# Chapter 14: Controlling the Effects of a Blast ## Learning Objectives - Describe flying material, its causes, and techniques to control it. - Explain what a blasting mat is and how to use it properly. - Explain the purpose of a test shot. - Discuss the different types of ground vibration, how they are measured, and the damage they can cause. - List and describe factors affecting ground vibration. - Describe how ground vibration can be controlled. - Explain scaled distance and the formula used to measure it. - Describe the relationship between shock waves and fragmentation. - Describe air blast and the damage it can cause. - Explain the factors that affect air blast. - Describe techniques used to control air blast. - Describe backbreak and how to control it. --- ## Overview Explosives used in rock blasting produce both desirable and undesirable effects. The desirable effects include breaking up and moving rock. The undesirable effects include flying material, ground vibration, and air blast. This chapter focuses on how to control the undesirable effects of a blast. --- ## Flying Material Flying material is the undesirable throw of debris from an explosion. High-pressure gases can propel materials a considerable distance with great force. Flying material can cause serious injury and property damage. All blasting operations are capable of producing flying material. The blaster should know the causes and the techniques necessary for controlling it. The blaster and the employer are responsible for protecting people and property from flying material. ### Types of Flying Material Flying material is any material (such as rock, pieces of blasting mats, and overburden) disturbed by the blast. The most common type of flying material is rock (or "fly rock"). ### Causes of Flying Material There are many causes of flying material, including the following: - Geological seams, planes, and cracks that cause the rock to break unevenly - Geological cavities that collect an excessive amount of explosives - Poor pattern design with insufficient or excessive burden and spacing - Improper distribution of explosives in the rock - Shallow holes or "crater" blasting without sufficient containment - Blast holes that have been overloaded - Improper delay timing that does not provide adequate burden relief - Collar priming, which can create greater throw than bottom (toe) priming - Failure to use covering material, blasting mats, or sand to contain the flying material - Inadequate or insufficient stemming - Reduced burden due to improperly placed angle holes or wandering holes at the face To summarize, flying material can be produced if: - The rock is abnormal. - The drill pattern is inaccurate. - Explosives are overloaded. - The sequence of initiation is improper. - Effective containment, such as blasting mats and cushion (ground material placed in front of the open face), is not used. ### Techniques to Control Flying Material To control flying material effectively, the blaster should do the following: - Select the most appropriate drill pattern. This is usually determined by the type of explosive, the depth of the cut, the type of rock, and other factors such as the proximity of structures. - Consult with the driller about the nature of the material. Drilling the material should reveal the location of any abnormalities (e.g., slips, cavities, and groundwater). Drillers must log and communicate these abnormalities to the blaster, who may then make adjustments while loading the blast. - Double-check the burden of face holes to ensure it is adequate, especially when re-blasting a misfire. - Choose the most suitable explosive for the conditions, with an energy factor that is adequate but not excessive. - Properly load each hole. Should a cavity, fault, or slip exist, load accordingly. Beware of cavities, and do not overload a hole. Monitor column rise in the blast hole. - Ensure the hole is properly stemmed. The minimum depth of stemming is generally 1.0 to 1.1 times the burden distance. Ideally, the stemming material is clear, crushed rock. - Choose the most suitable initiation system. Delay-sequence blasting must allow for adequate burden relief. Generally, bottom (toe) priming does not create as much throw as top (collar) initiation, and is not as likely to result in a misfire. - When blasting near potentially occupied buildings or structures, blasting mats must be used to contain flying material. In remote areas, if blasting mats are not available, the blaster should cover the blast with a layer of sand or other soft fill material. ### Blasting Mats Blasting mats are usually constructed of rubber tires. These mats contain small gaps to retain debris but allow the escape of explosive gases. Woven-steel blasting mats are also available. These mats provide flexibility and effective cover. These mats are considered more environmentally friendly because they do not release rubber into the environment. Solid coverings (such as steel plates) should not be used. They can be thrown by the expanding gases. #### Using Blasting Mats Matting a blast requires a skilled operator and non-verbal communication between the blaster and the machine operator. Hand signals for rigging and heavy equipment operation are set out in section 15.20 of the OHS Regulation. It is essential that the blaster and the machine operator discuss and confirm these signals prior to matting a blast. In these discussions, do the following: - Determine blind spots and establish clear hand signals. - Discuss where the blaster wants