Unit 2 Excavation Equipment-11

UNIT 2 EXCAVATION EQUIPMENT-11 Structure 2.1 Introduction \ Object~ves 2.2 Power Shovels 2.3 Hoes 2.4 Draglines \ 2.5 Clamshells 2.6 Motor Graders 2.7 Trenchers 2.8 Bucket Excavators 2.9 Dredges 2.10 Bulldozers 2.1 1 Tree Dozers 2.12 Rippers 2.13 Scrapers 2.14 Crawler vs Wheeled Equipment 2.15 Summary 2.16 Answers to SAQs 2.1 INTRODUCTION I 'The type of foundation to be used for any particular structure will have to be determined at the planning stage. The type chosen will depend to a large extent on the surface or subsurface conditions encountered at the site. Soil or subsoil in its natural state is often sufficiently stable to support the foundations of light structures. Foundations for heavy structures, on the other hand, will be brought to a level with sufficient bearing strength, or to bedrock (unless the bedrock occurs at a very great depth). To build or clear a foundation, an excavation is required, and this operation will usually be carried out by some type of power excavation equipment. This unit describes the different type of excavating equipment used in construction. In this unit, various types of equipment, their application and production rates in carrying out earthwork are described. A,Rer studying this unit, you should be able to understand the working of power shovels, the factors affecting their output, the procedure for selecting the type and size of shovel and calculate the production rates, understand the operations and determine the output of hoes and draglines, a explain the size of clamshell buckets and work out the output of clamshells, a understand the operation and find the output of motor graders, differentiate between the types of trencher and the factors affecting their selection and preduction, I 23 Construction Equipment explain the types of bucket excavators and compute their production rates, differentiate between various types of dredges, I explain the operations of a bulldozer, a tree dozer and scrapers, I explain the use of and the method to assess output of rippers, and explain the relative merits ofcrawler or track based equipment and wheel based equipment. 2.2 POWER SHOVELS Power shoveis are used mainly to excavate earth and load into trucks or tractor-drawn yagons. They can excavate all types of earth except solid rock without prior loosening. They may be mounted on crawler tracks (hence called crawler-mounted shovels), in which case they have very low travel speed and give low soil pressures and so suited to soft ground. The rubber-tyre-mounted shovels which have higher travel speeds are useful for small ipbs where considerable travelling is involved and where the road surfaces and the ground are firm The bask parts of pr parer shove! include the mounting, cab, boom, dipper stick and dipper (Figure 2. I(B)}, QthGr n#ochmmta to the rhoval include hoe, dragline, 'PTal?nah4ttarndarndafmifPtg%W22~)). , . " . .+ ..:. ,.... < ""... .,,..., -..*--4-.+-2 ..- s- * Size of Power ~'havels' The size of power shovels is denoted by the size of the dipper in m3. In measuring the size of the dipper the earth is struck even (giving "struck volume") with the contour of the dipper. This is referred to (as said above) as the struck volume, to distinguish from heaped volume which a dipper may pick up in loose soil. Power shovels are commonly available in dipper size of 0.29,0.38,0.57,0.76,0.95,1.14, 1.33, 1.53 and 1.91 m3. Figure 2.1 : (a) Power Shevets Excavation Equiptnent-11 (b) Altrchmeab of shovel oaiL Figure 2.1 : (b) Attachments of Shovel unit Operations of Shovels 1 Positioning the shovel near the face of the earth to be excavated, the dipper is 1 lowered to the floor of the pit, with the teeth pointing into the face. A crowding force is applied through the dipper shaft and at the same time tension is applied to the hoisting line to pull the dipper up the face of the pit. If the depth of the face (called the depth of cut) is just right, the dipper will be filled as it reaches the top of the face. If the depth is shallow it will not be possible to fill the dipper completely without excessive crowding and hoisting tension. If the depth of cut is more than is required to fill the dipper, the depth of penetration of the dipper into the face must be reduced if the full face is to be excavated or to start the L excavation above the floor of the pit. Optimum Depth of cut for Power Shovels ik ik ; The optimum depth of cut is that