Big Ben Clock Tower: Ticking with a Tilt

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The Big Ben Clock Tower is the British cultural icon and was renamed "Elizabeth Tower" in 2012 to celebrate the diamond jubilee of Queen Elizabeth II. Upon its construction in the year 1858, it was the tallest and the largest clock tower in the world. The Big Ben, situated in London, is famous for its precision and accuracy and is known as the most accurate four-faced chiming clock in the world.

The tower was constructed from load-bearing brickwork and stone cladding of 11 m² area installed up to a height of 61 m. The Clock Tower was founded on a 3 m thick concrete raft, covering a total area of 15 m² within terrace gravels overlying London clay. The weight of the Clock Tower is about 8400 tons imposing an average foundation bearing pressure of approximately 400 kPa.

The clock face is 55 m above the ground level and is out of plumb towards the north-west by 220 mm. Thus, the inclination is about 1/250, an amount which is often quoted as being just discernible to the tourists. Likely so, most of the tourists are often seen debating the verticality of the Clock Tower.

The Clock Tower is structurally connected to the four-story east wing of the Palace of Westminster, as shown in Figure-1. The Palace houses the Parliament of the United Kingdom. Both the Clock Tower and the palace have a single-level basement of vaulted brickwork.

The construction of the New Westminster station for the London underground metro project caused significant movements in the Clock Tower. Protective measures, primarily in the form of compensation grouting beneath the Clock Tower, were implemented during the construction period to control the settlement and tilt of the monument.

In this article, we have discussed the tilt, settlement, and protective measures taken to protect this iconic monument of the United Kingdom.

1. Geology of the Clock Tower 

The following points describe the geology of the Clock Tower: 

1. The upper ground depth between 5 m and 8 m consists of an alluvium gravel. 
2. Further, the London clay is located till 35 m depth. The Lambeth-Group lies below the London clay. Thanet-Beds (geological formation made of grey sand and mostly found in London) lies below the Lambeth-Group and resting above the chalk rock. 
3. The Lambeth-Group is 18 m thick. It is predominantly clayey soil comprising 8 m of upper mottled clay dissected by a thin layer of laminated beds over lower mottled clay about 5 m thick. 
4. The lowermost 5 m includes a thin layer of the pebble bed over sand.
5. The Thanet-Beds are about 8 m thick on the top of the chalk and located at 73 m below the ground level.
6. The groundwater level in the nearby aquifer is about 9 m below ground level with a little tidal variation. 
7. Pore-water pressures are close to hydrostatic equilibrium with the overlying aquifer throughout the London clay.
8. The water pressure in the Thanet-Beds and chalk at the Clock Tower site is substantially reduced because of the continuous pumping from the nearby aquifer.

2. Tilt of the Clock Tower and Remedial Measures

The first activity to cause the tilt of the Clock Tower was the westbound running London’s underground new jubilee line tunnel drive. The metro tunnel construction was undertaken in March 1995 with limited compensation grouting. During the tunnel drive, a tilt of 4 mm to the north was observed. The northward tilt continued to increase significantly following the tunnel drive and reached 9 mm in July 1995.

By close monitoring of the tilt, it was observed that the volume losses of the area were about 3%. It was significantly larger than the allowed value in the design. 

The observed increase in the tilt after the tunnel drive also demonstrated that substantial time-dependent movements would be expected both during the construction period and later on. Therefore, to reduce the substantial movement of the Clock Tower, the following actions were taken:  

1. The tunnel advancement was coordinated with the implementation of grouting to allow settlements to be fully compensated. This is referred as concurrent compensation grouting. 
2. A trial grouting phase below the Clock Tower was undertaken to demonstrate that the control of the tilt of the tower could be exercised. 
3. An expert review panel was set up to advise on geotechnical and construction issues relating to the Clock Tower. 
4. The finite element analysis undertaken at the design stage was updated to take account of revised construction methods and sequences. 
5. The results of finite element analysis assisted in identifying potential mechanisms of movement of the Clock Tower and allowed variations in the excavation and construction procedure. 
6. Further independent measurements of horizontal and vertical movements of the Clock Tower were made.  

Figure-2 shows the measured north-south tilts of the Clock Tower from the optical plumb throughout the construction period and for six months thereafter. It also depicts the timings of the main construction activities. The passage of the four tunnel drives is shown across the top of the figure. The timing at various excavation depths is shown across the bottom. The thick vertical line at December 1995 indicates the start of compensation grouting to control the tilt of the Clock Tower.

2.1 Principal of Compensation Grouting

The principle of compensation grouting is to inject the grout under pressure into the ground at chosen locations to counter the subsidence that an overlying building might be experiencing. This is done by installing steel tubes into the ground at regular intervals, typically about 0.3 m.

Each hole is covered by short rubber sleeve which acts as a one-way valve. Thus, the grout is pumped in under pressure without flowing back. Any hole can be selected for grout injection and the system allows repeated grouting through the same hole if required. 

