Article_Impact of Blast Induced Vibrations on Buildings

7/23/2019 Article_Impact of Blast Induced Vibrations on Buildings

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Report on impacts of blasting activities in the vicinity of

Nakatooke quarry.

An Assessment of the baseline survey.



1. INTRODUCTION .......................................................................................................................................... 3

1.1. A

BSTRACT

................................................................................................................................................. 3

1.2. EARTH AS A BUILDING MATERIAL: ..................................................................... .............................. 3

2. WHY THE ASSESSMENT? ................................................................................................................ 4

3.

THE BLAST AND ITS INTERACTION WITH STRUCTURES

........................................... 5

3.1. T

HE NATURE OF THE BUILDING MATERIAL

..................................................................................... 5

3.2. THE

G

ROUND

V

IBRATION AND

A

IR

B

LAST

..................................................................................... 6

3.3. MEASUREMENT OF DAMAGE POTENTIAL .......................................................................................... 7

3.4. SAFE LEVELS OF GROUND VIBRATIONS AND AIR BLAST. ......................................................... 8

4. FINDINGS -VIBRATION MONITORING RESULTS .............................................................. 11

5. CONCLUSIONS ...................................................................................................................................... 11

6. ACTIONS TAKEN ................................................................................................................................. 12

7. RECOMMENDATIONS .......................................................................................................................... 12

7.1. I

MPLEMENTATION OF

L

IMITS

............................................................................................................. 12

6 1 1

Existing Recommendations

......................................................................................... 13

6 1 2 Recommendations for Isimba quarry site ..................................................... 13

8. REFERENCES ........................................................................................................................................ 13

9. APPENDICES ......................................................................................................................................... 14

9.1. A

PPENDIX

-1

:R

ECORDS OF

V

IBRATION

M

ONITORING

. .............................................................. 14

9 1 1

Vibration Readings 01

.................................................................................................... 14

9 1 2 Vibration Readings 02 ................................................................................................... 16



1. Introduction

1.1. Abstract

The purpose of this report is to assess the impact of the Construction

and blasting activities at Nakatooke quarry on the structures of the

residents in the vicinity of Isimba HPP and to make recommendationsaimed at mitigating or limiting the possibility of damages due to

operation of heavy machinery or blasting activities.

The aim of this article is to assess the impact of blasting activities on

the structural integrity of mud and wattle structures vernacular to

Uganda. The report also covers recommendations for operation of

machinery. The structures assessed are located within a 2 km radius of

the Nakatooke quarry located in Kayunga district.

1.2.

Earth as a building material:

In scientific terms, earth is referred to as loam. It is a mixture of

clay, silt (very fine sand), sand and occasionally, larger aggregates. It is

used all over the world in several forms, these being handmade

unbaked bricks (mud bricks/adobes); compressed unbaked bricks (soil

blocks) or compacted within formwork (rammed earth). In Uganda, the

most common structures within rural communities are mud and wattle

constructions. Owing to the fact that loam is not a standardized

construction material, information on its performance as a construction

material has been scarce. However, the need for an analysis of its

performance has been brought about by the opening up of quarries for

large scale industrial constructions. The aggregates were excavated by

blasting, which was carried out within the framework of conditions that

take into account standardized structures built with the conventional

materials such as burnt clay bricks or concrete blocks. However, the

performance of mud and wattle structures does not seem to have been

factored into the framework of considerations that govern the

compliances required for the attainment of permits for blasting

activities.In light of this, the need for an assessment of the performance of mud

and wattle structures under the cited conditions cannot be understated

since blasting activities can have a significant vibration output that has

been known to damage structures.

In order to assess the performance of the subject structures, a baseline

survey was carried out on the structures within a 2 km radius of the

blasting epicenter so as to assess their initial conditions. During the

initial survey it was noted that the structures varied in terms of thecharacteristics. This was attributed to the differences in the amounts



and types of clay used for the construction, the silt content, the

workmanship as well as the form of construction, be it adobes, soil

blocks or rammed earth.

