Evaluation of blast induced ground vibration for minimizing negative effects on surrounding structures

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Abstract

The present paper mainly deals with the prediction of blast-induced ground vibration level in Bakhtiary formation at intake of waterway system in Gotvand dam, Iran. For this research the ground vibration components were recorded carefully by means of 3 sets of vibration monitors for 32 blast events during the bench blasting in front of tunnels. Then, the data pairs of scaled distance and particle velocity were analyzed by using the USBM equation. At the end of statistical evaluations, a relationship between peak particle velocity and scaled distance for this site was established with good correlation. Again, other data measurements during tunnel excavation near concrete structures were used to validate the predicted PPV and optimize the blasting patterns to omit the effects of resonance and vibration in USBM (RI-8507) standard. Based on the vibration tests done in Bakhtiary conglomerate, constant dynamic factors of the rock mass related to vibration velocity are 159.07 and 1.077.

Highlights

► Velocity of 50 (mm/s) presents as a maximum particle velocity with no damage. ► In Bakhtiary conglomerate, dynamic factors are 160 and 1.078. ► Vibration from underground blasting is at least 3-times more than surface blasts. ► Frequencies of blasts with different detonators in conglomerate are higher than 20 Hz. ► Potential of damage risk and self structural frequencies were analyzed.

Introduction

Blasting is generally inevitable for hard rock excavation activities not only in mining and quarrying but also in tunnel, subway, highways and in dam constructions. The ill effects of blasting (ground vibrations, air blasts, fly rocks, back breaks, and noise) are unavoidable and cannot be completely eliminated but can certainly be minimized up to permissible level. Among all the ill effects, ground vibration is a major concern to the planners and environmentalists. The level of ground and structure vibration caused by construction work depends on the construction methods, soil and rock medium, heterogeneity of soil and rock deposit at the site, distance from the source, characteristics of wave propagation at a site, dynamic characteristics of soil and rocks, response characteristics of fractures and susceptibility rating of the structures [2]. Many of these parameters especially geological and geotechnical conditions of rocks cannot be altered, but the quantity of explosive detonated per delay can be estimated with empirical formula and proposed for blast design [4]. Various research studies were carried out in the past in order to isolate environmental issues produced from blasting; a general reliable approach or a formula has not been established yet because of the complexity of the matter. In addition to the wave and ground motion characteristics, the complexity of blasting parameters and site factors restrict the development of a general criterion and experimental site-specific studies should be still performed in order to predict and control blasting effects [5], [6]. Therefore vibrations must be monitored at the beginning of construction and continued during construction to measure geological factors and blast data and to ensure serviceability of vulnerable structures [7]. By selecting the right blasting methods and correct drilling and firing patterns, the magnitude of ground vibrations can be controlled. Present paper mainly deals with the prediction of blast-induced ground vibration level and dynamic site factors and will discuss excavation designing of intake waterway system in Upper Gotvand dam by focusing on the reduction of negative effects of blasting on concrete of gate shaft structure that had been constructed at the back of them. These measurements were undertaken to determine the relationships between peak particle velocity (PPV) and square root scaled distance (SD) to predict PPV and to estimate dominant frequency. Both PPV and frequency are necessary to determine the response of the neighboring buildings and structures. This relation was used to calculate the site factors and design blasting patterns, contains of vibration controlling method by delay blasting with NONEL and electrical detonators and pre-split blasting.

Section snippets

Dynamic waves from blasting

Source of construction vibration generates body and surface waves in soil and rock medium. Body waves propagate through the soil deposits and rock. Compression and shear waves are the main types of body waves that should be taken in to consideration at relatively small distance from the construction sources. Surface waves, of which Rayleigh waves are the primary type, propagate along the upper ground surface. Rayleigh waves have the highest practical interest for structural engineers because

Scaled distance modeling and prediction of ground vibrations

The literature search revealed a plethora of papers on blasting physics and measurement techniques, many of which correlate structural damage to peak particle velocity. Seismic waves die out or decay with distance in a fairly regular manner which makes them predictable with the acceptable accuracy and allow restriction on blasting vibrations to be regulated either by means of mathematical expressions. The peak particle velocity (PPV), experienced at some distance (D) from an explosive source of

Existing vibration standards and criteria to prevent damage

Peak particle velocity has been traditionally used in practice for the measurement of blast damage to structures. In this criterion the shape of the wave formed and duration of dynamic loading are not taken in to account. Some of the suggested damage criteria that base solely on the peak particle velocity (PPV mm/s) are listed in Table 1.

These recommendations are based on author experiences for vibration limit in blasting near various types of structures in urban area and is different for the

Case study: designing excavation pattern of intake waterway system, near concrete walls

This dam is located on the Karun River in south west of Iran (Fig. 3). The 178 m height and 730 m length embankment dam, regulating the water of the Karun River, also serves power generation, flood control and irrigation needs. The final design of the dam has been completed and construction works has been started at 2000.

The waterway system of powerhouse consists of huge collections of underground and surface excavations. One of the particular traits of this waterway system is the existence of

Measurements and statistical analysis

To reach the time schedule for filling the reservoir, excavation of rectangular tunnels and vertical shafts were done at first and then during concreting of these parts, excavating bugle shape tunnels was done by head and bench blasting from entrance of tunnels same as Fig. 5, Fig. 6.

To reduce negative effects of blasting on concrete of neighbor structures the USBM relation on the basis of peak particle velocity was adopted to design a safe blast for excavation of bugle shape tunnels. At first

Designing of blasting patterns and verification

A major geological fault intersecting the path may largely prevent propagation in a particular direction. At first for reduction of ground vibration induced from bench blasting of burden in front of tunnels (part (0) in Fig. 5), pre-splitting blast was used to create artificial joint and separate this part of rock mass. By using this method wave propagation from surface was omitted. This part was excavated by bench blasting with vertical blast holes with 76 mm in diameter and 10 m length for all

Results and conclusion

  • 1.

    The measurement of ground vibration induced by blasting is significantly important in controlling and eliminating the blast damages to structures. Since the particle velocity is still one of the most important grounds vibration predictors for regulating the blast design, an empirical relationship with good correlation has been established between peak particle velocity and scaled distance.

  • 2.

    Based on the vibration tests done in Bakhtiary conglomerate, constant dynamic factors of the rock mass

Acknowledgments

The author would like to thank the Sepasad Engineering Co. for partial financial support for carrying out this research work.

References (19)

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