End of Unit Assignment – Site Surveying
1.) Explain the procedures for setting out and levelling of foundations for steel framed and pre-cast concrete buildings.
Setting out involves the transfer of positions and levels of buildings and associated features from those dimensions and co-ordinates marked on a drawing or plan or computer printout to the ground by a variety of methods and instruments designed for that purpose. Precision is required in ensuring that the building and placement level match the position shown on the drawings relative to Ordnance survey references expressed by reference to longitude and latitude, plus established boundary points for the site; also ordnance bench marks (OBM) that indicate levels relative to ordnance datum i.e. mean sea level at Newlyn Cornwall.
OBM’s are frequently found chiselled into buildings and can be transferred to site with temporary benchmarks (TBM) suitably marked and protected from damage or disturbance. The purpose of setting out is to ensure correct positioning and dimensional control during construction. A good reference guide to the degree of accuracy required in setting out buildings is covered by BS5606: 1990 Accuracy in Building.
Levelling is the establishment of a level height plane for the site from which measurements can be taken. This will be referenced from a datum known as a benchmark (BM), ideally linked to an OBM or a TBM.
Steel framed and pre-cast buildings have particular requirements for high accuracy in setting out as the limited tolerances available during the construction stages with much of the dimensional constraint applied off-site. In masonry built buildings a degree more tolerance is available due to the smaller components being assembled.
Setting out usually follows a set procedure albeit that most projects provide for a degree of uniqueness that requires skill and forethought to be applied before setting out proceeds.
A typical approach would include:
1.) An appraisal of the setting out documents to obtain the fundamental setting out data and this would include dimensions of buildings and boundaries also finished levels of the works.
2.) Site inspection would identify obstacles to setting out, ground conditions would help guide the type of markers to be used for pegs and site rails. Existing TBM could be confirmed.
3.) Verify works will match the planned use of the site by checking dimensions.
4.) Plan ahead by calculating any offsets required due to obstructions, level changes and other calculations needed during setting out that will improve site efficiency.
5.) Proceed to set out the plan of the works using tape to measure distance and theodolite for angles having first checked equipment for accuracy.
6.) It is usual to establish site lines and profiles and other markers outside of the area of construction to enable quick and easy reference during construction and avoid damage occasioned by the activities involved.
7.) Proceed to establish levels for the vertical dimensions of the works.
A grid is set up using a theodolite and marked out with marker pegs; it is essential that the theodolite is level. Calculation of diagonals by the means of triangulation is an effective means of checking the accuracy of the setting out. Triangulation calculates the length of lines by extrapolation from the angles between them, applying Pythagoras’ theorem. Following the completion of the grid; sight rails can be set up for the excavation, allowing the use of a traveller to assist in the levelling of the ground. Excavation would usually be done with machinery but may be done manually. Figure 1 illustrates how a grid appears once setting out is complete.
Figure 1: Surveying for Construction, Third Edition (Page 242)
2.) Explain the procedures for checking the verticality of the frame as above.
It is vital the frame of a building is perpendicular. Larger buildings require greater accuracy regarding verticality, as a minor error on the first storey can become a major error the higher up the building you get. A theodolite, a plumb-bob and other special instruments can be used to check for verticality.
The theodolite is set up so that it is in line with the wall of the building usually at the lowest point visible; however you can use the highest point but any result taken from the top of the building will be the inverse of what it would be from the bottom. It is essential the theodolite is level for this task to work. You need to ensure the theodolite is fixed so it cannot be turned horizontally, you do however require it to move in a vertical plain as you are measuring for deviations regarding the verticality. Once you have recorded your results, you will have to conduct this procedure again at a right angle facing the same corner. You need to repeat this process for every corner of the building. Figure 2 illustrates the procedure for one corner of a building; you have to site from P1 through to P2 and then repeat the process sighting from P3 to P4.
Figure 2: Surveying for Construction, Third Edition (Page 243)
A plumb-bob and string may be used if the building is only a few metres tall; therefore it is unlikely to be used in this scenario. The plumb bob is generally heavily weighted and the wire should be damp, to reduce movement by factors such as wind. Gravity makes the plumb bob hang vertical, this can then be referred to, by staging surrounding it. The piano wire falls into a baseline from which measurements can be made. This is a tried and tested solution used throughout the ages to asses verticality, the word ‘plum’ coming from the Latin ‘plumbum’ the word for lead, the material frequently used for the heavy weight.
An Autoplumb resembles a theodolite but automatically sets out the verticality line. An optical plummet is used for sighting downwards and a telescope is used for viewing sighting upwards. The Autoplumb being centred and levelled in a predetermined location at ground level establishes the vertical line; the horizontal level being set to zero follows this. The altitude bubble is centred and the telescope points vertically as the vertical bubble is set to zenith. The instrument is then turned through 90ï¿½, 180ï¿½ and 270ï¿½ whilst recording results. You need to have holes in each floor so you can sight upwards without any obstruction being in the way. A good example of an Autoplumb is the ‘Hilger & Watts’, Figure 3 Shows a cross section of a typical Autoplumb.
