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Field Study – intrusive visual and physical activity to:
- Establish site characteristics from the desk study
- Access potential hazards to health and safety
- Appraise surface conditions
- Appraise ground water conditions
- Take subsoil samples and conduct in-situ tests
- Consider the need for subsoil exploration, trial pits and bore holes
- Appraise existing structures
(Roger Greeno – Construction Handbook)
1.2 Scope of works
- Specification of soil and ground investigation.
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To identify the different types of foundations and superstructures available for both the 2no. single storey buildings and the 1no. four storey building, and provide a suitable solution for each.
- SUMMARY OF SITE BACKGROUND
- Site Description and Location
The site is Bexley College, Holly Hill Campus which is located on Upper Holly Hill Road, Belvedere, Kent DA17 6HF and is centered on national grid reference TQ497783. This site falls in the category of D1 – Non-Residential institution, according to the Local Council (Bexley County Council).
[Figure 1 – Site Location Plan]
[Source: www.multimap.com]
The site consists of the following:
- GROUND INVESTIGATION
3.1 Summary of Activities
The field work was undertaken on 19th February 2002 and completed on 20th February 2002. This investigation will analyse the type of subsoil in the ground which will then determine the type of foundation needed as well as the load bearing of the superstructure. This will ensure any difficulties are avoided during the construction process to produce a safe and economic building.
- Both boring and trial pits can be carried out on the ground to obtain subsoil samples and identify, classify and ascertain the subsoil’s characteristics and properties. They may also be used to establish the presence of any geological faults and the upper and lower limits of the water table. Boring holes are carried out sporadically across the proposed site on the gridlines specified on the footprint of the building. Trial holes are used for a more specific location, next to the grid lines where the most load bearing will be. In this instance, light cable percussion (shell and auger), using chiseling techniques from 0.5m to 6.0, which took 2 hours. This was carried out by Albury S.I. Ltd, report no. 02/6021/KJC.
(Roger Greeno, Building Construction Handbook)
- Sampling Results
The following results came out of the above bore hole testing:
0.25m (0.15) - Made ground (tarmacadam)
0.25m (0.50) - Light brown sans with pebbles
1.00m – 6.00m - Very dense brown sand and pebbles
KEY TO BOREHOLE AND TRIAL PIT LOGS
(source: www.bgs.ac.uk)
No water strikes made, therefore the water table is in deep ground.
The ground results show that the subsoil is coarse grained and compact. This type of subsoil is fairly good ground to be built on.
The particle size range is more than 2mm. This ground requires a pick for excavation. 50mm peg hard to drive more than about 150mm.
This type should be kept above water wherever possible. Slopes on sand are liable to erosion. Foundations are to be 450mm below ground level on ground susceptible to frost heave.
Trees are in the vicinity and will need to be left a distance of at least 9m for large trees, from the proposed buildings. This is dependable on type and size. Further specialist investigation will be taken out if required to do so.
- FOUNDATIONS SUMMARY
4.1 Strip Foundation
Types:
Suitable for most subsoils and light to medium structural loadings i.e. low to medium rise buildings where mass concrete can be used. A pre-cast concrete frame is ideal as it is light and ideal for single storey buildings. The foundation will be cast before the superstructure is put in place.
(Roger Greeno, Building Construction Handbook)
- Pad Foundation
Types:
- Isolated Column foundation
- Continuous column foundation
- Combined column foundation
- Cantilever foundation
- Balanced base foundation
Suitable for most subsoils except loose sands, loose gravels and filled areas. Pad foundations are usually made out up of reinforced concrete.
Pad foundations are designed to take the load and spread it in two directions to keep the structure of the building well balanced. The design of this is to span in two directions, therefore main bars are placed in the bottom both ways.
(Roger Greeno, Building Construction Handbook)
- Raft Foundation
Types:
- Solid slab raft
- Beam and slab raft
- Cellular raft
A large combined slab foundation designed to cover a whole or large part of the site. Used when:
- The soil is weak
- Columns are so closely spaced in both directions
- Overlapping isolated column foundations bearing a high load
A raft reduces the ground pressure immediately. Is made up of alot of re-bar. Is an alternative to piling.
(Roger Greeno, Building Construction Handbook)
- Pile Foundation
Types:
- Friction piles
- End bearing piles
Used when the ground conditions are poor. They are used to dig deep to find good ground through poor ground. The load of the building is taken down to a low level though columns in the soil on which the building rests. Commonly used to depths of 18m and will go to 30m or more in exceptional circumstances. Can be used in deep beds of clay and silt as an alternative to raft. Piling produces a lot of noise and this will need to be taken into consideration if when working in built up areas.
(Roger Greeno, Building Construction Handbook)
- SUPERSTRUCTURE
5.1 Framed Superstructure
Made up of (or in combination):
- Steel columns and beams
- In-situ concrete and beams
- Pre-cast concrete columns and beams
Used mainly for high rise building, which enables the reduction of the dead weight of the building. The design layout of the building can be flexible with this option.
The disadvantages of this option are that any alterations to the load path through the frame will need to be checked down to foundation level resulting in time and money loss. A framed structure also requires shear walls.
(Handout, Bexley College, Les)
5.2 Non-framed Structure
Made up of:
- Masonry (Brick & Block work)
Non-framed structures are more suited to smaller footprint buildings where the shear stress is taken in by the masonry work. Due to the small size of the building, any structural alteration will be easier to accommodate than a framed structure.
An appropriate foundation option will need to be acquired due to the high dead weight of the building causing an increase on the load. The flexibility of the layout is limited due to this.
(Handout, Bexley College, Les)
6.0 CONCLUSION AND RECCOMMENDATIONS
6.1 Single Storey Block
Foundation
The strip foundation is recommended due to the small footprint and light load of the buildings. The subsoil report of the site shows that the site has good ground making the strip foundation ideal in this instance,
Structure
A non-framed structure is recommended as a single storey building has a light to medium load. This coincides with the strip foundation which, supports light to medium loads mainly consisting of masonry work.
6.2 Four Storey Block
Foundation
From the subsoil report, the ground is made up of dense sand and pebbles. Therefore, pad foundation is ideal for this good ground.
Structure
A framed structure of the three choices (as mentioned previously in report), is ideal for this multi-storey building. There is extra design flexibility whilst reducing the dead weight.
Another option for the four-storey building will be to use both cantilever foundation and pad foundation to hold a framed structure that can accommodate an underground car park whilst using steel columns to uphold the building. This option will be more costly that the former, but, will be able to accommodate a number of vehicles of the staff and students at the college.
7.0 REFERENCES
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Alan C. Twort, J Gordon Rees (2004) Civil Engineering Project Management, Fourth Edition, Oxford
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Jack Stroud Foster, Raymond Harington and Roger Greeno (2007) Mitchell’s Structure & Fabric, 7th Edition – Part 2, Essex
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N.J. Seward (2006) Structural Foundation, Designers Manual, Oxford
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R.F. Craig (2004) Craig’s Soil Mechanics, Oxon
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Roy Chudley & Roger Greeno (2009) Building Construction Handbook, 7th Edition
