Soil Contributions to Blasting Effects from Case Histories

Soil Contributions to Blasting Effects from Case Histories and Vibration Limits Mark R. Svinkin, Member, ASCE 1 1VIBRACONSULT, 13821 Cedar Road, #205, Cleveland, OH 44118-2376; PH (216) 397-9625; FAX (216) 397-9625; msvinkin@ Abstract The existing vibration limits were developed for 1-2 story houses surrounding coal surface mines, and these limits are applied for ground vibrations as the criteria for the possible crack formation in structures. However, other blast design, soil conditions and structures require different vibration limits. There are no regulations for multistory building vibrations from blasting and there are no limits to assess possible dynamic settlements of soil and structures triggered by ground vibrations from blasting. Nevertheless, numerous case histories are available in technical papers which describe tolerable and unacceptable soil and structural vibrations measured for different soil conditions and structures. These case histories are valuable sources of information to evaluate vibration effects on structures, analyze vibration levels that do not produce damage to structures, determine the areas of application of the existing regulations and assess tolerable vibration limits for various structures during blasting operations. Introduction Blasting operations conducted in urban areas are usually connected with potential problems of manifestation of vibration effects on adjacent and remote houses, buildings and other super and underground structures. Dynamic consequences of structural vibrations range from benign to harmful. It is common that two significant questions come up at neighborhood areas around sites with blasting operations: what is the cause of damage to structures and what vibration limits have to be used at specified areas. It is necessary to underline that similar damage to structures can be produced by different group of static and dynamic loads such as environmental forces (temperature, humidity, wind, rain, snow, etc.), geotechnical hazards (heave, settlement, stability, sliding and failure), and dynamic effects from blasting. Determination of actual causes of damage to structures withoutthe performance of a pre- and post-blast structure surveys is a complicated task, and 335D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .missing surveys are the basis for contradictory judgments of blasters and house owners. Counteraction between both sides sometimes extends in large scales. On the one hand blasters stand up for their correct blasting operations without any damage to structures. On the other hand residents who believe that their houses were damaged by blasting joint efforts to push forward the legislature for diminishing the vibration limits, increase areas of pre-blasting survey and choose larger scaled-distances to decrease vibration effects on houses from blasting. Such restrictions can create substantial obstacles for production blasts without insurance of safe blasting. Obviously, a number of resident complaints are related to public perception of blasting and public relations. However, there are case histories with real damage to structures from blasting. Vibration limits correctly determined can help to properly assess vibration problems during blasting. Case histories are large and valuable sources of information which can be used to determine vibration limits. In this paper, a number of case histories are analyzed with an emphasis on soil contributions to blasting effects on wave propagation and damage to structures. The Existing Vibration Limits Apparently, it is impossible to have universal regulations of blasting vibrations applied for different kinds of blasting, variety of soil conditions and types of structures. The available optional regulations, NFPA 495 (2006), include the vibration cosmetic cracking thresholds for assessment of surface coal mine blasting effects on 1-2 story residential houses, Figure 1. These criteria developed by the U.S. Bureau of Mines (USBM) have the following displacement and velocity values for the four ranges of the dominant frequencies: 0.76 mm (0.03 in) for 1-4 Hz, 19 mm/s (0.75 in/s) for 4-15 Hz, 0.2 mm (0.008 in) for 15-40 Hz, and 50.8 mm/s (2.0 in/s) for 40-100 Hz. The limit of 19 mm/s (0.75 in/s) for 4-15 Hz is used for drywall while the limit of 13 mm/s (0.5 in/s) for 2.5-10 Hz is applied for plaster, Siskind et al. (1980). The derivative version of the USBM safe limits shown in Figure 1 was included as the Chart Option into the surface coal mine regulations by the Office of Surface Mining Reclamation and Enforcement, OSM (1983). The USBM vibration limits were built up on the basis of the two decades research studies of a correlation between ground vibrations and observations of blast damage to low-rise houses, and these limits are applied for ground vibrations as the criteria of possible crack formation in various structures. The USBM research study has been recognized as the great achievement which provides the safety of 1-2 story residential houses from vibrations generated by surface coal mining blasting. Obviously, these vibration limits can be successfully used for adequate blasting loads, similar structures and ground conditions they were developed for, but different limits should be used for other combination of dynamic loads, soil conditions and structures.FORENSIC ENGINEERING 