Oaklands, Redevelopment Above HS2 Service Tunnel
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- Site Investigation
- Interpretative Reporting
- Remedial Method Statements
- Foundation Feasibility Scheme and Design
- PLAXIS 3D Finite Element Analysis
- HS2 Infrastructure Impact Assessments
- Network Rail Infrastructure Impact Assessments
- Temporary Works Design
CGL were commissioned to provide a detailed ground investigation and interpretative report for the proposed development at Oaklands House in West London. The £175 million mixed-use development will provide over 600 new homes, across three multi-storey blocks, varying in height up to 26 storeys.
The site is directly over the proposed alignment of a 6m diameter HS2 service tunnel some 17m below ground level, with a 6m vertical exclusion zone above. The tunnel alignment also has a considerable horizontal deviation zone, which extends some 30m across the site to the northern boundary. The heavily loaded cores associated with each development, which would normally require very long deep piles, were positioned within the deviation zone and above the proposed alignment of the tunnel. This posed a considerable risk to the development both from the perspective of potential impact on the tunnel from the proposed structures and loading, or impact from the tunnel construction and associated ground movements on the completed development, whichever comes first.
Due to these constraints a robust and somewhat non-conventional foundation solution was required to enable the construction of the development blocks so that stress increase and deformation along the tunnel alignment was within acceptable limits as agreed during meetings with HS2.
CGL developed a foundation system concept for the heavily loaded cores of each block above the deviation zone forming raft-enhanced pile groups. In this particular system, piles are utilised as settlement reducers and designed to be loaded close to their ultimate capacity, with the remaining load taken up as bearing on the underside of the raft/pile cap at formation.
3D finite element analysis
CGL carried out a preliminary feasibility analysis and assessment of the foundation system suitability using conventional analytical methods and detailed PLAXIS 3D finite element analysis, whereby the key construction stages, structural loading and tunnel construction were modelled within a single integrated sequential analysis model for each development block. The results showed stress increase and deformation at the critical tunnel alignments were found to be within acceptable limits as agreed with HS2, as were the settlements and differential movements of the proposed development blocks under combined structural loading and tunnel construction below.
As Network Rail (NR) infrastructure is located along the western boundary of the site adjacent to a basement wall. A 3D numerical analysis impact assessment was used to assess ground movements due to the construction of the adjacent Block A, with particular focus on pile installation effect, basement excavation and structural loading during construction and over the long term.
To manage risk during the works CGL provided details and recommendations for a monitoring strategy. The information obtained from this can be used to validate the analysis models and inform future works, particularly where HS2 and NR infrastructure constraints are present.
The development is currently under construction by Galliford Try with piling and basement excavations completed for each block. CGL has had an ongoing role during this work as temporary works designers and foundation design checkers. The HS2 service tunnel is now expected to pass below the site post completion of the majority of the superstructure blocks during 2020.