NEPAL ENGINEERING SERVICES
Showing posts with label Survey. Show all posts
Showing posts with label Survey. Show all posts

Sunday, May 26, 2019

PROFESSIONAL ROAD ALIGNMENT SURVEY AND GEOMETRIC DESIGN

ROAD ALIGNMENTS AND GEOMETRIC DESIGN

            An identifiable route, way or path connecting two or more places s called road. Roads are  smoothed, paved, or allow easy travel, though they need not be, and historically many roads were simply recognizable routes without any formal construction or maintenance.
            Before the construction of the road, preliminary survey is done. Road alignment is the preliminary stage of road construction. Selection of Intersection Points (IP) is the foundation of construction of the road. After that cross section, longitudinal section and formation level are required.

EQUIPMENTS & ACCESSORIES:

The equipment used in the survey-during the preparation of topography map are as follows:
1. Theodolite
2. Leveling Staffs
3. Ranging rods
4. Measuring Tapes 30m & 5m
5. Leveling instruments
6. Compass
7. Abney level
8. Pegs
9. Marker
HORIZONTAL ALIGNMENT:
Horizontal alignment is done for fixing the road direction in horizonal plane. For this, the bearing of initial line connecting two initial stations are measured using compass. Theodolite at each IP and then deflection angles are calculated.
                                Deflection angle = (360° or 180°) - observed angle
                   If +ve, the survey line deflects right (clockwise) with the prolongation of preceding line and deflects left if -ve (anti-clockwise). The radius is assume according to the deflection angle. Then the tangent length, EC, BC, apex distance along with their chainage are found by using following formulae,
                Tangent length (TL) = R x Tan (Δ/2)
                 Length of curve (LC) = 3.142 x R x Δ/180
                 Apex distance= R x 1/(Cos (A/2)-1)
                 Chainage of BC= Chainage of IP -TL
                 Chainage of MC= Chainage of BC+LC/2
                 Chainage of EC =Chainage of MC + LC/2
The BC and EC points are located along the line by measuring the tangent length from the apex and the põints were marked distinctly. The radius was chosen such that the tangent does not overlap. The apex was fixed at the length of apex distance from IP along the line bisecting the interior angle.


VERTICAL ALIGNMENT:

Vertical profile of the Road alignment is known by the vertical alignment. In the L-section of the Road alignment, vertical alignment is plotted with maximum gradient of 12 %. According to Nepal Road Standard, Gradient of the Road cannot be taken more than 12 %. In the vertical alignment, we set the vertical curve with proper design. Vertical curve may be either summit curve or valley curve. While setting the vertical alignment, it should keep in min whether cutting and filling were balanced or not.

LONGITUDINAL SECTION:

For the longitudinal section of the road the staff reading is taken at the interval of every 15m along the center-line of the road. Besides, these staff  readings at beginning of the curve, ending of the curve and apex were also taken. The RL of each point are calculated.
The profile is plotted on the graph at the horizontal scale of 1:1000 and vertical scale of 1:100; chainage of each point along the horizontal direction and RL in the vertical direction.


CROSS SECTION:

Cross section  run at right angles to the longitudinal profile on either side up to 15m distances wherever possible and the change in the slope is  directly measured using the staff intercept made by the horizontal tape i.e. the stepping method.
Horizontal scale-1:100
Vertical scale 1:100


 STRUCTURES:

The main structures provided for road constructions are retaining structures, cross drain, side-drain, bio-engineering structures etc. retaining  structures are provided where the slope is critical. Gabion structure, dry masonry structures are the example. The camber of the road is made perfectly by putting 4% of stage for gravel road so as to avoid any collection of water on it.

