How to draw road profiles with Autocad Civil 3D and how to set up

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QUESTION : EXPLAIN BASIC RULES IN PRACTICE WHEN CONDUCTING A LEVELING

1.     EXPLAIN BASIC RULES IN PRACTICE WHEN CONDUCTING A LEVELING

    

                Levelling is the process of measuring the difference in elevation between two or more points. In engineering surveying, levelling has many application and is used at all stages in construction projects from the initial site survey through the final setting out. In practice, it is possible to measure heights to better than a few millimeters when levelling  this precision  is more than adequate for height measurement on the majority of civil engineering project.

The basic rules in practice when conducting a levelling fieldwork should be adhered to if many of the sources of error are to be avoided. Levelling should always start and finish  at points of known reduced level so that misclosures can be detected. When only one bench mark is available, levelling lines must be run in loops starting and finishing at the bench mark. 

Where possible, all sights length should below 50m. The staff must be held vertically by suitable use of a circular bubble or by rocking the staff and notong the minimum reading. Backsight and fortsight length should be equal for each instrument position. For engineering application, many intermediate sight readings may be taken from each set- up. Under this circumstances it is important that the level has no more than a small collimation error. 

Reading should book immediately after they are observed and important readings, particularly at change points, should be checked. The rise and fall method of reduction should used when heighting reference or change  points and the HPR method ( height of collimation) should be used for contouring , sectioning and setting out applications.

Describe the different surveys to be carried out for the highway projects.

Describe the different surveys to be carried out for the highway projects.

i) Reconnaissance

ii) Preliminary Survey

iii) Location Survey

i) Reconnaissance:

During the reconnaissance survey the following factors have to be taken into consideration.

Ø Obstructions along the route.

Ø Gradients and length of curves.

Ø Cross drainage works.

Ø Soil type along the route.

Ø Sources of construction materials and

Ø Type of terrain.

ii) Preliminary Survey:

The preliminary survey in a highway project is done with the main objectives

Ø Various alternate arrangements

Ø Estimate the quantity of earth work materials and other construction aspects

Ø Compare different proposals.

The following surveys are constructed

Ø Primary transverse

Ø Topographical surveys

Ø Levelling work

Ø Hydrological data

Ø Soil surveys.

iii) Location Survey:

The final alignment decided after the preliminary survey is to be first located on the field by

establishing the centre line. Next the detailed survey should be carried out.

The detailed survey involves:

Ø Fixing temporary bench marks along the route for every 300 m.

Ø The cross sectional details are taken for 30 m on either side of the central line.

Ø All details of cross drainage works are taken.

Ø Topographical details are taken.

Ø Detailed soil survey is carried out.

Explain the methods of transferring reduced levels from surface to underground in a tunnel setting out work.

Explain the methods of transferring reduced levels from surface to underground in a tunnel setting out work. 

i) Setting out central line of tunnel

ii) Setting out inside tunnels

iii) Transferring of alignment through shafts

i) Setting out central line of tunnel:

The centre-line of tunnels are fixed on the surface along with shaft locations.

Generally the surface control points of the tunnels are not visible from each other. However, by the method of reciprocal ranging points on the summit can be established which can be joined to get the central line. The measurements should be made accurately. Linear measurements are made using invar substance bars with an accuracy of 1 in 10000. Angular measurements are made using 1 second theodolite with an accuracy 0f 15√ N where N is the number of angles. In case of N tunnels in hilly regions it is neither feasible to align the tunnel ends by direct ranging or reciprocal ranging. In such cases precise triangulation has to be used.

The figure shows a scheme of triangulation network with QR as base line for a tunnel

project. Here all the angles are measured accurately by one second theodolite. Usual corrections for length, temperature, terrain, sag and reduction of levels with respect to sea level are all followed in arriving at the values of the coordinates. The traverse is adjusted for angles and coordinates. The proposed tunnel axis is shown in figure as HR.

ii) Setting out Inside Tunnels:

After the coordinates of portals and shafts are finalized, setting out is started. Centre line of

tunnel is done as shown in figure from various portals and shafts.

Back sighting on the pillar, aligned and constructed as far as practicable on the extended

centre line such as pillar C and then by transiting. Reference points are constructed on the roof of tunnels or slightly below the invert for every 300 m.

iii) Transferring of alignment through shafts:

Transfer of alignment is done through shafts by adopting any one of the following methods:

i) By hanging two or more plumb lines down the shaft.

ii) By lighting directly from edge of shaft where shaft diameter to depth ratio is high.

Co-planning is done by hanging two or more plumb lines down the shaft and determining

the bearing of the plumb planes so formed which are connected to the surface. The plumb lines

should be well apart as for as possible. The plumb lines are of special type. The line shall be of

fine steel wire and carrying a symmetrical weight of 35 kg or more. The wire should be well

stretched to keep it tight. In order to keep the wires vertical, the bob should be contained in a

canister with a hood. This arrangement will shield the bob and will reduce oscillations set up by air currents or by water dropping down the shafts. The canister can be filled with water or oil to reduce the vibrations. The bearing of the plumb plane underground is assumed same as at the surface.

This forms the starting direction for the underground survey work.

The following procedure is adopted for transferring the centerline from top.

Ø A theodolite is set up on top of the hill at a suitable position to maintain the centre line of theshaft.

Ø The RLs of both the ends of the shaft are determined by a level. Knowing the bottom RLs ofthe ends the depth of the shaft is found.

Ø Excavation of the shaft is started and verticality is maintained with the help of the plumb-bob which is suspended from wires from top through pulleys.

Ø The excavation is continued until the required bottom level is reached. The depth of the shaft is measured by measuring the length of suspended wire.

Ø The centre line inside the tunnel is maintained by a precise theodolite. This type of

theodolites are provided with an artificial illumination system to enable work at night and in

the darkness of the tunnel.

Ø It should be properly taken care to see that the centre line is maintained from both ends and one transferred from top coincide.