2. Why the assessment?

The need for the assessment stemmed from the fact that the areas in

the vicinity of the quarry were previously populated by several rural

communities for whom the only option in terms of building material

was and still is, the use of mud and wattle. This is because mud and

wattle is the most abundantly available building material, requiring little

or no technology for processing. This lead to a significant population

675 within the proximity of the dam area alone. These Project Affected

People were compensated and the land was handed over to the

Contractor for commencement of works on the Construction of Isimba

Hydro Power Plant. However the communities adjacent to the Projectboundaries are still impacted by the blast waves that occur due to the

detonation of between 500 and as much as 3500 Kg of ammonium

Nitrate emulsion explosives. These blasting activities carried out at the

quarry are approved and a license for the activity has been obtained by

the Contractor. However, the permit conditions only oblige the permit

holder to clear a 300m radius within the vicinity of the blast epicenter.

The blasting activities have been noted to propagate the effects of the

blast wave as far as 5 km away, where the clients offices are situated.

Whereas these may not necessarily damage the concrete block buildingsof the Contractor and the Client, the homes of the locals are mostly

mud and wattle and as such require an independent assessment of the

impacts of the blasting activities.

With Projects of this scale and magnitude, there are stringent measures

put in place to protect the interests of the local communities.

Community sensitization has been conducted to inform the locals of the

potential impact of the blasting activities and of the notice and warning

system that warns of imminent blasting activities. However, this does

not change the fact that structures cannot be shifted and the localcommunities will have to bear the impacts of the blasting activities.

Despite incentives by the EPC Contractor, getting the local communities

to temporarily moved further from the epicenter of the blasting

activities has not been easy to accomplish. The number of households

was just too numerous and the locals will still have to return to their

homes after the blasting activities are completed. A simple disturbance

allowance does little or nothing to restore or maintain a structure

damaged by the blast waves.



3. The blast and its interaction with structures

3.1. The nature of the building material

It has been observed that the impacts of the blasting activities are not

so easily ascertained mainly due to the varying degrees of workmanship

during the construction of the mud huts. However, there have beenclaims that some brick houses have developed cracks as a result of the

blasting at the quarry. In view of the claims the EPC Contractor has

been requested to conduct vibration monitoring at varying distances

from the epicenter of the blasting activities.

The main inherent difficulties with regards to working with adobe are:

1. Loam is not a standardised building material

Depending on the site where the loam is dug out, it will be composed

of differing amounts and types of clay, silt, and aggregates. Its

characteristics, therefore, may differ from site to site, and thepreparation of the correct mix or a specific application may also

differ. In order to judge its characteristics and alter these, when

necessary, by applying additives, one needs to know the specific

composition of the loam involved.

2. Loam mixtures shrink when drying

Due to evaporation of the water used to prepare the mixture (moisture

is required to activate its binding strength and to achieve workability),

shrinkage cracks will occur. The linear shrinkage ratio is usually

between 3% and 12% with wet mixtures (such as those used for mortarand mud bricks), and between 0.4% and 2% with drier mixtures (used

for rammed earth, compressed soil blocks). Shrinkage can be minimised

by reducing the clay and the water content, by optimising the grain

size distribution, and by using additives.

3. Loam is not water resistant

Loam must be protected against rain and frost especially in its wet

state.

The above stated have made it difficult to determine the actual impact

of the blasting activities. In addition, some of the structures had

evident pre-existing damage most likely due to poor workmanship and

possibly due to the shrinkage of the soil as well as the poor water

resistance. However, it was said that the pre-existing damage was

worsened by the impacts of the blasting activities.

Given the potential for damage to property and the substantial nuisancecaused to the local population it is vital to have the impacts quantifiedso as to determine limits and mitigation measures where applicable andpossible. The damage to property can be caused directly by ground

wave movements or indirectly via potentially unstable soil or rockconditions in the vicinity of the quarry site (e.g. soil liquefaction, slope



failure). Air blast is not considered to be a significant factor in causingdamage to structures but is a significant nuisance to the local

communities in the vicinity of the quarry.