The maximum gradient of the road is about 9% and the minimum gradient of road is about 0% so as to facilitate the flow of drainage to specified direction. However the maximum of 12% is taken wherever not possible. Longitudinal drain is provided on the sides of the road. Retaining walls are
provided on required places.
Construction of hill roads involves many special structures. These may include wide range of structures which are used to retain soil mass, to increase stability of road embankment slopes as well as natural hill slopes, to accommodate road bed in steep slope, to penetrate deep through mountain
pass and so on. Integration of bio-engineering measures with engineering structures is yet another sector requiring special attention. The following types of structures are used normally on the hill road:
1. Retaining structures
2. Drainage structures
3. Slope protection structures

RETAINING STRUCTURES:

A retaining structure is usually a wall constructed for the purpose of  supporting or retaining a vertical or nearly vertical earth bank, which in turn may support, vertical loads along with the self-weight of it. It provides adequate stability to the road way and to the slope. Retaining walls are
constructed on the valley side on the roadway and also on the cut hillside to prevent slide towards the roadway. Types of retaining wall are:
1. Gravity walls
2. Semi gravity walls
3. Cantilever walls
4. Counter fort walls
5. Buttressed walls
6. Crib walls
7. Breast walls
8. Reinforced soil walls

 CONCLUSION:

Survey of the road alignment is done to make most economical, comfortable, and durable. Extra case is taken to avoid any soil erosion and any other ecological damage. Vertical and horizontal curves are set according to Road Design Standards for comfort and other factors.
While setting the road alignment, it should be kept in mind that the minimum IP points should be taken as far as possible and deflection angles should be minimal as far as possible. The task may  be challengeable and tough due to the route high altitude.

The format for road curve calculation is here

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Topographical Survey: Detailed Explanation

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TOPOGRAPHICAL SURVEY

Topographical surveying is the process of determining the positions of natural and artificial features of the locality by means of conventional signs up on a topographical map. Topographic surveys are three-dimensional; they provide the techniques of plane surveying and other special techniques to establish both horizontal and vertical control.
               Hence the fieldwork in a topographical surveying consists of three parts.
                 1. It establishes both horizontal and vertical control.
                  2. It locates the contours.
                  3. It also locates the details such as rivers, streams, lakes, roads, houses,
and trees etc.


MATERIALS REQUIRED

1. Theodolite
2. Leveling Staffs
3. Ranging Rods
4. Measuring tapes 30m and 5m
5. Leveling Instrument
6. Plumb Bob
7. Nails,Pegs
8 Compass
9. Marker Pen
10. Umbrella

METHODOLOGY

The methodology of surveying is based on the principle of surveying. They are as follows:
                    1. Working from whole to a part
                    2. Independent checik
                    3. Consistency of work
The different methodologies were used in surveying to solve the problems arise in the field. These methodologies are as follows:


RECONNAISSANCE (RECCE):

Recce means the exploration or scouting of an area. In survey, it involves walking around the survey area and roughly planning the number of stations and the position of the traverse stations. Recce is primarily done to get overall idea of the site. This helps to make the necessary observations regarding the total area, type of land, topography, vegetation, climate, geologyand intervisibility conditions that help in detailed planning. The following
points have to be taken into consideration for fixing traverse stations:
1. The adjacent stations should be clearly intervisible
2. The whole area should include the least number of stations possible.
3. The traverse station should maintain the ratio of maximum traverse leg to minimum traverse leg less than 2:1 for Major Traverse and 3:1 for Minor traverse.
4. The steep slopes and badly broken ground should be avoided as far as possible, which may cause inaccuracy in taping.
5. The stations should provide minimum level surface required for setting up the instrument.
6. The traverse line of sight should not be near the ground level to avoid
the refraction.
     Taking the above given points into consideration, the traverse stations are fixed. Then two way taping is done for each traverse leg. Thus permanent fixing of the control points completes recce.

TRAVERSING

Traversing is a type of surveying in which a number of connected survey lines form the framework. It is also a method of control surveying. The survey consists of the measurement of followings:
1. Angles between successive lines or bearings of each line.
2. The length of each line.

The directions and the lengths of the survey lines are measured with the help of an angle-measuring instrument such as theodolite and a tape. If the co-ordinates of the first station and the bearing of the first line are known, the co-ordinates of all successive points can be computed as follows:

.Xb=Xa+ LCosθ
 Yb=Ya+LSin
θ

 where, L=Length of traverse leg
            θ = Bearing of AB
There are two types of traverse. They are as follows:

1. CLOSE TRAVERSE
If the figure formed by the lines closes at a station i.e. if they form a polygon or it starts and finishes at the points of known co-ordinates then the traverse is called closed traverse.

2. OPEN TRAVERSE

If a traverse starts and finishes at points other than the starting point or point of known co-ordinates, then the traverse is called open traverse.


