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“RESPONSES OF LOW-RISE STRUCTURES TO DYNAMIC LOADING.”

VIEWS: 15 PAGES: 37

									Management of mine-related
structural hazards on nearby
          buildings


             • H.C. Uzoegbo
   • 2008 Built Environment Symposium
MAIN SOURCES OF PROBLEMS

• MINE-INDUCED EARTH
  TREMORS



• EFFECT OF BLASTING
  Southern African seismicity
• Seismic events are rare but can be
  catastrophic.

• Two types of seismic activity.
  – Tectonic activity.
  – Mining related seismicity (triggered, induced)
 IMPACT OF MAGNITUDE OF S/EVENT
< 4:     UNLIKELY TO CAUSE
         STRUCTURAL DAMAGE

5 – 6:   WELL DESIGNED
         STRUCTURES CAN RESIST

7:       CAN CAUSE SERIOUS
         DAMAGE TO STRUCTURES

8 – 9:   CATASTROPHIC DAMAGE
              Tectonic activity

• Significant events (> 5 on Richter Scale)
• - St. Lucia         1932 (6.3) on Richter Scale
   – Ceres-Tulbagh 1969 (6.3) and 1970 (5.7)
   – Welkom           1976(5.5); 1994(5.1); 1999(5.1)
   – Klerksdorp       1977(5.2); 1983 (5.1); 1984(5);
                      2005(5.3)
   – Carletonville    1993(5)
   – Mozambique       2006(7.5)
      Mine-related seismicity
• South Africa has very developed mining
  industry.

• Since 1971 there have been over 2000
  events measuring >3 on the Richter Scale.
Average peak velocity   Modified Mercalli Scale                                         peak acceleration)         Richter Scale
  (centimetres per                                                                                                  equivalent
      second)

                        I. Not felt except by a very few under especially favorable                          3.5 – 4.2
                        circumstances.



1-2                     IV. During the day felt indoors by many, outdoors                 0.015g-0.02g       4.3
                        by few. At night some awakened.

2-5                     V. Felt by nearly everyone,                                        0.03g-0.04g       4.8


          5-8           VI. Felt by all, many frightened and run outdoors.                 0.06g-0.07g       4.9 – 5.4


         8-12           VII. Everybody runs outdoors. Damage negligible                    0.10g-0.15g       5.5 – 6.1
                        in buildings of good design and construction;

         20-30          VIII. Damage slight in specially designed                          0.25g-0.30g       6.2
                        structures; columns, monuments, walls

         45-55          IX. Damage considerable in pipes broken.                           0.50g-0.55g       6.9


      More than 60      X. most masonry and frame                                     More than 0.60g        7.0 – 7.3

                        XI. Few, if any, (masonry) structures remain                                         7.4 – 8.1
                        standing.


                        XII. Damage total                                                                    Over 8.1
           Characteristics of Mine-
             induced Tremors
•   Relatively high frequency, (5 – 40 Hz).
•   Relatively low depth (1 – 3 km)
•   Relatively Small radius affected (20 -30 km)
•   Low-rise structures are particularly at risk.
•   Relatively short duration
typical damage to walls due to earthquake.
          CASE STUDY

•   Date: 2005/03/09
•   Time: 12:15:31.7
•   Magnitude: 5.3 ML
•   Region: Klerksdorp Area, SA
•   E – W direction
Out-of-plane failure (stilfontein)
Wall failure in Stilfontein earthquake
In-plane failure (stilfontein)
Cantilever wall
Failure at wall intersection
Zig-zag wall construction failure
Fixed vs hinged glass windows
Shear failure of toilet bowl
Response of Hydraform wall structure
       Cause of the 2005 event
• Dept of minerals & Energy/CSIR
  Report
• Partly due to past mining
• Mining activities responsible for
  most of the strain energy
• Caused by rejuvenated slippage on
  existing major fault
EFFECT OF BUILDING
    ORIENTATION
ON SEISMIC CAPACITY
Distribution of structural walls in
               plan
ORIENTATION OF SHEAR WALL
Sun’s movement in summer
      (A=28o, B=80o)
Sun’s movement in winter
     (A=25o, B=36o)
Neftegorsk, 1995: collapse of large-block
masonry buildings with transverse load-
    bearing walls (photo by G.Koff)
Effect of Mine Blasting on Nearby
            Structures
OPEN CAST MINE
         WHY BLAST?
• REMOVE ROCK OVERBURDEN

• EXPOSE COAL

• MECHANICALLY EXCAVATE COAL
  Possible adverse effects of
           blasting
• Fly rock: Up to 120 m from blast site: not more than
  half distance to nearest building

• Airblast: Airborne shockwave (decibels). Can break
  windows but no structural damage: Must not exceed
  134 decibels: windows break at 140+ decibels.

• Ground vibrations: Measured by Peak Particle
  Velocity (PPV).

• Human perception
      Ground vibrations
• FREQUENCY RANGE:

2 – 40 Hz for soil

10 – 100 Hz for Rock
       Ground vibrations
• Peak Particle Velocity

• Human Perception: Clearly
  Perceive 1 mm/s

• PPV to cause visible
  damage to residential
  building: 50 mm/s
  (frequency dependent)
RECOMMENDED PPV FOR
   HUMAN CONFORT
ESTIMATING PPV
         Test in Australia (Coal Mine)
•   PPV                No of Blasts
•   <5                 9
•   5 – 10             8
•   10 – 20            12
•   20 – 30            4
•   30 – 50            6
•   50 – 100           2
•   > 100              2
          CONCLUSIONS
• NEED PPV > 100 mm/s
• FOR STRUCTURAL DAMAGE
• HUMAN PERCEPTION IS A PROBLEM

								
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