Introduction
Claridges Hotel is currently undergoing major civil engineering works but to those residing in one of the five-star suites or relishing their afternoon tea in the foyer, it appears very much business as usual with only a small gantry located to the rear of the hotel that might indicate that there was any form of construction works taking place.
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Figure 1[/caption]
However, while the hotel has continued to operate at full occupancy and carry out all of the operations involved to run a hotel of its size and stature, a new basement has been created under the entire footprint of the 1920s art deco wing, approximately 1,250m2, 4.5m high. The reinforced concrete raft, which was once the foundation to the eight-storey steel frame extension, is now a suspended slab supported on 61 circular reinforced concrete columns (Fig 1). Furthermore, by the end of 2018, another four levels of basement will be excavated and constructed, providing an additional five levels beneath the hotel without it closing for a single day.
This had been the main criteria set out by the client, the Maybourne Hotel Group, as part of the brief to redevelop the premises. Closing the hotel was not considered an option and many schemes proposed could not satisfy this objective. McGee Group Limited (MGL) put forward a conceptual design and was appointed the design and build contractor in November 2015 and embarked on the ambitious project.
Preliminary investigations
A number of trial pits, opening up works and soil investigations were commissioned to gain further insight into the structural and geotechnical detail of the building. The basement extension was proposed only beneath the 1920s art deco wing as the reinforced concrete raft was deemed more suitable to be able to cope with any alterations, unlike the strip footings of the Victorian building. It was later discovered that the Victorian building was also founded on a raft, however with no reinforcement and very poor concrete with brick aggregate, it was considered structurally too weak.
The early opening works confirmed that the building comprised of 61 steel columns erected on a grillage of steel beams and founded on the reinforced concrete raft slab, 1,100mm thick, which appeared to be in good condition and of good quality construction. Unfortunately the SI report did not glean the same confidence. The geological strata comprised of London Clay, overlain with gravel, overlain with a silty sandy clay immediately below the raft and it was this material that was somewhat of a show stopper.
In its contained state, it had a high bearing capacity and hence the building could be supported with no adverse effects. However once disturbed and released, the silt would run like a watery sand that could potentially undermine the existing foundations due to its high water content. However, early tests carried out on the material determined that once dry, it was actually very stable and had similar properties to clay.
Experts consulted to propose a solution to stabilise the material
Several experts were consulted to propose a solution to stabilise the material. Methods such as grout injection and ground freezing were not suitable as the material was too dense and clay-like to inject with grout, and expansion from a freezing option would have damaged the building. One potential option involved installing a series of wells connected to a vacuum dewatering pump which could, in theory, extract the water from the ground. However, this relied on the water not being recharged.
Therefore, the primary objective in order to make the project viable was to produce a water cut-off on all four elevations. Further investigations determined that Larssen steel sheet piling had been installed along the entire length of the western elevation in 1929 to facilitate the excavation for the art deco raft as the new formation level was much lower than the bearing level of the adjacent Victorian foundations. On the eastern elevation, the building occupying the northern half had been underpinned along the party wall line to again facilitate the excavation of the new raft.
The underpinning extended just into the London Clay and as such provided reasonable water cut-off. On the southern half of the eastern elevation, the adjoining site had been redeveloped in 1985 and included a new basement which extended below the Claridges raft level and as such provided an excellent earth and water retention in the form of a secant pile wall with a reinforced concrete lining to its basement. This left two challenges to the northern and southern elevations.
Construct a new hard/soft secant wall along this elevation
It was discovered that the raft slab extended beyond the southern facade by 2.4m into Brooks Mews. Following agreement with Westminster City Council to temporarily suspend the pavement and parking bays, MGL was able to construct a new hard/soft secant wall along this elevation. This would not only retain the water along this elevation but also provide an earth retention system when excavating beneath the hotel at a later stage.