2009336D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S C E . F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .Figure 1. Safe level blasting criteria from USBM RI 8507 and the derivative version, the Chart Option from OSM surface coal mine regulations. Shaded area shows maximum velocities of resonant structural vibrations with amplification of 4.5. Data were modified from Siskind (2000). Plot was adapted from Svinkin (2008). Siskind (2000) presented the accumulated results of research studies accomplished by the USBM and others on vibrations from blasting and their effects on low-rise houses, Figure 2. Ground vibrations were measured near 1-2 story residential structures from 718 blasts and 233 documented observations were obtained. Non-damaging blasts are not shown although some of them produced relatively high level of ground vibrations even exceeding 51 mm/s (2 in/s). These data indicate different vibration effects on structures depending on the dominant frequency and the peak particle velocity (PPV) of ground vibrations. It is necessary to point out that the USBM criteria are conservative for assessment of direct blasting vibration effects on structures in the non-resonant frequency zones of structural vibrations when ground vibrations do not trigger plastic soil deformations under structures (Figure 2), but they cannot protect low-rise structures from appearance of cosmetic cracks by amplification of ground vibrations higher than 4.5x and beyond the 4-12 Hz frequency range, Svinkin (2003, 2004). Furthermore, the application of these limits to different super and underground structure is incorrect. AASHTO Designation: R 8-96 (2004) stated the application of the USBM limits to markedly different types of structures is common and inaccurate. Besides, the USBM limits do not actually take into account construction blasts with much higher frequency content. Dowding (1996) demonstrated examples where the dominant frequencies of ground vibrations from surface mining and construction blasting would lie between 12 and 18 Hz and 70 and 100 Hz, respectively. It means that the USBM limits cannot beused for construction blasts.FORENSIC ENGINEERING 2009337D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .Some publications have stated that there are no problems with blasting vibration limits. For example, Aimone-Martin (2005) has asserted, that the safe blasting criteria defined by the U.S. Bureau of Mines in 1980 are widely used in the U.S. rock blasting industry. “This safe criteria is based on over 40 years of research and crack observations and has been scientifically supported since 1980 by experts in the field of structure response to rock blasting. There has been no scientific data to date that disputes this criteria.” On the one hand this statement is correct because the USBM criteria without doubt are good for the specific blast design, soil conditions and the types of structures they were developed for. On the other hand this statement is confusing because in a number of cases, these vibration limits are used for different blast, soil and structure conditions. Verification is necessary for the application of theUSBM criteria for other blasts, soil and 1-2 story houses. Figure 2. Ground vibrations from blasting and structural damage summary grouped in three zones. Dashed lines define USBM safe limits. Data were modified from Siskind (2000). Plot was adapted from Svinkin (2008). Siskind (1991) wrote, “Vibration criteria for nonresidential cases are needed: concrete, power poles, pipelines, bridges, etc. Without reliable criteria for such cases, regulation is often based on relatively strict levels required to prevent cosmetic damage to residences”. Also, Siskind (2000) stated that most regulatory bodies, including OSM, recognize “that a more rigorous treatment may be needed in special cases, such as that outlined in RI 8507”. Some states use single vibration limits between 6.4-51 mm/s (0.25-2.0 in/s) for PPV of ground vibrations that only aggravates problems with assessment of damage to houses because of lack of objectivity and ignorance of possible resonant structural vibrations. The British Standard BS 7385 (1993) and the USBM criteria are depicted in Figure 3. It can be seen that the BS 7385 considers two lines of the safe limits depending on a type of buildings. Line 1 with the frequency-independent criterion of 51 mm/s (2 in/s) isapplied to industrial and heavy commercial buildings. Line 2 with the frequency-based safe limits is similar to the USBM criteria and applied to residential or light commercial FORENSIC ENGINEERING 2009338D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S C E . F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .type buildings. It seems the values presented in Figure 3 are small to represent structural vibrations. On the basis of similarity of two standards, it can be assumed that the BS 7385 requires measurement of ground vibrations as the criteria of possible structural damage. If so, the BS 7385 has similar restrictions of its application as the USBM criteria, Svinkin (2007).Figure 3. Vibration guidelines - USBM RI 8507 (solid line) compared to BS 7385 (dashed line). Line 1: reinforced or framed structures, industrial and heavy commercial buildings; line 2: unreinforced or light framed structures, residential or light commercial type buildings. From AASHTO Designation: R 8-96 (2004). The German Standard DIN 4150 (1986) and the USBM criteria are depicted in Figure 4. DIN 4150 suggests three lines of time-dependent vibration limits for different types of structures. Lines 1-3 represent different kinds of structures described in the figure caption of Figure 4. Lines 2 and 3 are located below the USBM vibration limits. Line 1 is close to the upper boundary of the USBM vibration limits in the 4-100 Hz frequency range. Comments are needed regarding structures with particular sensitivity to vibrations. Such terminology is appropriate for devices and technology sensitive to vibration, but structure sensitivity is different. Resonant vibrations are dangerous for any kind of structures: from buildings under the preservation orders to industrial buildings. Dynamic settlements can also be devastated for any kind of buildings. However, industrial buildings have much higher resistance to direct vibration effects than 1-2 story residential houses. The basis for the guideline level of DIN 4150 is unknown, AASHTO Designation: R 8-96 (2004). Obviously, these criteria intended to minimize human perceptions and complaints. This approach would be sensible if construction operationsFORENSIC ENGINEERING 2009339D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .constantly affect houses and even at night. However, if blasts are performed 1-2 times per day, such an approach is unreasonable, Svinkin (2007).Figure 4. Vibration guidelines - USBM RI 8507 (solid line) compared to DIN 4150 (dashed line). Line 1: buildings used for commercial purposes, industrial buildings and buildings of similar design; line 2: dwellings and buildings of similar design and/or use; line 3: structures that because of their particular sensitivity to vibration, do not correspond to those listed in lines 1 and 2, and are of great intrinsic values (e.g. building that are under a preservation order). From AASHTO Designation: R 8-96 (2004). The Russian limits of 30-50 mm/s (1.18-1.97 in/s) for vibrations of sound structures were found by the Moscow Institute of Physics of the Earth to assess the safety of structures from explosive effects of various blasts in the air, on the ground and under the ground at the time of the Second World War, Sadovskii (1946). So, for multi-story residential, commercial and industrial buildings, the frequency-independent safe limit of 51 mm/s (2 in/s) can be chosen for the PPV of structural, not ground, vibrations. This criterion automatically takes into account soil-structure interaction for the whole building frequency range. In the support of the proposed criterion, it is necessary to underline that according to Siskind (2000), the PPV of 51 mm/s (2 in/s) is the highest vibration level generated inside homes by walking, jumping, slamming doors, etc. Besides, this vibration limit is compatible with the European Standards, and it does not exclude higher allowable vibration levels. It is easy to demonstrate compatibility of the new simplified safe criterion and the USBM vibration limits, Figure 1. To evaluate tolerable structural vibrations, the smallest vibration limits of 13 mm/s (0.5 in/s) and 19 mm/s (0.75 in/s) have to be multiplied by 4.5, and theirFORENSIC ENGINEERING 2009340D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S C E . F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .products of 57 mm/s (2.25 in/s) and 85.5 mm/s (3.37 in/s) are higher than the simplified criterion of 51 mm/s (2 in/s), Svinkin (2007, 2008). Soil and Structure Responses to Blasting Blasting operations generate different body and surface waves which travel through the medium of soil deposits and rock. Elastic waves travel from dynamic sources and induce elastic soil deformations (ground vibrations) which levels depend on intensity of propagated waves. The structural responses to ground vibrations depend on soil-structure interaction. Ground vibrations can produce direct vibration effects on structures and trigger resonant structural vibrations of adjacent and remote structures. However, under certain circumstances such as a combination of non-cohesive soil deposits, water table levels and ground vibrations, elastic waves can be the cause of plastic soil deformations, e.g. liquefaction, densification and soil settlements triggered by small ground vibrations. Soil-structure interaction will be different for soil failure. The structural response to ground excitation depends on the soil response to waves propagated from the source and soil soil-structure interaction. Because elastic and plastic soil deformations cause dissimilar structural responses and damage, diverse thresholds have to be used for assessment of direct vibration effects, resonant structural vibrations, and plastic soil deformations (Svinkin, 2008). The existing regulations were developed for assessment of elastic soil deformations and structural responses to ground vibrations from blasting. These criteria have nothing to do with plastic soil deformations. It is necessary to underline that there are no federal, state and local regulations of the critical vibration levels of ground vibrations which may trigger dynamic soil failure. Therefore, an experience from cases histories of soil failure contribution to structural damage from blasting is very important for assessment of vibration limits to cover plastic soil deformation effects triggered by blasting. Low Attenuation of