2 site and temporary of works r0riginal (stu.ed)
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2 site and temporary of works r0riginal (stu.ed) Document Transcript

  • 1. 2 SITE ANALYSIS AND SET-UPThe physical characteristics of the site and its immediate environment will influence the decisions to bemade about a buildings design and construction. The information gathered from a thorough site analysis is,therefore, a vital exercise and must be completed before any design or construction work commences. Thisinformation can then be collated in a performance appraisal document from which informed decisions aboutthe best way to proceed may be made.2.1 FUNCTION OF THE SITE ANALYSISPrior to any construction operations, the client/developer will want to know whether it is economically viableto build on the proposed site. Because the nature and condition of the ground and soil below the surface ofthe site are an unknown quantity they pose a considerable risk to the construction project, with the potentialto cause delays and additional costs, both of which can be substantial. Inadequate soil investigation andhence inappropriate foundation design can lead to structural problems at a later date, which is a problemfor the building owners and users as well as the building insurers.Many projects are built on brownfield sites (land that has been previously built on and used) and it may benecessary to seal, stabilise or remove any contaminated ground, toxic waste or other dangeroussubstances before commencing the main construction works. The extent of contamination must beestablished before any work commences on the site.The main purpose of site analysis is to identify and hence reduce the risks associated with the developmentby recording site features and soil characteristics, helping to determine the design and cost of suitablefoundations and structure. A thorough site analysis is an essential first step that will assist development,design and construction decisions. The site analysis helps: The client to assess whether the project is viable (best done in consultation with professional advisors) The client, designer, structural engineer and contractor to locate the best position for the building, avoiding or accommodating identified problems where possible, while making the best possible use of physical features and environmental conditions The engineers to design the most suitable foundation system The mechanical and electrical consultants to design the service provision The designers and contractor to ensure that safe construction methods are usedThe environmental consultants to identify the most suitable way of dealing with any contaminants and problem materials, e.g. remediation works, material reuse, on-site treatment and disposal optionsSequence of activitiesThe site analysis comprises three interrelated research activities (Figure 2.1); the desk-top study, the sitereconnaissance and the ground and soil investigations. The soil investigation may also involve laboratorytests on liquids and gases found in soil samples. The order in which these activities are carried out willdepend to a large extent on the nature of the development and the timescales involved. The preferredsequence of overlapping activities is shown in Figure 2.1 and described in Sections 2.2-2.4. This shouldresult in a comprehensive appraisal document, as described in Section 2.5.2.2 THE ‘DESK-TOP STUDYThe desk-top study is a vital element in any site analysis exercise. The study involves the collection of alldocuments and materials that can be obtained without having to visit the site. There is a considerableamount of information available from local and national authorities, museums, private companies andresearch groups. The client or previous owners may also have relevant information to hand.Although the different site investigation operations often overlap, care should be taken not to commencewith expensive ground exploration and soil tests before the desk-top study is completed. This is partly toavoid unnecessary work and expenses — for example, information from the desk-top study may reveal thatrecent site and soil investigations are available — and partly a health and safety issue since theapproximate location of services and potential hazards must be known before carrying out any physicalinvestigations. Early and thorough research may also show that the site is unsuitable for development, orthat special measures are required before proceeding further. 1
  • 2. Information requiredOwnership(s) and legal boundariesThe client should provide, via a legal representative, information pertaining to the exact location of the siteboundaries and responsibilities for maintaining them. These will need to be checked against a measuredsurvey and any areas of uncertainty checked by the legal representative. Other issues to be determined bythe clients legal representative include: Rights of way Rights of light Rights of support (for adjoining properties) Legal easements Ownership of land (essential where parcels of land are being assembled to make a larger site) Rights of tenants, etc.Ground conditionsAs a first step it is usual to collect information on soil and subsoil conditions from the county and localauthority. This includes knowledge from maps, geological surveys, aerial photography and works forbuildings and services adjacent to the site, which may in itself give an adequate guide to subsoil conditions.Geological maps from the British Geological Survey, information from local geological societies, OrdnanceSurvey maps, mining, river and coastal information may also be useful.ServicesAll suppliers of services should be contacted to confirm