the machine to be positioned to swing the mats. - Discuss where the rigger will be positioned to hook the mat. - Ensure that the machine operator never lowers a mat onto blast holes without direction from the blaster of record. - Ensure that mats are never dragged over blast holes. - Discuss where the blaster would like the mats to be placed after the blast for the next shot. - Discuss cushion material and placement. - Inspect rigging and blasting mats for holes, embedded rock, or other defects before placement. For large blasting mats, aim for a 1 m (3 ft.) overlap between mats. Blasters must be aware of potential backbreak and allow for adequate cover, especially on sloped terrain. In some cases, survey paint can be used to show the area around the blast that the mats should cover. On steep slopes, it may be necessary to drill a few holes for pins and to secure mats in place using chains or cables. This prevents the mats from sliding away. Blasting mats are not failproof, and there have been many cases where blasting mats ended up in power lines and other unwanted places. The first line of defence always involves the following: - A good drill pattern - Proper explosive load - The right type and amount of stemming - Effective burden relief - A cushion of muck or earth in front of the open face prior to covering the blast #### Hand Mats For small blasts, such as boulders and in places with limited machine access, hand mats may be used. These mats are usually made from conveyor belts cut and woven into squares or rectangles. Hand mats are moved into place by hand. Multiple mats are needed to adequately cover a small blast, and they should overlap. At times, covering a blast with mats is impractical. To protect property from damage, it may be more effective to place a substantial guard or covering material directly over the object requiring protection. However, this is not acceptable practice in close proximity blasting. > **In the Regulation:** Requirements for close proximity blasting are detailed in sections 21.86 to 21.93 of the OHS Regulation. ### Test Shot A small test blast (or test shot) should always be conducted first. A test shot is especially valuable in the following cases: - Close proximity blasting - Whenever there is doubt as to the effect of the explosives on the material A test shot allows the blaster to assess the material and make any adjustments prior to engaging in larger blast operations. --- ## Ground Vibration When an explosive detonates, rock fracturing occurs due to the pressure of the high-density and high-temperature gases in the blast hole. When the gases enter these fractures, bending occurs and causes the fractures to open and expand outward from the hole. Although the explosives in a blast are designed to break and move rock, some energy is wasted as ground vibration. Excess ground vibration is normally the sign of a poorly designed blast pattern or a lack of adequate delays. ### Types of Vibration Vibrations move in three different ways or directions: - **Vertical** (up and down) - **Lateral** (forward and backward) - **Transverse** (left and right) ### Measuring Ground Vibration In blasting, ground particles move in response to vibration. To measure ground vibration, a seismograph is used. This device can measure waves in four different ways: - **Displacement** is the distance the particle moves from its position of rest. - **Acceleration** is a measure of the maximum change in speed of the particles. - **Frequency** is the number of times the particles move back and forth in 1 second. Frequency is measured in hertz and is one of the most important factors controlling the response of structures. (Blasters normally want high-frequency readings and aim to avoid low-frequency blasts.) - **Particle velocity** measures the speed at which the particles in the ground are vibrating during the blast. This is measured in inches per second (in./s) or millimetres per second (mm/s). Particle velocity (along with frequency) is primarily used to determine damage potential for structures. ### Ground Vibration Damage Excessive ground vibration (or ground motion) caused by blasts can lead to damage to structures. The most common type of damage associated with excessive ground motion is the cracking of plaster or drywall walls in residential homes. > **Note:** When blasting near buildings and other structures, the blasting contractor should have pre-inspections done on those structures before starting the work. Some municipalities require pre-inspections and blasting permits. The U.S. Bureau of Mines (USBM) carried out research to determine the effects of various ground vibration levels on nearby residential buildings. The findings are shown in the following table. #### Effect on Nearby Residential Buildings by Ground Vibration Level | Ground Vibration Level (PPV) | Effect on Nearby Residential Buildings | |------------------------------|---------------------------------------| | 2.0 in./s | Safe level | | 4.0 in./s | Threshold of damage: Opening of old cracks, formation of new cracks, dislodging of loose objects | | 5.4 in./s | Minor damage: Fallen plaster, broken windows, fine cracks in masonry, no weakening of structures | | 7.6 in./s | Major damage: Large cracks in masonry, shifting of foundation-bearing walls, serious weakening of structures | ### The Z-Curve Later research by the USBM further examined safe criteria for residential structures. These criteria were based on the impact of ground vibration on plaster and drywall, some of the weakest