Slide 1

Blasting Vibration Assessment of Slopes

Keith WK KongFICE FIMMM MHKIE RPE(GEL)

Rock Excavation Excavation Methods (e.g.)Underground:Drill and splitMechanical (e.g. hydraulic breaker/hammer)Tunnel Boring MachineDrill and Blast

Surface:Drill and break / Diamond-saw cutsMechanical (e.g. hydraulic breaker/hammer; WBE)Drill and Blast

Blasting Mechanism

Blasting Mechanism

Blasting Video Clip

Body Waves

P-wave (Longitudinal)S-wave (Transverse or shear)R-wave (Rayleigh)Love wave

Body Waves

P-wave (Longitudinal)Resultant vibration

S-wave (Transverse)Rayleigh wave

Wave Motion (Undamped Free Vibration)

Considered as a Simple Harmonic Motion (SHM) A=Initial displacement from equilibrium 0 at time t = 0x=Displacement from equilibrium 0 at time t=ACos (t)( = angular velocity or frequency)=Velocity of body (or particle)=-A Sin (t)= -A Cos (t + /2)=Acceleration of body (or particle)=-A Cos (t)= -A Cos (t + )T=Period of Oscillation=2 / f=Frequency of Oscillation= / 2 (i.e. = 2 f)

Risks of Ground VibrationProperty lossDamage to buildings/structuresDamage to underground services or utilitiesFailure occurred of geotechnical featuresNuisanceComplaints

Rock Slope Failure Due to Rock Blasting

Guidance Documents forBlasting Assessment in Hong KongGEO Circular No. 27 Geotechnical Control of Blasting

Mines Division Practice Notes (Nos. 1 - 4 )

Mines Division Guidance Note No. 1 on Vibration Monitoring

Project Administration Handbook for Civil Engineering Works, 2014 Edition

General Specification for Civil Engineering Works Vol. 1 (Section 6)

Buildings Department Practice Note for Authorized Persons and Registered Structural Engineers 178

GEO Report No. 15 Assessment of Stability of Slopes Subjected to Blasting Vibration

GEO Report No. 45 Gravity Retaining Walls Subject to Seismic Loading

GEO REPORT No. 102 A Study of the Effects of Blasting Vibration on Green Concrete

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Slope Stability of Vibration AnalysisAnalytical Methods:

Pseudo-static Approach (GEO Report 15 suggested method)(for soil slope)

Dynamic Approach

Energy Approach (GEO Report 15 suggested method) (for rock slope)

Slope Stability Analysis Conventional limit equilibrium methods to be used [e.g. Bishop(1955), Janbu (1972), Morgenstern & Price (1965), etc] :

Limit equilibrium analyses assume the Factor of Safety (FoS) is the same along the entire slip surface.

If FoS is greater than unity (i.e. FoS > 1.0), the available shear resistance will exceed the required for equilibrium; and hence the slope will be stable with respect to sliding along the specified slip surface as analysed.

If FoS is less than 1.0, the slope will be unstable.

(Note: There are no rules for acceptable factors of safety under seismic conditions)

Slope Stability Analysis

Standard slice in limit equilibrium methods (Fredlund and Krahn, 1977)

Pseudo-static ApproachWong & Pang, 1992 (i.e. GEO Report 15) suggested:

PPVc = Kcg / (Ka)

where:Kc=the critical acceleration (m/sec) at which the slope hasa Factor of Safety of 1.0 against failure; g=the acceleration due to gravity (m/sec);=the circular frequency of the ground motion (2f). f is the frequency of ground vibration during blasting;Ka =the magnification factorPPVc =Critical Peak particle velocity of ground mass

Pseudo-static ApproachKc - can be acquired from some of the geotechnical computer programmes such as SLOPE/W and OASYS-SLOPE

- Ground vibration frequency of 30Hz is adopted as recommended by Wong & Pang (1992).

National Institute of Rock Mechanics (NIRM, 2005) study, the most common frequency of ground vibration () reported for construction blasts varies from 10 to 200 Hz, typically greater than 20Hz.

Pseudo-static ApproachKa - A ratio of the maximum of the net response acceleration at the mass to the maximum of input acceleration at bedrock.

The response acceleration at the mass is subjected to the geometry of the slope and the failure slip.

Determination of magnification factor (Ka)

Inclined Bedrock FormationSoil mass shear wave velocity, S = 300 m/s (GEO Report 15)

Determination of Soil Shear Wave Velocity

SPT: 20 to 50 (typical CDG)S = 240 m/s to 440 m/sAverage = 340 m/s

Determination of magnification factor (Ka)Horizontal Bedrock Formation

Soil mass shear wave velocity, S = 300 m/s

Ground Vibration Prediction of BlastingHong Kongs 84% Confidence Average-line (Li & Ng 1992)

Where:

PPV peak particle velocity (mm/s)R distance between blast and measuring point (m)W maximum charge weight per delay interval (kg)

Worked Example

25m

q = 39 degreet = 9 + sn tan fSPT N-value = 40blasting site

Pseudo-static Approach

Determine the magnification factor, Ka Shock wave Velocity of the soil, S = 300 m/secTotal horizontal thickness of the deposit, D = 6mS / D = 50

PPV = 644 W 0.61 R -1.22(W)Slope/w model

Dynamic ApproachBy considering Hookes law in the uniaxial conditions, andNewtons second law, the compressive stress waveequation can be written as:

= CV Where: = compressive stress of waveC = wave velocity of materialV = peak particle of material

Noted that longitudinal (P-) wave velocity, Cp in a material is alwaysgreater than transverse (S-) wave velocity, Ct (or Cs).

A list of typical Cp and Cs values (Press, 1966)MaterialCp (m/s)Cs (m/s)Density (kg/m)Sandstone1400 45002450Shale, Slate2300 47002350Limestone2650Soft1700 4200Hard2800 6400Crystalline5700 6400Dolomite3500 69002840Granite, Granodiorite4600 - 60002800 32002670Gabbro6400 - 67003400 36002980Basalt5400 64002700 32003000Schist4200 - 49002500 32002800Gneiss3500 - 75003300 37002650Water14501000Air335-

Dynamic Approach Worked Exampple (1/2)

sn of Slip#2at restUnstableStable

snshsv

tq

Dynamic Approach Worked Exampple (2/2)

PPV = 644 W 0.61 R -1.22(W)

UnstableStable

sn of Slip#2at restNotes:Youngs modulus of soil (E) = 1.1 x SPT-N (Davies,1987)

wave velocity of soil

= CV

Case Study (1)

Source: GeoInfo Map, LandsD

Case Study (1)

Case Study Slope Section

31 m8m span tunnel

Case Study (1)Slope PortionPotential Failure Calculated allowable PPV (mm/s)Wt. of Explosive, W (kg) when PPV