the position of pipes and cables and the nature ofthe existing supply, i.e. its capacity. This includes gas, mains water, sewage and surface drainage pipes aswell as electricity, broadband and telephone cables. It is usual for a representative of the supplier to visitthe site to identify their equipment (which might be in a different location to that shown on plans).Contaminated land and methanePrevious use of land can give some clues as to the likely contaminants to be found and the local authoritymay have records that can help in this regard. However, extensive soil testing should be carried out toascertain the nature and extent of any contamination. Methane is associated with landfill sites, and localknowledge about the position of old tips/landfill can prove useful at an early stage.RadonIn certain areas of the UK, radon gas occurs naturally within the underlying ground and poses a healththreat to the inhabitants of buildings. Local authorities provide advice on the likely level of contaminationand will advise on the extent of protection required to prevent the radon gas from entering the building. 2
  • 3. FloodingDamage to property and possessions and the associated disruption to businesses and family life hasbecome a serious concern in recent years as the frequency of flooding has increased. Factors includeheavy rainfall, buildings sited on, or too close to, flood plains and inadequate maintenance of rivers andwatercourses. Thorough checks should be made about previous flooding of the site (if any), proximity toflood plains and any special requirements suggested or required by the various authorities and insurers.Typical sources of informationInformation will always be site-specific; however, the following list of information sources serves as ageneral guide:  Ordnance Survey — detailed maps in many different formats are available from Ordnance Survey, Romsey Road, Southampton S016 4GU (www.ordnancesurvey.co.uk)  Historical maps (www.old-maps.co.uk) and libraries local to the site  Geological maps — the British Geological Survey is the national repository for geosciences data in the UK. Information provided includes maps, records and materials, including borehole cores and specimens from across the UK. Address: London Information Office, British Geological Survey, Earth Galleries, Natural History Museum, Exhibition Road, London SW7 2DE (www.bgs.ac.uk)  Hydrogeological maps — soil reports and publication lists are obtainable from soil sur- vey and Land Research Centre, Cranfield University, Silsoe, Bedford MK 45 4DT  Meteorological information — monthly and annual reports are available on air temperature, wind speed, rainfall and sunshine. Such information is useful when designing the building and for scheduling construction operations. Statistics on averages and extremes are also available. The Met Office, Room JG6, Johnson House, London Road, Bracknell, Berks RG12 2SY (www.metoffice.gov.uk)  Hydrological information — surface water run-off data are collected by water authorities, private water undertakings and local authorities Site history:  Previous owners and developers  Site surveys and drawings used for previous development  Records held by Building Control  Local newspaper archives  Records held by the local planning authority Gas supplier — location of gas mains Electricity supplier — location of electricity cables Electricity generating board — mains electricity cables Water suppliers — water supply mains Mains sewers Local authority — local sewers Telecommunications authority — telephone and optical cables Rail authority — railways British Water Board — canals British coal — underground working building inspector Aerial photographs — there are many collections of aerial photographs dating back over many decades.A directory of organisations and agencies that hold aerial photographs can be obtained from PublicationsDepartment, Aslib, The Association for Information Management, Information House, 20-24 Old Street,London EC1V 9AP2.3 SITE RECONNAISSANCEWritten approvals from the client and/or the property owners must be in place and a thorough riskassessment exercise must be carried out before entering the site, especially before any invasiveinvestigations are carried out (which may require separate written permission). Obvious considerations arerelated to trespass and criminal damage, although the prime concern must be for the safety of those doingthe investigations. The majority of sites will have been used previously (and may still be in use) and mightcontain buildings that are structurally unsound (which may be redundant or still in use despite theircondition). Specialists should be appointed to establish the condition and safety of existing structures andwhether or not asbestos is present. Figure 2.2 provides an overview of the type of information that can be 3
