The geology at the site consisted of clay with interbedded silts and sands. High piezometric pressures meant that the design of the new structures would need to overcome the risk of uplift forces from groundwater beneath the ASP and FST structures.
This presented challenges for both temporary and permanent works.
The design to remove uplift pressure beneath the ASP slab required a structural drainage layer. A composite drainage layer of Deckdrain 1200S (incorporating non-woven general purpose membrane: NW8), was chosen for this purpose, so eliminating the need for large volumes of expensive type 3 stone. Furthermore, the Deckdrain is quick and easy to install and reduces the volume of material required to be excavated.
In the permanent condition, and with the tanks full of liquid, there is no risk from uplift pressure beneath the slab. However, when the tanks are emptied during maintenance, this situation changes. For this reason the drainage layer beneath the ASP was connected to the FST drainage system to provide permanent condition groundwater pressure reduction.
The FST permanent works design was required to counter uplift forces from groundwater under normal rainfall conditions, heavy rainfall and 1:100 year flood conditions. OGI designed a passive pressure relief system that intercepts groundwater beneath all five tanks. This system is integrated to accommodate groundwater drained from the neighbouring ASP tank.
To increase the resistance against uplift pressure an extended toe was added to the base of the FST structure, together with the inclusion of pressure relief valves, to protect against groundwater uplift pressures brought about by 1:100 rainfall flooding events.
The resisting forces against uplift were calculated. These forces comprised weight of the structure, weight of the soil supported by the toe and additional shearing resistance of the soil along a shearing plane between the toe and the ground surface.
OGI applied a finite element mathematical model to analyse elasto-plastic stress deformation in order to determine the extra soil resistance against uplift due to the addition of the structural toe.
Benefits of the Pressure Relief System Design:
- Deckdrain as a structural drainage layer beneath the ASP and FSTs resulted in a considerable cost saving over the alternative approach requiring deeper excavation and backfilling with a Type 3 drainage layer.
- The passive drainage system to lower groundwater uplift pressure removed the requirement for ground anchors.
- The combined drainage system enabled both the ASP and the FSTs to be constructed without risk from uplift Pressures during Temporary Works.
- The installed system provides a permanent, passive solution to resist groundwater uplift pressure risk for the ASP and the FSTs.