To the north, the challenge was much more difficult in that occupation of the high way of Brook Street would not be granted by WCC nor did the hotel owners wish to see a major civil engineering project outside their main entrance. The solution derived was what came to be known as the water cut-off beam, a reinforced concrete structure measuring 1.2m wide, 1.8m high and 25m long. The heading for the beam was excavated utilising the vacuum dewatering strategy along the edge of the raft slab and subsequently filled with reinforced self-compacting concrete.
With the completion of the water cut-off beam, the secant piled wall, the existing sheet piles and existing contiguous piled wall, the ingress of water into the site had essentially been cut off over the entire footprint of the art deco wing. In reality, there were still some areas of ingress but the rate of flow had been significantly reduced and therefore only the water contained within the silt itself remained.
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(Fig 2)[/caption]
Construction sequence: Remain a live hotel throughout the project
Unlike typical basement construction under existing basements, Claridges was to remain a live hotel throughout the construction project with only two rooms and a connecting corridor located on the lower ground floor allocated to service the works. The hotel still occupied the rest of the lower ground floor with a number of back-of-house facilities including laundry, staff facilities and banqueting kitchens operating as normal.
The vacuum dewatering wells were installed in each of the rooms while a gantry was erected to service the site. Arup Consulting Engineers were appointed by MGL as the structural and geotechnical engineers for the project to realise the conceptual scheme.
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Figure 3[/caption]
Initially, only the removal of a small section of the raft slab was permitted in one of the rooms. A service shaft was excavated and a network of headings (Fig 3), designed to support the existing structure, were formed to the location of each of the 61 columns in the building.
The headings were formed using a series of steel frames designed by RKD consulting - employed as MGL’s temporary works engineers. The frames were relatively lightweight, made from 152 UC 23 structural steelwork and bolted together to form a 1.8x1.8m opening.
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Figure 4[/caption]
The excavation of the headings progressed in 500mm increments with the frame constructed on precast concrete planks bedded on sand and cement. Three mechanical jacks were placed between the head tree of the frame and the raft soffit and preloaded. Each frame was then grouted before progressing to the next frame to ensure there would be no settlement should the ground move into the space created by the excavation.
The load was transferred from the raft into the frames and concrete planks and back into the ground to temporarily support the structure above. However, at each column location, the sill beams of the frame needed to be removed in order to excavate a shaft which would disturb this load path.
Installing a pair of temporary steel trusses
This was overcome by installing a pair of temporary steel trusses along the length of the heading at the column location. Typically these headings at the central columns were made up of eight frames and the four central sill beams were to be removed to allow a steel caisson ring to be installed. The truss was installed spanning across all eight frames and preloaded to allow the load to be transferred to the outer frames while the sills were removed.
The first of the steel rings was excavated. These measured 1.8m in diameter, 6mm thick and were 1m high when assembled but were divided into three segments and a key piece which allowed them to be easily manoeuvred into position and transported.
Reinforcement was fixed to the outer diameter of the ring and bolts installed from the side trees of the frames which would be cast into the concrete collar around the first ring. Once the ring was set in place, concrete was poured around it capturing the bolts hanging down from the side trees. Once cured, the truss could be removed as the load path for the steel frames of the heading was now diverted into the concrete collar around the first steel ring.
The rest of the caisson could now be excavated one ring at a time down to the new B5 level basement.(Fig 5) The excavation for each steel ring was approximately 2.9m3 allowing for 50mm of grout to be pumped around the annulus between the back of the ring and the clay.
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Figure 5[/caption]
The steel caisson rings were designed to be the maximum permissible diameter specified by Arup as the span of the raft that could be opened up was limited. However, the nature of the project working in such restricted headroom meant the caissons had to be hand-dug and therefore needed to be large enough so it was comfortable for two miners to work in.
Accommodate what would be the new pile foundation
This was only a concern immediately below the raft but at depth the diameter could be increased to accommodate what would be the new pile foundation. At the new B5 level, a steel transition piece was installed that allowed the caisson diameter to be increased from 1.8m to 2.44m. The caisson proceeded below this level using standard concrete segmental linings varying in depth depending on the column load and level of the clay across the site (Fig 6).
