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Unit 4 Maintaining Dimensional Accuracy of the Works

Introduction.

4.1 Obtaining Information

 

4.2 Establish lines and levels

 

4.3 Inaccurate or Missing Information




Information and Guidance is available by clicking on the ‘Student Area’ above.

 

Learning outcome:  On completion the learner will know how to establish lines and levels for the works.

 

Contents

4.2.1 Relate the setting out information to the circumstances of the site.
4.2.2 Set out lines and levels for construction operations.
4.2.3 Determine the lengths of travellers for excavation and concreting.



    Book


Foster, G (1989) Construction site studies production, administration and personnel (2nd Ed), Longman; Harlow, Chapter 11


 

4.2.1 Relate the setting out information to the circumstances of the site


This section will be considered in its entirety during the site field course. 



4.2.2 Set out lines and levels for construction operations


This section will be covered in practical exercises during the two day practical section of the unit. Although it will be an advantage if you can understand the content and processes before you attend the workshop.

 

The equipment you will need to be able to use is:

 

·         Tapes and rule

·         Spirit levels and plumb bobs

·         Optical surveying equipment.

 

The first two you will be familiar with one which may be new to you is optical surveying equipment and its use.

 

The following outlines the levelling process and the instruments that will be used during the workshop. It introduces you to the levelling exercise and how levels are booked and recorded.

 

Don’t worry too much if you are having problems understanding this, it will be explained to you on the workshop.

 

 

The Levelling Process

 

Optical Level

 

An automatic optical level is essentially a telescope which is used to sight on to a levelling staff. It must be set up so that it is level at all points when swung through 360º. It has controls to adjust focus, slowly traverse/clamp and to increase the definition of the crosshairs (black lines on eyepiece used to read the staff).

 

An example of an optical level is shown below.

Optical_Level



















Setting up an Optical Level


Most modern tripods are made of alloy and may have straps tied around the legs for easy transportation. Setting up the tripod involves standing it upright, releasing the screws clamps on the legs and extending the top of the tripod to about the forehead level of the user. The screw clamps are tightened; the legs are spread and pushed firmly into the ground. Selected clamps may now be loosened to allow rough levelling of the top of the tripod and these are of course re-tightened prior to fixing the instrument. The tripod plate should be reasonably level at this point.

 

The level is attached to the tripod using a brass screw thread and handle, which is part of the tripod assembly. The screw housing on the base of the instrument ensures that the instrument is securely fixed, but should never be over tightened.

 

           

Key points in levelling the instrument ready for use:

 

  • Level the instrument by adjusting the levelling screws.
  • The bubble (not visible on the above image) should be central in the circle.
  • Turn the telescope through 180º
  • The level is accurate if the bubble remains in the centre of the circle.
  • If the level is inaccurate the bubble will move outside the circle
  • Parallax is the apparent movement of the cross-hairs over the levelling staff when the eye is moved up and down while sighting through the instrument.
  • To eliminate parallax the eye piece should be perfectly focused on the cross-hairs and the Telescope should be perfectly focused on the levelling staff.

 

 

The procedure for setting up a level can be seen by watching the video linked to below.


Levelling Staff

 

A levelling staff or rod is a graduated wooden or aluminium rod, which is graduated to allow the differences in elevation to be determined: An example is shown below.

Staff

















 




Reading the Staff

 

A reading is taken through the lens of the instrument onto the rod and the reading of the level is recorded. This is done using the follow steps:

 

  • Sight onto the levelling staff.
  • Focus the telescope on the staff.
  • Always ensure the bubble is central
  • Take the reading of height.
  • Each square is 1cm (10mm)
  • First metre is black and second metre is red
  • Each E is 100mm (0.1m) apart 

 

Booking the Levels


The booking of the levels is done at the same time that a reading is taken. How this is done can be seen by watching the video ‘Recording Level Readings’ in the multimedia box below.


There are two methods of booking levels:

 

  • Collimation Method: (The height of collimation is the height of the horizontal line of sight above the datum, and is sometimes called the height of instrument). In order to understand this you should watch the ‘Calculating the Height of Collimation’ video below.
  • Rise and Fall Method:

 

Advantages of the two methods of booking:

 

The advantage of the collimation method is that it is ideal for setting out reduced levels on site. The reduced level of points/pegs on site can easily and quickly be found by measuring down (or up, for inverted reduced levels) from the height of collimation.

 

The advantage of the rise and fall method is that it is the simplest method of booking and checking the calculations on site.

