15 – 1915 Excavating the main basin

 
While work continued on the entrance lock and the emergency entrance, the rest of the sea-walls round the main basin were completed. The foundations for the west and south walls were created by sinking concrete monoliths, while the foundations of north wall and the east wall from the emergency entrance to the shore were formed in wooden-lined trenches. The walls were all built from reinforced concrete.

Work could commence on excavating the main basin. This could be done in the dry, because the entire inner basin and the entrance lock had been sealed off behind temporary clay dams.

Before August 1914, 1.3 million cubic yards (1 million cu m) of material had already been excavated, leaving a further 800,000 cubic yards (610,000 cu m) to be removed. This was mostly clay, which could be removed by steam shovels. The final 6% was underlying rock which was broken up by blasting. The clay was used for land reclamation on site, and the rock as aggregate for concrete.

This work was completed by 16th September 1915, thanks to Admiral Beatty who lent 2,000 sailors from his ships to overcome the labour shortage.

Flooding the main basin – September 1915

By September 2015, the inner basin walls were complete. Graving docks No 1 and 2 were complete, and the entrance and caisson chamber for graving dock No 3 was complete. The emergency entrance was in place, as were the basic elements – walls floor and caisson cambers – of the entrance dock.

At 9.30 pm on 16th September the sluice valves were opened and flooding of the main basin began. By high tide at 9 pm the following evening the basin had filled to a depth of 37ft (11 m). The sluice valves were then closed, and more water was pumped into the basin to temporarily increase the depth to 39ft 6 inches (12 metres) and allow the sliding caissons to be floated into position.

Sluice Valves

There are six sluice valves at the south-east corner of the main basin. When open, they allow water to flow in or out of the main basin – depending on the state of the tide. For example if the emergency entrance is to be used around high tide level, the basin sluices can be opened to adjust the water level in the basin to match the current state of the tide. This allows the emergency entrance to be used for some time each side of the time of the high tide mark.

When a ship uses the emergency entrance it pushes in or pulls out a volume of water from the basin creating a current of water that impedes smooth passage. Opening the sluice valves minimises this current and makes for a smoother transit of the entrance.