, crushed rock below the water table taken place, crushed rock below the water table exhibits the properties of a water-bearing sand. exhibits the properties of a water-bearing sand.

Squeezing rock Squeezing rock slowly advances into the tunnel slowly advances into the tunnel without perceptible volume increase.without perceptible volume increase.

Swelling rock Swelling rock advances into the tunnel chiefly on advances into the tunnel chiefly on account of expansionaccount of expansion

According to Terzaghi’s According to Terzaghi’s classification for tunnels excavated classification for tunnels excavated in stratified rock the maximum expected overbreak, if no in stratified rock the maximum expected overbreak, if no support is installed, is ranging from 0.25B to 0.5B, where B is support is installed, is ranging from 0.25B to 0.5B, where B is the tunnel span. The lower estimate is assigned to vertically the tunnel span. The lower estimate is assigned to vertically stratified rock (Fig. 2.2b) while the higher is assigned to stratified rock (Fig. 2.2b) while the higher is assigned to horizontally stratified rock (Fig 2.2a). For tunnels excavated horizontally stratified rock (Fig 2.2a). For tunnels excavated in moderately massive jointed rock the maximum expected in moderately massive jointed rock the maximum expected over break is 0.25B. For tunnels excavated in blocky rock over break is 0.25B. For tunnels excavated in blocky rock mass the expected over break ranges from 0.25B to mass the expected over break ranges from 0.25B to 1.1(B+Ht), where Ht is the height of tunnel, pending on the 1.1(B+Ht), where Ht is the height of tunnel, pending on the degree of jointing. This estimate is valid for tunnels at depth degree of jointing. This estimate is valid for tunnels at depth of up to 1.5(B+Ht), for deeper tunnels the expected over of up to 1.5(B+Ht), for deeper tunnels the expected over break is constant at 1.15(B+Ht). Chapter 2 – Stability analbreak is constant at 1.15(B+Ht). Chapter 2 – Stability anal

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

DESIGN OF OPENINGSDESIGN OF OPENINGS

Division of area around Division of area around an an 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

Requirements for Stress Requirements for Stress 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

Design ApproachDesign Approach

• • Analytical solutionAnalytical solution • • Empirical SolutionEmpirical Solution • • Physical ModelingPhysical Modeling • • Numerical ModelingNumerical Modeling

Circular OpeningCircular Opening

Prediction of the stresses and Prediction of the stresses and displacements around a circular opening in displacements around a circular opening in the rock mass at great depth is an the rock mass at great depth is an important problem in geotechnical, important problem in geotechnical, petroleum and mining engineering such as petroleum and mining engineering such as the design of tunnels, boreholes and mine the design of tunnels, boreholes and mine shafts.shafts.

Importance of Elastic Importance of Elastic analysisanalysis

Maximum and minimum stresses on the Maximum and minimum stresses on the boundary of the opening.boundary of the opening.

Boundary displacement induces by the Boundary 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 the The increase in strain energy, and the

dynamic energy released, when an dynamic energy released, when an excavation is generated.excavation is generated.

Design methods for Design methods for single single openingopening

Any opening will be stable if the maximum Any opening will be stable if the maximum stress occurring around the opening is less stress occurring around the opening is less than the failure strength of the rocks than the failure strength of the rocks defined in a failure criterion.defined in a failure criterion.

Simplest method Simplest method of designing is to of designing is to determine what type of opening in what determine what type of opening in what geometry produces the maximum stress geometry produces the maximum stress and compare it withthe failure strength of and compare it withthe failure strength of the rocks. the rocks.

ConclusionConclusion

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