Balancing the traverse

During the computation of the traverse, we need to balance the traverse because of the different errors in the field measurement. There are different methods of adjusting a traverse such as:
1. Bowditch's method
2. Transit method
3. Graphical method
4. Axis method
The basis of these methods is on the assumptions that the errors in linear measurements are proportional to L' and that the errors in angular measurements are inversely proportional to 'L' where L is the length of a traverse leg. The Bowditch's Rule is commonly used to balance a traverse in latitude and angular measurements are of equal precision. The total error and in the departure is distributed in proportion to the lengths of sides. Bowditch rule gives the correction as,
Correction-To-Lat"-or-Dept 
={Total Error in- Lat(or Dept) *(Length of That_Leg)}/{Perimeter of_that Traverse}



ADJUSTMENT OF ANGULAR ERROR AND BEARING:

 The error (e) in a closed traverse due to bearing may be determined by comparing the two bearings of the last line as observed at the first and last stations of traverse. If the closed traverse, has N number of sides then,
                       Correction for the first line= e/N
                       Correction for the second line= 2*(e/N)
                        And similarly, correction for the last line= N*(e/N) = e
                In a closed traverse, by geometry, the sum of the interior angles should be equal to (2n-4)*90° where n is the number of traverse stations. If the angles are measured with the same degree of precision, the error in the sum of the angles may be distributed equally among each angle of the traverse.

DISCREPANCY AND LINEAR MİSCLOSURE:

In order to measure the lengths of the sides of the traverse, two ways taping (forward and backward) is done. In difficult areas where taping is not possible, other methods like the subtense bar is used. The difference in values obtained by forward and backward taping is called discrepancy. The reciprocal of mean of the two measurements divided by the discrepancy is called precision. Both the discrepancy and the precision for each traverse leg should be within the given limits.
Mathematically,
Discrepancy= | Forward length -Backward length |

& Linear Precision= 1/ (Mean length / Discrepancy)

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Reciprocal leveling

Two Peg Test


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Reciprocal leveling: A Useful method For finding level difference

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Reciprocal leveling



Reciprocal leveling is used to find accurate relative elevations of two widely separated intervisible points.This procedure is used for either differential or trigonometric leveling when along sight across a wide river, ravine, or similar obstacle must be made. 
The errors may arise out of the curvature of the earth or intervening atmosphere (associated with variation in temperature and refraction) or instrument (due to error in collimation) or any combination of these.

Steps1. To find the difference in elevation between two points,  X and Y
2. A level is set up at L near X and readings (X1 and Y1) are observed with staff on both X and Y respectively.
3. The level is then set up near Y and staff readings (Y2 and X2 ) are taken respectively to the near and distant points. If the differences in the set of observations are not same, then the observations are fraught with errors.
4. The errors may arise out of the curvature of the earth or intervening atmosphere (associated with variation in temperature and refraction) or instrument (due to error in collimation) or any combination of these.
The format for reciprocal levelling is given below::

Reciprocal Levelling Format

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Check your levelling machine with Two Peg Test.

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Two Peg Test

 The two-peg test is simple test that provides a way to test the accuracy of a level. And thus we can adjust it to remove the error. 

The basic principle it works on is " if you place the instrument exactly midway between two rod sightings, the vertical error reading on the rod is the same for each, thus the difference in reading between the rods will still give you an accurate elevation difference. "

Steps
1. Establish 2 points approximately 50 metres apart on level ground
2. Set the level half way between the 2 points.
3. Take the 2 staff readings
4. Move the level as close as possible to one of the peg. Take the 2 staff readings again.
5. If the difference in height is the same the level is okay. If not,the instrument needs to be serviced.
The format for two peg test is::
Two peg or Collimation Test Format

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Saturday, May 25, 2019

Professional Bridge Site Survey: Technical Norms and Specifications

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Bridge Site Survey:

1. Carry out reconnaissance survey of the bridge site area. Establish necessary triangulation stations to determine Bridge Axis Length, as well as horizontal and vertical control of the area. Well condition triangles should be formed while selecting the triangulation stations.

