Of all the surveying operations used in construction, levelling is the most common. Practically every aspect of a construction project requires some application of the levelling process. The more general are as follows.
This type of levelling is used to produce ground profiles for use in the design of roads, railways and pipelines.
In the case of such projects, the route centre-line is set out using pegs at 10 m, 20 m or 30 m intervals. Levels are then taken at these peg positions and at critical points such as sudden changes in ground profiles, road crossings, ditches, bridges, culverts, etc. A plot of these elevations is called a longitudinal section.
When plotting, the vertical scale is exaggerated compared with the horizontal, usually in the ratio of 10 : 1. The longitudinal section is then used in the vertical design process to produce formation levels for the proposed route design (Figure 3.26). Whilst the above process produces information along a centre-line only, cross-sectional levelling extends that information at 90◦ to the centre-line for 20–30 m each side. At each centre-line peg the levels are taken to all points of interest on either side. Where the ground is featureless, levels at 5 m intervals or less are taken. In this way a ground profile at right angles to the centre-line is obtained.
Longitudinal section of proposed route
When the design template showing the road details and side slopes is plotted at formation level, a cross-sectional area is produced, which can later be used to compute volumes of earthwork. When plotting cross-sections the vertical and horizontal scales are the same, to permit easy scaling of the area and side slopes .
From the above it can be seen that sectional levelling also requires the measurement of horizontal distance between the points whose elevations are obtained. As the process involves the observation of many points, it is important to connect to existing BMs at regular intervals. In most cases of route construction, one of the earliest tasks is to establish BMs at 100 m intervals throughout the area of interest. Levelling which does not require the measurement of distance, such as establishing BMs at known positions, is sometimes called ‘fly levelling’.
A contour is a horizontal curve connecting points of equal elevation. Contours graphically represent, in a two-dimensional format on a plan or map, the shape or morphology of the terrain. The vertical distance between contour lines is called the contour interval. Depending on the accuracy required, they may be plotted at 0.1 m to 0.5 m intervals in flat terrain and at 1 m to 10 m intervals in undulating terrain. The interval chosen depends on:
(1) The type of project involved; for instance, contouring an airstrip requires an extremely small contour interval.
(2) The type of terrain, flat or undulating.
(3) The cost, for the smaller the interval the greater the amount of field data required, resulting in greater expense.
Contours are generally well understood so only a few of their most important properties will be outlined here.
(1) Contours are perpendicular to the direction of maximum slope.
(2) The horizontal separation between contour lines indicates the steepness of the ground. Close spacing defines steep slopes, wide spacing gentle slopes.
(3) Highly irregular contours define rugged, often mountainous terrain.
(4) Concentric closed contours represent hills or hollows, depending on the increase or decrease in elevation.
(5) The slope between contour lines is assumed to be regular.
(6) Contour lines crossing a stream form V’s pointing upstream.
(7) The edge of a body of water forms a contour line.
Contours are used by engineers to:
(1) Construct longitudinal sections and cross-sections for initial investigation.
(2) Compute volumes.
(3) Construct route lines of constant gradient.
(4) Delineate the limits of constructed dams, road, railways, tunnels, etc.
(5) Delineate and measure drainage areas.
If the ground is reasonably flat, the optical level can be used for contouring using either the direct or indirect methods. In steep terrain it is more economical to use other heighting, as outlined lat