The provision of grouting tubes below one of the London’s busiest areas was not a simple task. The horizontal array of grouting tubes was installed by drilling radially outwards from a vertical shaft located in the middle of the Clock Tower street. The tubes were about 50 m long and were drilled beneath the north foundation of the Clock Tower.

The elevations of the tubes were chosen such that they were just within the London clay stratum. This was done to avoid encountering the groundwater in the overlying gravel stratum. 16 tubes were installed beneath the foundation with a maximum spacing of 2.5 m.

Without any compensation grouting, the cumulative increase in the tilt of the Clock Tower would have been at least 120 mm. Therefore, it would certainly have resulted in significant cracking of the Clock Tower. By the end of construction, no further grouting was undertaken. Thus, the time-dependent tilt had continued at a decreasing rate. This was consistent with computer predictions and is still being monitored very closely.

2.2 Remedial Measures

The trial grouting was carried out in December 1995, when the northward tilt was 14 mm. After the trial grouting, the tilt was reduced by about 10 mm. Thus, the method was suitable and a significant control of tilt could be exercised.

The next grouting phase below the Clock Tower was concurrent grouting associated with the enlargement to form the westbound station tunnel. Grouting within the settlement trough was fully coordinated with tunnel advances. It was augmented by additional injections below the Clock Tower. The aim was to produce full compensation for the tunneling-induced settlements together with a reduction in tilt. This was successfully achieved and the tilt of the Clock Tower was reduced by 5 mm. 

A total of 24 phases of grouting were undertaken between January 1996 and September 1997. Over that period, a total volume of 122 m³ of the grout was injected beneath the Clock Tower. Mostly, grouting was confined to the northern half of the raft foundation. 

3. Settlement of the Clock Tower during Construction of Underground Metro Tunnel

The vertical displacements of the Clock Tower showed the cycles of heave associated with the grouting phase and the subsequent settlement. The following points describe the settlement of Clock Tower during construction:

1. The settlement at the start of grouting in December 1995 had a maximum value of 9 mm on the north-west corner of the Clock Tower. This had increased to 11 mm at the time of the second successful trial in February 1996.
2. During the 20-month period of grouting to control the tilt of the Clock Tower (February 1996 to September 1997) the settlement was in the range of 7-14 mm. In other words, during the period of grouting the vertical movement was controlled between 4 mm heave and 3 mm settlement.
3. Immediately before the final grouting phase on 28^th August 1997, the maximum settlement had reached 14 mm. After completion of this phase, it had reduced to less than 9 mm. i.e., a heave of about 5 mm was induced.
4. In the five months following the final grouting phase, the settlement increased to about 15 mm, thereby returning approximately to the value before the final grouting phase.
5. The maximum heaves associated with individual grouting phases were generally 3 mm or less. However, in the final phase it was 5 mm.  
6. Settlements extended southwards to a distance of 30 m from the north face of the Clock Tower.
7. The shape of the settlement profile was similar to the resultant estimated settlement trough, but the magnitude was substantially less.
8. Although the compensation grouting only extended beneath the northern half of the Clock Tower, but its effects extended southwards which is about 25 m from the planned extent of the grout injections.

4. Post Construction Behavior of the Clock Tower

After the completion of construction in September 1997, the tilt of the Clock Tower continued to increase because of the ongoing consolidation of the London Clay. By the end of 1997, the rate of tilting was decreasing, but the performance control level (PCL) exceeded within a month.

The compensation grouting facilities were still in place. However, it was felt that a further phase of grouting might accelerate the movements again and could be counterproductive. Therefore, it was decided to continue to monitor the PCL to ascertain the further tilting of the Clock Tower. 

Further, an increase in tilt was observed in March 1998, six months after the end of construction and the final compensation grouting phase. The tilt observed was about 40 mm. 

Examination of the crack width measurements revealed the mechanism by which an increase in the tilt of the Clock Tower was reflected in increased crack widths within the structure. A detailed analysis showed that for each 1 mm increase in tilt over the vertical gauge length of 55 m, the average crack width at a high level in the building would increase by 0.07 mm.

Recent measurements indicated that the long-term tilt has almost stabilized at around 35 mm. Also, an opening of the existing cracks by up to 3 mm was observed. However, it didn’t significantly affect the ease and cost of repair. 

5. Future of Clock Tower

As reported by researchers, the construction of the underground metro caused the tower to tilt about 35 mm to the south at a height of 55 m. The measurements show that in the 22 years following the completion of the metro tunnel, the Clock Tower underwent a background rate of tilt to the north of about 0.65 mm per annum. The fluctuations around this trend of about ± 2.5 mm are due to the seasonal and daily thermal effects mentioned previously. Therefore, the important role in further movement of the Clock Tower would be decided by the seasonal and daily thermal effects.  

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