  • 4. collected during a site reconnaissance.The visual inspection of the siteA visit to the site and its surroundings should always be made to record everything relevant to the proposeddevelopment. The site reconnaissance is often referred to as the visual inspection or the `walkover. Fromexperience we have found that two pairs of eyes (or more) are always better than one and so the visualinspection should be undertaken by at least two, and preferably three, people, e.g. the architect, engineerand contractor, with each taking their own notes but discussing features as they come across them. Carefulobservation should be made of the nature of the subsoil, vegetation, evidence of marshy ground, signs ofgroundwater and flooding, irregularities in topography, ground erosion and ditches and flat ground nearstreams and rivers where there may be soft alluvial soil. A record should be made of the foundations of oldbuildings on the site. Cracks and other signs of movement in adjacent buildings should be noted. Whenundertaking site reconnaissance on contaminated land, ensure as far as possible that all hazards havebeen identified and that correct safety procedures are followed. In preparation for the site reconnaissance,all of the maps and records should be assembled so that any differences or omissions found when walkingover and observing the site can be recorded. A visual inspection of physical site boundaries should bemade and compared with any legal documents that show boundaries.British Standard procedure for walkover surveysWhen conducting a walkover survey, the British Standard for site investigations BS 5930:1999) suggeststhat the surveyor should:  Traverse the whole area on foot (if possible and safe to do so)  Establish the proposed location of work on plans  Identify and record any differences on the plans and maps  Record details of existing services, trees, structures, buildings and obstructions 4
  • 5.  Check access and determine capability of sustaining heavy construction traffic  Record water levels, fluctuations in levels, direction of flow and flow rate  Identify adjacent property and the likelihood of it being affected by proposed works  Identify any previous or current activities that may have led to contamination  Record mine or quarry workings, old structures and other features  Record obvious features that pose immediate hazard to public health and safety or the environment  Record any areas of discoloured soil, evidence of gas production or underground combustionIdentification and physical location of servicesBefore undertaking any digging, e.g. for trail holes, it is necessary to clearly identify the nature of theservices on the site and their actual position. Unfortunately, the majority of the plans provided by theservices providers only give an approximate location of their pipes and cables; therefore, some detectivework is required on site. The first task is to identify all inspection covers and the nature of the service, andcompare their positions with those on the drawings. Handheld sonic and magnetic detecting devices to helplocate the position of services are available from most plant hire firms. Exact position and depth can beestablished by carefully hand digging trial pits to expose pipes, cables and conduits. The service providerswill also be keen to establish exact positions in an attempt to prevent damage to their pipes and cables.The organisations responsible for particular services should be invited to the site to help to establish theexact position, size and capacity of their supply and to resolve any uncertainty.In order to develop the site to its full potential services may need to be re-routed, if the appropriate authoritywill give permission. This is usually an expensive option, which could threaten the viability of the project.Alternatively, the proposed position of the building may need to be adjusted to enable the project toproceed without undue disruption to major service routes.SurveysMeasured surveyA land surveyor should conduct a topographical survey to establish the physical boundaries, existingfeatures and variations in level. Most land surveyors have a standard list of features to be establishedduring the land survey, although it is not uncommon to direct the land surveyors to particular areas torecord additional information.Condition surveyCondition surveys are used to record the condition of boundaries and adjoining property prior to workcommencing. Condition surveys are also employed to record the state of existing buildings on the site thatare to be protected, refurbished or altered. Before commencing any work that is likely to result in vibration(e.g. demolition, excavations, piling, heavy construction traffic, and so on) it is important to undertake a fullcondition survey of surrounding and adjoining properties and structures. This serves as a record for anysubsequent claims for damage and also serves as a good source of design information, for example,indicating how particular materials have weathered. Detailed drawings and written descriptions of theproperty should be supported with photographic evidence.Photographic and video surveysPhotographic and video surveys are useful tools to prompt ones memory when back in the office. Theyalso provide a record of the original condition of the site and adjoining land/property in case of any disputeor claim for damage. Photographic surveys should be conducted in a systematic and thorough manner, withthe position of the photographer and direction of view noted on a site plan to avoid any future confusion.2.4 SOIL INVESTIGATIONSDetails of the subsoil should include soil type, consistency or strength, soil structure, moisture conditionsand the presence of roots. From the nature of the subsoil the bearing capacity, seasonal volume changesand other possible ground movements are assumed. To determine the nature of the subsoil belowfoundation level it is necessary either to excavate trial pits some depth below the assumed foundation levelor to bore in the base of the trial hole to withdraw samples.The common methods of obtaining samples include: 5
