BHP Billiton

Project:

Slope stability is a major concern if the only thing between your mine and the Suriname River is an embankment of sand, especially if critical reserves require blasting in close proximity to the embankment. To keep the ore flowing out of the mine, GeoSonics/Vibra-Tech was brought in to establish vibration criteria and develop blast design recommendations to minimize the impact on the sand embankment.

Problem:

After completion of a Coffer Dam and sand buttressing of a natural levee, BHP Billiton’s mine management became concerned about the effects of blast vibrations on the new earthen structures designed to hold back the Suriname River. The mine plan called for the removal of critical reserves within several meters of the toe of the embankment. The expected vibration levels at this distance caused great concern about the potential failure of the slope.

Solution:

Based upon a review of boring logs and CPT data previously collected at the site, GeoSonics/Vibra-Tech determined that further study of the sand dyke and Coffer Dam was warranted. The following objectives were outlined in order to gather additional information for the purpose of the study:

  • Determine the shear wave velocity of the subsurface materials contained in the embankment to be used in the finite element model
  • Simultaneous measurement of vibration at the toe and the top of the slope to determine the natural frequency and amplification of the earthen structures
  • Develop finite element model of sand dyke and calculate shear stress induced by theoretical high amplitude ground vibration
  • Establish a site-specific blast criteria based upon modeling results
  • Determine the ground vibration attenuation characteristic from the mine to the earthen structures
  • Determine appropriate blast designs to minimize energy at the natural frequency of the earthen structures and reduce vibration amplitudes
  • Develop a blast vibration monitoring program and install equipment to monitor advancing blasting operations

Result:

GeoSonics/Vibra-Tech utilized Multiple Analysis of Service Waves (MASW) to determine the shear wave velocity of the materials comprising the earthen embankment. The shear wave velocity is one of the critical soil properties needed for developing the finite element model of the embankment. Single hole and production blast waveforms were also collected through simultaneous measurement at the toe and the top of the slope. These data were utilized to determine the natural frequency and amplification of the earthen structures for verification of the finite element model. Theoretical high amplitude time histories were also derived from these data and utilized as source input for the finite element model. The shear stress induced by these time histories was calculated to determine the corresponding particle velocity where failure would occur. Once a specific blast vibration criterion was determined for the stability of the earthen structures, recommendations for blast design were required to achieve the criteria. Linear regression analysis was utilized to determine the site-specific equation of attenuation. This equation allowed for the calculation of the appropriate charge weight to be utilized for a given distance to the earthen structures. Vibra-Map analysis was also utilized to determine the optimum timing sequence to minimize vibration energy at the natural frequency of the earthen structures. As blasting advanced toward the earthen structures, seismographs were installed by GeoSonics/Vibra-Tech for continued measurement and analysis of ground vibrations.