The location of the casting basin was determined by the available site geometry and the economic assessment of a large number (more than ten) of alternative options. Casting basin sites on both the east and the west bank of the Medway were considered, together with one off-site option and one ‘in line’ option involving unit s cast on both banks. A combination of open-cut excavations, sheet pile, diaphragm, pinapile, secant and combipile options was considered to enable the required excavation area to be provided within the site area avail able.
It was evident from site excavation trials and earl y site investigation results that excavation slopes in the alluvial clays would need to be between 1 vertical to 3 horizontal and 1 vertical to 4 horizontal. It was also evident from an earl y stage that a retaining wall at either the crest or the toe of the excavation would be required to enable the casting-basin footprint to be constructed within the available site area . The casting-basin excavation was also required to tie in with the temporary cofferdams at the interface between the tunnel units and the in situ works. The major constraints to the casting-basin layout were the location of potential obstructions at a slipway location 50 m south of the tunnel portal and a substantial listed building just north of the road alignment (Fig. 6).
Figure 7: Design of casting basin slope.
Hydrostatic groundwater levels were taken at or close to the ground surface to approximate rapid-drawdown conditions. Piezo metric pressures within the gravel horizon were varied in order to determine pore pressure limits for stability. It should be noted that once the tunnel units were constructed , the casting basin was to be flooded for float-out and then redrained to enable the approach works to be constructed. The excavation slopes were as follows:
(a) made ground: 1 vertical to 2 horizontal
(b) alluvial clays: range 1 vertical to 2·5 horizontal to 1 vertical to 4 horizontal (depending on location and soil conditions)
(c) chalk: 1 vertical to 2 horizontal.
The casting basin was fully instrumented with piezometers and in clinometers and a positive management system was adopted to enable excavation slopes, the retaining wall and groundwater levels to be monitored during the works.
Figure 6: Casting basin layout required showing location of the portal, slipway and listed building.
A 15 m deep excavation was required to enable float-out of the tunnel units and their passage through the portal cofferdams. It was decided to cast three units , each 127 m long. This minimized the number of construction joint s in the submerged section of the tunnel. The chosen design combined an open excavation with a 6 m high retaining wall at its toe as shown in Fig. 7. Soil data from additional site investigations enabled a three-dimensional mod el of the anticipated soil conditions to be plotted. Slope analyses were carried out using circular (Bishop) and non-circular analyses for both long-term and short-term parameters . A minimum factor of safety of 1·3 was adopted in all cases in view of the serious risks to the project in the event of a slope or retaining-wall failure, which could have physically dislodged or damaged the tunnel units.