  • 6.  Trial pits  Boreholes — Cable percussive boreholes and rotary drilled borehole  Window sampling and dynamic probe testingWhen proposing work to existing buildings (e.g. adding another floor) it will be necessary to expose theexisting foundation in a number of places to check if it was built as detailed on drawings (if available) andalso to check the subsoil below the foundation. Whichever system is adopted will depend on economy, theproposed building works and the nature of the subsoil. Trial pits or boreholes should be sufficient in numberto determine the nature of the subsoil over and around the site of the building and should be at most, say,30 m apart.Ground movements that may cause settlement are:  Compression of the soil by the load of the building  Seasonal volume changes in the soil  Mass movement in unstable areas such as made up ground and mining areas where there may be considerable settlement  Ground made unstable by adjacent excavations or by de-watering, for example, due to an adjacent road cutting.Trial pitsTo make an examination of the subsoil trial pits and/or boreholes are excavated. Trial pits are usuallyexcavated by a mechanical excavator, or in some cases by hand tools, to a depth of 3-4 m. The nature ofthe subsoil is determined by examination of the sides of the excavations. Soil and rocks can be examinedin situ on the faces of the excavated pit, and samples taken for further laboratory tests. The trial pit alsoprovides an indication of the ease of dig (or excavation), trench stability and groundwater conditions. Forexploration of shallow depths (up to 3 m) this is usually more economical than boreholes. The pits areusually rectangular, being approximately 1.2 X 1.2 m in plan. The pits should be excavated in the vicinity ofthe proposed structure; if the pit is located under a proposed foundation particular attention needs to begiven to the material used to backfill the hole. In such situations material should be of sufficient strengthand well compacted. As each trial pit is excavated and inspected a report should be made of the inspection.Typical information contained in the inspection, the trial pit log, is shown in Figure 2.6.Boreholes`Borehole is often used as a generic term that represents the various methods used to excavate andextract disturbed and undisturbed soil samples. All samples taken from bore-holes should be sealed assoon as possible to minimise any loss of moisture before testing. Some of the most commonly usedmethods are as follows.Auger boring — rotary boring methodsThese holes are usually made by hand or powered auger into the ground. Auger holes are typically 75-150mm in diameter. Short helical augers are used and disturbed samples of soil are collected as they arebrought to the surface. Such methods are not widely used as they do not allow the soil to be examined insitu and are not capable of penetrating to the depth of boreholes. Rotary drilling is used where theboreholes are being cut into very dense gravel or bedrock. Samples or bedrock are recovered in seamlessplastic tubes, are logged by and engineer, and then taken for laboratory testing (Photograph 2.1). 6
  • 7. Window samplersA window sampler is a steel tube about 1 m long with a hole cut into the wall of the tube allowing thedisturbed sample to be viewed or soil samples taken from the tube. The tube is driven into the ground usinga lightweight percussion hammer and extracted with the aid of jacks. A range of tube diameters areavailable. In practice the large tubes are driven in first and removed leaving a hole for smaller tubes to beinserted and driven in further. Samples can be obtained down to a depth of 8 m. 7
  • 8. Window sampling can be carried out using either handheld pneumatic samplers or tracked percussivesamplers (Photograph 2.2). The samples are retrieved in seamless plastic tubes. A qualified engineer logsthe samples (Photograph 2.3). Window sampling is suited to sites with restricted access, where disturbanceis to be kept to a minimum and contamination investigation. The percussive samplers are also normallycapable of doing penetrometer testing. Penetrometer testing is a continuous soil test procedure which ena-bles the relative density or strength of the ground to be determined. Further information can be found at theStructural Soils website, www.soils.co.uk. 8
  • 9. Ground and soil testsThere are a wide variety of on-site and laboratory tests that can be used to establish the characteristicground and soil conditions. The extent of soil investigation will be based on the nature of the building andcharacteristics of the site. More detailed site and laboratory studies will provide more information, reducingthe risks inherent in building on unknown ground. Laboratory and on-site tests that can be used include:On-site test Plate load test Vane shear test California bearing ratio (CBR) test Dry density/moisture relationship SPT (Standard Penetration Test) Lightweight dynamic penetrometers Cone penetration tests (CPT) Methane/oxygen/carbon dioxide/barometric pressure testLaboratory work  Triaxial compression tests  Liquid and plastic limit tests  Sieve analysis — particle size and distribution  Moisture content  pH value tests2.6 SITE SET-UP AND SECURITYSite planning is important, especially for sites with limited space and/or those located in busy areas.Although this is usually covered under the literature on construction management a brief summary of themain issues to be considered are presented here.Site set-upAccessAccess to the site is required for personnel, construction plant and delivery vehicles during construction.Firefighting equipment must also have clear access in case of a fire or emergency occurring during theworks. Temporary vehicular access may be allowed in consultation with the appropriate Highways, Policeand Town Planning departments. Safe access for large cranes and wide, heavy and/or long loads will alsoneed careful planning. 9
