ROCK SLOPE ENGINEERING

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ROCK MASSROCK MASSRock mass is a non-homogeneous, anisotropic and discontinuous medium ; often it is a pre-stressed mass

ROCK MECHANICSROCK MECHANICS

Rock mechanics is defined as “ the theoretical and applied science of mechanical behavior of rock; it is that branch of mechanics concerned with the response of rock to the force field of its physical environment “

- As per ASEG (American Society of Engineering Geology )

Applications of Rock MechanicsApplications of Rock Mechanics

Rock mechanics is primarily applied in- Civil Engineering- Mining Engineering- Petroleum Engineering

CIVIL ENGINEERING CIVIL ENGINEERING APPLICATIONSAPPLICATIONS

The Civil Engineer is mainly concerned with

• Competency of the rock mass to carry the loads of the structures built on it

• Stability of the excavations undertaken involving a rock mass, whether surface or underground

STRENGTH OF ROCKSTRENGTH OF ROCK

• Rock should be classified and its strength to be assessed in its simple state of existence i.e unconfined condition

• Rock can be either “intact “ or “ jointed”• Parameters for assessing the rock

strength/stability In-situ stress/confining conditions Environmental factors eg. seepage

pressure etc.

INTACT ROCK Vs ROCK MASSINTACT ROCK Vs ROCK MASS

INTACT ROCK• No through going fractures

ROCK MASS• Intact rock + Discontinuities

INTACT ROCK Vs ROCK MASSINTACT ROCK Vs ROCK MASS(Contd.)(Contd.)Discontinuity• Joints• Fractures• Faults• Shear zones

Makes the rock discontinuous Makes the rock anisotropic Makes the rock stress dependent

FACTORS AFFECTING ROCK FACTORS AFFECTING ROCK STRENGTHSTRENGTH• Nature of discontinuities• Location of discontinuities• Orientation of discontinuities

Deformability Strength Permeability

ROCK MASS DESCRIPTIONROCK MASS DESCRIPTION

MASSIVE ROCK• Rock mass with few discontinuities• Excavation dimension < discontinuity spacingBLOCKY/JOINTED ROCK • Rock mass with moderate number of

discontinuities• Excavation dimension > discontinuity spacingHEAVILY JOINTED ROCK • Rock mass with a large number of discontinuities• Excavation dimension >> discontinuity spacing

DISCONTINUITY PARAMETERSDISCONTINUITY PARAMETERS

Spacing & frequencyOrientation & dip/dip directionPersistence, size & shapeRoughnessApertureDiscontinuity setsBlock size

AFFECT OF DISCONTINUITIESAFFECT OF DISCONTINUITIES

Permeability – GroutingBlast designStability of slopes

ROCK CLASSIFICATIONROCK CLASSIFICATION

Intact rocks are classified on the basis of

i) Uni axial compressive strength (UCS)ii)Modulus of deformationiii)Modulus ratioRocks are classified as of very high

strength, high strength, medium strength, low strength and very low strength based on the above classifications

ROCK MASS ROCK MASS CLASSIFICATIONSCLASSIFICATIONSTerzaghi’s in situ rock

classification◦Intact◦Stratified◦Moderately jointed◦Blocky and seamy◦Crushed◦Squeezing◦Swelling

ROCK MASS ROCK MASS CLASSIFICATIONSCLASSIFICATIONS(Contd.)(Contd.)Rock quality designation (RQD)

◦Very Poor ( 0-25)◦Poor (25-50)◦Fair (50-75)◦Good (75-90)◦Excellent (90-100)RQD , expressed as % , is the

summation of all the cores larger than 10 cm from the preferred 150 cm drilled core run.

RQD = 110.4 -3.68 Jn

ROCK MASS ROCK MASS CLASSIFICATIONSCLASSIFICATIONS(Contd.)(Contd.)Geomechanics classification

(RMR)Based on the following parametersi) UCS of Intact material/rockii)RQDiii)Spacing of discontinuitiesiv)Condition of discontinuitiesv)Ground water conditionvi)Orientation of discontinuities

ROCK MASS ROCK MASS CLASSIFICATIONSCLASSIFICATIONS(Contd.)(Contd.)Q-System (Norwegian geo.tech

classification)Based on i) RQD ii) No.of joint setsiii) Joint Roughness iv) Degree of

alteration or filling v) Water inflow vi) stress condition.