components of a residential structure. This research developed what is known as the Z-curve, which shows the allowable limits of vibration frequency and intensity for blasting. The Z-curve is today's standard for ground vibration on residential structures. The Z-curve established lower ground vibration limits than those shown in the table above. In the Z-curve, allowable levels of vibration frequency and intensity fall below the lines shown. Vibration levels that are higher than the limits appear above the lines. Key Z-curve thresholds: - Drywall: 19 mm/sec (0.75 in./s) - Plaster: 12.7 mm/sec (0.5 in./s) - Upper limit: 50.8 mm/sec (2 in./s) The Z-curve is solely for residential structures. Industrial structures can withstand much higher levels of ground vibration. ### Factors Affecting Ground Vibration All structures close to a blast site will respond to ground vibration depending on the following: - Distance - Charge weight per delay - Frequency of the vibration - Shot design - Confinement of the blast Differences in geology and building construction can mean some structures are more susceptible to vibration damage than others. #### Factors That Influence Ground Vibration | Factor | Significant | Moderately Significant | Insignificant | |--------|-------------|------------------------|---------------| | Charge weight per delay | ● | | | | Delay interval | ● | | | | Burden and spacing | | ● | | | Stemming (amount) | | ● | | | Stemming (type) | | | ● | | Charge length and diameter | | ● | | | Angle of blast hole | | | ● | | Direction of initiation | | ● | | | Charge weight per blast | | | ● | | Charge depth | | | ● | | Bare vs. covered detonating cord | | | ● | | Charge confinement | | ● | | | Wind and weather conditions | | | ● | ### Controlling Ground Vibration To keep ground vibration under control, do the following: - **Reduce the charge weight per delay.** This is easily done by reducing the number of holes fired on a given delay. If using only one delay per hole, go to a smaller or shorter hole. An alternative is to deck the holes and use more than one delay per hole. - **Be sure to use the correct amount and size of stemming.** Stemming blowout can lead to energy loss. Energy loss leads to overconfinement, which causes ground vibration. - **Do not exceed subdrilling required to break the rock to grade**, as this can increase ground vibration. - **Make sure that the toe burden is not excessive.** Too much burden results in excessive pressure on the next row of holes, causes severe vibration, and may not break the rock to grade. - **If there is not enough relief time for the first row of holes to efficiently move**, consider increasing the time delay between rows. - **The length of delay between holes and rows may also be increased.** However, this may result in complaints from nearby residents and businesses because long blasts (1 s or 1000 ms, for example) tend to increase air blast. To reduce the risk of complaints, the blast duration should be as short as possible while allowing enough time for each hole to fire and move the rock before the next hole fires. ### Measuring Scaled Distance Scaled distance gives a blaster an estimate of effects that a blast may have on a structure at a given distance. Scaled distance compares blast effects (e.g., ground vibration) from various sizes of charges of the same explosive at various distances. Scaled distance provides a good starting point to determine: - An allowable charge weight of explosives per delay - What hole diameter to use - If decking may be required Check local bylaws to find out if scaled distance is approved for use in blasting operations. In the absence of seismographs, scaled distance can be calculated with the following formula. #### Scaled Distance Formula The imperial formula for scaled distance is: ``` Ds = D ÷ √W ``` Where: - Ds = Scaled distance (ft./lb.) - D = Distance (ft.) to nearest structure - W = Maximum weight of explosives (lb.) per delay - √ = Square root The following variations of this formula can be used to calculate the distance to the nearest structure and the maximum weight of explosives per delay: ``` D = Ds × √W ``` ``` W = (D ÷ Ds)² ``` #### Example: Calculating Maximum Weight of Explosives per Delay A house is 300 ft. away. The local bylaws allow a scaled distance of 50. What is the maximum weight of explosives per delay that could be fired without causing damage? ``` W = (D ÷ Ds)² W = (300 ÷ 50)² W = 6² = 36 lb. ``` If loading up to 18 lb. of explosives per hole, two holes could be fired on one delay. If a hole contains 34 lb., only one hole could be fired per delay. If a hole contains 72 lb., the blaster would need to deck and use two delays per hole. ### Predicting Peak Particle Velocity (Ground Motion) The ground vibration from a construction blast can be determined before the blast is fired. The prediction equation for a typical construction blast is as follows: ``` PPV = 51 × Ds⁻¹·¹⁵ ``` Where: - PPV = Peak particle velocity (in./s) - 51 = A constant - Ds = Scaled distance (ft./lb.