In this article, the third in a series of six, David Rhodes of Traction Sports considers the initial phase of ground preparation, site drainage, kerbs, gripper rails and the various layers available
The initial phase of ground preparation is very often an exciting time, as it truly marks the start of the contract, with machines on the ground carrying out excavation works etc. At this point, the site itself should have been made safe, in line with health and safety recommendations, so that contractors are working within a safe construction footprint, protected typically by temporary Heras style protective fencing to segregate the contract operatives from members of the public etc. The extent and type of this protection would normally be agreed with your health and safety consultant as part of the overall health and safety strategy.
The initial site set-up would have erected this fencing and installed any temporary haul roads, site signage, and prepared site access and egress routes in a safe and timely manner. Also, as part of the site set up, the area itself would have been weed killed if surface vegetation is present so that, when the topsoil is stripped and stored, the vegetation will die down quickly. It is then easy to reuse when carrying out final landscaping or recycling in some manner. Most contractors will also rotovate dying vegetation to enhance organic breakdown whilst the top soil is temporarily stored for potential re-use.
On occasions, a lack of consideration is made as to where topsoil can be stored on site, either on a temporary or permanent basis. You will be surprised how much topsoil comes off a site strip, and the difficulty you can have in storing it temporarily, permanently or the costs associated with removing it from site. The cost of these movements is significant, and this must be calculated and considered in detail beforehand as part of the project planning and budget preparation. Some topsoil will always be retained for final landscaping, and topsoil can be recycled and used elsewhere, but there are considerations and calculations to make in this regard, as the material may need pre-use testing to ensure it is compatible and clean.
If the topsoil is stored in temporary stockpiles, I would advise that these are not stacked too high and would normally recommend something less than four metres, in order that the internal temperature does not increase significantly, and cause issues with microbiological populations and, therefore, render the topsoil inert biologically speaking in the future.
The site footprint itself would normally be larger than the final facility to deal with any site gradients and banks or batters that need to be installed for future maintenance.
Once the topsoil has been stripped, the subsoil is exposed and is worked on further to create a formation footprint to build upon. It is at this point that you find out whether your extensive ground searches, planning and preparations have borne fruit, or if there is something in the ground that you will now expose that you didn't know about!
Fingers crossed, it is as you envisaged and there are no surprises.
When the formation has been exposed, it will be prepared to the correct gradients in order to develop new facilities upon it. The formation itself would normally be consolidated and can be stabilised in addition to this, which is described in more detail below. The consolidation process, after initial grading, is carried out by specialist equipment nowadays to ensure significant consolidation and compaction of this layer so that it is strong enough to build up. Some contractors have made significant investment in compaction rollers with on-board compaction meters to measure the level of compaction achieved with repeated passes of the roller itself. There are alternatives to this technique, and in situ compaction testing can be carried out as a key stage test.
What is interesting here is the crossover between engineering and the geotechnical status of the soil and its influence and impact on any outcomes achieved. This, again, is a good example of where thorough research beforehand can start to bear fruit during the construction process.
The contractor will not ordinarily remove subsoils from site, but will carry out a balanced cut and fill whereby soils are moved around to create plateaus on which to build, and the design solution is normally calculated with this principle in mind. Most contractors nowadays, if not all, will have specialist laser equipment attached to machinery to carry out this work in a fairly exacting manner.
If the weather is in your favour, you will be surprised at how quickly this process takes place and you will start to see the initial site footprint taking shape. Ongoing climatic conditions can affect this outcome significantly, so timing of this element should be considered carefully as part of the overall project development and programme.
In previous years, I have started jobs in the autumn or spring months and endured very dry prevailing conditions and, conversely, started works in the so called dry summer months and endured very wet summer holiday periods. You never quite know what the prevailing weather conditions will be.
I think there is a useful process described below, that I have used on several occasions now to expand the traditional construction calendar and give a lot more certainty with regards to ground conditions on which to build, and I personally think it has real merit in sports construction.
To be honest, I am surprised that this technique is not used more often in the sports construction industry, particularly for synthetics and artificials where exacting finished tolerances are required. In my opinion, what it does is minimise risk on site, by solidifying the formation on which the pitch is built. There are some pre-requisites for successful stabilisation and it is not unusual for a series of chemical tests to be required to understand, in detail, the chemical status of the soil and how this would react to any lime or cement integration.
In practical terms, I believe it extends the traditional construction season and gives contractors more certainty that the formation base will not move over extended periods of time which, in turn, allows them to extend and increase warranties for the sub-base construction works. There are one or two contractors within the industry who have latched on to this, and possess their own equipment, or specialist sub-contractors who can carry the work out.
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 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.
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 on surface gradients to remove water from the surface to the surrounding drainage system. Specialist advice, in this instance, would be necessary, and this design solution would also impact on post installation maintenance techniques required to render the surface suitable for use on an ongoing basis.
I have seen a design solution recently that has a recycling emphasis that relies on water movement into a specially developed kerb/drain. Watch this space, it may have legs!
Kerbs and Gripper Rails
Typically, precast concrete kerb edges are used to retain the construction footprint when suitably haunched in a concrete bed to hold the kerb in place. I presume, due to market pressures and historical design solutions, so called pin kerbs or 50mm kerbs are commonly found and used as part of the build process.