Based on these parameters Q is expressed as (RQD/Js)x (Jr/Ja) x (Jw/SRF)

ROCK MASS ROCK MASS CLASSIFICATIONSCLASSIFICATIONS(Contd.)(Contd.)RMR (Rock mass rating ) proposed

by Bieniawsky (1973,19888 & 1993) based on shear strength parameters (Cohesion c and Ф (angle of friction) ). The rock mass is classified at five levels.

ROCK QUALITYROCK QUALITY

(Massive Rock) (Jointed Rock)

(Massive Rock) (Heavily Jointed Rock)

ROCK QUALITYQ=100(Massive Rock)• Rock mass with few discontinuity

• Excavation dimension< discontinuity spacing

Q=3 (Jointed Or blocky Rock)

• Rock mass with moderate nos. of discontinuity

• Excavation dimension> discontinuity spacing

Q=0.1(Heavily Jointed Rock)

• Rock mass with large nos. of discontinuity

• Excavation dimension >> discontinuity spacing.

SLOPE FAILURE MECHANISIM

CONTINUUM MANY CONTINUTIES WEAK ROCK EFFECTIVELY CONTINUUM

DISCONTINUUM FEW CONTINUTIES STRONG ROCK DISCONTINUUM

TRANSITION FROM INTACT ROCK TO HEAVILY JOINTED MASS

IDEALISE ILLUSTRATION OF TRANSITION FROM INTACT ROCK TO HEAVILY JOINTED MASS WITH INCREASING SAMPLE SIZE (AFTER HOEK AND BRON,1980)

JOINTS PROBLEM IN CIVIL ENGINEERING ROAD CUTTING ARE CONSTRUCTED, WHERE POSSIBLE AT RIGHT ANGLE

TO STRIKE OF THE MAIN, GENTLY TO MODERATLY DIPPING PLANES OF WEAKNESS IN THE ROCK.

JOINTS PROBLEM IN CIVIL ENGINEERING (Cont.) RESERVOIR AND DAMS: THE AXIS OF THE DAM SHOULD BE

CONSTRUCTED PARALAL TO MAIN FACTURE SET AND THE LATER SLOULD DIP UPSTREAM TOWARDS THE RESERVOIR.

ROCK ENGINEERING

Rock “Material”-STRONG, Stiff, Brittle WEAK ROCK> STRONG CONCRETE. STRONG IN COMPRERSSION, WEAK IN TENSION POST PEAK STRENGTH IS LOW UNLESS CONFINED.

Rock “Mass”-behavior controlled by discomfitures. ROCK MASS STRENGTH IS ½ TO 1/10 OF ROCK

STREGTH.

Discontinuities give rock masses scale effects

ROCK ENGINEERING (Contd.)

ROCK STRESSES IN SITUVERTICAL STRESS ≈ WEIGHT OF OVERLYING

ROCK ~27 KPA / M => 35.7 MPA AT 1300 M

HORIZONTAL STRESS CONTROLLED BY TECTONIC FORCES (BUILDS STRESSES) & CREEP (RELAXES STRESSES)

AT DEPTH, σv ≈ σh UNLESS THERE ARE ACTIVE TECTONIC FORCES

ROCK SLOPESROCK SLOPES

• SLOPES CAN BENATURAL SLOPESMAN-MADE OR CUT SLOPES

EXCAVATED FOR ENGINEERING CONSTRUCTION LIKE

BUILDINGS, ROADWAYS, WATER WAYS, WATER RESOURCES/HYDRO-ELECTRIC PROJECTS ETC.

OPEN CAST MINING

FAILURE OF SLOPESFAILURE OF SLOPES

• Failure of natural slopes is common geological phenomenon

• Reasons for failure areImbalance between shear strength and

the shear stress in the ground/rock massFailure can be either slow-time

dependent process or by extraneous factors in an abrupt manner.

• The extraneous factors can be Increased shear stresses due to surface

loadingsseepage pressure due to built up of

hydro static pressure.