  • 10. Storage and wasteMaterials and plant must be stored so as to protect them from the weather and from damage from siteoperations. This applies equally to materials stored on site for a short period and those stored for a longerperiod. Space for the construction and subsequent protection from damage of sample panels is a relatedconsideration. The reduction of waste on site is linked to good site management and to good detailing.When detailing a building, attention should be given to reducing the amount of cutting and hence wastegenerated on the site (which is expensive to dispose of).Services and accommodationAs a minimum a suitable, metered supply of electricity and water will be required; so too will foul drainage.Site personnel require office space and comfort facilities. These are usually provided in specialistprefabricated site units that are hired by the contractor for the duration of the project. The well-being ofconstruction workers and also visitors to the site is an important factor.Security and safetySite security is required for two reasons. First, to protect the materials and plant left on the site overnightfrom theft and malicious damage. Second, to stop members of the public from inadvertently wandering onto the site and hence endangering their safety. Perimeter fencing must be secure and all access pointsmonitored. In addition to fencing and physical barriers to unauthorised entry, many contractors also employsecurity firms to provide additional protection at night and at weekends. Monitoring of materials enteringand leaving the site is also required to prevent pilfering by site workers.Levelling and setting outOnce all of the information on the site has been collected, appropriate levels and positions for the buildingsmust be established. To determine a level for the building it is necessary to find the levels and gradients ofthe land, the amount of material that will need to be removed, e.g. unstable or degradable material such astopsoil and waste. Once the lie of the land is mapped out then the loadbearing strata on which thefoundations and ground floor construction will bear is determined. In most cases the ground floor level ispositioned slightly higher than the existing ground level. By positioning the floor level above the externalground level unnecessarily deep excavation is avoided and problems of groundwater penetration arereduced.Setting out for excavation and constructionOnce the contractor has obtained legal possession of the site, and has made the site safe and secure, theland must be taken down to workable levels. Building and grid lines should be set out and before any workcan commence a temporary benchmark should be established. Temporary benchmarks will provide a fixedlevel on the site. All other levels for footings, floor levels, road levels, etc. can be determined by working outthe difference in levels from the temporary benchmark.In order to establish the on-site temporary benchmark a fixed reference point off-site needs to be found, i.e.Ordnance Survey record benchmarks that have been established on existing structures. Benchmarks canoften be found on churches, bridges and other large structures (and should be shown on the landsurveyors drawings). Once the level has been found it can be transferred to site. Site-based temporarybenchmarks should be placed on a sturdy structure in positions where they are not liable to be knocked ordisturbed. The benchmark should be recorded on a site plan for reference during construction.Reference points on the ground also need to be determined to position the building and associated workscorrectly. North- and east-based coordinates are used to position grid lines; these may be obtained using aGPS (global positioning system) or using two reference points with known coordinates. Alternatively, thebuilding or grid lines can be simply set out off existing structures.Reduced level digOnce a level has been established and grid lines or building lines marked out the reduce level dig cancommence. Generally, areas of the site where major excavation will take place are reduced. Underbuildings the reduced level dig is excavated to the underside level of the floor construction; this is thelargest volume of excavation that should take place. Reducing the level of the site where buildings arepositioned helps to provide a level site for ease of work. Foundations that go deeper than the underside ofthe floor construction are excavated independently from this level. 10
  • 11. 2.7 SITE WORKS AND SETTING OUTWhen a builder is given possession of a building site the contractor will have been provided with the sitelayout plan and the detail drawings necessary for him to construct the building(s). Under most forms ofbuilding contract it is the builders responsibility to see that the setting out is accurate.The site having been taken over, the task of preparing for and setting out the building can be commenced.These operations can be grouped under three headings:• clearing the site;• setting out the building;• establishing a datum level.Clearing the siteThis may involve the demolition of existing buildings, the grubbing out of bushes and trees, and the removalof soil to reduce levels. Demolition is a skilled occupation and should be tackled only by an experienceddemolition contractor. The removal of trees can be carried out by manual or mechanical means. Theremoval of large trees should be left to the specialist contractors.Building Regulation Cl, `The ground to be covered by the building shall be reasonably free from vegetablematter. This is in effect to sterilise the ground, because the top 300 mm or so will contain plant life anddecaying vegetation. This means that the topsoil is easily compressed and would be unsuitable forfoundations. Topsoil is valuable as a dressing for gardens, and will be retained for reinstatement when thesite is landscaped. The method chosen for conducting the site clearance work will be determined by thescale of development, and by consideration for any adjacent buildings. 11
