CASE STUDY: RESTRICTED ACCESS PILING, STOKE BARDOLPH WWTW, NOTTINGHAM.
Project: The piling to form the foundations for a new ASP (Activated Sludge Plant) including 8 lane aeration and 8No. FST (Final Settlement Tanks).
Location: Stoke Bardolph, Nottingham
Client: Severn Trent Water.
Principal Contractor: NMC Nomenca.
Project Overview
Having successfully completed the Waste Water Treatment projects at both Minworth and Stoke Lyme Valley for North Midland Construction, Van Elle were approached to discuss possible design and installation options for the piling at Stoke Bardolph, a huge plant on the east side of Nottingham.
Although a large open site with the works spread over a considerable area, loose granular material had been discovered down to about 4m and the requirement for such a thick and extensive piling platform made it more cost effective to use some of our lighter fleet to carry out the work. This also enabled the Final Settlement Tanks to be excavated prior to commencement of the piling as the rigs could drill safely whilst sat on the incline.
The project consisted of piles for a new 8 lane aeration, Activated Sludge Plant (ASP) and 8No. Final Settlement Tanks (FST).
We opted for two different rigs which were ideally suited to the task at hand; the Soiltec S45 and the Hϋtte HBR205. Both have proven themselves on similar projects in the past and are reliable with relatively low bearing pressures yet are extremely powerful and versatile, able to safely work from a sloped location which wouldn’t have been possible for a larger rig.
Once the site had been de-watered, we first carried out 12No. tension pull-out tests on working piles to check both the rock level and that our design performed to the project specification. The results were excellent and proved that our proposed solution was ideal for the project requirements.
We were then able to begin the piling for the Activated Sludge Plant.
This is a series of huge tanks which were piled on a grid pattern as can be seen in the diagram on page 1.
In the photo to the left, the base has been cast and the bars to reinforce the walls can be seen protruding up to 15m into the air.
In these pictures only the first lane has been built up above ground level and the steel is being laid ready for the slab to be cast for the other 7 aeration lanes.
The design for the anchor piles at Stoke Bardolph required consideration of the ground conditions, levels, anticipated pile performance and global stability.
Once the ground conditions had been assessed from the site investigation data provided, performance of ground anchors and tension piles in similar ground conditions was checked against case studies and published guidance. It was felt that the ground conditions represented competent material and that a good working pile/rock bond was likely. Confirmation of the likely performance would also be available from the numerous preliminary tension tests that were proposed.
Advice was sought from Jim Martin of independent geotechnical Engineers, Byland Engineering who has many years experience of design and overseeing installation of such anchor systems. Concerns were raised regarding the water levels and potential for smearing along the bore which led to instruction on the best way of installing to minimise any risk.
As the piles were to act in pure tension, the structural tendon was provided by employing a central 40mm Dywidag pre-stressing steel threadbar which could be anchored into the slab by utilising a plate detail threaded onto the main bar. Once again, Byland Engineering provided some advice on how this may be achieved whilst providing a stronger connection and benefiting the slab design.
The reinforcement was checked for corrosion and a 120 year design life criteria was achieved.
Due to the variation in piling platform levels on the FST bases, the design had to provide clear and precise detail of founding datum level for the toes with corresponding length of pile so that any last minute change in platform levels could be easily accommodated during installation and also meant that costs were minimised with no unnecessary length of pile.
Finally, checks were performed for global stability ensuring that a cone pull check for each of the proposed piles was satisfactory.
In total 1056No. 350mm diameter piles were installed with a single bar and a rocket socket of 3.6m.
The value of the project was roughly £650k which was split equally between the ASP and FSTs.
The FSTs, shown in the images to the right were piled in a concentric circle and the smaller rig, capable of working on a sloped platform, enabled the excavations to be carried out prior to the piling. This worked very well for the programme and for the sequence of works allowing the groundworks to be undertaken whilst we were piling the ASP.