Propagated Waves in Saturated Soils C ase 1. Siskind and Stagg (2000) have made a comprehensive study of structural vibration damage to the ten typical houses from quarry blasting. The houses were selected at five areas in Miami-Dade County. Vibration measurements of ground and house structure vibrations were made from 42 quarry blasts during about a 2-month period from late February to late April 2000. The maximum distances between the active quarries and the test sites were in the 3.2-4.8 km (2-3 mi) range. Pre- and post-blasting condition surveys were made before and after vibration monitoring. Additional 756 vibration records from other reports were considered in the study. The soil at the area consisted of 0.3-0.9 m (1-3 ft) of fine grained to fibrous peat followed by 0.6-1.2 m (2-4 ft) of sand followed 0.6 m (2 ft) of marl underlain by weakly to well cemented limestone. The groundwater table is virtually coincidentwith the surface of peat. The specific condition of wave propagation has been found FORENSIC ENGINEERING 2009341D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .in the Miami-Dade County: attenuation with distances is less than found on similar sites in the north. The report has revealed other interesting findings: (a) structure oscillations are superposition of vibrations with various frequencies including low frequencies of about 8 Hz, which are close to the natural frequencies of house horizontal vibrations, and very low frequencies of 2 to 4 Hz; (b) vibrations are of long duration at the houses with maximum duration about 17 second; (c) ground vibrations with the PPV of 19 mm/s (0.75 in./s) and with structure response of 6.1x would produce a global wall strain that is 1.5 times higher of the strain limit for crack formation. On the basis of vibration monitoring in the west and south-west Miami area, the authors concluded that the highest vibrations there would likely be under 3 mm/s (0.12 in/s). Liquefaction in Sands C ase 2. Sanders (1982) has performed an assessment of liquefaction danger from explosively creating breaches in the Bird’s Point – New Madrid fuze plug levee. Creation of two large crevasses would use tens tons of explosives that would cause liquefaction hazards to structures in the area. A rock layer was located at the depth greater than 30 m (100 ft). There were problems with the scarcity of borehole data below 4.5 m (15 ft) and the scarcity of data bearing on the investigation of blast-induced liquefaction. A data base from row charge experiments in soil was used in the assessment of the liquefaction hazards in the area. This study had to determine a possible liquefaction PPV threshold, the maximum distance from a blast where the threshold could be expected, and availability of structures within the distance. The safety criterion of 10 cm/s (3.9 in/s) was found out as the conservative threshold for assessment of liquefaction hazards. The maximum distances at which liquefaction might occur from blasting were ascertained in the 300-383 m (1000-1275 ft) range. The report was reviewed by Dr. H.B. Seed, one of the founders of Earthquake Engineering, who used a totally different line of approach and concluded that the threshold of 10 cm/s (3.9 in/s) was reasonable. Settlement in Sand and Clay Soils C ase 3. Charlie et al. (2001) have reported the potential responses of earth-fill and tailing dams, and other saturated earthen structures subjected to blast vibrations. Eight explosive tests were conducted on a 2.25 m (7.4 ft) high dam constructed of loose dilative sand, and they showed that significant increases in a residual pore pressure occurred when the PPV exceeded 1.5 cm/s (0.59 in/s) at shallow depths to 3.5 cm/s (1.38 in/s) at greater depths. Limited crest settlement occurred when the PPV exceeded 2.5 cm/s (1.0 in/s). Results of research studies and investigation of the field behavior of full-scale earth-fill and tailing dams indicate that the PPV below 2.5 cm/s (0.98 in/s) and 10 cm/s (3.94 in/s) are reasonable thresholds to limit pore pressurebuildup in these dams sensitive and not sensitive to vibrations, respectively.FORENSIC ENGINEERING 2009342D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S C E . F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .C ase 4. Edwards and Northwood (1960) have presented the results of controlled blasting tests of six buildings on two different sites. Damage was correlated with explosive weight, distance, PPV of ground or structure vibrations, settlement and strain measurements in the buildings. The selected buildings were old but in good conditions except for quite localized areas in one or two of them. All had basements constructed with heavy stone masonry walls and frames with brick superstructures. The soil at the first site consisted of 6.1 m (20 ft) loose wet sand underlain by soft marine clay. The water table was about 2.1 m (7 ft) below grade. Sand soil was frozen to a depth of about 0.3 m (1 ft) at the time of the test. The buildings at the first site: church, school and house with 305 mm (12 in) brick walls and plaster inside. Large low-frequency (2-5 Hz) ground vibrations in the sand-clay soils were obtained within hundred feet of the source, and they did not occur at all at 457 m (1,500 ft) and beyond. Damage inspected in the buildings on sand-clay deposits: cracks were mainly vertical and some diagonal cracks developed in the basement and upper