depth which produces the greatest output and at ' which the dipper comes up with a full load. Angb of Swing of Power Shovels The angle of swing of a power shovel is the horizontal angle, expressed in degrees, between the position of the dipper when it is excavating and the position when it is discharging the load. The total cycle time includes digging, swinging to the dumping position, dumping, and returning to the digging position. Output of Power Shovels ,I The actual output of a power shovel depknds on : (a) Class of material (b) bepth of cut (DOC) (c) Angle of swing (AOS) Conetruetion Equipment (d) Job conditions (e) Management conditions (f) Size of haul units (g) Skill of the operator (h) Physical condition of the shovel. Selecting Type and Size of Power Shovels If the work areas are scattered, the mobility of the rubber-type-mounted shovel is advantageous. If the work is concentrated in large jobs, mobility is less important and a crawler-mounted shovel is desirable. A crawler shovel is cheaper than a rubber tyred shovel. In selecting the size of a shovel, the two primary factors are : the cost per m3 of the material excavated and the job conditions under which the shovel will operate. In estimating the cost per m3, the factors considered are : (a) The size of the job, as a larger job may justify the higher cost of a large shovel, (b) The cost of transporting a large shovel will be higher than a small one, (c) The depreciation rate for a large shovel may be higher than for a small one. Disposing off a large shovel at the end of a job is more difficult than a small one, (d) The cost of downtime repairs for a large shovel may be more than for a small one, due to delays in getting the parts of a large shovel, (e) The cost of drilling, blasting and excavating rock for a large shovel may be less than for a small shovel as a large ,machine can handle bigger rocks. This may effect some saving-in drilling and blasting, and 3 (f) The cost of wages per m will be less for a large shovel than a small one. The following job conditions need to be considered in selecting the size of a shovel : ) (a) High lifts from the working face to the havl units will require the long reach of a large shovel, \ (b) If blasted rock is to be excavated, the large sized dipper will handle bigger rocks, (c) If the material to be excavated is hard and tou~&>a larger shovel will handle the material more easily, if (d) If the time allotted for completing the job requires a high hourly output, a large shovel must be used, (e) If large haul units are available, than a large shovel should be used, and (f) The weight limitations and clearance of bridges and underpasses may restrict the size. Example 2.1 Determine the expected production of a power shovel given the following data : 3 Size of bucket = 1.14 m Actual depth of cut = 3.15 m Actual angle of swing = 75" Class of material : sand and gavel Job and management conditions : excellent-good Working hour = 50 min, per hr. Solution Excavation Equipment-I1 3 I [deal production = 206 m (bank)/hr. I Optimum depth of cut = 2.1 m Job and management factor = 0.8 1 100 Percent of actual to optimum depth of cut = 3.15 x - = 150% 2.1 Conversion factor for DOC and AOS for a power shovel = 0.935 3 Average production = 206 x 0.935 = 192.61 m (B)/hr. Expected production = 50 x 192.61 x 0.81 = 130 m3 (B)/hr. 60 SAQ 1 (a) Explain the operations of a power shovel. (b) How do you define the optimum depth of cut for a power shovel? I (c) How is "angle of swing" defined for a power shovel? (d) On what factors does the selection of type and size of a power shovel depend? I 2.3 HOES r The term hoe applies to an excavating machine of the power-shovel group. It is referred to by several other names, such as, backhoe, back shovel, and pull shovel. Figure 2.2 shows a typical cable operated backhoe. A power shovel is converted into a backhoe by installing a'dipper stick and a dipper at the end of the shovel boom. A hoe is frequently equipped with a gooseneck boom to increase the digging depth of the machine. Hoes are used primarily to excavate below the level at which the machine r&sThey are adapted to dig trenches, pits and basements. Due to their rigidity they are superior to draglines in operating on close-range work and dumping into trucks. Because of the direct pull on the dipper, hoes may exert greater tooth pressure than power shovels.

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