2. In triangulation, distance of Base Line must be measured in an accuracy of 
1: 2000.

3: Observed two sets of horizontal circle reading by the theodolite to measure the angle of base triangles. Note that the difference between the mean angles of two sets should be within a minute. Angular misclosure for base triangle should be
±30" (N^0.5) and other triangle ±1'(N^0.5)

4. Compute the length of proposed bridge axis by triangulation survey from two adjacent base triangles by using sine law and determine the length of bridge axis by taking average length. Computed mean length of bridge axis from two base triangles should be >=1:2000.

5. Conduct fly levelling to transfer the R.L. from given BM to the nearest triangulation station of the bridge axis and make circuit close for checking error of closure.

6. Carry out reciprocal levelling to transfer level from one bank to other bank of the river / stream within a precision of ±25(K^0.5) mm. Determine the RL of the other triangulation stations by fly levelling from the end point of bridge axis.


7. Plot a topographic map indicating contour lines at suitable interval (contour interval =1m). Interpolate the Index Contour lines precisely by arithmetic calculation method with the help of the guide points and then interpolate remaining contour lines either by graphical method on by estimation method. Do not erase RL of guide points in the original sheet even after plotting of contours.
Note: Value of Index Contour Multiple of (5* Contour interval)
8. Draw longitudinal section along the river bed up to 150m up-stream and 50m down-stream. Assuming 0.00 at centre of bridge axis. Draw cross section at 25m interval from topographic map and one at the bridge axis.


Scale for plotting
a. Topographic map: 1:500
b. L-section: Horizontal Scale 1:500 and Vertical scale 1:50
c. Cross section: Horizontal scale 1:50 and Vertical scale 1:50

9. Carry out hydrological survey of river/stream. Collect hydraulic data such as velocity of flow, high flood level, normal water level and low water level. All these should be shown in map and cross section of river. All these should be shown in map and cross section of river.

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Professional Road Alignment Survey: Technical Norms and Specifications for

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Road Alignment Survey

 1. Carry out reconnaissance survey and alignment selection of a road corridor about 700m or more.

2. Starting and end point of road, location of bridge site will be provided at the site.

3. Road alignment selection i.e. IP selection shall be carried out considering the obligatory
points, permissible gradient, bridge site, balancing cut and fill, shape of the valley and cross drainage, lateral slopes, geometry of horizontal and vertical curves, etc.

4. Alignment must not be selected beyond 12% gradient of the existing ground surface.

5. Radius of the horizontal curve should not less than 12 m while assuming the radius of the
horizontal curve, select the radius in the multiple of 5 or 10.

6. Point of commencement (T1) and point of tangency (T2) must not be located within the
bridge axis. Start and finish of curves must be totally outside the bridge axis end points.

7. Avoid subsequent reverse curve in road alignment.

8. Deflection angle should not be greater than 90
°.  
9. Two successive curves must not be overlapped.

10. Measure Bearing of the starting leg. To compute bearing of next leg, record deflection   angle with respect to preceding leg.

11. Setting out of horizontal curve is not necessary for  less than 3 degree deflection angle but compute the chainage as usual manner

12. Theodolite team shall carry out the detailed survey of the road alignment. Theodolite team will mark pegging at 15m interval along the centre line of road and at curve points BC, MC and EC for longitudinal section as well as X-section. Set horizontal curve by fixing BC, MC and EC. Theodolite team should fill up three field book (FB) separately such as tacheometry FB, horizontal curve FB and detail sketch.


13. Carry out levelling survey for longitudinal section along the centre line at 15 m interval, at abrupt change point and at the curve point BC, MC and EC. Establish TBM at approximately 500m interval and near cross drainage. Close the levelling survey and check the RL at job site immediately. Permissible error of closure for levelling must not be greater than
±25 (K)^0.5 mm.

14. Perform cross section survey either by levelling instrument or by levelling staff and tape along the road alignment at 15m interval and at abrupt change of the topography as established by the theodolite team. During the field survey, draw neat and clean free hand dimensional the cross section. Coverage width of the cross section must not be less than 10mm on  either side (left and right) from proposed centre line of the road and at 5m regular interval as well as other salient points from centre line.

15. Prepare a road corridor plan in 1:500 scale showing 5m formation width, 20m right of way (corridor width 10m left and 10m right) from proposed centre line, location of intersection point (IP), geometry of horizontal curve with chainages of BC, MC, EC chainages of drainages, chainages of centre line details and other details such as vegetation, forest, cultivation, barren land etc.

