Tollgate Gardens, Maida Vale, London

Mark Creighton

Mark Creighton


Tel: 020 3096 7567 Email Mark


Keepmoat (on behalf of Affinity Sutton)


  • 3D Finite Element Modelling
  • Ground and Sewer Movement Analysis
  • Infrastructure Asset Protection

Innovative use of density compensation to control sewer confinement

The project at Tollgate Gardens is a £50m rebuild of a former 1960’s estate to deliver 148 new and refurbished homes, half of which will be affordable.

The new development will include five new multi-storey blocks. At 9 storeys, the tallest block involved the excavation of a new lower  ground floor level that straddles the North Western Storm Relief Extension Sewer (NWSRES).

The NWSRES is a 2.2m diameter concrete lined sewer belonging to Thames Water (TW). It alleviates the flooding risk of significant parts of north London and it surcharged during floods in 2002. TW set protection criteria for the sewer that required ground movements to be calculated to demonstrate that sewer curvature and joint opening limits were not exceeded in response to changes of loading through the build cycle.

The sewer was located some 3m below the proposed Structural Slab Level of the proposed new lower ground floor. During construction, excavation for the lower ground floor would reduce the ratio of soil weight to potential hydrostatic pressure within the sewer (Confinement Pressure Ratio or CPR) to significantly lower than the required value. Furthermore, in the long term TW would not allow building weight to be taken into account in the calculation of CPR.

It was therefore not possible to construct the basement as originally designed. Without significant geotechnical input, the basement would have been removed, relocated or reduced in depth. None of these options were acceptable to the client.

Analysis-based solution

CGL used PLAXIS 3D FE analytical software and continuum analysis to develop an excavation sequence suitable for TW limits. In order to achieve the required CPR a super dense concrete was used with a unit weight of 38kN/m3 containing ‘Magnadense’, a natural magnetite aggregate.  Iesis Special Structures, the structural engineer, incorporated CGL’s analysis to determine a feasible construction sequence with a slab thickness necessary to minimise excavation depths whilst remaining beneath the level of the proposed structure.

The contractor excavated in bays over the sewer, thereby limiting the overburden stress relief to be compatible with available emergency measures. In the temporary condition the risks from flooding were controlled by implementation of a weather monitoring programme with a set of rules to be followed by the groundworks contractor.


Through open communication with stakeholders including Thames Water, the client plus the design and construction team, the solution was agreed in principle, then a design was detailed and a working method agreed to allow successful scheme implementation.

The main achievement of CGL’s work was to allow the basement to be constructed. Without the innovative approach adopted, the  basement in its designed form would not have been possible and would have required changes in layout (with a requirement to go back to planning) or the removal of the basement (removing valuable space from within the development).