In view of the above, records of vibration monitoring will be used toassess the impact and determine measures of mitigation if need be.

3.2. The Ground Vibration and Air Blast

The detonation of an explosive charge in a blast hole results in intense

dynamic stresses around the blast hole. This is caused by the sudden

acceleration of rock mass by the detonation gas pressure exerted on

the hole wall. Consequently, a wave motion is set up in the ground as

the strain waves are transmitted through the surrounding rock mass.

Different mechanisms of breakage fragment the rock mass (crushing,

radial cracking etc) and this impact is localized to the fragmentationzone. The rest of the energy is propagated through the strain waves of

lower intensity, unable to cause permanent deformation to the rock

mass. The Strain waves propagate through the medium as elastic waves,

oscillating the particles through which they travel. These waves in the

elastic zone display visco-elastic behaviour and as such, the strain

waves are attenuated over distance since a fixed amount of energy gets

spread over a larger mass of material with increase in distance.

However, larger amounts of explosives can still result in the propagation

of ground vibrations large enough to cause damage to structures bycausing dynamic stresses that exceed the material strength.

The following are some of the vibration predictor equations:

where v is the peak particles velocity (mm/s), QMAX the maximumcharge per delay (kg), R the distance between blast face to vibrationmonitoring point (m), and K and B the site constants, which can bedetermined by multiple regression analysis.



3.3. Measurement of Damage Potential

Particle velocity is generally adopted worldwide as the best criterion forrelating ground vibrations to building damage. Levels of groundvibrations are also determined by measurement of displacement oracceleration of a particle at the site.

The following equations show the relationship between velocity,displacement and acceleration:

Eventually however, the performance of a structure will depend on amultitude of factors, some of which include the type of foundation,underlying ground conditions and the building construction as well asthe state of repair of the structure.Guidance on the levels of vibrations above which buildings could bedamaged is mainly derived from BS 7385*, however, for detailed

engineering analysis, criteria other than the vibration levels may needto be considered.So as not to overlook the impact of human exposure to blast induced

vibrations, reference is made to BS 6472-2:2008.

Typical damage that can be expected in relation to the threshold valueof the peak particle velocity experienced in the ground waves from theblasts are indicated in table 1.(Reference 5), from which it is evident that theonset of plaster cracking in a house occurs at a threshold peak velocityof 50mm/s (2in./s). This criteria is universally accepted in NorthAmerica.

table 1:

Type of

Structure Type of Damage

Peak Particle Velocity

Threshold at Which Damage

Starts

mm s kv per sec

Rigidly MountedMercury Switches

Trip Out 1225 0.5

Houses Plaster Cracking 50 2Set initial Limit of125 mm/s (5 in. persec) Maximum at theCrusher



Concrete Blockas in a NewHouse

Cracks in Blocks

200 8

Cased Drill HolesRetaining Walls.Loose Ground

MechanicalEquipmentPumps

Horizontal Offset 375 15

CompressorsPrefabricatedMetal Building onConcrete Pads Shafts Misaligned

Cracked PadsBuilding Twistedand

1000 40Beyond 250 rwn/»(10 In per sec) MaforDamage Starts. Suchas Possible Crackingof Cement Block

Distorted 150060

3.4.

Safe levels of Ground Vibrations and Air Blast.

This report's recommendations for reducing ground vibrations and airblast levels are aimed at minimizing distress to people as well asavoiding damage to buildings. Since humans respond to levels of groundvibrations and air blast considerably lower than those necessary toinduce structure damage, the limits recommended construction projectsare quite conservative. As mentioned in Section 3.3, particle velocity is

used as a parameter for damage assessment. It should be noted that ata Construction site such as Isimba HPP, there are multiple sources ofthese vibrations and therefore the limits recommended depend on thevibration source as defined below:

Blasting Pile Drivers, vibratory rollers and traffic

Air blast.The current acceptable levels of ground vibration from blasting arerecommended as in Australian Standard AS 2187-983 (Reference 1).These are shown in Table 2 .

