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The upgrading of a Sewage Treatment Works in Staffordshire required the construction of an Activated Sludge Plant (ASP). The structure required a deep excavation within very tight space constraints.


The geology over the new ASP footprint consisted of clay with inter-bedded silts and sands. The geology was variable locally with high piezometric pressures. The Contractor’s original intention was to construct the tank within costly retaining wall structures, in combination with a dewatering system to control the high groundwater level.


From the available S.I., OGI identified that the ASP structure could potentially be constructed using an open battered approach, removing the requirement for temporary or permanent retaining walls.

Strongford 1


OGI specified additional S.I. and testing. From this information a conceptual model was produced and the values were input into the mathematical slope stability model which confirmed that the steep batter approach was feasible.


OGI was able to demonstrate that dewatering was not required so long as water entering the excavation could be adequately drained (which would also add strength to the soils within the batter) and provided that a drainage layer was installed below the slab.


Geological logs at all relevant borehole locations were used, together with in situ and laboratory testing, to construct a strength profile of the ground in and around the proposed excavation.


This information, along with existing structure and site vehicle loadings were used to design safe 1:1 batter slopes for each side of the excavation.

Strongford OGI

The slope design, documentation and checks were carried out to comply with the criteria of Eurocode 7: Geotechnical design. OGI then issued design certificates for the batter slopes which documented and justified slope design criteria, assumptions and limitations.


An Inspection, Testing and Monitoring (ITM) Plan was developed to enable OGI’s design assumptions and ground strength parameters to be tested and certified as excavation progressed.


The Client was able to use the ITM Plan during excavation to provide guidance as to requirements and appropriate responses, depending on the geology or ground strength conditions encountered.


OGI’s engineer inspected the site periodically to ensure that the Client understood the ITM Plan and was complying with the ITM Plan requirements, thus validating the design.

Strongford Slopes

Upper Diagram showing geology around ASP
Lower Diagram showing stone drainage requirements.
Strongford Key
The design of the batters required specific drainage measures to protect the batter slope where silt and sand layers were encountered, including a series of horizontal and vertical drains leading to a perimeter drain.

Strongford Slopes