Knowsley Rail Loading Transfer Station (SITA) - Secant Pile Wall & Passive Ties

Byland Engineering Limited were appointed by Van Elle Limited to undertake the design of a restricted access secant pile wall and passive ties at Knowsley Rail Transfer Loading Station, Merseyside.

The proposed facility will enable up to 500,000 tonnes of residual waste to be transferred to a ‘waste to energy’ facility in Teesside by rail each year, as well as creating 25 permanent jobs.  The facility is contained within an existing building to prevent dust and odours from escaping. 

The ground conditions comprised of:- 

• 0.0m to 1.2m   – Made Ground (concrete slab over hardcore)
• 1.2m to 5.0m   - Cohesive Glacial Till
• 5.0m to 7.0m   – Weathered Sherwood Sandstone
• 7.0m to depth  - Sherwood Sandstone
• Ground water level was ~2.9m depth.

The detailed design was undertaken in accordance with Eurocode Standards and complimentary British Standards BS 8002 (2015), BS 8004 (2015) and BS 8081 (2015).

The existing portal frame warehouse was converted to its new use by the construction of 2 No 35m x 18m x 7.5m deep waste transfer pits formed using embedded secant pile walls and the installation of an overhead crane and associated compaction equipment.

Each pit contains 110 No female piles, 106 No male piles and 32 No passive ties. All male and female secant piles were formed using rotary piling techniques using 816mm Φ temporary steel casing down to seal into sandstone bedrock at 4.5 to 5.0m depth, and 750mm Φ open hole thereafter. The passive ties comprised Dywidag Systems International (DSI) T76 self-drilled hollow bars installed using grout flush.

The male and female piles were installed through a scalloped reinforced concrete guide wall from existing slab level to control the positional and verticality tolerances at the design 1050mm centres to ensure suitable interlock (bite). The passive ties were installed through every 3^rd female pile, i.e. at maximum 3m centres, at 40 degrees below the horizontal founding in the Sherwood Sandstone. The passive ties were terminated in the capping beam with a 260 x 260 x 50mm steel head plate, secured above and below with full strength nuts, to provide the necessary restraint to the secant pile wall. No stressing operations or ongoing maintenance is required with this type of permanent wall restraint.

Following excavation to dredge level, Van Elle returned to site to install 50 No. 790kN SWL Antiflotation Tension Micropiles (ATM’s) in each pit to restrain long term hydrostatic uplift forces, which was temporarily lowered by the Main Contractor’s dewatering scheme. The ATM’s comprised of 7m deep x 150mm Φ open hole rock sockets into sandstone bedrock, each containing a full depth 40mm Φ Dywidag pre-stressing bar and formed using a 1:1 sand cement grout.  This was connected to the 500mm thick r/c slab with a 220 x 220 x 50mm thick steel head plate secured with full strength nuts above and below.

The works were successfully completed on time and on budget during late 2015.