Big Ben Clock Tower in London: History, architecture and working mechanism

The Big Ben Clock Tower, a symbol of British culture, was renamed “Elizabeth Tower” in 2012 to honour Queen Elizabeth II’s diamond jubilee. It was the world’s highest and biggest clock tower when it was built in 1858. Big Ben, a London landmark, is renowned for its accuracy and precision and is considered the most precise four-faced chiming clock in the whole world.

The Clock Tower was built on a 15 m² concrete raft that was three metres thick and covered 15 m² of terrace gravel on top of London clay. The Clock Tower weighs roughly 8,400 tonnes with an average base bearing pressure of about 400 kPa.

The clock face is 220 mm out of plumb to the northwest and 55 metres above the ground. Hence, the incline is around 1/250, which is sometimes stated as being barely perceptible to tourists. Most tourists are probably observed arguing about whether the Clock Tower is vertical or not.

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See also: Tower Bridge: Grade I listed, neo-Gothic designed bridge in London

Big Ben Clock Tower: Earth science

The geology of the Clock Tower is described as follows:

1. Alluvium gravel comprises the top 5 to 8 metres of the earth.
2. The London clay is also found 35 metres below the surface. The London clay is underneath the Lambeth Group. Lambeth Group is located below, and Thanet-Beds, a geological formation composed of grey sand predominantly found in London, is perched above the chalk rock.
3. The thickness of the Lambeth Group is 18 metres. It is mostly clayey soil, with a lower layer of mottled clay about 5 m thick and an upper layer of higher mottled clay 8 m deep, divided by a thin layer of laminated beds.
4. A thin layer of the pebble bed over sand may be found in the bottom 5 metres.
5. The Thanet-Beds are located 73 metres below the earth and are around 8 metres thick on top of the chalk.
6. With some tidal volatility, the groundwater level in the surrounding aquifer is around 9 metres below ground level.
7. The London clay has porous water pressures near hydrostatic equilibrium with the aquifer underneath it.
8. The constant pumping from the neighbouring aquifer has significantly decreased the water pressure in the Thanet Beds and chalk at the Clock Tower site.

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Big Ben Clock Tower: Settlement of the clock tower during underground metro tunnel construction

The clock tower’s vertical displacements demonstrated the heavy cycles connected to the grouting phase and the following settling. The settling of the Big Ben Clock Tower during construction is described as follows:

1. The northwest corner of the Clock Tower had a maximum settlement value of 9 mm when grouting began in December 1995. At the time of the second successful study, which took place in February 1996, this had climbed to 11 mm.
2. The settlement was in the range of 7–14 mm throughout the 20-month grouting period (February 1996–September 1997) used to regulate the tilt of the clock tower. In other words, the vertical movement was controlled during the grouting process between 4 mm heave and 3 mm settlement.
3. The maximum settlement had reached 14 mm just before the last grouting process on August 28, 1997. It had decreased to less than 9 mm by the time this phase was through. In other words, a 5 mm heave was generated.
4. The settlement climbed to around 15 mm five months after the last grouting phase, roughly reverting to the figure before the final grouting phase.
5. The highest heaves connected to each grouting process were typically 3 mm or less. But at the last stage, it was 5 mm.
6. Thirty metres south of the north face of the clock tower, settlements may be found.
7. While the magnitude was far smaller, the settlement profile’s shape was comparable to the projected settling trough that was the outcome.
8. Although the compensating grouting only covered the northern half of the clock tower, its effects could be seen to the south, roughly 25 metres beyond where the grout injections were supposed to go.

Big Ben Clock Tower: Post-construction behaviour

Due to the continuous consolidation of the London Clay after construction was completed in September 1997, the clock tower’s tilt kept worsening. The tilting rate was reduced by 1997, but the performance control level (PCL) was already surpassed. The facilities for compensating grouting were still in operation. Nevertheless, it was believed that adding another grouting process might make the motions worse. It was decided to keep checking the PCL to determine whether the Clock Tower is leaning further. Also, six months after the completion of construction and the last phase of compensating grouting, an increase in tilt was seen in March 1998. Around 40 mm of tilt was visible.

The process by which a rise in the tilt of the clock tower was mirrored in higher crack widths inside the structure was discovered via examination of the crack width data. Thorough research revealed that the average crack width at a high level in the building would grow by 0.07 mm for every 1 mm increase in tilt across the vertical gauge length of 55 metre. According to recent measurements, the long-term tilt has virtually stabilised at 35 mm. Moreover, up to 3 mm of the existing fissures were expanding.

Big Ben Clock Tower’s future

According to studies, the tower tilted around 35 mm to the south at a height of 55 m as a result of the underground metro’s construction. The data reveal that the Clock Tower had a baseline tilt rate to the north of around 0.65 mm per year in the 22 years following the construction of the metro tunnel. The previously indicated seasonal and daily heat impacts cause oscillations around this trend, roughly 2.5 mm. Consequently, seasonal and daily temperature influences would significantly influence the Clock Tower’s future movement.

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