CHAPTER 14 DIRECT LEVELING AND BASIC ENGINEERING SURVEYS

Figure 13-31.-Kinds of angles measured at the occupied station.

Figure 14-1.-Common types of bench mark construction and application.

Engineering Aid 3 - Beginning Structural engineering guide book
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CHAPTER 14 DIRECT LEVELING AND BASIC ENGINEERING SURVEYS Leveling is an operation that is used for determining the elevations of points or the differences in elevation between points on the earth’s surface. This operation is extremely vital for deriving necessary data required for various engineering designs, mapping, and construction. Data from a finished level survey are used to (1) design roads, highways, and airfields; (2) develop maps, showing the general configuration of the ground; (3) calculate volume of earthwork; and (4) lay out construc- tion projects. In this chapter, we discuss the basic principles of DIRECT LEVELING and the types of methods used; the duties and responsibilities of the leveling crew; field procedures used in differential leveling; precision in leveling; and proper ways of handling leveling instruments and equipment. INDIRECT LEVELING, such as barometric and trigonometric leveling, adjustment of level network, and end areas and volume of earth’s computations, is not covered in this book. In this chapter, you will find a general description of basic engineering surveys and various construction-site safety hazards commonly associated with the EA survey party. Other types of engineering and construction surveys— particularly those for curves and earthwork—will be presented at the EA2 level. BASIC TERMS USED IN LEVELING OPERATIONS Generally, the basic vertical control for topographic survey and mapping is derived from first- and second-order leveling. For many construction projects and for filling gaps between second-order bench marks (BMs), less precise third-order leveling is acceptable. In leveling, a level reference surface, or datum, is established, and an elevation is assigned to it. This datum may be assigned an assumed elevation, but true elevation is required for the establishment of a BM. A series of properly established BMs is therefore the framework of any vertical control. Although further discussion will follow, fundamentally, direct leveling describes the method of measuring vertical distances (dif- ferences in elevation) between the plane of known or assumed elevation (datum) and the plane of a point whose elevation you are seeking. Once these distances are known, they may be added to, or subtracted from, the known or assumed elevation to get the elevation of the desired point. These vertical distances are obtained by use of a leveling rod and, usually, an engineer’s level. Some of the basic terms commonly used in leveling operations are defined in the following paragraphs. BENCH MARK ABM is a relatively permanent object, natural or artificial, bearing a marked point whose elevation is known. BMs are established over an area to serve as (1) starting points for leveling operations so the topographic parties can determine other unknown elevation points and (2) reference marks during later construction work. BMs are classified as PERMANENT or TEMPORARY. Generally, BM is used to indicate a permanent bench mark and TBM, to signify a temporary bench mark. TBMs are established to use for a particular job and are retained for the duration of that job. Throughout the United States, a series of BMs have been established by various government agencies. These identification markers are set in stone, iron pipe, or concrete 14-1

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