CCTV showed lateral drains were undulating significantly
heave and damage was significant. Being unable to smooth and re-level the surface prevented the scarring from being satisfactorily repaired until ground conditions became softer the following autumn. Some sections of pipe were installed by thrusting, but this was an expensive process (four times the cost of moling), and could only be used on small sections of pipe in order to keep within budget.
Drainage
The condition of the original lateral drains installed during the early 1990s was investigated by CCTV. They were found to undulate significantly, a result of progressive settling of the material used to reclaim the area. However, the gravel backfill in the drains was found to be clean. During excavation of the lateral drain backfill, the original slit drains were found to be heavily capped, but the underlying gravel backfill was also clean. The only method available to minimise
trench excavation and subsequent generation of spoil, while effectively connecting the surface to the clean
lateral and slit drain backfill, was by injected gravel banding. By creating a narrow slot in the soil with an expander leg and immediately filling it with permeable material, it was judged that the existing drainage scheme could therefore be utilised.
Although not universally used as a secondary drainage option in New Zealand, gravel banding has been used successfully in the UK for many years. Careful research was required to select the most appropriate backfilling material in order to maximise the life span of the injected bands on this site. In total, 55km of narrow gravel bands were installed.
In some areas of the park, lateral drains were not installed during the original 1990s upgrade in order to avoid having to trench through the existing cricket pitches. Wider and deeper gravel bands were therefore installed using conventional slit trenching equipment to drain areas that were not drained during the 1990s. This worked by creating a gravel-to-gravel connection with the backfill over the existing lateral drains.
The cost of spoil disposal was factored into the budget.
Worker safety
The level of soil contamination at Victoria Park presented a low-risk to workers. Nevertheless, overalls, dust masks, overshoes and gloves were required to be worn. A detailed site visit and operations log was also required to be kept and be available on-demand to visiting inspectors. Finally, wash down areas and wheel wash bays were established in designated areas. No soiled machinery was permitted off-site and worker protective clothing was disposed of after each working day. Overall, the restrictions on Victoria
Park were complex and far-reaching. The techniques used were somewhat untested in New Zealand and, therefore, increased the risk that the investment of capital would not result in a sportsfield of similar quality to a conventional renewal. But, what drainage solutions are available for situations where there is no outlet or, at best, significant restrictions
An AFT 45 on a compact tractor has two applications
1 Fitted with its chain and boom for wider and deeper trenches 2 Fitted with its slitting wheel for narrow slit drainage
AFT Trenchers Tel: 01787 311811 email:
info@trenchers.co.uk www.trenchers.co.uk
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