Setting Out - Levelling

Levelling ~ the process of establishing height dimensions, relative to a fixed point or datum. Datum is mean sea level, which varies between different countries. For UK purposes this is established at Newlyn in Cornwall, from tide data recorded between May 1915 and April 1921. Relative levels defined by benchmarks are located throughout the country. The most common, identified as carved arrows, can be found cut into walls of stable structures. Reference to Ordnance Survey maps of an area will indicate benchmark positions and their height above sea level, hence the name Ordnance Datum (OD).

On site it is usual to measure levels from a temporary benchmark (TBM), i.e. a manhole cover or other permanent fixture, as an OD may be some distance away.

Instruments consist of a level (tilting or automatic) and a staff. A tilting level is basically a telescope mounted on a tripod for stability. Correcting screws establish accuracy in the horizontal plane by air bubble in a vial and focus is by adjustable lens. Cross hairs of horizontal and vertical lines indicate image sharpness on an extending staff of 3, 4 or 5m length. Staff graduations are in 10mm intervals, with estimates taken to the nearest millimetre. An automatic level is much simpler to use, eliminating the need for manual adjustment. It is approximately levelled by centre bulb bubble. A compensator within the telescope effects fine adjustment.

Application ~ methods to determine differences in ground levels for calculation of site excavation volumes and costs.

Setting Out Reduced Level Excavations

Setting Out Reduced Level Excavations ~ the overall outline of the reduced level area can be set out using a theodolite, ranging rods, tape and pegs working from a base line. To control the depth of excavation, sight rails are set up at a convenient height and at positions which will enable a traveller to be used.

Setting Out a Framed Building

Setting Out a Framed Building ~ framed buildings are usually related to a grid, the intersections of the grid lines being the centre point of an isolated or pad foundation. The grid is usually set out from a base line which does not always form part of the grid. Setting out dimensions for locating the grid can either be given on a drawing or they will have to be accurately scaled off a general layout plan. The grid is established using a theodolite and marking the grid line intersections with stout pegs. Once the grid has been set out offset pegs or profiles can be fixed clear of any subsequent excavation work. Control of excavation depth can be by means of a traveller sighted between sight rails or by level and staff related to site datum.

Setting Out Trenches

Setting Out Trenches ~ the objective of this task is twofold. Firstly it must establish the excavation size, shape and direction and secondly it must establish the width and position of the walls. The outline of building will have been set out and using this outline profile boards can be set up to control the position, width and possibly the depth of the proposed trenches. Profile boards should be set up at least 2„000 clear of trench positions so they do not obstruct the excavation work. The level of the profile crossboard should be related to the site datum and fixed at a convenient height above ground level if a traveller is to be used to control the depth of the trench. Alternatively the trench depth can be controlled using a level and staff related to site datum. The trench width can be marked on the profile with either nails or sawcuts and with a painted band if required for identification.

NB. Corners of walls transferred from intersecting cord lines to mortar spots on concrete foundations using a spirit level

Setting Out the Building Outline

Setting Out the Building Outline ~ this task is usually undertaken once the site has been cleared of any debris or obstructions and any reduced level excavation work is finished. It is usually the responsibility of the contractor to set out the building(s) using the information provided by the designer or architect. Accurate setting out is of paramount importance and should therefore only be carried out by competent persons and all their work thoroughly checked, preferably by different personnel and by a different method.

The first task in setting out the building is to establish a base line to which all the setting out can be related. The base line very often coincides with the building line which is a line, whose position on site is given by the local authority in front of which no development is permitted.

PROTECTION ORDERS FOR TREES AND STRUCTURES

Trees ~ these are part of our national heritage and are also the source of timber - to maintain this source a control over tree felling has been established under the Forestry Act 1967 which places the control responsibility on the Forestry Commission. Local planning authorities also have powers under the Town and Country Planning Act 1990 and the Town and Country Amenities Act 1974 to protect trees by making tree preservation orders. Contravention of such an order can lead to a substantial fine and a compulsion to replace any protected tree which has been removed or destroyed. Trees on building sites which are covered by a tree preservation order should be protected by a suitable fence.

Trees, shrubs, bushes and tree roots which are to be removed from site can usually be grubbed out using hand held tools such as saws, picks and spades. Where whole trees are to be removed for relocation special labour and equipment is required to ensure that the roots, root earth ball and bark are not damaged.

Structures ~ buildings which are considered to be of historic or architectural interest can be protected under the Planning Acts provisions. The Department for Communities and Local Government lists buildings according to age, architectural, historical and/or intrinsic value. It is an offence to demolish or alter a listed building without first obtaining `listed building consent' from the local imprisonment. It is also an offence to demolish a listed building without giving notice to the Royal Commission on Historical

Monuments, this is to enable them to note and record details of the building.

TIMBER ROT - CAUSES, TREATMENT AND PRESERVATION

Causes -
* Defective construction, e.g. broken roof tiles; no damp-proof course.
* Installation of wet timber during construction, e.g. framing sealed behind plasterboard linings; wet joists under floor decking.
* Lack of ventilation, e.g. blocked air bricks to suspended timber ground floor; condensation in unventilated roof spaces.
* Defective water services, e.g. undetected leaks on internal pipework; blocked or broken rainwater pipes and guttering.

General treatment -
* Remove source of dampness.
* Allow affected area to dry.
* Remove and burn all affected timber and sound timber within 500mm of fungal attack.
* Remove contaminated plaster and rake out adjacent mortar joints to masonry.

Note: This is normally sufficient treatment where wet rot is identified. However, where dry rot is apparent the following additional treatment is necessary:

* Sterilise surface of concrete and masonry.
Heat with a blow torch until the surface is too hot to touch.
Apply a proprietary fungicide† generously to warm surface.
Irrigate badly affected masonry and floors, i.e. provide 12mm
diameter bore holes at about 500mm spacing and flood or
pressure inject with fungicide.

- 20:1 dilution of water and sodium pentachlorophenate, sodium orthophenylphate or mercuric chloride. Product manufacturers' safety in handling and use measures must be observed when applying these chemicals.

Replacement work should ensure that new timbers are pressure impregnated with a preservative. Cement and sand mixes for rendering, plastering and screeds should contain a zinc oxychloride fungicide.

Further reading -
BRE: Timber pack (ref. AP 265) † various Digests, Information
Papers, Good Repair Guides and Good Building Guides.
In-situ timber treatment using timber preservatives † HSE Books.

Ref: Bldg. Regs. Approved Document C, Site preparation and resistance to contaminants and moisture.