Supply Chain - Site Preparation - Ground & Slope Stabilisation
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Case Studies
Summary chart from the reservoir drawdown hydraulic analysis - Courtesy of MMB[/caption]
Pipes and thrust restraint
Hoops stresses within the DN700 mild steel siphon pipes were checked and a DWI Regulation 31 coating specified. The stainless steel pipe supports were analysed for bending and shear and concrete thrust blocks were designed for the system test pressures. The pipe wall thickness was reduced from 10mm to 8mm as an efficiency. The pipework arrangement drawing was uploaded into the federated model in BIM360 to check for clashes and when installed on site was found to fit perfectly.
Ground investigation and geotechnical analysis
Following the original construction of West Hallington Reservoir (between 1884 and 1890) the Resident Engineer Charles Henzell wrote a report to the Institution of Civil Engineers describing the works and detailing a number of challenges which included:
(left) Cable percussive borehole rig on the crest of the embankment and (right) scaffold decked area required for rotary bore works on the slope - Courtesy of MMB[/caption]
A desk study and intrusive in situ ground investigation (cable percussive and rotary bored boreholes, standard penetration tests (SPTs), piezo installation, lab testing) were carried out to classify and determine the thickness of the superficial and solid geology along with groundwater levels, identify any contaminated or aggressive ground and determine the engineering properties of the native soils. Slope stability and ground bearing pressure checks were carried out to ensure large plant would not compromise the stability of the embankments.
The clay core and embankment material of the Pennine-type dam was found to be very silty and exhibit poor engineering qualities and so was disposed of, and a source of good quality puddle clay and Series 600-2C material was found locally at a brick factory in Birtley, Gateshead. This was installed in compliance with the test and inspection plan (T&IP) to a good standard.
Thankfully the three springs in the west embankment mentioned in the Henzell report were not encountered.
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Extreme weather conditions during the construction stages: (left) winter 2022 and (right) flooding from Storm Babet (October 2023) - Courtesy of MMB[/caption]
Temporary works
A borehole at the toe of the embankment on the dry side indicated the presence of water-bearing sands and gravels and so interlocking sheet piles were specified that punched through the sands and gravels into the underlying clays and so cut off groundwater flows into the excavation.
A Portadam supplied and installed by OnSite Central was employed to provide a dry area to construct the intake bay. The Portadam was erected in the wet by operatives wearing buoyancy suits, and then pumped out and an excellent seal was achieved. The Portadam also allowed plant access around the pipework whilst the stone pitching was reinstated.
Flood risk assessment
A flood risk assessment (FRA) was carried out to support the planning permission and, as due diligence, to ensure MMB would not flood a property by accident. The assessment looked at the extent of flooding during operation of the siphon along with fluvial, surface water, groundwater and finally reservoir flood risk due to an uncontrolled release (which of course the siphon is designed to help prevent). The FRA concluded that:
A screen dump from the SYNCHRO 4D simulation - Courtesy of MMB[/caption]
The simulation highlighted a number of pinch points when materials must be delivered or when water levels must be lowered or raised and helped drive efficiencies. SYNCHRO 4D was also useful for explaining the construction process to the client, to site operatives and to visitors.
Solar power
The mechanical equipment associated with the siphon is housed within a high security kiosk. Due to the remote site location, the cost of a permanent power supply for the kiosk building services was prohibitive. A solar powered solution has been installed utilising roof mounted PV cells, DC/AC converter and battery storage.
This sustainable solution provides sufficient energy for lighting and heating within the kiosk with negligible running costs.
Solar panel display - Courtesy of MMB[/caption]
Siphon intake bay constructed within a Portadam barrier supplied and installed by OnSite Central - Courtesy of MMB[/caption]
Stone pitching
Reinstatement of the original stone pitching (to resist wave action) took seven weeks longer than planned; mainly down to availability of stone masons. Productivity increased as the works progressed and the three man gang went from replacing 3 to 5m2 per day. The stone pitching was reinstated to a good standard, but it was felt that the detailing around one of the pipe supports would be built differently next time.