Booking_levels



 










Diagrams showing booking Procedure

This survey is booked as shown below. It starts and ends on the same OBM 

Table_1












 Table 1

 

Rules of Booking

  • Back sight - the first reading from a new instrument position. The survey starts with a known level. This will be an Ordnance Bench Mark (OBM) or a Temporary Bench Mark (TBM). It is good practice to make the final reading for the complete survey to be at this point to check accuracy.
  • The Back sight is added to the reduced level to give the height of collimation, entered on the same line.
  • An  Intermediate sight will occur between the Back sight and Foresight
  • The Foresight is always the last reading from an instrument position
  • The Foresight or intermediate sight is subtracted from the height of collimation to give the reduced level, entered on the same line as the foresight or intermediate sight.
  • The height of collimation only changes when the instrument is moved to a new position.
  • Every Back sight reading gives a new height of collimation, entered on the same line.
  • All readings referring to the same point on the ground are entered on the same line.

 

The reducing of the levels using the above rules is shown in Table 2 

Height_of_Col



















Table 2   Height of Collimation calculations



Rise and Fall
 


Rules used when booking levels 

  • The first (and last) reduced level is on an Ordnance Bench Mark (OBM) or a Temporary Bench Mark (TBM)
  • Following line by line down the page calculating the rise or fall between consecutive staff readings.
  • A rise occurs if the first staff reading is greater than the second staff reading in any consecutive pair of staff readings.
  • A fall occurs if the first staff reading is less than the second staff reading in any consecutive pair of staff readings.
  • Add the rise or subtract the fall from the preceding reduced level to obtain the new reduced level, entered on the same line as the rise or fall.

 

Rise_Fall


















Table 3 Rise & Fall Calculations


To work through this process you should watch the video 'Calculating Rise and Fall' in the multimedia box.


Accuracy Issues

For all types of survey the accuracy of level values should be as follows: 

·         Site TBM relative to Ordnance Survey bench mark ± 10mm

·         Spot levels relative to TBM within 10mm on hard surfaces 90% should be to ± 5mm.


 

If the closing error exceeds these values the survey should be repeated.

 

Checks on Calculations

Check on reduced levels obtained from back sights and foresights

Sum of back sights - sum of foresights = first reduced level - last reduced level.

 

Rise & Fall

Sum of back sights - sum of foresights = first reduced level - last reduced level = sum of rises - sum of falls = first reduced level - last reduced level. 



Height of Collimation

 

The Height of Collimation is the height of the line of sight of the instrument over the station above which it is centred, such as the specified datum level or Ordinance Bench Mark (OBM) the way this is calculated is explained in the multimedia presentation entitled ‘Calculating Height of Collimation’ in the multimedia box below.




 

Recording Level Readings

Calculating Rise and Fall

Calculating Height of Collimation





Task 1 Unit 4.2 Taking Readings

With the use of bullet points, explain the process used in taking readings on a site.


Word Guide - 400 

See ‘Submitting Tasks’ below




4.2.3 Determine the lengths of travellers for excavation and concreting

Where it is intended that the sight rails produced will be used for excavation or construction purposes Rather than the use of laser or optical levels the use of a traveller (sometimes referred to as a boning rod) will be required. Accuracy in establishing the length of the boning rod to the depth below the line of sight is essential and relatively easily achieved; (as illustrated below).

 






















Travellers (Click on drawing above to open in a separtate window)

 

Where excavations will take place which are designed to facilitate sloping construction as in the case of drainage excavations, it is simply a matter of ensuring that the sight rails mirror the fall required and hence the excavation becomes shallower or deeper as the work progresses along the excavation by sighting through the sight rails and leaving the traveller unaltered in length.

 

The basic concept in the use of the boning rod or traveller is that by establishing fixed points at known levels it is a simple procedure to fix a third point by sighting through to others.

 

The traveller is always of a known length, and can be reduced in length as subsequent layers of construction are added.



Additional information

If you would like additional information you can visit the constructionsite unit listed below.

 
constructionsite

 

Basic Principles of Surveying 






Task 2 Unit 4.2 Practical Exercise

This will be carried out during the attendance workshop




Submitting Tasks

When you have completed the Tasks you should send these to us.  This can be done in three ways:

 

1. Inserting it into the Online Task Submission Form, which you can open by clicking on the link below. It is suggested that you produce it in a word processing package and copy and paste it into the Form.

2. By printing off the Task Submission Sheet and the Task Assessment and Feedback Form and posting them with your work. Details are given on the Submission sheet.

3. By copying and completing Task Submission Sheet and Task Assessment and Feedback Form and emailing them with your work.

These forms and instructions are accessible from the Student Area.

 

Always keep a copy of your work.

 


Section Complete

 

You have now completed this section of unit 4. You will be notified as soon as the tasks you have submitted have been assessed. Though you may now move on to the next Section by clicking on the Section 4.3 link below.






Section 4.3 Inaccurate or Missing Information


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