Table 2 – acceptable levels of g round vib ration from b lasting

Notes:



1. In a specific instance, where substantiated by careful investigation, a value of peak

particle velocity other than that recommended in the table may be used.2. The peak particle velocities in the table have been selected taking no consideration of

human discomfort and the effect on sensitive equipment within the building. In

particular, the limits recommended for buildings types 2 and 3 may cause complaints.

Blasting:

The control of blasting procedures to limit ground vibration

levels to those outlines in Table 1 should automatically limit air blastoverpressures to safe levels with respect to building damage. Theproposed maximum levels are shown in Table 3 below .

Table 3 -maximum air blast levels

Pile drivers, Vibratory Rollers and Traffic:The Construction Site has heavy machinery many of which could serveas a source of Vibrations. Ground vibrations caused by these sourcesare of a continuous nature usually lasting for extended time periods.Because of this, it is proposed that vibration limits should be set atlower levels than from blasting. A peak particle velocity (VRmax) limit of5 mm/sec is therefore recommended. Tynan (Reference 2) contains ahandy user guide applicable to vibrating rollers which approximates the

recommended limit. It is shown in Table 4. The actual results attached herein as appendix - 1 indicate forinstance that at a distance of 400m, the vibration readings are 13

mm/s, which is close to the range indicated in table 2. However, itshould be noted that the charge used was over 2000 kg, Withadjustments to the quantities of charge used, the vibrations can easilybe brought down to well within more tolerable limits.



Table 4a: user guid e applicable to vibrating rol lers

* Values in brackets are those suggested to keep claims and complaints to an acceptably low level. For complaints to be stoppedcompletely in residential areas, these values would possibly be needed to be increased still further.

Table 4b:A ir Blast:

Fig. 3.4 - 1 Response of the Human body to mechanical vibration (Goldman, 1948)



Fig. 3.4 - 2 Human and Structural response to sound Pressure levels.

4. Findings -Vibration monitoring results

The vibration monitoring is conducted using a seismograph with analysissoftware. The seismograph consists of a 3-axis velocity transducer anda data acquisition storage device. The blasting analysis software provides

features for graphical output of the wave forms in each of the 3 axesand a comparison of the measured peak particle velocities andfrequency content. The 3 axes correspond to the radial, transverse andvertical components to the velocities measured. The blast recordsanalyzed are taken from data of 5 blasts. The derived data is attachedas Appendix-1.

5. Conclusions

The nature of the building material used by the majority of the locals

has certain inherent characteristics which make identification of actualimpact of the vibration caused by blasting somewhat problematic. The

shrinkage which occurs as the adobe dries, causes crack which areeasily mistaken for cracks caused by vibrations due to blasting. That isnot to say that cracks due to blasting do not occur, but rather, itimplies that the material is not as strong as the Concrete or burntbrick buildings that are within the same range from the blastepicenter. Claims have also been made by owners of houses made ofburnt brick, however, in some instances, visual investigations of thesecracks seemed to indicate that the damage was a pre-existing conditionsince the surfaces of the cracks showed signs of aging. These were

compared with cracks on buildings were it was evident that the cracks



had only recently been formed. The types of structures and the qualityof workmanship was also analyzed during the baseline survey.

It can be concluded from the findings and from comparisons withinternational practices, that the determining factor in setting up a limitfor the vibrations is the human factor and its response to thevibrations. The Effects of vibrations become intolerable to humans at alevels appreciably lower than the levels at which structural damageoccurs. It is therefore only fair that the limits should be set based onthese limits as is common practice internationally.It should also be noted as an example that limits used in the US ofpeak particle velocity of 12.5 mm/s (0.5 in./s) have been known to

reduce the number of complaints by a factor of three compared to50mm/s (2 in./s). In comparison, the United States Bureau of Mining(USBM) recorded complaints on one construction site as high as 30% at50 mm/s, 10% at 12.5 mm/s and 1% at 2 mm/s, which is just theperceptible range. The current blasting code for Ontario, Canada calls

for a maximum peak particle velocity of 10 mm/s.

