A group of 14 keen CivSoc members were fortunate enough to be blessed with sunshine as they went on a site visit, organised by Halcrow, to the Woolwich Arsenal DLR Station on Wednesday 21st Nov, despite it having been raining continuously for the previous two days.
Woolwich Arsenal station is part of the latest DLR extension connecting from the London City Airport branch just across the Thames, which was previously designed by Halcrow too.
When the station opens in 2009, it will not only provide an alternative means of crossing the river, it will also make an inter-change with the existing Network Rail station at Woolwich Arsenal contributing to Mayor Ken Livingstone?s ?railway network master-plan?.
Halcrow has been employed as the consultant on this ?design and build? project, working for the contractor Morgan Est (previously AMEC) on a Public-Private-Partnership with DLR.
Although this extension only has one stop, it has been a challenging project in its own right as a pair of twin tunnels had to be bored through chalk underneath River Thames, plus most parts of the rest of the line were built with two underground through urban area, vibration effects due to boring on the nearby and adjacent structures had to be closely monitored.
A guided tour was given by the Halcrow civil engineer, Paul O?Regan, which the I.C. undergraduates ? including a number of freshers ? all gratefully benefited from. The excavation work carried out can be summarised in 5 key points:
A Continuous Flight Auger was used for the pile walls in shallow excavations, whereby the auger (a screw-like device) drills through the soil and as it is retrieved, a hollow pipe within it pumps concrete to fill the hole, thus retaining the sides. Before the concrete sets, the steel reinforcement cage is pushed gradually into the concrete, ensuring the steel cage doesn?t buckle. However, the current technology only allows it to go accurately for 20 metres.
2. Diaphragm Wall
As a result of the limitation on the CFA, Diaphragm-walls were used for deeper excavations.
The filling method for the walls is slightly different to CFA. The trench is first filled with viscous bentonite slurry to temporarily retain the soil. Then the rebar cage is placed within the panel and, finally, concrete is poured, displacing the bentonite, to form the panel wall.
Once the pile walls were installed, excavation of the soil bounded by the walls can proceed. Bottom-up construction is when the base slab is constructed first before the roof is installed. This is a type of ?cut & cover? construction, and its opened-top allows materials to be lifted in and out easily by cranes. As the soil gets excavated, there would need to be temporary horizontal props, which will be removed once the permanent ones are in place, to support the retaining walls on both sides.
This is the exact opposite to the ?bottom-up? construction. In fact, it involves installing the roof first to act as a prop to the retaining wall while excavation beneath is yet to be completed. This is commonly adopted if the road surface needs to be restored as soon as possible to minimise the disruption caused at the ground level. On this project, the client had particularly opted for this method on building the station to reduce the spending on temporary props. Note that there needs to be gaps between the roof slabs in order for materials to be lifted in and out.
This was the main feature of the ?cut & cover? structure which Paul highlighted.A sump is the lowest elevation point for which water can be collected and pumped away. Its primary function is to collect all water that enters the station and open-cut structures and to pump this water into an existing drainage system.
CivSoc would also like to thank Thomas Varghese in Halcrow Group for acting as the co-ordinator for the visit.
Message from Industrial Liaison Officer, Agnes Law
Apart from the site visit on 21st Nov, Halcrow had also co-organised with CivSoc on a ?Graduates Sharing Session? for the 3rd and 4th years on 14th Nov at their Hammersmith office. During the event we had the valuable chance to meet senior members of staff in Halcrow, a senior HR officer and chat with fellow graduate engineers throughout the afternoon. The group leaders in last year?s constructionarium also gave a presentation to Halcrow, which is one of the sponsor companies. CivSoc would like to express gratitude to Sam El-Jouzi and Thomas Varghese in making the event successful.