The chosen substructure option and outline

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1.0 Introduction.

This report advises the promoters of a portal framed building as to which substructure would be of best use. This report is firstly going to identify the chosen substructure option and outline as to why this method has been chosen in relation to the project. Further discussion will identify alternative options available and reasons as to why these were not selected. Secondly this report will give drawings, annotated and to scale, showing the form of construction chosen. Where required, references will be quoted within this report.

2.0 Substructure Chosen.

The first issue that this report deals with is the choice of substructure for the site. It is this report's findings that a pile foundation, in this case an end bearing pile would be the best option available to the project.

2.1 Reasons for choice.

A pile foundation, form of construction has been selected, as a site survey shows that the proposed site for the project is an abandoned, back filled sandstone quarry. The project would be constructed upon back filled and graded rubble, which is approximately 6m deep, at the bottom of which is a 200mm thick layer of clay fill, resting on the sandstone bed rock beneath. Therefore, a pile foundation, would best, as unlike for instance, a raft foundation, which would sit upon the existing rubble and would be more susceptible to failure in that movement and settling could crack in the floor. Pile foundation's on the other hand, would overcome this, as its not be constructed on the substructure, but on, as the name says a piles, which means the structure would have no load bearing points upon the surrounding strata

2.2 Description Of Substructure

Test pits being bored in to the earth, find solid strata, and determine the depth of the solid ground, in this case being 6 meters deep. There are variations of pile foundations, these being timber, steel, reinforced concrete and pre-tensioned concrete. Wikipedia suggests that, Pile foundations can be installed by either driving them into the ground or drilling a shaft and filling it with concrete, mass or reinforced. This report suggests that the driven-pile be the most appropriate for this project. Hammering the piles through the back-fill would provide the accuracy needed to produce a stable platform, where as drilling and then filling with reinforced concrete would also provide the stability needed, but the extra time and cost involved with this process make this, an unjustified option. The particular choice made for this project is a reinforced concrete pile. The reason for this being, that the timber form of pile would be unsuitable due to the water table as in time would rot and attract wood eating insects, which would compromise the integrity of the pile. The same water table issues make this report disregard the steel pole form of piles, as they would rust and disintegrate over time. Further discussion in latter sections will go into greater depth on Cast-in-drilled-hole piles (CIDH piles) and End-bearing piles.

2.3 Site Preparation.

For health and safety reasons, before work gets underway, the site will need preparing; the reason for this is that the strata, at present would probably not be able to support the weight of the heavy plant, which would be required on site. Preparation would be completed in two stages, both of which will involve extra ‘fill' being brought on to site. The first fill the will be a 6f2 grade backfill that will be followed by a grade 1 fill, both of which being compressed to provide an adequate and stable base for contractors to work upon. Ideally just a grade 1 fill would be used, but this being an expensive method, this report finds that using the 6f2 fill material in conjunction with an optimal 100mm thick layer of grade 1 would do just an effective, and cheaper job.

3.0 CIDH piles

The online resources website, Wikipedia makes the suggestion, that rotary boring techniques offer larger diameter piles than any other piling method and permit pile construction through particularly dense or hard strata. Boring is finished when the hard rock or soft rock layer is reached, in the case of end bearing piles. If the boring machine is not equipped with a rock auger, then socketing of the hard rock layer is done with the help of a heavy chisel which is dropped from a height of about 1.5 meters (depends on the weight of the chisel and design requirements) by suspending it from a tripod stand attached to a winch crane. The socketing is carried out until the desired depth within the rock layer has been attained. Usually, the required depth within the rock layer is considered, equal to the diameter of the pile in hard rock layers and is taken to be equal to 2.5 times the diameter of the pile in soft rock layers.

4.0 End-bearing Piles.

A pile foundation comprises two basic structural elements, the pile and the pile cap. A pile cap is a structural base, similar to a spread footing, which supports a structural column, wall, or slab, except that it bears on a single pile or group of piles. A pile can be best described as a structural stilt hammered into the ground, each pile carries a portion of the pile cap load and transfers it to the strata near the pile tip, located at the bottom of the pile. (see illustration).

End-bearing pile.

The pile and pile cap configuration has provided the basic design solution to the difficult problem of obtaining deep foundation support below areas where poor soil conditions prevail. Poor soil conditions may be difficult to excavate through, and are incapable of supporting structural loads. They are typically characterised by the presence of a soft, compressible layer of clay, high ground-water levels, loosely filled soils, uncontrolled landfills, boulders, abandoned underground structures, and natural bodies of water. By supporting a structure on piles in lieu of spread footings, any adverse soil condition may be over come and virtually bypassed. Foundation support can be obtained at any depth, without the need to perform deep excavation, dewater, or the need to install temporary sheeting and bracing. Piles are available in a variety of sizes, shapes, and materials that enable a particular type of pile foundation to be viable both economically and structurally. Materials that can be used are timber, concrete, and steel. Pile foundations are primarily used in locations where poor soil conditions exist.

5.0 Factory Flooring.

This report suggests, as the concrete flooring will be laid upon back-fill it needs to be thicker than average to prevent failure, therefore, the recommendations to overcome this is a 200mm thick slab, as this would give the extra strength needed for possible activity carried out by the occupying firm. For example, forklift trucks, and cranes needed for the production of gearboxes. A gas membrane will also be fitted before the instillation of the flooring, the following section gives more information about gas membranes.

6.0 Gas membrane.

Gas membrane products are specifically designed to protect against hazardous gas emissions of which Radon, Methane and Carbon Dioxide gas are the most common. In normal atmospheric conditions these gases would dissipate harmlessly, however when these gases seep in to buildings through cavities, pipe entries and construction joints they become a serious health and safety risk. Radon in particular, is a naturally occurring radioactive gas, which is now accepted to be carcinogenic (A carcinogen is any substance or radiation that is an agent directly involved in the exacerbation of cancer) and toxic. As radon comes up through the ground, the best protection can be achieved by installing radon barrier gas membranes before construction. (see illustration)

7.0 Project Site.

The project is located on Turner Street, near the A61, close to all the major motorways therefore, making the transportation of piles and necessary machinery particularly easy.

As the piles will be driven into the ground, there will be a large amount of noise pollution, as the project site is not located close to any residential developments there should be little, if any disruption to residents. That said, there are local government restrictions as to when loud construction noise can be made. Sheffield council state ‘Sometimes there are practical limits in reducing construction noise and the restriction of working hours is often the most practicable option for reducing the impact on local residents and businesses.' In regards to where residential occupiers are concerned, the enforced hours of work are prohibited to 8.00 am to 6.00 pm Monday to Friday and 8.00 am to 1.00 pm on Saturday with noworking on Sundays or Public Holidays. As the area for construction is classed as industrial, these regulations are not applicable.

8.0 Alternate Construction methods.

As with any project, there are always alternatives. The alternatives for this project were a raft foundation, strip foundation, and pad foundation. These forms of construction were looked at and will be explained in-depth as to why they were discredited for this project.

8.1 Raft Foundations.

Firstly, raft foundations, these reinforced concrete slabs extend fully under the structure. They provide bridges over weak areas and can be used to bridge over soft spots, which would be good for getting over the voids, which may be created between the back-fill. This method was disregarded, mainly due to the cost to the client, due to the large amount of concrete needed to complete the foundation.

8.2 Strip Foundations.

Secondly, strip foundations. These are most commonly used for house building has it provides the simplest and cheapest form of foundation. This method was also disregarded due to the weak filled ground, which the site has, but it is unsuitable for framed buildings, the main influence was due to the massive amount of work, concrete and cost needed to make this a viable option.

8.3 Pad Foundations.

Finally, pad foundations. Again, this would not be suitable choice as it also is not suitable for filled or weak strata.

For the three foundation methods mentioned above, they could be used, but it would mean have to use different method for preparing the ground to suit the foundation. But the overall cost to the client and time taken to just prepare the strata, to take such methods is not an economical option.

9.0 Conclusion.

It is this reports conclusion that the report appears to have provided suitable advice to the promoters of a portal framed building, as to which substructure would be of best use. This report firstly identified the end bearing pile as the chosen substructure option and outlined as to why this method was chosen in relation to the project. Further discussion then went on to give local government policies regarding construction work noise. In addition, outlined as to why a gas membrane should be used, in conjunction to the project. Secondly this report gave drawings, annotated and to scale.

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