Blast Vibration Basics Post #7

USBM RI 8507 Blasting Level Criteria

This is the seventh post in a series on blast vibration basics. Blast vibrations are an important concept to understand because all blasts produce vibrations and, in some cases, those vibrations will annoy neighbors or potentially damage nearby structures. However, in today’s environment, blast vibration damage is extremely rare due to the regulatory limits placed on blast vibrations from blasting and other construction activities.

This post focuses on the safe levels recommended by the US Bureau of Mines in RI 8507. This information is an excerpt from a study I coauthored for the Florida State Fire Marshal on whether or not the US Bureau of Mines (USBM) RI 8507 vibration limit recommendations apply to Florida construction materials mining activities.

Report of Investigations 8507 Safe Levels of Blasting Vibrations for Residential-Type Structures [Siskind et al., 1980]

Introduction

The United States Bureau of Mines produced a study called Report of Investigations (RI) 8507, which forms the basis for most vibration regulations in the United States. It was not originally meant to be used as the basis for regulations, rather the intent was to provide guidelines and show that the 2.0 in/sec blast vibration limit of the time was not restrictive enough at some frequencies.

The USBM completed the study by collecting a lot of vibration recordings from blasting and coupled those recordings with residential inspections to identify which vibration amplitudes and frequencies cause damage. The dataset was split into frequencies above 40 Hz and below 40 Hz because vibrations below 40 Hz were understood to be more likely to cause damage.

The USBM identified three safe vibration limit levels, based on frequency [Siskind et al., 1980]:

• The USBM identified a limit of 0.5 in/sec for frequencies under 40 Hz for older (pre-1950s) plaster-on-lath structures. No sign of cracking was observed at or below 0.5 in/sec during the USBM study. The USBM calculated a 5 percent probability of superficial cracking below 0.5 in/sec.

• Modern construction is stronger than pre-1950s plaster-on-lath, so the limit should understandably be higher. The USBM suggested using 0.75 in/sec as the safe level criterion for modern construction with frequencies below 40 Hz. The lowest vibration level identified by the USBM that corresponded to minor crack extensions in modern construction was 0.79 in/sec. Of note, many nondamage observations were made at vibrations of over 2.0 in/sec for drywall construction.

• The USBM justified that 2.0 in/sec was still a safe level for high-frequency (over 40 Hz) blasts. They calculated the 5 percent threshold damage probability value was actually 3.2 in/sec, but chose 2.0 in/sec because the USBM observed damage at 2.2 in/sec in the over 40 Hz frequency range once.

Note that these values were calculated based on threshold damage to the weakest building materials. Threshold damage can be defined as “the occurrence of cosmetic damage; that is, the most superficial interior cracking of the type that develops in all homes independent of blasting” [Siskind et al., 1980]. Threshold damage is the most important level of damage because this level is the difference between nondamage and damage. Examples of threshold damage include loosening of paint, small plaster cracks at joints between construction elements, and lengthening of old cracks.

 Safe Blasting Level Criteria

The table shown at the begging of this article is the safe level criteria developed by the USBM for RI 8507 (The table is a summary of the three limits listed in the introduction). The limits are appropriate for seismographs buried in the ground as recommended by the International Society of Explosives Engineers because the values account for structural response and amplification factors of blast-induced ground vibrations.

RI 8507 states that the recommended limits have the following requirements [Siskind et al., 1980]:

The structures are sited on a firm foundation, do not exceed 2 stories, and have the dimensions of typical residences, and that the vibration wave trains are not longer than a few seconds.

Note, over the last 40+ years, that the USBM, NIOSH, and third party studies have continually proven that the USBM RI 8507 limits successfully protect residential structures from blast vibration damage.

Alternative Blasting Level Criteria

The USBM also developed an alternative blasting level criteria (ABLC) that combined the particle velocity-frequency limits shown in the table above with displacement limits (left figure below). The ABLC decreases particle velocity limits at frequencies lower than 4 Hz with a constant displacement of 0.03 in. Between 14 and 40 Hz, the ABLC gradually increases the particle velocity limit at a constant displacement of 0.008 in.

In the right figure, the safe levels from the table at the top of this article (blue lines in the figure) are plotted on the ABLC. This figure illustrates the differences between the initial blasting level criteria and the ABLC.

Conclusions

The following points are quoted conclusions from RI 8507 [Siskind et al., 1980]:

1. Particle velocity is still the best single ground motion descriptor.

2. Particle velocity is the most practical descriptor for regulating the damage potential for a class of structures with well-defined response characteristics (e.g., single-family residences).

3. Damage potentials for low-frequency blasts (< 40 Hz) are considerably higher than those for high-frequency blasts (> 40 Hz), with the latter often produced by close-in construction and excavation blasts.

4. Home construction is also a factor in the minimum expected damage levels. Gypsum board (Drywall) interior walls are more damage resistant than older, plaster on wood lath construction.

5. Practical safe criteria for blasts that generate low-frequency ground vibrations are 0.75 in/sec for modern gypsum board houses and 0.50 in/sec for plaster-on-lath interiors. For frequencies above 40 Hz, a safe particle velocity maximum of 2.0 in/sec is recommended for all houses.

6. All homes eventually crack because of a variety of environmental stresses, including humidity and temperature changes, settlement from consolidation and variations in ground moisture, wind, and even water absorption from tree roots. Consequently, there may be no absolute minimum vibration damage threshold when the vibration (from any cause, for instance slamming a door) could in some case precipitate a crack about to occur.

7. The chance of damage from a blast generating peak particle velocities below 0.5 in/sec is not only small (5 percent for worst cases) but decreases more rapidly than the mean prediction for the entire range of vibration levels (almost asymptotically below about 0.5 in/sec).