The construction of the shaft base of a pumping station, as part of the Environment Agency Sea Defences, Canvey Island, was an engineering challenge due to its proximity to the tidal waters of the Thames Estuary along with the fine but permeable sands located beneath the shaft.

During the building of the Environment Agency Sea Defences, the Civil Engineer Contractor chose to construct the reinforced concrete base slab of the pumping station under dry conditions.  This decision ensured safe, unobstructed working within the shaft and created an environment in which the risk of artesian groundwater ingress to the shaft was removed.

To enable these works to proceed, groundwater specialists OGI were engaged to design and install a system to lower the water table and remove the artesian pressure beneath the shaft.

 

Due to the potential impact of the dewatering on the surrounding soil stability, together with the proximity of the shaft to surrounding properties, strict constraints were imposed on the dewatering operation.

Firstly, the dewatering wells were required to be designed and installed such that there was no removal of soil silts or fines.  This constraint was imposed to avoid any collapse of the soil matrix due to the removal of solids.

Secondly, and related to the above constraint, the groundwater abstraction system needed to be designed and installed so that the abstracted groundwater was free from solid material for direct disposal without any further treatment.

Thirdly, despite the pressure of tidal conditions 30m from the shaft, the dewatering system needed to be designed such that:

1. The target drawdown beneath the shaft was maintained throughout a range of tidal and weather conditions but,

2. The target drawdown was not to be exceeded beneath the shaft (as a maximum drawdown beneath surrounding properties was to be maintained).

3. Finally, due to the congested nature of the site that included existing buildings, pipelines and cables, the position of any borehole was restricted.

These constraints on the system required a precision approach to the dewatering operation as summarised by the following plan:

(a) Gathering of the available site investigation in the area.

(b) Developing a conceptual model of the local hydrogeology.

(c) Installing pressure transducers complete with data loggers in a number of existing standpipes to monitor ambient conditions.

(d) Conducting a pumping test to calculate the wider permeability of the ground.

(e) Designing pre-gravelled well casing to ensure no movement of soil lines through the well filters.

(f) Development of OGI’s groundwater computer model to the local hydrogeology to provide a design tool which would optimise the design of the dewatering system.

(g) Install a pumping system that has groundwater sampling points so that the water from each well could be regularly tested.

The OGI groundwater model was used to optimise the design of the dewatering system to work within the above constraints.

Installation of the wells used a cable percussive technique with wells installed to 12.5m below ground level.  During the installation, OGI’s site engineer communicated each drilling record back to the office based design team to enable the conceptual and computer models to be upgraded during the installation operation.  This process produced an evolving but continually closer representative picture of the actual ground conditions.

Submersible pumps were installed within each of the eight boreholes such that the location of the pump inlet could be varied.  All pump levels were set to an equal level Ordnance Datum.  However, to ensure the pressure level was achieved, the level of the pumps was calibrated against the observation well.

The target level of the observation well was in turn calibrated against the computer model which, when it matched precisely, would result in the target level at the shaft with no excessive drawdown at the properties, together with minimal differential drawdown.

In conclusion, precision dewatering is a challenge and to get it right requires a range of disciplines from a technical team together with a strong working relationship between the contractor and groundwater specialist.

At the Pumping Station, the working relationship between the Civil Engineering contractor and OGI in pursuit of a precision dewatering system produced the rewards of a smooth construction of the shaft base with no surrounding structure or environmental impact, together with a construction completed safely and on time.