London: Royal Arsenal Woolwich – FAS

DAWSON-WAM was contracted by Berkeley Homes to design & construct the fluvial flood defence measures & load bearing piles for Phase Nine of their Royal Arsenal Riverside development in Woolwich.

The project is located on the site of the former Woolwich Coal Fired Power Station built in 1893 & adjacent to the River Thames. The power station has long since been demolished, however the old structure had left many challenges behind for the new development of the site.

Whilst the power station had been demolished in the 1970’s, the foundations and structure below the current car park level had not been removed at the time of demolition. Little or no information existed on the true nature or position of obstructions. It was considered impracticable to remove all of these for piling works so a strategy was developed to attempt to core out these obstructions to a depth of up to 11mtrs into the underlying natural ground and then backfill with granular material prior to piling proper being carried out unhindered.

This project involved the use of numerous different piling techniques (sheet pile, secant & CFA) all which were provided by in-house resources. Final detailed design of sheet piled flood wall & foundation piles by DAWSON-WAM.

DAWSON-WAM constructed a piled earth embankment (160m) adjacent to the River Thames to protect the site from potential fluvial flooding until the sheet piled flood wall was installed.

The obstructions required removal to allow for the installation of the sheet piled wall which provided a flood defence from the adjacent River Thames. Following on from this, the bearing piles which would support the four 18 storey residential blocks involved in the development would be installed. To remove these obstructions DAWSON-WAM employed three rotary type Delmag Rigs.

To remove the obstructions around the 390lm perimeter length, a specialist 880mm diameter overlapping guide was designed and fabricated in house. This allowed a secant pattern to be followed by the drilling rigs ensuring no obstructions were missed. When each of the 590 holes were drilled out, they were then backfilled with imported material to ensure the sheet piles could be installed unobstructed. 1070t/5890m2 of AZ40 interlocking steel sheet piles with waterproofing clutch sealant installed in-river by DW’s ABI Mobilram TM 18/22B leader system on a Sennebogen SR35TB chassis.

Reinforced Concrete insitu capping beam constructed, 800x1000mm deep, for total length of sheet piled wall. DAWSON-WAM installed 600lm of back drainage along the length of the new flood wall and connected to the existing storm outfall pipe.

In addition to the difficulties posed by the presence of the obstructions further restrictions were applied from the Crossrail Tunnel which passed beneath the site. Restrictions were placed on the vibration, movement and depth allowable within the site in these areas. This meant it was key to control the removal of the obstructions and later the installation of sheet piles and vibro-concrete columns.

Flood defence measures carried out under Flood Risk Activity Permit (FRAP) approved by Environmental Agency. Boat with sorbent boom/float barrier, provided & monitored by DAWSON-WAM, ensured any potential spillages into the river were contained & allowed for its recovery, removal, or dispersal.

Once the flood wall was completed works commenced on the load bearing pile positions. These were drilled out again using the Delmag rigs at 1000mm diameter. In all 205 load bearing piles were installed consisting of depths ranging up to 32mtrs. All pile cages were de-bonded and then plunged to the required depth using an additional leader service rig. This reduced the amount of concrete breakdown, reducing the risks of hand arm vibration for operatives as the top section of the piles breaks easier from the lack of bonded reinforcement.

Following completion of the residential blocks, DAWSON-WAM constructed 180m of brick clad RC property boundary walls, 120m of wooden fences and completed garden reinstatement to each property. 3,400m2 of hard and soft landscaping constructed from HRA pathways, Granite effect paving slabs/brick and semi-mature trees with cast iron tree pit grilles.

The site was located in the heart of Woolwich, occupying prime location along the River Thames. During construction of boundary walls and pathways, we were working within the public open space and ensured that suitable traffic/pedestrian management was in place including advanced warning signage. Significant numbers of pedestrians were in close vicinity to our working area due to the locality of the National Rail & DLR stations only 300m away. Secure timber hoarding was erected around the working area whilst ensuring adequate space was left on the footpaths for pedestrians/cyclists. Project notice board erected on hoarding to inform public of proposed development with details of DAWSON-WAM contacts in case of emergency.

Flood wall sheet piles and load bearing piles were installed successfully and within tolerance following the removal of obstructions and the client, Berkeley Homes East Thames, have been very satisfied with all works completed on this scheme within time and budget.