ground

  • Damp-proof courses above ground

    There should be a continuous horizontal dpc above ground in walls whose foundations are in contact with the ground, to prevent moisture from the ground rising through the foundation to the wall above ground, which otherwise would make wall surfaces damp and damage wall finishes. The dpc above ground should be continuous for the whole length and thickness of the wall and be at least 150 mm above finished ground level to avoid the possibility of a build up of material against the wall acting as a bridge for moisture from the ground as illustrated in Fig. 35.
    dmc

  • Oversite concrete

    When Portland cement was first continuously produced, towards the end of the nineteenth century, it became practical to cover the site of buildings with a layer of concrete as a solid level base for floors and as a barrier to rising damp. From the early part of the twentieth century it became accepted practice to cover the site of buildings with a layer of concrete some 100 mm thick, the concrete oversite or oversite concrete. At the time, many ground floors of houses were formed as raised timber floors on oversite concrete with the space below the floor ventilated against stagnant damp air.

    With the shortage of timber that followed the Second World War, the raised timber ground floor was abandoned and the majority of ground floors were formed as solid, ground supported floors with the floor finish laid on the concrete oversite. At the time it was accepted practice to form a continuous horizontal damp-proof course, some 150 mm above ground level, in all walls with foundations in the ground.

    With the removal of vegetable top soil the level of the soil inside the building would be from 100 to 300 mm below the level of the ground outside. If a layer of concrete were then laid oversite its finished level would be up to 200 mm below outside ground level and up to 350 mm below the horizontal dpc in walls. There would then be considerable likelihood of moisture rising through the foundation walls, to make the inside walls below the dpc damp, as illustrated in Fig. 24.
    Oversite concrete
    It would, of course, be possible to make the concrete oversite up to 450 mm thick so that its top surface was level with the dpc and so prevent damp rising into the building. But this would be unnecessarily expensive. Instead, a layer of what is known as hardcore is spread oversite, of sufficient thickness to raise the level of the top of the concrete oversite to that of the dpc in walls. The purpose of the hardcore is primarily to raise the level of the concrete oversite for solid, ground supported floors.
    The layer of concrete oversite will serve as a reasonably effective barrier to damp rising from the ground by absorbing some moisture from below. The moisture retained in the concrete will tend to make solid floor finishes cold underfoot and may adversely affect timber floor finishes. During the second half of the twentieth century it became accepted practice to form a waterproof membrane under, in or over the oversite concrete as a barrier to rising damp, against the cold underfoot feel of solid floors and to protect floor finishes. Having accepted the use of a damp-proof membrane it was then logical to unite this barrier to damp, to the damp-proof} course in walls, by forming them at the same level or by running a vertical dpc up from the lower membrane to unite with the dpc in walls.

    Even with the damp-proof membrane there is some appreciable transfer of heat from heated buildings through the concrete and hardcore to the cold ground below. In Approved Document L to the Building Regulations is the inclusion of provision for insulation to ground floors for the conservation of fuel and power. The requirement can be met by a layer of insulating material under the site concrete, under a floor screed or under boarded or sheet floor finishes to provide a maximum U value of 0.45 W/m2K for the floor.

    The requirement to the Building Regulations for the resistance of the passage of moisture to the inside of the building through floors is met if the ground is covered with dense concrete laid on a hardcore bed and a damp-proof membrane. The concrete should be at least 100 mm thick and composed of 50 kg of cement to not more than 0.11 m3 of fine aggregate and 0.16 m3 of coarse aggregate of BS 5328 mix ST2. The hardcore bed should be of broken brick or similar inert material, free from materials including water soluble sulphates in quantities which could damage the concrete. A damp-proof membrane, above or below the concrete, should ideally be continuous with the dpc in the walls.

    It is practice on building sites to first build external and internal load bearing walls from the concrete foundation up to the level of the dpc, above ground, in walls. The hardcore bed and the oversite concrete are then spread and levelled within the external walls.

    If the hardcore is spread over the area of the ground floor and into excavations for foundations and soft pockets of ground that have been removed and the hardcore is thoroughly consolidated by ramming, there should be very little consolidation settlement of the concrete ground supported floor slab inside walls. Where a floor slab has suffered settlement cracking, it has been due to an inadequate hardcore bed, poor filling of excavation for trenches or ground movement due to moisture changes. It has been suggested that the floor slab be cast into walls for edge support. This dubious practice, which required edge formwork support of slabs at cavities, will have the effect of promoting cracking of the slab, that may be caused by any slight consolidation settlement. Where appreciable settlement is anticipated it is best to reinforce the slab and build it into walls as a suspended reinforced concrete slab.

  • Foundations on sloping sites

    The natural surface of ground is rarely level to the extent that there may be an appreciable slope either across or along or both across and along the site of most buildings.

    On sloping sites an initial decision to be made is whether the ground floor is to be above ground at the highest point or partly sunk below ground as illustrated in Fig. 15.

    Fill and cut and fill

    Where the ground floor is to be at or just above ground level at the highest point, it is necessary to import some dry fill material such as broken brick or concrete hardcore to raise the level of the oversite concrete and floor. This fill will be placed, spread and consolidated up to the external wall once it has been built.

    The consolidated fill will impose some horizontal pressure on the wall. To make sure that the stability of the wall is adequate to withstand this lateral pressure it is recommended practice that the thickness of the wall should be at least a quarter of the height of the fill bearing on it as illustrated in Fig. 16. The thickness of a cavity wall is taken as the combined thickness of the two leaves unless the cavity is filed with concrete when the overall thickness is taken.

