Artificial Surfaces
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Precise calculations can be made on both the attenuation of the stone sub-base, subsequent layers and the run off into an outlet or soakaway
Stone sub-base on stabilised formation - note PCC kerb Whereas, from the client’s perspective, it
allows you to be more certain with costs, refine and predict construction programmes with more accuracy and gives greater confidence to the outcome that can be achieved. This is particularly important with sports such as athletics where you are constructing facilities to precise gradients over very large areas. The exact methodology and volumes of lime and/or cement incorporated into the sub-soil will vary depending upon the chemistry of each site but, in essence, these materials are added, rotovated in, watered and rolled, then allowed to set and become hard. Typically, a 300mm layer of stabilisation will be incorporated into the formation and land drains will be cut through the stabilised and solidified layer using a whizz wheel type trencher. The base itself does not drain, but is
Where is your drainage outlet?
The drainage pipes themselves would be a conventional dimension ordinarily, but would need to be installed in an appropriate manner
” 112 I PC JUNE/JULY 2015
contoured so that water moves across the plateau to the nearest installed land drain. Incidentally, it is a useful technique if precise engineering calculations are required for the drainage system as part of a planning condition. Precise calculations can be made on both the attenuation of the stone sub- base, subsequent layers and the run off into an outlet or soakaway. This information may allow you to dispense with drainage calculations that may be attached to a successful planning application as part of a pre-commencement condition. The stone sub-layers and surface layers are
installed on top of the solidified layer speedily and effectively because, in simple terms, the ground does not stand any chance of any movement. Historically, this technique has been employed on poor ground, but I am using it more and more on a range of sites for all advantages described above. It can be cost neutral, but typically would involve a small uplift in overall costs. I feel that, when compared to the advantages, it is a price worth paying. I would certainly encourage anybody to investigate it further as part of a successful design solution, although appreciate that it would not be adopted in every instance and sometimes small footprints do not justify any extra uplift in
cost, so it must be analysed on a case by case basis.
Site Drainage
This is one area where traditions seem to have held on the basis that is ‘if it isn’t broken then don’t fix it’. Traditional drainage techniques are used and a conventional ten metre land drainage system is typically installed connecting with a surround main drain to some form of drainage outlet. Normally, this drainage outlet is a soakaway or more positive connection, such as a drainage ditch or outlet system of some sort. As already stated, a stabilised sub-base can
facilitate water attenuation in the layers above, but there would be more emphasis on the drainage system in the ground, as the ground itself cannot absorb water and remove it from site, gravitationally speaking. The drainage pipes themselves would be a conventional dimension ordinarily, but would need to be installed in an appropriate manner, i.e. on an even gradient to a true line and backfilled appropriately. It is important to inspect the drainage
system installation as part of the site monitoring and project management duties, and the contractor must avoid any significant landslip into the drainage lines themselves, as contamination can be an issue. You must ensure that all connections and junctions are positive and are constructed in a recognised manner, using appropriate junction boxes and sleeves etc. It is always worthwhile trying to connect
to a positive outlet, such as a ditch or water course, so that water moves freely and quickly from site, however, in some instances, this is not feasible as a positive outlet is unavailable to the project development team. In this instance, more specialist calculations and techniques may well need to be employed, but a good drainage engineer and/or project manager should be able to advise you on this. Most surfacing systems installed in this
country are porous and, therefore, a sub- base drainage system is an important aspect of the design solution. There are one or two surface types that are non-porous and rely
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