SLOPE FAILURE MECHANISMSSLOPE FAILURE MECHANISMS

• Rock falls due to dislocation of blocks (Mostly occur on steep slopes - with out sliding)

• Fractures/weathered rock slope fails along a curved surface with a rotational slide

• Translational slide to be planar along weak bedding/shear plane/fault zone

• Movement initiates rotational slides where as imbalance in forces results in translational movements.

MODES OF FAILURESMODES OF FAILURES

Rotational or circular/arc failureSliding /planar failureWedge failureToppling failure (without sliding

mechanism)Buckling failure

In any open/surface rock excavation, one mode or a combination of several modes of failures can occur

FAILURE MECHANISMS HAVE GENERALLY FAILURE MECHANISMS HAVE GENERALLY BEEN DESCRIBED AND ANALYZED IN TWO BEEN DESCRIBED AND ANALYZED IN TWO DIMENSIONSDIMENSIONS

MAIN LANDSLIP TYPESMAIN LANDSLIP TYPES

SLOPE MASS RATINGSLOPE MASS RATINGA practical approach proposed by

ROMANA (1985) to evaluate the slope stability. SMR is expressed as

SMR = RMRbasic – (F1,F2,F3) + F4WhereRMR basic is evaluated according to

Bieniawsky (1979,1989)F1= Square (1-Sin A)Where A denotes angle between

strikes of slope face and that of the joints

SLOPE MASS RATING SLOPE MASS RATING (Contd.)(Contd.)F2 = Tan (βj) where βj is the joint dip

angle in planar failure mode.

Both F1 and F2 vary from 0.15 to 1.0. For toppling mode of failure value of F2 becomes 1.0

F3 = Measure of relationship between the slope face and joint dips.In planar failure mode F3 refers to the probabilty of joints day-lighting in the slope face.

SLOPE MASS RATING SLOPE MASS RATING (Contd.)(Contd.)F4 pertains to adjustment for the

method of excavation. Values of F4 are as follows:

i) Natural slope + 15ii)Pre splitting + 10iii)Smooth blasting + 8iv)Normal blasting or mechanical excavation 0v)Poor blasting - 8SMR value ranges from 0 to 100

FOUNDATIONS ON ROCK

STABILITY OF SLIDING BLOCK RELATED TO DIP OF SLIDING SURFACE.

POTENTIAL FAILURE PATH

ANALYSIS OF ROCK SLOPESANALYSIS OF ROCK SLOPES

Rock mass or the proposed slope needs to be analyzed for the possible mode of failure

Rock slopes can be analyzed byConventional limit equilibrium methods/closed

form solutionsNumerical approximation methods

• Discrete element methods• Finite element methods

SLOPE STABILITY / PROTECTIONSLOPE STABILITY / PROTECTION

• Decreasing the seepage pressure • Flattening the rock slope as much as

possible• Reducing the height of slope/excavation

depth (may not be possible in some situations)

• Rock support measuresRock bolts/rock anchors/soil nailingShotcrete with or without wire mesh

(mainly to improve/protect the surface stability)

SLOPE STABILITY / PROTECTIONSLOPE STABILITY / PROTECTION(Contd.)(Contd.)

Toe protection – Retaining walls/butress with weep holes

Tree plantation/grass turfing Catch water drains Nailing wire mesh/geo-grids ( in to steep

slopes)Drainage gallery behind toe ( in special

circumstances)

Rock Slope Stability Problems Rock Slope Stability Problems In HimalayasIn Himalayas

Slope Protection Using Soldier Slope Protection Using Soldier Piles/Shotcrete Retaining WallPiles/Shotcrete Retaining Wall

Slope Protection Using Soldier Slope Protection Using Soldier Piles/Shotcrete Retaining WallPiles/Shotcrete Retaining Wall

Use of Soil Nail in Slope Use of Soil Nail in Slope ProtectionProtection

Use of Soil Nail in Slope Use of Soil Nail in Slope ProtectionProtection

Use of Soil Nail in Slope Use of Soil Nail in Slope ProtectionProtection

Use of Rock Anchors/Shotcrete in Use of Rock Anchors/Shotcrete in Slope ProtectionSlope Protection

POINT TO PONDER“…… Care has to be taken that

the design is driven by sound geological reasons and rigorous Engineering Logic rather by the very attractive images that appear on the Computer screen.”

by Hoek, 1999

POINT TO PONDERPOINT TO PONDER