Wisdom from Blast and Construction Vibration Monitoring Data
By C. H. Dowding
Declining cost and miniaturization of digital hardware and nearly universal cellular coverage have reduced the overall cost of vibration monitoring and the increased subdivision of monitoring activity. In some respects manufacturers of blasting seismographs have led in the way in integration of monitoring hardware and communication software with strong competition by the home security, defense, and earthquake engineering industries.
During the over half century of my involvement in monitoring of construction induced vibration and air over pressure, seismographs have declined in size, complexity and become more interconnected. In the 1960’s they were the size of a large brief case with over 700 electro-mechanical parts. Today they are solid state and the size of half of a brick with no moving parts. Much of that volume is occupied by batteries. In the 1960’s coordination of seismographs and common time base were non-existent. (Figures 1 and 2 below compare 1960’s instruments I employed in my PhD thesis with 2010’s seismographs with equivalent capabilities.)
Today cellular communication allows instantaneous remote data access to multiple seismographs and GPS timing signals make possible a shared time base of separated seismographs to the nearest 1000th of a second. If battery problems could be solved seismographs could be reduced to the size of a deck of playing cards.
From data to wisdom
Increased hardware sophistication and capability of both monitoring hardware and analytical technique leads to a subdivision of monitoring activity and required skill. These subdivisions can be thought of as increasing levels of understanding. Level one or data is seismograph installation and maintenance. This skill level is generally supplied by the field technician. Level two or aggregation of data for informationrequires skills in vibration data analysis along with blast fragmentation and should be supplied by an experienced blaster or contractor. Communication of this information for project and public use to develop knowledge, level three, requires skills in remote data acquisition, data base maintenance and web based communication. This skill level is generally supplied by the seismograph manufacturer or third party monitoring company. The forth level or comparison of the now known ground motion relations (knowledge) with requirements or regulations to develop wisdom is generally supplied by the engineer of record or project vibration consultant.
State of the art in monitoring varies widely in all four levels in the United States. Activity in these four levels varies by state and is harmonized by differing national organizations. Thus summarizing the state of monitoring in the United States is complex, as it is similar to summarizing the state of monitoring in the European Union.
Seismograph operation and installation activities (data) in the United States rely on guidelines published by the International Society of Explosive Engineers. The primary documents are 1) ISEE Field Practice Guidelines for Blasting Seismographs (ISEE, 2015) and 2) ISEE Performance Specifications for Blasting Seismographs (ISEE, 2017); both of which are published by the ISEE Standards Committee. At this time there is no national program to certify field installation personnel; however, the ISEE Seismograph Section is discussing development of such a certification program. Status of this project is documented by a survey of the Seismographic Section and minutes of the ISEE Seismograph Section, which are available through ISEE.
Harmonizing blasting operations
The fourth level or monitoring wisdom is discussed next as it is the next most harmonized in the United States. However the wisdom is that of compliance for response of residential and commercial structures with similar dynamic response properties to similar types of ground motions. For other structures and considerations determination of strain may be helpful. There are two national organizations that regulate allowable blasting vibration levels for typical structures and thus harmonize blasting operations in the United States. There is no national organization that regulates other construction vibrations, even though blasting vibration experience is equally applicable to construction vibration. Blasting for surface mining of coal is regulated by the Office of Surface Mining (OSM) but carried out by states. Blasting in general is regulated by the National Fire Protection Association (NFPA) Standard 495, except where superseded by local regulation. Both of these regulations are based upon the findings of the US Bureau of Mines (USBM) RI 8507 embodied in the frequency sensitive “Z” curve.
Relative to the other two levels of monitoring skill, there is less harmonization of levels two and three, or use of vibration information or knowledge for design of blasts and communication of the results. Intricacies of relief, initiation sequence, type of detonators, type of explosive, geology, etc. contribute to the monitored vibration resulting from blast designs by the blasting specialist. Because of the complexity of blast drilling and design many large mining and construction firms outsource blasting to firms that sell explosives. Communication software and protocols employed to communicate monitoring information and knowledge for project and public use are in the domain of the seismograph manufacturers. They all have proprietary software, but provide software for translation of time histories to ASCII text files for the wisdom level of monitoring.
There are two area’s that deserve greater consideration at all four levels of monitoring skill; measurement of crack response to vibrations and weather and the importance of strain. Monitoring issues not discussed here are multiple and are fertile grounds for further harmonization. They include but are not limited to:
- Human Response vs Structural Response
- Cosmetic cracking from multiple sources
- Need for more simple communication with neighbors
- Observation of cracking at time of ground motion and sound
- Annoyance vs cracking (strain)
- Measurement and significance of ground response
- Relevance of earthquake and protective structure research
- Measurement of propagation velocity
Three reasons why crack response is important
Measurement of crack response to vibratory and weather effects (shown in Figures 3 and 4 below) is important for three reasons. First it addresses the wisdom level of monitoring, as it provides a means to directly address concerns of neighbors of construction activities about the cracks they see. Secondly these measurements support the conclusion that regulatory levels recommended in USBM RI 8507 prevent cosmetic cracks in the weakest of wall coverings in residential and commercial structures with similar dynamic properties.
The nineteen case studies in my book, Micrometer Crack Response to Vibration and Weather and the additional 10 case studies in the book’s web site all demonstrate the same crack response. Ground vibration at regulatory levels produce less crack response than do daily changes in temperature and humidity. Seasonal environmental changes produce even greater crack response. Thirdly measurement of crack response to both blasting vibration and weather supplies an independent measure of structural response against which various regulatory guides can be compared. How does crack response at various regulatory levels of vibration control compare with crack response induced by temperature and humidity effects?
The nineteen case studies in my book, Micrometer Crack Response to Vibration and Weather, and the additional 10 case studies in the book’s web site all demonstrate the same crack response. Ground vibration at regulatory levels produce less crack response than do daily changes in temperature and humidity. Seasonal environmental changes produce even greater crack response.
Strain is a fundamental parameter
Blast induced strain is important because it is the fundamental parameter that causes cosmetic cracking. Thus blast vibration monitoring in the future is likely to move toward increased reliance on structural strains to control the effects of blasting and construction vibrations. While peak particle velocity is a relatively easily measured index parameter that correlates with the observed occurrence of cosmetic cracks, strain is a more fundamental but more difficult to measure parameter. Increased sophistication and declining cost of seismographs now allows them to be deployed so that structural strains can be calculated from pairs of velocity time histories. Ground strains can also be calculated from pairs of measured velocity time histories or from particle velocities divided by the propagation velocity.
Every month or so I publish/distribute a 2 page CONST-VIBRATIONS Newsletter that summarizes an important topic in the area of construction vibrations through my Listserv. These Newsletters, most of my publications, and other important documents can be found on my webpage; http://www.civil.northwestern.edu/people/dowding/acm/
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Charles H. Dowding,
Professor of Civil & Environmental Engineering
at Northwestern University.
Figure 1: Author with 1960’s era recording equipment (encircled in blue) employed for PhD thesis that shows the volume and complexity of the equipment necessary to measure blasting vibrations compared to that in Figure 2.
Figure 2: Author with 2010’s era seismograph (encircled in blue) that replaces all of that encircled in Figure 1.
Figure 3: Micro-meter displacement sensor that measures both long-term, climatological and vibratory crack response is shown placed across a crack in the inset.
Figure 4: Miniscule vibratory response (red) compared to the climatological response (black) challenges graphical comparison. Even in this case where ground motions were as high as 11.9 mm/s (0.45 ips), vibratory crack response was still only 1⁄6 that of the daily temperature response.