Civil Engineering: Supervision and Management BOOK DOWNLOAD
Civil Engineering: Supervision and Management BOOK DOWNLOAD
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setting out of triangles
Setting out larger sites Triangulation from a measured baseline is the usual method adopted, the triangles being as well proportioned as possible. This method is usually better than a lengthy closed traverse when the weather is changeable, as a closed traverse rep resents more work to be done at one time and may be interrupted by bad weather.
If a closed traverse has to be interrupted by bad weather and is left for a day, there is always a danger that one or other of the last two traverse pegs is disturbed by site plant. Even if the pegs have not been disturbed, a large closing error may cause the surveyor to think this has happened, and he will feel it necessary to do the whole job again. for more book refer this site
It is worth going to some trouble to find a suitable baseline, which, for preference, should be level and horizontal. If this is not available it is worth while cutting the grass or removing any humps from a piece of horizontal ground so that the tape may be laid flat and given the standard pull required by means of a spring balance.
A new, tested steel tape should be used, the maker's corrections being known and allowed for. A thermometer is also necessary. The temperature should be fairly steady: for example, the work should not be done if there is intermittent hot sunshine between periods of cloud and cold.
A quiet, still, cloudy day is best. Accurate measurement of angles with a theodolite is easier than accurate measurement of distances by tape, so it is worth finding a good baseline at the expense, possibly, of not having the best angled triangles for setting out other points. Preliminary calculations before deciding on the baseline will indicate whether a satisfactory degree of accuracy can be obtained.
- A 1 s theodolite should give an accuracy of ±l.S mm at 300 m. Distances can also be measured by EDM (electromagnetic distance measurement) equipment, or by electro-optical instruments.
- The former are used for geodetic surveying over long distances and are not applicable to most site work.
- The latter can be medium- or short-range instruments, and may be useful for large sites.
- However, as the average construction site only needs the measurement of one baseline, and most of the rest of the work is set out by theodolite and tape work, the expense of this distance-measuring equipment is seldom justified.
Excavating and earth-placing machinery Bulldozers ('dozers') are used for cutting and grading work, for pushing scrapers to assist in their loading, stripping borrowpits, and for spreading and com pacting fill.
The larger sizes are powerful but are costly to run and maintain, so it is not economic for the contractor to keep one on site for the occasional job. Its principal full-time use is for cutting, or for spreading fill for earthworks in the specified layer thickness and compacting and bonding it to the previously com pacted layer.
It is the weight and vibration of the dozer that achieves compaction, so that a Caterpillar 08 115 h.p. weighing about 15 t, or its equivalent, is the ma chine required; not a 06 weighing 7.5 t, which is not half as effective in com paction.
The dozer cannot shift material very far; it can only spread it locally. A dozer with gripped tracks can climb a 1 in 2 slope, and may also climb a slope as steep as 1 in 1.5 provided the material of the slope gives adequate grip and is not composed of loose rounded cobbles. On such slopes of 1 in 1.5 or 1 in 2 the dozer must not tum, but must go straight up or down the slope, turning on flatter ground at the top and bottom.
It is dangerous to work a dozer (and any kind of tractor) on sidelong ground, particularly if the ground is soft. Dozers can not traverse metalled roads because of the damage this would cause, and they should not be permitted on finished formation surfaces. Sometimes a flat-tracked dozer (that is, with no grips to the tracks) can be used on a formation if the ground is suitable. Motorized scrapers are the principal bulk excavation and earth-placing ma chines, used extensively on road construction or earth dam construction.
Their movement needs to be planned so that they pick up material on a downgrade, their weight assisting in loading; if this cannot be managed or the ground is tough, they may need a dozer acting as a pusher when loading.
This not only avoids the need for a more expensive higher-powered scraper, but reduces the wear on its large balloon tyres, which are expensive. The motorized scraper gives the lowest cost of excavation per cubic metre of any machine, but it needs a wide
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