Figure 3: Engineering Surveying, Fifth Edition (Page 474)
The theodolite method is the most common, it is not dependable on weather nor does it have potential restraints in the design of holes to allow an Autoplumb to work, it is however expensive but can be hired if not owned. Laser equipment now makes everything a lot simpler to do with self-levelling bases etc.
3.) Describe the procedure and instrumentation for transferring control points to upper floors of multi-storey in-situ reinforced concrete frames.
Vertical transfer can be achieved using a variety of techniques; optical plumbing tends to be the most popular method for large buildings. The theodolite is installed directly above the control points on the ground level; the instrument is then directed vertically upwards by assistance from theodolite attachments, such as a 90ï¿½ prism, which can be connected to the top of the telescope. The 90ï¿½ prism allows the theodolite to be vertically aligned when it is horizontally aligned, as the prism projects the sight line vertically, 90ï¿½ adjacent.
Reference points are collated on each level when sighting upwards with the theodolite; these points are then used to help create reference lines for construction elements on that floor. Holes which are ‘about 200mm square’ as stated in Basic Surveying (Page 268) are usually located near corners on each floor to help vertical transfer; this is particularly common in buildings which have floors laid as construction rises, i.e. multi-storey in-situ reinforced concrete frames. Figure 4 shows how a theodolite would be set up below sight holes in the floors of a building. It also displays the sight holes in the corners of the building as explained earlier on the ‘Base Figure’.
Figure 4: Engineering Surveying, Fifth Edition (Page 475)
Optical plumbing should be carried out no further than about 30 metres, therefore every 30 metres you should set up the instrument again at the new level; this does however depend on the range of the instrument. Specialist equipment such as lasers may also be used to provide a vertically visible sight line, however this equipment is expensive.
An alternative method for transferring control points vertically is by two-theodolite inspection. A ground point is viewed in from two different directions, commonly 90ï¿½ apart from each other. The points are then located at higher levels by elevating both theodolite and establishing where the collimation lines intersect. Lasers which are talked about in Question 5 can also be used to transfer control points.
4.) Describe the procedure for setting large radii horizontal curves for road construction.
The setting of curves requires a more complex effort than the marking of straight lines. There are also several ways of doing this, using theodolites and tapes to transfer coordinates potentially derived from a computer program or manually calculated.
Tangential Angles Method – this is the most common method and uses a tape and theodolite.
Figure 5 illustrates the tangent as T. The curve is calculated with a series of chords: BC and CD; the first and final sub-chords are labelled TB (c1) and DT1 (c4). Calculating the angles ?1, ?2, ?3 and ?4 (the tangential or deflection angles) allows the curve to be set out.
Prior to the calculation of angles the following procedure takes place:
1.) First, the theodolite is set facing I and zeroed (00ï¿½ 00′ 00″).
2.) Turn the theodolite through ?1 degrees, at that distance (chord c1) point B is pegged in.
3.) Repeat the same procedure as (2), however angle ?2 is added and chord c2 is formed at the correct length from B until it intersects angle ?2.
The above procedure is repeated until T1 is established. T1 should be ?1 + ?2 + ?3 + ?4 degrees from point I.
Figure 5: Surveying for Construction, Third Edition (Page 232)
In the situation of a large radii horizontal curve for road construction, obviously more pegs are going to be required than illustrated in Figure 5 however the same procedure must be applied. You can calculate the number of chords required by applying the following formula:
Two Theodolite Method – this method is useful for difficult ground conditions.
Theodolites are placed at points T and T1 (see Figure 6) and both are sighted directly towards I and zeroed (00ï¿½ 00′ 00″). The angles ?1 and ?2 are taken from each theodolite. Where the angles meet points C and D are marked out. To complete the curve further angles are taken and points marked out. This method has the advantages that each point is based on independent measurements so cumulative error can’t build up.
Figure 6: Surveying for Construction, Third Edition (Page 234)
A complication that is sometimes encountered when setting out curves for road construction is where the origin point is obstructed and cannot be accessed. Two methods of overcoming this obstacle are ‘Plot and Transfer’ and ‘Calculate and Transfer’; both methods require a chord between the two tangent points of the arc.
Plot and Transfer
This method requires offset points to be plotted on a drawing, which are then replicated on site. CAD (Computer Aided Design) software for computers is commonly used to assist in plotting the information out.
Calculate and Transfer
This is where offset points are calculated using Pythagoras’ theorem. You have to create a right angled triangle intercepting the mid-point of the chord, from this you can then calculate a reference point; this procedure is repeated as many times as required to create more reference points.
Another solution, which may be used when setting out where the origin point is obstructed is creating an arc directly opposite, projecting the inverse radius with a chord intercepting both tangents, directed the same as before. The results from the inverse arc can be plotted on the chord, which can then be shifted to the other side enabling reference points to be available yet again.
5.) Explain the use of a variety of modern electronic surveying instruments including lasers and their application to construction work.
