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Fig.2.3 Blasthole section view (A) showing termionlogy used in design and ( B) pattern array for layout of holes
greater.
Many blast designs use decked charges formed by dividing the explosive column into two or more individual charges, initiated on the same or different time delays, separated by inert stemming material. Decking is employed to ①conserve explosive use adjacent to weak rock zones, faults, or clay seams ；②reduce the charge quantity detonated on one time delay, lowering ground vibrations ；or (3) bring the powder column up higher in the hole to assure good breakage near the collar. Decked charges should be separated by stemming materials at a length beyond which two adjacent decks do not affect one another. If interdeck stemming is too small，the deck designed to initiate on the earlier time delay may prematurely initiate the second deck. This situation is referred to as sympathetic detonation and may lead to excessively high ground vibrations or flyrock. A rule of thumb for the design of interdeck stem length is to employ the hole radius dimension in feet.
2.Hole Loading Practice
Once drilled, all holes should be plugged or covered to prevent rocks and drill cuttings from filling the hole. Prior to loading, the driller’s log should be checked for
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hole depths, subdrilling, and indications of hard or soft seams, voids, and the presence of water. The blaster must check each hole for any change in conditions prior to final design. This is to ensure that all safety precautions, are considered. Abandoned
holes must be filled with cuttings to prevent flyrock.
Transporting the blasting crew and explosives to the blasting site is done once the site has been identified with markers and the site cleared of all personnel not involved in hole loading. Initiators，cap-sensitive explosives，and noncap-sensitive agents must be transported in accordance with regulations in approved vehicles，carrying classification signs. Proper tools for loading include a cloth measuring tape, a wooden tamping pole, a pair of wire cutters and strippers, a mirror, grappling hooks, and a powder punch. If the blastholes are filled with water, and a water-resistant explosive of a density greater than one is not used，the holes must be pumped or blown free of excess water.
Once the delay sequence is designed, in-hole or surface delays are placed adjacent to respective holes. Each hole is then loaded after it is measured for correct depth. If primers are used for noncap-sensitive explosives, they are made up at the time of loading. Caps or detonating cords are inserted and wrapped around the primer in accordance with manufacturer’s recommendations.
A small amount of main charge is added to the hole bottom, then then the primer carefully lowered. The main charge is poured (for free-running explosives) or carefully lowered (if packaged) , while measuring the loading depth to ensure that each deck or column rises to the designed length (based on known loading density) 3. Surface Delay Blasting
Delay blasting techniques are employed to improve fragmentation, control of rock movement, overbreak, and to reduce ground vibrations. Delays are incorporated into the blast design using electric or nonelectric caps or delay connectors with detonating cord. The delay patterns used in design will determine the sequence of hole os desk initiations，thereby，dictate the overall direction of blasted rock movement and resulting fragmentation. Depending on the S/B ratio, the actual timing (in millisconds) between detonating charges will determine muck pile displacement height and distance from the bench. Fig. 2. 4 shows variations of timing patterns used for surface blasting.The effective spacing is the distance between holes in a row defined by adjacent time delays (e.g. delays by rows). Effective burden is the distance in the direction of resultant.
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mass movement. The V and echelon (diagonal) patterns are used when rock placement is restricted. Designs using two free faces usually provide improved fragmentation an throw control over those using a single face.
The design of initiation timing for multiple-hole blasting is critical to the blasting effectiveness. If the interhole delay is too short, the movement of row burdens is restricted and fragmentation is poor. High ground vibrations result, and backbreak- along the new highwall may persist, jeopardizing the stability of the slope. If interhole delays are too long, cutoffs of surface delays may occur. The minimum time for design is controlled by the stress wave travel distance in order for radial cracking to begin to develop, contributing to the detachment of the rock mass in the vicinity of the hole. This detachment forms an internal free face (or relief) to which successive detonations will interact with the reflection of stress waves.
Vocabulary
explosive n. 爆炸物，炸药 vibration n. 震动
hardness n. 硬，硬度，艰难，难度 fragmentn.碎片；片断
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blast hole n. 炮孔，炮眼 tensile adj. 拉力的，张力的 detonatevt.& vi. (使）爆炸，引爆 Burden] n. 抵抗线 chargevt. 装药 powder factor 单位耗药量，炸药比 stemming n. 填塞物，堵塞物 subgrade n. 地基；路基 fine-tuning 精调谐，微调 Borehole n. 钻孔 rig n. 装备；钻探设备 Breakagen. 破损；破坏 Dictate n. 指示；命令 deck adj. 分层的 sympathetic adj. 赞同的，支持的 flyrock n. 飞石 rule of thumb 凭感觉的方法；单凭经验的方法 subdrilling n. 超深 initiator n. 引发剂，起爆药 cap-sensitive adj. 雷管感度的 primer 火帽；雷管，起爆具 wrapvt. 包，裹；卷 overbreakn. 超爆，超挖 jeopardize] vt. 危及，损害 detachment n. 分离，分开，超脱 2.3 Stripping and Pit Development 露天矿剥离与生产 1. Surface Mining Method Select
The subject of surface mine development is large. This section covers the fundamental concepts involved in both the open pit mining of thick or steeply dipping deposits as well as the strip mining of thin，shallow，and relatively flat-lying seams or beds.
The surface mining method selected for a specific property is generally keyed to the overburden removal system, since this unit operation usually involves the greatest
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capital and operating cost components. Once the stripping method and complementary equipment have been selected, compatible equipment systems are selected for the other unit operations ( Fig. 2. 5).
Standard engineering practice dictates that various alternatives be investigated， not only for the stripping system, but also for the other unit operations. The goal is to provide maximum resource extraction at a minimum cost including both capital and operating cost considerations.
2.Pit Development
(2) Clearing and grubbing. This involves clearing the property of trees and shrubs and then removing the stumps and roots to insure a homogeneous topsoil. Grubbing is often done with rake-like grubbing attachments on agricultural tractors or dozers.
(3) Topsoil removal. Regulations dictate that the topsoil be removed and ultimately replaced upon graded spoils. Topsoil can either be stockpiled at the side of the pit area for later redistribution or hauled immediately to the graded area for redistribution.
In the latter case，the topsoil can either be hauled around the pit or across the pit on spoil bridges. The decision is made on the basis of economics considering topsoil quantities and haul distances.
Self-elevating scrapers are the most common means of removing and redistributing topsoil, although loader/truck or wheel excavator/belt systems find application where topsoil quantities are large and/or haul distances are long.