Construction Sub-Structures Essay

First it is assumed that houses number 10 and 14 are already built before deciding how to approach the site investigation report. ‘A site is typically characterized by undertaking site investigations (Desk top study, walk over survey and intrusive investigations) in association with a risk assessment, to develop a conceptual site model.

Before purchasing a site a builder must establish whether there are any hazards on or below site, which could result in expensive house foundations. You could end up paying for a site investigation whether you have one or not, meaning the consequences of not having one could end up costing just as much and in some cases a whole lot more.

Site investigations should always begin with a desk study so that consideration can be given to health, safety and environmental hazards prior to fieldwork commencing.’[1]This involves collecting as much information as possible about the site in the form of geological maps, ordinance survey maps, old mining records or even past site investigations. Desk studies can also show anomalies with the ground that we may not be able to see when carrying out field research. These can be found from historical maps of the site aerial photographs, maps and drainage layouts. There may be drainage running right underneath our proposed building so we can’t just go digging holes where we like. Desk study will also involve consultations with the local authority environment agency. The local authority and the environment agency will have a duty to inspect the area for contamination and such like and to make this information readily available. We also need to determine the site boundaries and who will be affected when we start to carry out the development.

It is then important to carry out a field study and visit the site; this is an essential part of a site investigation and should always be carried out. This will require us to carry out 2 tasks, a walk over survey. This needs to be done in order to establish the access to the site, adjacent buildings and how they will be affected and the compatibility of the site, is it feasible to build the building.

‘Access to the site in which the works are being constructed is necessary to allow transport and delivery of materials, plant and equipment as well as to allow services and the workforce to reach the site. In order to be effective the access must be suitable for the type of transport required and must be available to the contractor at the appropriate time. Even if access is physically available local permissions and custom clearances may be necessary to allow legal use of access’[2]. For the site in question it will be necessary to gain access to the site over a public footpath and therefore will need the councils permission to use this but also temporary signs will be needed to warn pedestrians. A walk over survey is also carried out to check and supplement information gained from the desk top study, i.e. Groundwater and springs, Railway cuttings, Manholes, Existing Dwellings, Trees and Bushes and the like.

The second field study task that will need to be carried out is a soil investigation to determine the ground type and what we have to work with. The most ideal ground type we would hope to find would be clay. For this type of low housing I would consider that simple trial pits are the most useful and economic way of determining the ground conditions. ‘When excavated they should be deep enough to confirm that the strata below the proposed foundation level remain competent and adequate for at least a further meter below the standard-width footing of at least 600mm. The minimum depth of the trial pits should range from 25.m to 3m where possible and should be at least 3m from proposed dwellings’[3] When carrying out my trial holes I will need to note the following information:

Depth and the nature of the strata

Description of the various strata, i.e. loose material or stiff clays

The depth that the water entered the pit

Whether the sides collapsed or not

I will also take clay samples for testing in the lab to determine the ph, moisture content and elasticity.

In order for to carry out the investigation and dig the trial holes I will require a 5 tonne excavator and also a dumper to clear the spoil off site. These will also be used further in the development.

There are a number of options available for foundations on a site like this and these are typical strip foundations, Trench Fill Foundations, Wide Strip Foundations and maybe even raft foundations, depending upon the soil type.

A typical strip foundation ‘consist of a continuous strip of concrete designed to spread the load from uniformly loaded walls of brick, masonry or concrete to a sufficient area of subsoil. The spread of the strip depends on the foundation loads and the bearing capacity and shear strength of the subsoil.’ [4] If the ground is a little softer than we like then in this case we may have to use wide strip foundations to spread the load that bit more. If a wide strip foundation is used then it will have to be reinforced this is because ‘where the edges of the a foundation project beyond the faces of the wall it supports, bending due to cantilever action will occur as a result of the resistance of the soil, causing bending and shear stress in the foundation.’[5]

Compression

150mm

Tension 1200mm

The load and the ground type determine the overall thickness of a foundation and as our initial soil survey indicates a sub soil of stiff clay there would be no reason for us to build our foundations wider than 600mm. For this reason on a typical strip foundation with out reinforcement the dimensions would have to be as so:

150mm

300mm 150mm

600mm

To use a trench fill foundation is also an option.

[6]

‘These are only suitable when a good bearing stratum is known to be present at an economic and practical depth. The stratum below the base of the trench fill must remain competent for at least 1.5times the foundation width’[7] However when using trench fill foundations we have to consider where the services will go and it then becomes complicated, also it is important that the foundation is level to within 10mm for us to start building our brickwork.

Our final option would be to use a raft foundation. These are generally used where there tends to be no firm bearing strata of soil existing at a reasonable depth below the surface, therefore a maximum are of foundation is required to spread the pressure. Raft foundations, however can also be used in firmer soils also. ‘A raft foundation is a large slab foundation covering the whole building area, through which all the loads from the building are transmitted to the soil. These foundations when used for the purposes described here they are laid on, or just below the surface of the ground and are termed surface rafts.’[8]

In my opinion and with the initial sub soil report indicating a sub soil of stiff clay I would recommend using a trench fill foundation. They are completed very quickly and also because of this it means there is less chance of the clay swelling or shrinking due to the speed. It also is cost effective due to there being less need for planking and strutting but also because there will be no need for brickwork below dpc and therefore less labour time. It also means that there is huge reduction in the risk of the trench collapsing and damaging brickwork or injuring others. With good ground type there is no need for anything fancy and more time can be spent on the substructure.