^1/2) This is an industry standard that applies to all rock types encountered in construction blasting. It aims to predict the upper limit of ground vibration. Most blasts should have vibration levels below these predicted limits. ### Shock Waves and Fragmentation Studies have confirmed that shock waves have no effect on fragmentation. Shock waves decrease very rapidly and do not have enough energy within less than a metre from the blast hole to have any noticeable effects on the rock. In addition, data supports that shock waves have no effect on ground vibration. For example, when compared on a scaled-distance basis, black powder (a deflagrating low explosive) creates the same level of ground vibration as dynamite and ANFO (high explosives). However, black powder has no shock wave effects. --- ## Air Blast Air blast is an atmospheric pressure wave that transmits from the blast outward to the surrounding area. This pressure wave consists of an audible sound that can be heard and a concussion sound that can be felt but not heard. Weather plays a large part in how far away this air blast is heard or felt. This pressure wave, which is also known as overpressure, may be able to cause damage. However, air blast is mostly considered a nuisance, as it can rattle windows in nearby buildings. Part of the blaster's responsibility is to reduce air blast levels as much as possible. ### Air Blast Damage Air blast is measured in decibels (dB) and in pounds per square inch (psi). The following table illustrates some of the typical sound and pressure levels and the damage they can produce. #### Sound and Damage Levels | dB | psi | Damage Type or Noise Comparison | |----|-----|--------------------------------| | 180 | 3.0 | Structural damage | | 176 | 2.6 | Plaster cracks | | 164 | 0.5 | Windows break | | 128 | 0.007 | Maximum accepted by U.S. Bureau of Mines | | 120 | 3 × 10⁻³ | Jackhammer (10,000 impacts a day; complaints likely) | | 100 | 3 × 10⁻⁴ | Pneumatic hammer | | 60 | 3 × 10⁻⁶ | Conversational speech | | 0 | 3 × 10⁻⁹ | Threshold of hearing | ### Causes of Air Blast Common causes of air blast include the following: - Stemming blowout - Displacement of the rock face - Secondary or boulder blasting ### Factors Affecting Air Blast The most important factors that affect air blast are weather and atmospheric conditions. Temperature inversions and surface winds can affect air blast considerably. Temperature inversions are quite common in the early morning when cool ground air lies below warm air. In an early-morning blast, the pressure wave created by the blast passes up through the cool air and hits the warm air. Due to the change in temperature, refraction causes the pressure wave to bend. This can cause the pressure wave to travel back to the ground and may lead to higher sound levels than predicted. Wind direction and speed also affect the travel of the pressure wave. The wave will follow the direction the wind is blowing to a degree. Wind speed is usually lower at ground level than higher up. In addition, cloud cover can amplify noise due to refraction. Humidity differences can also cause refraction to occur. For these reasons, it is preferable to wait for a clear, sunny day to fire a larger blast. ### Air Blast Control Techniques Air blast does not commonly cause structural damage, but it may rattle windows and doors. Windows that are poorly mounted are more likely to rattle and crack (or even break) depending on the level of air blast. The following methods can help reduce air blast damage and complaints: - Stay below the maximum air blast levels outlined by the U.S. Bureau of Mines. If levels can be kept under 134 dB, the chance of any damage to a structure is slight. However, complaints may still occur. - Stem every hole to a proper depth. Two or three poorly stemmed holes in a blast can lead to complaints. - Use the proper stemming material (¼-in. to ½-in. clear, crushed stone). Ensure that there is no bridging of material, especially in small-diameter blast holes. - Make sure that burden and spacing are as planned, especially the burden at the face. - In the case of mud seams or voids, mark these on the drill sheet and avoid loading explosives near them. Decking may be required in these holes. - When possible, avoid firing the blast in the direction of highly populated areas. - Use the proper sequence of delays. Make sure to use sufficient delay times between holes and rows. - When carrying out secondary blasting (i.e., blasting boulders), use smaller-diameter holes and adequate stemming material. - Ensure that burden is sufficient when blasting angle holes. --- ## Controlling Backbreak (Wall Control) In some cases, it may be necessary to ensure a clean wall after the last row of holes is blasted. This is known as wall control. Blasters have a couple of wall-control options to choose from. ### Line Drilling Line drilling means drilling a single row of closely spaced, unloaded holes neatly along a desired excavation line. This provides a plane of weakness that fractures will break to, reducing the risk of backbreak. This results in less shattering and stressing of the finished wall. For many years, line drilling was the only technique used to control backbreak. Today, pre-splitting or pre-shearing is the most common technique. ### Pre-Splitting or Pre-Shearing Pre-splitting or pre-shearing is a method of blasting that involves drilling and loading a single row of holes designed to minimize or eliminate backbreak to produce a smooth rock wall. This row is initiated before the primary blast to produce a planar crack. The crack helps to screen the surroundings from ground vibrations during the firing of the main round. Detonating cord is often used to trace the loaded blast holes using this method. Detonating cord for this application is usually of much higher explosive core load (typically 150 to 400 grains/ft.) than detonating cord normally used to tie in a blast. > **Note:** For blast design calculations for precision pre-splitting, see Chapter 23 (page 386).