In some instances, these kerbs are substantial enough to support the construction layers but, in other instances, in my opinion, they are not. I have used more substantial kerbs measuring 150mm x 125mm, not only to hold the construction profile in place properly, but also to frame the construction footprint, which I think gives a more substantial long term aesthetic outcome. This is a fairly minor detail in the overall scheme of things, but I feel it supports a gripper rail well and presents a more sustainable solution.
Due to cost pressures in the industry and the competitive nature of the sports construction industry, these kerbs might well not be offered as part of a design solution. The use of such kerbs, and others like it, will depend on the job in hand, but I would advise that you consider these in the correct context as they can work really well.
Another contentious issue is the use of gripper rails for sand-dressed or non-dressed carpet systems, particularly in the hockey and multi-use schools market. I recognise that, for long pile 3G installations, gripper rails are not used, but the points relating to kerbs above still apply to 3G scenarios. Historically, only one or two companies would advise the use of gripper rails and, to this day, not all synthetic grass companies require the use of grippers as part of the warranty offer they make.
It is also true to say that, over the years, I have installed pitches with and without gripper rails, as the cost of installation is a consideration within the overall budget figures. These days, if building from new or refurbishing, I tend to come down on the side of using gripper rails for synthetic grass systems, particularly where cross-play pitches are marked on the facility.
Synthetic grass does not move like it used to in the past, however the additional detail of a gripper can enhance the scheme overall and ensure the carpet stays in place and minimises any distortion to inlaid lines etc. It is an item to consider carefully, especially the cost, but I believe a sustainable design solution should include grippers in certain situations; however, I am mindful that the industry at large takes a variable view of this.
Historically and traditionally, we have used suitably graded aggregate to act as an intermediate layer between the drainage layers and surfacing layer of synthetic pitch and artificial constructions. This is based on tried and tested methods of road construction and involves the selection and use of virgin or recycled materials delivered to site and appropriately consolidated and graded to support the layers that sit above it.
As the price of materials has fluctuated, more and more recycled materials have become cleaner and more readily available, and are being used nowadays, but there is still a significant transportation issue in delivering these materials to site. Further developments within the industry, in recent times, have focused on these intermediate layers and alternative materials that can be used instead of traditional stone materials.
The objective of many of these layers is to enhance the performance of the sports surfacing system, whilst reducing the need for this intermediate stone, either in terms of overall volumes or depth of this material. In very recent times, I have seen systems that advocate not requiring an intermediate layer at all, and can be installed straight over stabilised sub-soils, discounting the need for an intermediate stone section.
The success or otherwise of these systems remains to be proven, but the environmental argument and costs associated with the production, distribution and use of significant volumes of stone is one that the industry is constantly looking at.
It goes without saying that the selection and grade of stone must be suitable to maximise both surface stability and overall robustness, whilst optimising drainage potential through the profile to the drainage system underneath.
At this moment in time, my bias is to stick with what we know works, and it is certainly the case that appropriately graded stone can be purchased via the recycled route nowadays, without any significant contamination issues, and subsequently used to build sustainably facilities.
It is worth pointing out that, geographically speaking, some recycled materials are easier to get hold of than others, so a careful geographical analysis and consideration should be undertaken. The use of soil stabilisation also offers increased potential for reconsidering traditional construction depths and perhaps optimising the depth of material that is installed as a stone intermediate layer, whilst providing a traditional design solution of sorts.
It is one area of the market that is evolving at the moment and, eventually, alternative intermediate layers will probably be selected and offered as sustainable alternatives. I would always recommend that you view each layer in terms of its compatibility with the other layers of the profile, so as to create an integral system where each layer performs on its own and in conjunction with others to optimise system performance.
There has been much debate over the years about the relative merits of so called engineered layers, which typically consist of one or two layer tarmacadam installations. Historically, the alternative is a dynamic material laid and consolidated in a semi-loose state, but I have also seen attempts to bind this material with various spray coats that partially solidify. To this day, I am not sure there is relative agreement across the industry as to the best method to adopt, because some organisations are seeking to optimise cost efficiency using dynamic layers, whilst the more traditional engineering approach can offer less variation in terms of surface movement over time.
Recent industry research has suggested that dynamic systems can be, and are still, a problem over time, particularly when it comes to maintaining surface gradients as installed from the onset. However, I think due to cost pressures and wider industry analysis, the dynamic market is constantly being evaluated.
There are those who offer alternative intermediate layers as described above, and there are those who also offer a dynamic system, particularly in the 3G arena where longer grasses are used.
My personal approach, at the moment, is to stick with engineered layers as I am confident that I can offer my clients a warranty, via the contractor, and a sustainable design solution over time. The chances of an engineered system moving over time, if installed correctly, are minimal and, therefore, the refurbishment of these areas is far easier when you have to strip the shockpad and/or synthetic grass layer after ten or fifteen years or so.
At the moment, when trying to manage risk I would advise that serious consideration of an engineered system be made. If one considers all the pros and cons, I feel that this solution offers the most appropriate sustainable way forward, for the moment. There is a chance that this market will move and change over time, but sometimes better the devil you know!
The next article (No. 4 of 6) is part 2 of construction and considers surfacing materials and associated facilities to enhance an overall sports scheme.
For a more detailed overview of David's career to date, please see issue 59 of Pitchcare magazine.
Footnote - we advise you to look into the proposed CDM 15 changes as this may impact on any projects that you are planning in the future.