After opting for the trench fill foundation we need to design the substructure. For the brickwork blockwork I would recommend a 102.5mm outer skin facing brick with a 140mm Thermalite High strength 7 concrete blockwork inner leaf (strength 7N/mm2) with 100mm cavity and cavity fill (up to ground floor level). For the ground floor I would advise 150mm well compacted class 6N hardcore with a 50mm thick clean sandblinding. On top of the sandblinding I would lay a 2000gauge visqueen damp proof membrane turned up at the edges then, on top of the damp proof membrane, I would place a 50mm thick Kingspan TF70 insulation. Finally on top of the insulation I would lay my floor slab. This should be 150mm thick RC35 concrete floor slab to standard BS 5328and incorporating one layer of A252 mesh reinforcement placed in the top part of the slab. It is also a good idea to place another layer of damp proof membrane and Insulation.

Concrete Floor Slab

Damp Proof Membrane

G.L Sandblinding

Hardcore

Top Soil

In order to carry out the construction of the substructure will need to use a variety of plant. These go as follows:

1No 5 tonne excavator – A Takeuchi TB145 Midi Excavator from H.E. Services

1No 3 tonne dumper – Thwaites 3 Tonne Dumper form H. E. Services

1No Flex and Poker Drive Unit, 2inch, code 18/0500 from Speedy Hire

1No Ride on Roller, Cat CB224D from H. E. Services

After commencement of the excavation it was brought to our attention that the sub-soil is highly shrinkable clay. This is probably because of the large tree on the site taking a lot of the moisture from the soil. This now means that the clay is moveable and we will have to change our foundations to something more suitable. Short bored piles would probably be the most practical. ‘In shrinkable clay this foundation has a number of practical advantages over strip foundations: a reduction in the amount of excavated spoil, faster construction and the fact that work can continue in weather that would make trenching digging impractical. When mechanically bored in sufficient numbers this type of foundation is competitive in cost with a traditional strip foundation of appropriate depth’[9] Bored piles are used when a soil replacement rather than a soil displacement method of piling is required and also when there is a need to minimise vibration. This also means that our ground floor construction will also have to change to a beam and block floor. ‘This arrangement provides the least deadweight of construction and therefore the most economical structure’[10]

References

M.F. Atkinson, Structural Foundations Manual For Low Rise Buildings Second edition, Page 4

M.F. Atkinson, Structural Foundations Manual For Low Rise Buildings Second edition, Page 33

R. Barry, The Construction Of Buildings, Crosby Lockwood & Son Limited, 1996, page 7

R. Barry, The Construction Of Buildings, Crosby Lockwood & Son Limited, 1996, page 70

J. S. Foster, Structure and Fabric Part 1, B. T. Batsford Limited, 1975, Page75

J.S. Foster, Structure and Fabric Part 1, B. T. Batsford Limited, 1975, Page 78

J.S. Foster, Structure and Fabric Part 1, B. T. Batsford Limited, 1975, Page 79

Web References

http://www.bradford.gov.uk/NR/rdonlyres/63E78764-012E-41EA-BBC6-80669FB69D5C/0/cont_land_leaflet.pdf , 30.11.2007

http://www.atkinson-law.com/cases/CasesArticles/Articles/Access_to_Site.htm, 04.12.2007.

http://www.beamlockbuilding.co.uk/content/view/full/5375 07.12.2007

http://www.blackwellpublishing.com/cooke/docs/samplechapter.pdf 07.12.2007

Bibliography

M.F. Atkinson, Structural Foundations Manual For Low Rise Buildings Second edition

Barry. R, The Construction of Buildings, Crosby Lockwood and Son Ltd, 1996

Foster. J. S, Structure and Fabric Part 1, B. T. Batsford Limited, 1975

Foster. J. S, R. Harrington, Structure and Fabric Part 2, The Anchor Press, 1997

Marshall.D. & Worthing.D., The Construction of Houses – 3rd Edition, Estates Gazette, 2000

http://www.bradford.gov.uk/NR/rdonlyres/63E78764-012E-41EA-BBC6-80669FB69D5C/0/cont_land_leaflet.pdf , 30.11.2007

http://www.atkinson-law.com/cases/CasesArticles/Articles/Access_to_Site.htm, 04.12.2007.

http://www.beamlockbuilding.co.uk/content/view/full/5375 07.12.2007

http://www.blackwellpublishing.com/cooke/docs/samplechapter.pdf 07.12.2007

http://books.google.com/books?id=TastxSGEEGIC&pg=PA33&lpg=PA33&dq=trench+fill+foundations&source=web&ots=HrAXf1jdPZ&sig=xTez80h9xpX8YU4_phUinFUb8F0#PPA4,M1, 07.12.2007

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[1] http://www.bradford.gov.uk/NR/rdonlyres/63E78764-012E-41EA-BBC6-80669FB69D5C/0/cont_land_leaflet.pdf , 30.11.2007

[2] http://www.atkinson-law.com/cases/CasesArticles/Articles/Access_to_Site.htm, 04.12.2007.

[3] M.F. Atkinson, Structural Foundations Manual For Low Rise Buildings Second edition, Page 4

[4] R. Barry, The Construction Of Buildings, Crosby Lockwood & Son Limited, 1996, page 7

[5] J. S. Foster, Structure and Fabric Part 1, B. T. Batsford Limited, 1975, Page75

[6] http://www.beamlockbuilding.co.uk/content/view/full/5375 07.12.2007

[7] M.F. Atkinson, Structural Foundations Manual For Low Rise Buildings Second edition, Page 33

[8] J.S. Foster, Structure and Fabric Part 1, B. T. Batsford Limited, 1975, Page 78

[9] J.S. Foster, Structure and Fabric Part 1, B. T. Batsford Limited, 1975, Page 79

[10] R. Barry, The Construction Of Buildings, Crosby Lockwood & Son Limited, 1996, page 70

[11] http://www.blackwellpublishing.com/cooke/docs/samplechapter.pdf 07.12.2007