    Solid filling

    To reduce the amount of fill necessary under solid floors on sloping sites a system of cut and fill may be used as illustrated in Fig. 15. The disadvantage of this arrangement is that the ground floor is below ground level at the highest point and it is necessary to form an excavated dry area to collect and drain surface water that would otherwise run up to the wall and cause problems of dampness.

    Fill and cut and fill

    To economise in excavation and foundation walling on sloping sites where the subsoil, such as gravel and sand, is compact it is practice to use a stepped foundation as illustrated in Fig. 17, which contrasts diagrammatically the reduction in excavation and foundation walling of a level and a stepped foundation.

    Foundation on sloping site

    Figure 18 is an illustration of the stepped foundation for a small building on a sloping site where the subsoil is reasonably compact near the surface and will not be affected by volume changes. The foundation is stepped up the slope to minimise excavation and walling below ground.

    The foundation is stepped so that each step is no higher than the thickness of the concrete foundation and the foundation at the higher level overlaps the lower foundation by at least 300 mm.
    Stepped foundation
    The load bearing walls are raised and the foundation trenches around the walls backfilled with selected soil from the excavation. The concrete oversite and solid ground floor may be cast on granular fill no more than 600 mm deep or cast or placed as a suspended reinforced concrete slab. The drains shown at the back of the trench fill are laid to collect and drain water to the sides of the building.

  • Pad foundations

    On made up ground and ground with poor bearing capacity where a firm, natural bed of, for example, gravel or sand is some few metres below the surface, it may be economic to excavate for isolated piers of brick or concrete to support the load of buildings of some four storeys in height. The piers will be built at the angles, intersection of walls and under the more heavily loaded wall such as that between windows up the height of the building.

    Pits are excavated down to the necessary level, the sides of the excavation temporarily supported and isolated pads of concrete are cast in the bottom of the pits. Brick piers or reinforced concrete piers are built or cast on the pad foundations up to the underside of the reinforced concrete beams that support walls as illustrated in Fig. 11. The ground beams or foundation beams may be just below or at ground level, the walls being raised off the beams.

    Pad foundations

    The advantage of this system of foundation is that pockets of tipped stone or brick and concrete rubble that would obstruct bored piling may be removed as the pits are excavated and that the nature of the subsoil may be examined as the pits are dug to select a level of sound subsoil. This advantage may well be justification for this labour intensive and costly form of construction.

  • Unstable ground

    There are some extensive areas of ground in this country where mining and excavations for coal and excavations for taking out chalk for use as a fertiliser and making lime and others for extracting sand and gravel may have made the ground unstable. The surface of the ground under deep and shallow excavations below the surface may well be subject to periodic, unpredictable subsidence.

    Where it is known that ground may be unstable and there is no ready means of predicting the possibility of mass movement of the subsoil and it is expedient to build, a solution is to use some form of reinforced concrete raft under the whole of the buildings, as illustrated in Fig. 5.

    The concrete raft, which is cast on or just below the surface, is designed to spread the load of the building over the whole of the underside of the raft so that in a sense the raft floats on the surface.

    There are some extensive areas of ground in this country where mining and excavations for coal and excavations for taking out chalk for use as a fertiliser and making lime and others for extracting sand and gravel may have made the ground unstable. The surface of the ground under deep and shallow excavations below the surface may well be subject to periodic, unpredictable subsidence.Where it is known that ground may be unstable and there is no ready means of predicting the possibility of mass movement of the subsoil and it is expedient to build, a solution is to use some form of reinforced concrete raft under the whole of the buildings, as illustrated in Fig. 5.The concrete raft, which is cast on or just below the surface, is designed to spread the load of the building over the whole of the underside of the raft so that in a sense the raft floats on the surface.

  • Made up ground

    Areas of low lying ground near the coast and around rivers close to towns and cities have been raised by tipping waste, refuse and soil from excavations. Over the years the fill will have settled and consolidated to some extent. Areas of made up ground are often used for buildings as the towns and cities expand. Because of the varied nature of the materials tipped to fill and raise ground levels and the uncertainty of the bearing capacity of the fill, conventional foundations may well be unsatisfactory as a foundation.
    An example of made up ground is the area of  Westminster now known as Pimlico where the soil excavated during the construction of the London docks was transported by barge to what was low lying land that was usually flooded when high tides and heavy rainfall caused the Thames river to overflow. The raised land was subsequently heavily built on.

    A uniformly stable, natural, sound foundation may well be some 3 or more metres below the surface of made up ground. To excavate to that level below the surface for conventional strip foundations would be grossly uneconomic. A solution is the use of piers on isolated pad foundations supporting reinforced concrete ground beams on which walls are raised, as illustrated in.

    An example of made up ground is the area of Westminster now known as Pimlico where the soil excavated during the construction of the London docks was transported by barge to what was low lying land that was usually flooded when high tides and heavy rainfall caused the Thames river to overflow. The raised land was subsequently heavily built on.A uniformly stable, natural, sound foundation may well be some 3 or more metres below the surface of made up ground. To excavate to that level below the surface for conventional strip foundations would be grossly uneconomic. A solution is the use of piers on isolated pad foundations supporting reinforced concrete ground beams on which walls are raised, as illustrated in Fig. 4.

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