  • 12. Setting out the siteThe first task is to establish a baseline from which the whole of the building can be set out. The position ofthis line must be clearly marked on-site so that it can be re-established at any time. For on-site measuring asteel tape should be used (30 m would be a suitable length). Linen and plastic-coated tapes are alsoavailable. The disadvantage with linen tapes is that they are liable to stretch.After the baseline has been set out, marked and checked, the main lines of the building can be set out,each corner being marked with a stout peg. A check should now be made of the setting-out lines for rightangles and correct lengths. There are several methods of checking whether a right angle has beenestablished, and in fact the setting out would have been carried out by one of these methods. A check muststill be made, and it is advisable to check by a different method to that used for the setting out. Thesetting-out procedure and the methods of checking the right angles are illustrated in Fig. 1.1.1.After the setting out of the main building lines has been completed and checked, profile boards are set upas shown in Fig. 1.1.2. These are set up clear of the foundation trench positions to locate the trench,foundations and walls. Profile boards are required at all trench and wall intersections.Establishing a datum levelIt is important that all levels in a building are taken from a fixed point called a datum. This point should nowbe established; wherever possible it should be related to an ordnance benchmark. This is an arrow with ahorizontal mark above the arrow as shown in Fig. 1.1.3. The centreline of the horizontal is the actual level 12
  • 13. indicated on an Ordnance Survey map. Benchmarks are found cut or let into the sides of walls andbuildings. Where there are no benchmarks on or near the site, a suitable datum must be established. A sitedatum or temporary benchmark could be a post set in concrete or a concrete plinth set up on site. ,Taking levelsThe equipment used is an engineers level and a levelling staff. The level is simply a telescope fitted withcross-hairs to determine alignment. The telescope rotates on a horizontal axis plate, mounted on a tripod.The staff is usually 4 m long in folding or extendable sections. The `E pattern shown in Fig. 1.1.4 isgenerally used, with graduations at 10 mm intervals. Some staffs may have 5 mm graduations. Readingsare estimated to the nearest millimetre.Levelling commences with a sight to a benchmark from the instrument stationed on firm ground. Staffstations are located at measured intervals such as a 10 m grid. From these, instrument readings are takenas shown in Fig. 1.1.5. The level differentials can then be combined with plan area calculations todetermine the volume of site excavation or cut and fill required to level the site.From Fig. 1.1.5:Rise and fall method:Staff reading at A = 2.500 m Staff reading at B = 0.750 mGround level at A = 100 m above ordnance datum (AOD)Level at B = 100 m + rise (— fall if declining)Level at B = 100 m + (2.500 — 0.750) = 101.750 m.Alternative height of collimation (HC) method: HC at A = Reduced level (RL) + staff reading= 100 m + 2.500 = 102.500 (AOD) Level at B = HC at A — staff reading at B= 102.500 — 0.750 = 101.750 m 13
  • 14. Measuring anglesThe sitesquare shown in Fig. 1.1.1 is accurate for determining right angles in the horizontal plane. Whereacute or obtuse angles occur in the horizontal, or vertical angles are to be established or checked, atheodolite is used. This instrument is basically a focusing telescope with cross-hairs, mounted on horizontalindex plates over a tripod. A vertical measurement circle with index is attached to one side of the telescope.Figure 1.1.6 shows the outline features of a traditional vernier theodolite. Traditional theodolites requirevisual or manual measurement of angles on the micrometer index or scale. In contrast, contemporaryinstruments are far more sophisticated, with automatic settings, liquid crystal displays, and facilities for data 14
  • 15. transfer to computers.With the instrument firmly stationed, the telescope and horizontal (vertical if appropriate) plate are rotatedfrom an initial sighting through the required angle. A pole-mounted target may be used for location. A checkcan be made by rotating the telescope through 1800 vertically and the index through 180° horizontally for asecond reading. Angles are recorded in degrees, minutes and seconds, the extent of accuracy determinedby the quality of instrument and the skill of the user.Sloping sitesVery few sites are level, and therefore before any building work can be commenced the area covered bythe building must be levelled. In building terms this operation is called reducing levels. Three methods canbe used, and it is the most economical that is usually employed.  Cut and fill - The usual method because, if properly carried out, the amount of cut will equal the amount of fill.  Cut - This method has the advantage of giving undisturbed soil over the whole of the site, but has the disadvantage of the cost of removing the spoil from the site.  Fill - A method not to be recommended because, if the building is sited on the filled area, either deep foundations would be needed or the risk of settlement at a later stage would have to be accepted. The amount of fill should never exceed a depth of 600 mm.The principles of the above methods are shown in Fig. 1.1.7. 15