walls; cracks were from hairline to 13 mm (0.5 in) width; larger cracks were in walls closest to the blast. The authors concluded that the damage in the buildings on sand-clay deposits was caused by failure of the soil manifested in settlements under the building rather than by vibrations of building structures. The threshold level of 102-127 mm (4-5 in/s) was defined. Damage to Foundation Walls C ase 5. Edwards and Northwood (1960) have conducted experiments of controlled blasting operations near buildings with the objective of producing damage. General information is the same as in Case 4. The soil at the second site comprised well-consolidated glacial till which is a high shear strength material consisting of a highly compacted mixture of sand, clay, gravel and boulders. Till was frozen to a depth of about 1 ft at the time of the test. There were three houses at the second site: two with 305 mm (12 in) brick walls and plaster inside, and third with frame and vinyl sliding and wood fiber board inside. Most of the high-frequency components of ground vibrations were obtained in the till soils. Damage inspected in the buildings on till: some horizontal cracks; most of the windows in the ground floor longitudinal walls were broken while those in the walls normal to the blast remained intact. Major damage was inflicted on one building on till soil. Considerable section of the rear basement wall fell away and the masonry walls above ground level were bulged out approximately 76 mm (3 in). The threshold level of 102-127 mm (4-5 in/s) was defined. Vibration Limits and Surveys of StructuresCase histories presented above demonstrate various soil contributions to blasting effects on wave propagation and soil failure. Wave paths in saturated soils underlain FORENSIC ENGINEERING 2009343D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .by rock had low attenuation and as a result ground vibrations had higher PPV. Consequently, the vibration limit of ground vibrations was decreased to 3 mm/s (0.12 in/s). This criterion is four times less than the smallest USBM vibration limit. However, the vibration thresholds of soil failure like liquefaction, settlement and soil movement damaged foundation walls were 1.5-5 times higher then the highest USBM limit in most cases.The obtained results were determined for the specific cases, and the application of these criteria to other sites can be used only for preliminary assessment of blasting effects because each site is unique. It is necessary to keep in mind that changes of blast parameters, wave paths and structure types may yield different outcomes and require different safe criteria, Siskind et al. (1980), Siskind (1991), Siskind and Stagg (2000), Svinkin (2006, 2007). Such criteria are not always available. Therefore in addition to vibration measurements, it is imperative to use other approaches like the condition survey of houses and buildings that provides good opportunities for assessment of blasting damage to structures.Pre- and post-blast surveys of structures have to be professionally performed because different kind of static and dynamic loading like environmental forces, geotechnical hazards, and dynamic effects of blasting can be the causes of similar structural damage, and such damage can exist before the beginning of blasting. It is common to consider only the effects of environmental forces such as temperature, humidity, wind, rain, snow, etc. on structural integrity. However, it is particularly important to analyze the geotechnical natural hazards at various distances from construction sites such as heave, settlement, stability, sliding and failure which may damage structures prior to construction, during and after construction. There are case histories which show the major effects of environmental forces and geotechnical causes on crack formation and structural damage in spite of blasting at various distances from houses and buildings. For example, the writer analyzed the results of pre- and post-blast structure surveys of the building and found that the geotechnical hazard, a soil failure started prior to the beginning of blasting, was the cause of damage to structures.A pre-construction survey is the first step in the control of structural vibrations to ensure safety and serviceability of adjacent and remote houses, buildings and facilities. There are four goals of this survey: (1) document the existing cracks and other damage; (2) ascertain the cause(s) of the existing cracks and other damage; (3) assess susceptibility rating of structures; (4) determine mitigation measures of blasting effects on structures.At some sites, the condition survey can provide more information about benign and harmful blasting effects on structures than vibration measurements performed during blast operations because the vibration criteria cannot embrace the whole spectrum of the blast effects on structures. The condition surveys must always accompany measurements of blasting vibrations. Surveys of structures and analysis of damage to structures should be performed by engineers who have an experience in Soil-Structural Dynamics and evaluation of blast effects on soil and structures. For FORENSIC ENGINEERING 2009344D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y O c e a n U n i v e r s i t y o f C h i n a o n 11/19/13. C o p y r i g h t A S C E . F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .。