Underground Surveying Methods and Applications What is Underground Surveying?

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Underground Surveying Methods and Applications

What is Underground Surveying?

Underground surveying embraces the survey operations performed beneath the surface of the earth in connection with tunneling, exploration and construction in subterranean passageways. It is quite different from surveying on the surface.

The following peculiarities of underground surveys indicate how they differ from surface surveys:

• Artificial illumination is required to view instrument crosshairs, to read verniers, to sight targets etc. Because of poor lighting.
• Working space in passageways is often cramped.
• Instrument stations and benchmarks for levelling must often be set into the roof of a passageway to minimize disturbance from the operations being carried on in the workings.
• Instrument stations are set with some difficulty since plugs must be driven into drill holes in rock.
• In many instances the underground workings arc wet, with considerable water dripping from the roofs of passage ways and running along the floors.

Applications of Underground Surveys

The major application of underground surveysis in the construction of tunnels and other underground utilities. The tunnel is constructed when open excavation becomes uneconomical usually when it is more than 20 m. It

• Reduces the grade
• Shortens the distance between given points separated by a dividing mountain or ridge.
• Meets the demand of-modern rapid transit in a city.
• Engineering operations to be performed
• Exact alignment
• Proper gradient
• Establishment of permanent stations marking the proposed route.

The survey work in connection with tunneling can be divided into:

1. Surface survey
2. Transferring the alignment underground
3. Levels in tunnels

Surface survey:

Surface survey connects points representing each portal of a tunnel. A traverse connecting the portal points determines the azimuth, distance and differences in elevation of each end of the proposed tunnel. Based on the local conditions and proposed length of the tunnel the methods of working are adopted.

It is always advisable that the survey is based on the suitable local coordinate system. The alignment is permanently referenced by a system of monuments within an area outside each tunnel portal. And here is sketch regarding how the tunneling work proceeds.

Central line and grade stakes within the tunnel are usually set in the roof to avoid displacement and destruction by the constant flow of people and machinery as construction proceeds. If the stakes are set on the floor they should be offset into an area along the tunnel’s edge.

Transferring the alignment underground

For long tunnel excavation is carried inward from both parties. But vertical shafts are also sunk up to the required depth along the alignment of the tunnel at intermediate locations along the routes. The vertical alignment can be done by

• Plumb bob
• Optical collimator
• Laser

A heavy plumb bob (5 to 10 kg) is suspended on either a wire of heavy twine. Oscillations of the bob can be controlled by suspending it in a pot with high viscosity oil. The bob is suspended from a removable bracket attached to the surface side of the shaft.

Optical plumbing becomes important with the increase in depth of internal shaft. Various types of plummet are available for upwards and downward sighting to allow the establishment of a vertical line and these are normally manufactured so as to be interchangeable with theodolites on their tripods.

As the line of sight of a theodolite in adjustment will transit in a vertical plane, it can also be used to check perpendicularity.

The advantages of an optical collimator are:

• More convenient than a plumb bob.
• Can be used to set marks directly on the floor of a completed shaft
• No wires as in case of plumb bob.

A laser equipment can be, used to, provide a vertical line of sight. The laser generates a light beam of high intensity and of low angular divergence and can be projected over long distances since the spread of the beam is very small to provide a visible line for constant reference

Levels in tunnels:

In transferring levels underground, little difficulty is encountered at the ends of the tunnel, but at the shaft use is made of

• Steel tape
• Chain
• Constructed rods
• Steel wires

Now a days EDMI is also used. But in all cases the main idea is to deduct the height of the shaft measured from the top of a benchmark of known value.

Figure showing how the depth is measured by steel tape

Figure showing Depth measured by EDM

The important features of EDM are:

• EDMI unit and reflectors should be in the same vertical line.
• Both are mounted on stable support.
• Visibility should be good for EDMI to operate.

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Surveying for New Railway Line Construction

Proper surveying is required for construction of new railway line between two places. Several stages are involved to propose suitable alignment for new railway line.

Stages in Surveying for New Railway Line

Various steps involved in railway line surveying are as follows.

• Validation of New railway line construction
• Marking of tentative alignments
• Reconnaissance survey
• Selection of Good alignments
• Preliminary survey of alignment
• Final alignment survey
• Final survey report

Viability of New Railway Line Construction

When there is a demand for new railway line, the first and foremost point is to justify the viability of railway line by taking some important points into consideration. Those are

1. The total number of people living in the area across which a railway line is proposed are taken into record.
2. The population habits and living standards, economic conditions are observed to make sure that the department will earn revenue by this line.
3. To record the number of bridges, culverts, tunnels etc. a topographical map of that area should be studied.
4. To study the alignment and gradients, contour map of that area should be studied.
5. To connect all the industries by railway line, industrial map of that area should be studied.
6. New railway line laid should not affect the valuable land in agriculture so, agricultural map should also be studied.
7. Presence of agricultural, natural, industrial resources should be recorded.
8. Presence of religious places, business centers, markets etc. should be noted.
9. An estimate should be made on amount of revenue that may accumulate from passengers, goods, etc.

Marking of Tentative Alignments

Once, justification is shown to construct a new railway line then the next step is marking of different alignments which are likely to support the new railway line. But when marking is done there are some considerations should be followed as follows.

1. The alignment marked should be as short as possible.
2. The alignment should pass through less valuable lands and it should not pass through high value lands like through center of villages or cities, valuable structures etc.
3. The alignment should be as straight as possible and number of curves should be minimized.
4. The alignment should be marked on ridge lines to eliminate the earth filling cost. So, if there is any valley or depressions then it is better to avoid them. If it is necessary to mark across them, then proper attention is taken on this area in reconnaissance survey.
5. The alignment should not pass through religious places, temples, churches, mosques, burial ground etc.
6. The alignment should cross the river perpendicularly.

Reconnaissance Survey

Among the marked alignments, only one alignment is finally needed. So, reconnaissance survey is performed in which so much data is collected from every alignment. The information obtained is

1. Using prismatic compass find out the magnetic bearings of lines of alignments. And find the distances by stepping. Both measurements should be recorded in the field book.
2. The surrounding area of alignment about 100 meter radius is studied thoroughly and locate the positions of different objects.
3. Presence of water table level along the alignment is determined using boring.
4. Soil survey should be done along the alignment.
5. Note down the number of bridges, culverts etc. and their details along the alignment.
6. Note down the number of crossing points like roads, rivers, etc. along the alignment.
7. Note down the number of curves present along the alignment.
8. Determine the slope of ground along alignment using abney level.
9. Construction material availability and its transportation way should be observed.
10. Availability of labor and places for them to live, needs etc. should be observed.
11. If Depressions and high summits are unavoidable then, note down the details of those.
12. Note down the past recorded values of rainfall, discharge by the rivers etc. by considering past 10 year period.
13. If the alignment is passed through a private property or land or structure, then record its details along with owner details. Then the owner will receive compensation from the government.
14. An index map is prepared for all the alignments with details like no. of curves, bridges, approximate longitudinal section etc. which will help to select the good alignment in the next stage.

Selection of Good Alignments

When reconnaissance survey is completed for all the marked tentative alignments, some good alignments are selected in this stage. Its selection is done based upon the following points.

1. A good alignment is always short and economical.
2. A good alignment can generate considerable amount of revenue.
3. A good alignment consists minimum number of bridges or culverts in its way.
4. Earth filling or earth excavation is as minimum as possible along good alignment.
5. Alignments consisting vertical curves should be eliminated.
6. Construction material and labor availability is as near as possible.
7. A good alignment should connect all the important and rush areas in that region.
8. Location of station yards should be easily reachable for Passengers.
9. Cost of construction and expected revenue should be estimated along the good alignment.

Preliminary Survey of Alignment

After selecting good alignments, then it’s time to decide only one alignment finally. But before this final step, preliminary survey is conducted for all the good alignments. The preliminary survey consists:

1. A pillar is constructed at the starting point of alignment and this point is connected to nearby GTS benchmark using fly leveling.
2. Longitudinal leveling with an interval of 20 or 40 meter is carried out along the alignment.
3. Cross leveling with an interval of 100 meter is carried out.
4. Magnetic bearing of each line of traverse is noted in level book.
5. A route survey map is prepared which give the details of 100 m land on both sides of alignment. This can be done by plane table surveying or prismatic compass surveying.
6. At every 2km or with regular interval permanent bench marks are established.
7. Bearing capacity of soil and water table level is recorded.
8. A map is prepared for the marked station yards using plane table surveying.
9. At river crossings, the details of river like its cross section, width, water level, HFL, scour depth etc. are noted for a distance up to 1000 meters on the both sides crossing with an interval of 100 m.
10. The river bed is bored to find out the depth of foundation required.
11. With all the recorded readings, a drawing is prepared for the whole alignment.
12. For the drawings, an approximate estimate sheet is prepared for each alignment. Which includes earth works, compensations, cost of culverts, bridge structures etc.

Final Survey Alignment

After completion of preliminary survey, final alignment among all good alignments is selected. This is purely based on the economy and final alignment is economical than the others. But before its approval, final location survey is conducted for one final time which involves in the following steps.

1. Masonry pillars are constructed along the center line of alignment with an interval of 1000m. The pillar position can define the width required for railway track. In between these pegs are provided with an interval of 30m.
2. Station yards are marked at required points.
3. Level crossings, culverts etc. are marked.
4. Bridge provision places are marked with these pillars.
5. Intersection points and tangent points of curve are marked.
6. Compensation of properties is estimated for the final time and marked those places.

Final Survey Report

When final survey alignment is completed then a report is prepared for the final alignment and it is submitted for approval. The report includes

1. Project introduction
2. Necessity of project
3. Justification of final alignment
4. Details of final alignment like its length, area, number of bridges, culverts, crossings etc.
5. Estimation sheet
6. Specification details
7. Revenue expected
8. Recommendation of project

Along with the report some maps with suitable scale are to be attached and they are

• Map of the area through which alignment will pass
• Route survey map
• Longitudinal section of alignment
• Cross section of alignment
• Map of station yards
• Drawings of culverts, bridges, tunnels etc. to be constructed along the alignment.
• Drawings of station buildings, yards etc.

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Terms Used in Leveling and their Uses in Surveying

Various terms are used in leveling which must be understood clearly before starting surveying. Leveling is conducted in the field to know the elevation difference between two points. To find the elevation difference, at least the elevation of one point must be known.

What if the elevations are not known or how to proceed with leveling with unknown elevation points? It is not a big task if basic terms of leveling are known. So, one must know these basic terms before studying the art of leveling.

Terms Used in Leveling

The general terms used in leveling are:

• Vertical line
• Horizontal line
• Level surface
• Datum
• Benchmark
• Mean sea level
• Reduced level

Vertical Line

It is the line which is indicated by plumb at required station. So, this is also called as plumb line. It’s just decided based on the consideration of earth’s gravity.

Vertical line connects the station point to the center of the earth. A plumb is released from the instrument height with the help of thread and instrument is set up at that point as its center.

Horizontal Line

Horizontal line is the line of sight of instrument which is tangential to the level surface and It is perpendicular the plumb line. The surface along horizontal line of sight is called as horizontal surface.

Level Surface

Level surface is the continuous surface parallel to the mean spheroid of the earth. The line representing the level surface is termed as level line.

The level line makes right angles to the vertical line or plumb line at any point. It means the any point on level line is equidistance from the center of earth. Even though it is a curved surface, it is considered as plane surface for smaller area works.

Datum

Datum line is the reference line with respect to which the levels of other station points are fixed.

Mean Sea Level

Mean sea level is established by taking the average height of tides in the sea over a very long period (generally 19years). Mean sea level or MSL is used as datum level for all important surveys of that zone.

Reduced Level

Reduced level is the vertical difference between the level point and datum line or the mean sea level.

Benchmark

Benchmark is the point laid above or below the datum line with a known elevation. Bench mark is considered as check for the other level points.

There are different types of bench marks are available in the surveying as follows:

• Great Trigonometrical Survey benchmarks
• Permanent benchmarks
• Temporary benchmarks
• Arbitrary benchmarks

Great Trigonometrical Survey Benchmarks

Great trigonometrical bench mark or shortly GTS benchmarks are very accurate, and they are established by conducting high precise surveys. GTS benchmarks are decided by taking mean sea level as datum. These are generally established by higher survey authorities of particular country in all points of the country.

A concrete pedestal with bronze plate at its top is provided at a point where GTS benchmark is located. The bronze plate consists the information or value of the benchmark on its top. These pedestals are generally protected by masonry structure build around it.

Here is some of the list of agencies for different countries, who established the GTS benchmarks in their respective countries.

Country	Agency / Authority
India	Survey of India
United Kingdom	Ordnance survey
United states	The national Geodetic survey The United States Army Corps of Engineers The United States Forest Service The United States Geological Survey
Japan	Geographical Survey institute
Canada	Natural Resources Canada
New Zealand	Land Information New Zealand
Pakistan	Survey of Pakistan

Permanent Benchmarks

Permanent benchmarks are established with reference to GTS benchmarks. They are established by local state government agencies or railways at railway stations, public buildings, at bridges etc. Permanent benchmarks are useful for future references also.

