ROCK SLOPE ENGINEERING www.powerpointpresentationon.blogspot.com www.powerpointpresentationon.blogspot.com

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

ROCK 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 Mechanics Rock mechanics is primarily applied in Civil Engineering Mining Engineering Petroleum Engineering

CIVIL ENGINEERING APPLICATIONS 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 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 MASS No through going fractures ROCK MASS Intact rock + Discontinuities

INTACT ROCK Vs ROCK MASS (Contd.) Discontinuity Joints Fractures Faults Shear zones Makes the rock discontinuous Makes the rock anisotropic Makes the rock stress dependent

FACTORS AFFECTING ROCK STRENGTH Nature of discontinuities Location of discontinuities Orientation of discontinuities Deformability Strength Permeability

ROCK MASS DESCRIPTION MASSIVE ROCK Rock mass with few discontinuities Excavation dimension < discontinuity spacing BLOCKY/JOINTED ROCK Rock mass with moderate number of discontinuities Excavation dimension > discontinuity spacing HEAVILY JOINTED ROCK Rock mass with a large number of discontinuities Excavation dimension >> discontinuity spacing

DISCONTINUITY PARAMETERS Spacing & frequency Orientation & dip/dip direction Persistence, size & shape Roughness Aperture Discontinuity sets Block size

AFFECT OF DISCONTINUITIES Permeability – Grouting Blast design Stability of slopes

ROCK CLASSIFICATION Intact rocks are classified on the basis of Uni axial compressive strength (UCS) Modulus of deformation Modulus ratio Rocks are classified as of very high strength, high strength, medium strength, low strength and very low strength based on the above classifications

ROCK MASS CLASSIFICATIONS Terzaghi’s in situ rock classification Intact Stratified Moderately jointed Blocky and seamy Crushed Squeezing Swelling

ROCK MASS CLASSIFICATIONS (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 CLASSIFICATIONS (Contd.) Geomechanics classification (RMR) Based on the following parameters UCS of Intact material/rock RQD Spacing of discontinuities Condition of discontinuities Ground water condition Orientation of discontinuities

ROCK MASS CLASSIFICATIONS (Contd.) Q-System (Norwegian geo.tech classification) Based on i) RQD ii) No.of joint sets iii) 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 CLASSIFICATIONS (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 QUALITY (Massive Rock) (Jointed Rock) (Massive Rock) (Heavily Jointed Rock)

ROCK QUALITY Q=100(Massive Rock) Q=3 (Jointed Or blocky 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

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 “Material”-STRONG, Stiff, Brittle 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 SITU VERTICAL 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 SLOPES SLOPES CAN BE NATURAL SLOPES MAN-MADE OR CUT SLOPES EXCAVATED FOR ENGINEERING CONSTRUCTION LIKE BUILDINGS, ROADWAYS, WATER WAYS, WATER RESOURCES/HYDRO-ELECTRIC PROJECTS ETC. OPEN CAST MINING

FAILURE OF SLOPES The extraneous factors can be Failure of natural slopes is common geological phenomenon Reasons for failure are Imbalance between shear strength and the shear stress in the ground/rock mass Failure 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 loadings seepage pressure due to built up of hydro static pressure.

SLOPE 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 FAILURES Rotational or circular/arc failure Sliding /planar failure Wedge failure Toppling 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 BEEN DESCRIBED AND ANALYZED IN TWO DIMENSIONS

MAIN LANDSLIP TYPES

SLOPE MASS RATING A practical approach proposed by ROMANA (1985) to evaluate the slope stability. SMR is expressed as SMR = RMRbasic – (F1,F2,F3) + F4 Where RMR 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 (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 (Contd.) F4 pertains to adjustment for the method of excavation. Values of F4 are as follows: Natural slope + 15 Pre splitting + 10 Smooth blasting + 8 Normal blasting or mechanical excavation 0 Poor blasting - 8 SMR 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 SLOPES Rock mass or the proposed slope needs to be analyzed for the possible mode of failure Rock slopes can be analyzed by Conventional limit equilibrium methods/closed form solutions Numerical approximation methods Discrete element methods Finite element methods

SLOPE 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 measures Rock bolts/rock anchors/soil nailing Shotcrete with or without wire mesh (mainly to improve/protect the surface stability)

SLOPE STABILITY / PROTECTION (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 In Himalayas

Slope Protection Using Soldier Piles/Shotcrete Retaining Wall

Slope Protection Using Soldier Piles/Shotcrete Retaining Wall

Use of Soil Nail in Slope Protection

Use of Soil Nail in Slope Protection

Use of Soil Nail in Slope Protection

Use of Rock Anchors/Shotcrete in Slope 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 PONDER The innocent rock mass is often blamed for insufficient stability that is actually the result of rough and careless blasting. Where no precaution have been taken to avoid blasting damage, no knowledge of the real stability of undisturbed rock can be gained from looking at the remaining rock wall. What one sees are the sad remains of what could have been perfectly safe and stable. by Holmberg and Persson,1980

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