6. Actions taken

The EPC Contractor has put in place programs to sensitize the localresidents of the possible impact of the blasting activities. This has beendone in accordance with the Explosives' management plan that wassubmitted by the EPC Contractor. The local communities also stand to

gain from transfer of skills that is going on as the local workforceinteracts with foreign Contractor. This transference of skills most ofwhich occurred during the initial stages of the Project as the EPC

Contractor Constructed the Camps, could serve as a template for thefurther development of the region as the locals lean to build better.This is even more pertinent in view of the fact that some of thestructures/Buildings within the 2 km radius of the Blast epicenter hadshowed signs of cracks. Some of these were attributed to poorworkmanship.

7. Recommendations

7.1. Implementation of Limits

Different limits may apply depending upon whether there are national

guidelines in use prior to the introduction of this Report. The EPCContractor is guided by the conditions pertaining to the permits

obtained for the blasting activities. It may turn out that the limits

recommended by this report may not be consistent with National

guidelines on blasting activities. In such cases the applicable limits are

those set down in the Licence or Authority.

Ground vibration and air blast levels are generally measured at the

nearest sensitive site. However, in the interests of minimising potential

negative impacts on the local communities, monitoring has been

conducted at various distances from the blast epicentre to establish

magnitude of ground vibrations propagated to given distances.



6.1.1 Existing Recommendations

In international practice, Work Authorities or Licence Conditions set

limits for air blast and ground vibration measured at sensitive sites

and these are set as follows:

Ground vibration at sensitive sites should be below 10mm/s (ppv*)

at all times, and Airblast at sensitive sites should be below 120dB (Lin Peak*) at

all times.

6.1.2 Recommendations for Isimba quarry site

The levels for vibrations and Airblast for the communities in the

vicinity of Isimba HPP's quarry are recommended as follows:

Ground vibration at sensitive sites should be below 5 mm/s (ppv)

for 95% of all blasts.

Airblast at sensitive sites should be below 115dB (Lin Peak) for

95% of all blasts.

In view of the above, the EPC Contractor is advised to limit thequantities of explosives used so as to ensure that the ground

vibrations and airblast are limited to the ranges indicated above.

Note:

In situations where the location or the nature of the operations

mean that this is not achievable, these standards may be varied,

subject to the relevant authorities being satisfied that all effected

people have given informed consent).

8. References

1. Standards Association of Australia (SAA). Explosives Code AS2187-983Part, Use of Explosives.

2. Tynan A.E. (1973). Ground Vibrations, Australian Road Research Board

Special Report.3. BS 7385-2:1993: Evaluation and measurement for vibration in

buildings. Part 2: Guide to damage levels from ground bornevibration.

4. BS 6472-2:2008: Guide to evaluation of human exposure to vibrationsin buildings. Part 2: Blast induced vibrations

5. Surface Mining. Second Edition, edited by Bruce A. Kennedy, Societyfor mining, metallurgy and Exploration (US).



9. Appendices

9.1. Appendix-1 :Records of Vibration Monitoring.

9.1.1 Vibration Readings 01

Distance from Epicenter 400m

Number of Holes 139

Total Charge Kg) 2034

i. Velocity Graph



ii. Results:

Maximum Velocity 0.13 cm/s

Frequency 22.3 Hz



9.1.2 Vibration Readings 02

Distance from Epicenter 310 m

Number of Holes 148

Total Charge Kg) 3456



i. Velocity Graph

ii. Results:

Maximum Velocity 0.4311 cm/s

Frequency 22.2 Hz

Article_Impact of Blast Induced Vibrations on Buildings

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