Winter Sports - Rugby Union
Primary drainage trenching
Installation of the 3-layer liner system
Rugby post sockets are installed
through Kempton gravel beds. Prior to tender, there had been extensive investigation and trial bore holing of the site, and all four of the companies tendering had access to this. An impermeable membrane was the main feature of the design and a firm of engineers, Elliot Wood, were engaged to design a drainage infrastructure to ensure that water was directed to soakaways. These basic plans went out to tender in February 2013 to four contractors, and it was MJ Abbott that won the contract. Immediately MJA took over the site last May, the entire area was sprayed to kill all of the existing sward, and the clubhouse and driving range of the golf club was demolished. Even on a murky January day, it was remarkable to see what has been achieved already. Reports on existing ground conditions
were pretty extensive, Nathan tells me. There were actually about fifty trial boreholes drilled all told, and he says that they were particularly anxious to highlight the areas of cut, such as an irrigation lake for the main complex plateau and the pitches there. This was going to be 2.5 to 3 metres into the sub- structure. The existing boreholes would only give a certain amount of
Completion of drainage following lining of the main pitch area
information, so further test holes were dug to get detailed samples across wider areas of the site, specifically in cut areas. This vital, what you might call, ‘filling in the blanks’ period lasted a couple of weeks right at the beginning of the project.
A huge stumbling block was a 2-metre deep water hazard cum irrigation lake on the existing golf course, right where the three main pitches were to be sited. Draining this was a day-one priority, followed by removal of its liner and a vast amount of slurry. The securest of filling operations had to be undertaken ahead of any pitchwork. A bit of sinkage or settlement, albeit unwelcome, can be remedied beneath a natural turf pitch, but under a 3G artificial it is unthinkable and potentially calamitous. It was essential, therefore, that this area was compacted effectively. Geo engineers had to select the right kind of material to achieve this. The crucial standard here is CBR (California Bearing Ratio) used for sub- surfaces in all manner of construction work. A level of 10 was required for the 3G pitch, the same level as for road construction. Where there were any soft spots, additional dry compacted material was
installed. General compaction was achieved bit by bit in six inch layers using a Padfoot Roller attached to the back of a bulldozer, making at least ten passes. When this had been done, as an extra precaution high-energy impact compaction was designed into this part of the project and achieved with a 14- tonne downward force by means of a special 3-sided heavy roller used by sub- contractor Land Pac. “We’d compacted the hell out of the
lake area, but this piece of kit took the level down another six inches,” said Nathan.
This pounding machine can work down to four metres, and an on-board computer screen, with the 3G pitch area marked, showed the operator the CBR at any given point. Anywhere at all where there was the slightest weak point showed and extra pressure was applied. This vital exercise apparently lasted about a week in all, and will be worth every minute. There are all sorts of aquifers beneath the site, being barely a mile from the Thames. From the outset, what was not wanted were the very porous pitches, with rootzones and gravel rafts being leached out. The idea of lining the entire main plateau to prevent outflow,
FEBRUARY/MARCH 2014 PC 33
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