Developments in electronic instrumentation have increased the ease and efficiency of surveying work. Equipment that would once have been set up manually can now automatically adjust itself; and the data it collects can be recorded on computers, reducing error and increasing the availability of information. Modern equipment combines greater ease of use with increased accuracy – aided by the integration of surveying instruments, communications and computers. The modern site can benefit from constantly updated information and near instant communication of revised plans.
Electromagnetic distance measurement (EDM) can make measuring distance much quicker. Tape measures, theodolites, chains, etc, can be replaced by a tool – that can even be handheld – that can take an accurate measurement in moments. Its ease of use and adaptability can eliminate problems of terrain such as hills, sharp rises and falls of land, and expanses of water or marsh; traditional methods would be tedious, prone to error – or even impossible. Used with a theodolite, EDM can be used to set out and check co-ordinates. Its accuracy makes it useful for establishing control points and measuring important lengths e.g. traverse lines. This is reliant on various kinds of electromagnetic radiation.
Lasers are controlled beams of light. Straight lines can be made, or a level plane marked out by rotation of the laser. Lasers can be used to measure distances as the speed of light is fixed (in ordinary circumstances).
Therefore lasers can be used in surveying in the stead of theodolites – replacing the optical line-of-sight with a light-beam. They’re useful for working in confined spaces where standard methods would be difficult, such as tunnels.
However, accuracy can be reduced – the beam can be affected by variations in temperature, and the beam can suffer from divergence over long distances. There is also the danger of eye damage from misuse.
Computers, which become ever cheaper, more powerful, and more robust, are used for data storage, and the calculation of areas and volumes. 3D modelling (integrated with GPS) is referred to in Question 7.
6.) Describe how a GPS operates and how it can be applied to construction work.
The global positioning system (GPS) uses a network of 28 satellites that orbit at high altitudes above the earth’s surface (approximately 11,000 miles, orbiting twice a day). It was developed and launched by the American military. The satellites are monitored and controlled from facilities around America and on British/American bases.
These satellites broadcast precise data such as location on earth and time 24 hours a day. Anyone with a receiver can get the ‘standard-positioning service’ (SPS) free – their rough position (within about10m now) and the time (based on atomic clocks within the satellites). The ‘precise-positioning service’ (PPS) also includes velocity data and is accurate to within cm. This service is available to the American military, its allies, and can also be paid for (by those deemed acceptable).
The EU is developing its own ‘Galileo’ system of GPS (with other countries), which promises greater accuracy when complete. The current American system has been upgraded to increase accuracy within the last few years, with the SPS signal no longer being deliberately degraded so much to reduce its accuracy, this makes it less effective if another country wish to use it against the USA.
GPS works by triangulation. GPS satellites are synchronised to send information at exactly the same time. Their orbital planes and positions are spaced so that any point on earth has direct line of sight with 4 satellites. A receiver picking up the signals from 4 satellites can calculate its position in 3 dimensions by measuring the tiny differences in time the signals take to arrive.
GPS is increasingly being applied in construction work today; it is providing increasingly accurate information which is can be used to help in setting out and checking reference points by using two GPS receivers.
GPS does not provide high enough accuracy for surveying in standard conditions but can be more useful in different circumstances. It was one of the methods used in the construction of the Channel Tunnel. It comes into its own for use in terrain such as mountain or jungle where normal surveying methods are not possible; it is also much simpler than standard surveying equipment to operate. It can also be used at night.
Major excavation works can now be carried out by the use of GPS equipment fixed to excavators linked to computer modelling systems providing direct control of levels and contours.
7.) Describe the type of information you might extract from a digital mapping database and discuss one aspect of how the information may assist you in producing a surveying solution.
Digitalised databases allow easy access to different types of mapped information, which should be up-to-date and available in 2 dimensions or 3 dimensions.
Different types of information may be needed, depending on the circumstances of the survey, such as: spatial information – height of terrain and boundaries for rivers, roads, land ownership etc. Geological data may be needed to establish bedrocks, soil types, load bearing ability etc. Flora and fauna may be marked as protected. Water run-off systems and flood risks, weather conditions such as mean temperatures and prevailing winds, may all need to be taken into consideration and may have been mapped. An up-to-date digital map of underground utilities would greatly aid urban work.
Digital maps can be manipulated on a computer, allowing the site to be studied at leisure. The software provides you with a variety of useful facilities enabling the operator to highlight or enlarge particular features.
Software is continually being modified and enhanced and it is therefore becoming more sophisticated whereby more options are available.
An image of the proposed construction can be superimposed on the map of the site allowing decisions and changes to the plan to be made before construction begins. In this way, problems can be seen, and solved, before they occur. Software also allows you to alter the angle from which you view the map; therefore you are able to gage a better understanding of the lie of the land in relation to potential construction.
Digital mapping databases can also be used to monitor changes in the layout of land over periods of time. In coastal areas with no sea defences, land is lost to sea erosion at an inconsistent rate; therefore these changes will be visible if you were to conduct a digital mapping every year or so. This would allow you to monitor what areas of the coast are eroding fastest and would assist you in making future projections and thereby help in deciding where to best install sea defences.