16. Draw longitudinal section of road centre line [horizontal scale 1:1000, Vertical axis 1:100]. Draw the working profile i.e. formation level assuming balancing of cut and fill in between economical haul distances. Draw cross section [horizontal scale vertical scale 1:100] of existing ground surface and show the formation level and width (5m) assuming side slope 2:1 in filling and 1.5:1 in cutting. (H:V)

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Friday, May 24, 2019

Steps, Technical Norms and Specifications for Professional Topographic Survey

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Topographic Survey

1. Conduct reconnaissance survey of the given area then formulate major traverse framework which must be enclosed the given area then formulate the minor traverse frame work within or outside the major frame work. Prepare an overall sketch in A2 size paper forming a closed minor traverse around the periphery of the area by establishing the traverse stations. While selecting the traverse stations, maintain the ratio of maximum traverse leg distance to minimum traverse leg distance less than 2:1 for major traverse and 3:1 for minor traverse


2. Referencing of each traverse station is utmost necessary and should be shown in sketch

3. Horizontal Control and Vertical Control of the given area must be conducted based upon the given Standard Horizontal Datum and Vertical Datum. Coordinate must be transferred to the instrument station from the National Grid System by observing to at least 3 known points (known coordinates). Resection station should be selected in the system of major framework. If it is not possible, resection station must be connected by making loop to the nearest Major traverse Station

4. Station pegs should be marked as 14Mı for Major Traverse and 14m1 for minor traverse Prefix number 14 is camp group number; letter M or m denotes the major or minor traverse and the suffix number 1 denote the traverse station number. Use the same notation in plotting except for common station. (Use CP1 and CP2 for common station).

5. Measured two way traverse leg distance (i.e. in the forward and backward directions) by means of tape calibrated against the standard length provided in the field or by total station. Note that the precision of tape measurements should be >= 1:2000 and in case measured by Total Station, the precision should be >=1:5000 6. Measure two sets of horizontal circle reading of major traverse station either by Theodolite or by Total Station. The difference between face left and face right reading is not  allowed more than ±180°±120"(twenty seconds). Note that the difference between angles of two sets reading should be within a minute


7. Measure only one set of angles of minor traverse. The difference between face left and face right reading is not allowed more than ±180°±120". Precision on forward and backward distance measurements by tape should be >= 1:1000 and >=1:3000 in case of distance measured by Total station.

8. Balance the traverse by proper way adjusting angular and linear closing errors. The sum of interior/exterior angles in a closed traverse should be equal to (2N±4) x 90°. The permissible angular error for the sum of interior angles of the Traverse should be less than ±30"(n)^0.5 for major traverse and ±1'(n)^0.5, in for minor traverse respectively. Relative precision or total error of closure should be less than 1:5000 for major traverse and 1:3000 for minor traverse respectively

9. For Vertical Control, perform two peg tests before the start of fly levelling. Perform fly levelling to transfer R.L. from the given B.M. to the allocated minor traverse. Take three wire staff readings and establish other TBMs as per field requirement. Note that collimation error should be less than 1:10000. To eliminate collimation error, levelling instrument be set up in such position so that fore sight and back sight distances are equal. B.M and its R.L. will be provided in the field. The permissible error of closure for fly leveling is ±25(k^0.5) in mm.

10. Plot the major traverse and minor traverse at site by coordinate method in scale 1:1000 and 1:500 respectively. Plotting of minor traverse should depict the major traverse. Orientation check will be performed at site.

11. Carry out the detail survey of the given sub area (minor) either by tacheometric system using theodolite or by total station with reference to the major and minor traverse stations. Take the detailed sketch of the minor area including natural and man made features.

12. Never drive wooden pegs at the frequent public walkway. Station pegs should be driven flushed completely on the ground surface to avoid walking disturbance as far as possible

13. All the details are to be plotted with respect to vertical and horizontal control from the respective station. Use conventional symbols for plotting the map.

14. Topographic map of the given minor area should depict contour lines at suitable interval (contour interval = 1m). Interpolate the index contour lines precisely by arithmetic calculation method with the help of guide points and then interpolate remaining contour lines either by graphical method or by estimation method. Do not erase those guide points from the original sheet even after plotting of the contours.