Temporary Benchmarks

Temporary benchmarks are created by the surveyors in the field to mark the point in the field up to which the survey is completed. Then, it is easier to continue the survey from that point after large gap or on the next day of work.

The temporary bench mark point should be decided in a way that the object or point should be permanently fixed in that position.

Arbitrary Benchmarks

In Small survey works or engineering projects it is important to know the difference in elevation of two points. When this happens, we cannot predict the exact reduced level of that area with respect to mean sea level.

So, some fixed points in that area are arbitrarily taken as bench marks and values are assumed. The assumed arbitrary bench mark values may be 50 m, 100 m, 200 m etc.

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Different Types of Levels Used for Leveling in Surveying

There are various types of levels for leveling in surveying. The process of measuring vertical distances in surveying is called leveling.

To perform leveling, we need some level instruments to focus or to read the object. Nowadays, the technology also introduced in surveying and so many easy measuring instruments are designed. Here we discuss about the different levels used in leveling.

Types of Levels Used in Leveling

Following are the types of different levels used for leveling in surveying:

• Dumpy level
• Y level
• Cushing’s level
• Tilting level
• Cooke’s reversible level
• Automatic level

Dumpy Level

Dumpy level is the most commonly used instrument in leveling. In this level the telescope is restricted against movement in its horizontal plane and telescope is fixed to its support. A bubble tube is provided on the top of the telescope.

But however, the leveling head can be rotated in horizontal plane with the telescope. The telescope is internal focusing telescope is a metal tube contains four main parts as given below.

• Objective lens
• Negative lens
• Diaphragm
• Eye-piece

Objective Lens

Objective lens should be made as the combination of crown glass and flint glass. Because of this some defects like spherical aberration and chromatic aberration can be eliminated. A thin layer coating which has smaller refractive index than glass is provided on the objective lens to reduce the loss due to reflection.

Negative Lens

Negative lens located co axial to the objective lens. So, the optical axis for both lenses is same.

Diaphragm

Diaphragm is fitted inside the main tube which contains cross hairs (vertical and horizontal) and these are adjusted by capstan headed screws. The cross hairs are made of dark metal as filament wires which are inserted in diaphragm ring in exact position. For stadia leveling purposes, extra two horizontal cross hairs are provided above and below the horizontal wire.

Eyepiece

Eyepiece lens enable the ability to sight the object together with cross hairs. The image seen through eye piece is magnified and inverted. Some eyepieces erect the image into normal view and those are called as erecting eyepieces.

Y Level

Y level or Wye-level consists y-shaped frames which supports the telescope. Telescope cane be removed from the y-shaped supports by releasing clamp screws provided. These y-shaped frames are arranged to vertical spindle which helps to cause the rotation of telescope.

Compared to dumpy level, adjustments can be rapidly tested in y- level. But, there may be a chance of frictional wear of open parts of level.

Cushing’s Level

In case of Cushing’s level, the telescope is restricted against rotation in its longitudinal axis and it is non-removable. But, the object end and eye piece end can be interchangeable and reversible.

Tilting Level

Tilting level consist a telescope which enabled for the horizontal rotation as well as rotation about 4 degree in its vertical plane. Centering of bubble can be easily done in this type of level. But, for every setup bubble is to be centered with the help of tilting screw.

The main advantage of tilting level is it is useful when the few observations are to be taken with one setup of level.

Cooke’s Reversible Level

Cooke’s reversible level is the combination of dumpy level and y-level. In this instrument, the telescope can be reversed without rotation the instrument. Collimation error can be eliminated in this case because of bubble left and bubble right reading of telescope.



Automatic Level

Automatic level is like the dumpy level. In this case the telescope is fixed to its supports. Circular spirit can be attached to the side of the telescope for approximate leveling. For more accurate leveling, compensator is attached inside the telescope.

Compensator can help the instrument to level automatically. Compensator is also called as stabilizer which consists two fixed prisms and it creates an optical path between eye piece and objective.

Due to the action of gravity, the compensator results the optical system to swing into exact position of line of sight automatically. But before the process of leveling, compensator should be checked.

To check the compensator, just move the foot screws slightly if the leveling staff reading remains constant then compensator is perfect. If it is not constant, then tap the telescope gently to free the compensator. Automatic level is also called as self-adjusting level.