The 2,500m perimeter of the reservoir had been faced with stone pitching down to original ground level; installation of which in the 1880s without an excavator and selector grab must have been a job and a half!
Ecology
Prior to the construction works, NWG employed OS Ecology to undertake trapping for white clawed crayfish, a native species that is in decline due to the introduced North American signal crayfish. No white clawed crayfish were found within West Hallington Reservoir but three were trapped within the adjacent East Hallington Reservoir. A pied wagtail nest was spotted by eagle-eyed site workers next to the engine of an excavator. The excavator was withdrawn from service until the eggs had hatched and the fledglings left the nest.
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Reinstatement of the stone pitching - Courtesy of MMB[/caption]
DN700 siphon pipework during installation - Courtesy of MMB[/caption]
View of tank with backfill almost completed. Pedestrian crane on the left - Courtesy of ESD JV[/caption]
(top left) Groundworks nearing completion, (bottom left) tank construction underway, and (right) view of tank interior - Courtesy of ESD JV[/caption]
The almost completed structure before backfill operations - Courtesy of ESD JV[/caption]
Mini CS & Company News
- Frydd Wood, Elan Valley Aqueduct
Frydd Wood, Elan Valley AqueductThe BNM Alliance (a joint venture between Barhale and North Midland Construction) awarded Aarsleff the contract to design, supply and install a soil nailed slope to allow, with associated works, the launching of a tunnel boring machine for the Severn Trent Elan Valley Aqueduct Rehabilitation project in Wales. The original aqueduct was built between 1893 and 1904. Fast forward over 100 years and the aqueduct still provides drinking water from the Elan Valley to Severn Trent’s customers in Birmingham. The aqueduct transports high quality water entirely by gravity. Having supplied water for over 100 years, the aqueduct is beginning to tire in three key locations and offline replacements are necessary. 8 (No.) test nails were installed on the 17th July. These were tested to destruction to confirm the design prior to production works starting. With its Klemm 806 rig, Aarsleff will then installed 670 (No.) 100mm diameter fully galvanised R32 -280 self-drilling soil nails of 6-16m in length. [caption id="" align="alignnone" width="1200"]
Courtesy of Aarsleff Ground Engineering Ltd[/caption]
In addition to the soil nails, approximately 670 (No.) head plate and nut were supplied and fixed. The soil nailed wall has an estimated 1250m2 area to which Aarsleff has supplied and fixed a steel mesh and geotextile facing. Aarsleff were on site from 24th July with completion 15th September 2017. Aarsleff showcased its previous work in soil nail projects of similar size to the main contractor before winning the contract to demonstrate its technical capabilities and expertise whilst simultaneously, assuring the client of cost, time and quality. Aarsleff has sought value engineering primarily through optimisation of the soil nail layout and facing mesh.
The self-drilling soil nails were procured from Barnsley-based Minova and possess a hollow bar, which is used for drilling ether with water flush, air flush or cement grout flush and are extended by using couplers. The standard couplers have a design which ensures end-to-end bearing of the hollow bars to transfer percussive energy from the top hammer to the drill bit whilst a sealing ring in the centre stop, minimised flush spillage during drilling. The soil nails also possesses a sacrificial drill bit.
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Courtesy of Aarsleff Ground Engineering Ltd[/caption]
The rig we employed is designed for the use of powerful double head drilling units, hydrualic drifters or rotary heads. It can also be equipped with drill masts either with feed gear or cylinder feed systems, with the option of fitting lattice mast extensions, the modularity of which, allows the adaption of a variety of options, which makes the drill rig ideal for anchoring, micro piling, jet grouting and even geothermal drilling. After completion, two percent of all working nails will be ‘Acceptance’ tested to validate the design and provide confidence as to the quality of the installation.
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Courtesy of Aarsleff Ground Engineering Ltd[/caption]
Scope of works
- 670 (No.) 100mm soil nails
- 670 (No.) head plate and nut
- 1250m2 steel mesh and geotextile facing
For more information: Aarsleff Ground Engineering: +44 (0)1636 611140 | www.aarsleff.co.uk