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Figure 6[/caption]
Each caisson was under-reamed, which again varied in diameter depending on the design load with the largest measuring 4.6m in diameter and 1.8m high. Concrete deliveries were timed to match the completion of the caisson allowing for inspection by Arup.
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Figure 7[/caption]
Approximately 35m3 of concrete was pumped from the gantry to a hopper with a tremmie pipe at each caisson location and cast to the level of the underside of the B5 raft slab.
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Figure 8[/caption]
The level was controlled using a laser distance meter mounted at the top of the shaft. Standard C35 concrete was used until the last load where a self-compacting concrete was used to give a smooth level surface to mark the base of the column the next day.
The centrelines of the existing column were transferred to the base using plumb lasers and string lines and a 1.6m square area of the new B5 level slab was marked, reinforcement fixed and concrete cast with starter bars for the new 600m diameter column and couplers to connect the rest of the B5 slab in the future.
At this point, the construction of the column back up to the raft level was relatively conventional except that the works were to be carried out within the confines of a 1.8m diameter space. All of the reinforcing bars were coupled and a ‘waffle’ of slab starter bars installed at each of the new slab levels.
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Figure 9[/caption]
As the slabs could not restrain the column at this time, temporary props were installed below the level of the new slabs to restrain the column in the temporary condition.
Two hydraulic flat jacks with a capacity of about 800 tons
Once at the sill level of the heading, the head trees were removed to allow the column head to be cast installing two hydraulic flat jacks with a capacity of approximately 800 tons stacked on top of each other.
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Figure 10[/caption]
On completion of the column head, a nominal 200kNs load was jacked into the column before proceeding to open up the next adjacent caisson.
There were 61 columns to complete and the real time monitoring of the settlements together with the finite element analysis allowed the headings and caissons to progress from both the northern and southern access shafts, which offered major programming advantages.
Prior to removing the temporary works and remaining soil between the headings, the columns had an additional load jacked into them, which was equivalent to 75 per cent of the estimated dead load of the building, approximately equal to 3,000kN.
The jacks are to remain under pressure from the hydraulic oil and not grouted until the basement has been constructed to allow for any adjustments that may be necessary during the excavation due to potential settlement and ground heave. Fail safe steel packers are installed to safeguard against hydraulic failure of the jacks.
The excavation to B1 formation level, approximately 5m below the existing raft soffit, was through the clay and created the headroom to allow the mechanical plant to operate. The water cut-off beam and existing underpinning were extended to provide earth retention.
The toe of the existing sheet piles was above the B1 formation and therefore a temporary works solution was required to extend these and allow the excavation to proceed below the toe. Locally the ground was excavated at each sheet pile and a steel ‘H-pile’ installed in the trough. The area was backfilled with a cement bentonite mix before opening the adjacent excavation.
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Figure 11[/caption]
The combination of temporary works provided the support to excavate to the B1level (Fig 11) but a further retaining structure was required to enable the excavation to be completed to the B5 formation level.
A contiguous piled has been installed using a modified piling rig with reduced height mast and Kelly bar capable of drilling to a depth of 20m (Fig 12).
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Figure 12[/caption]
At this stage of the project, it becomes a traditional top down excavation with respect to the construction methodology (see main image, Fig 13). However, the demand on logistics, with all plant and materials transferred to site via a single 10T hoist over the 16m2 access shaft, involves meticulous planning and co-ordination.
Conclusion
The methods employed in the basement construction at Claridges are not unusual when considered individually but the combination of techniques, working in restricted headroom under a live building, has made the project the first of its kind and potentially paved the way for future clients to consider concurrent development and occupancy as an option.
This scheme will ultimately allow the plant currently occupying the upper floors to be moved into the lower levels of the new basement which, in turn, permits the redevelopment of these floors to create an additional 40 rooms and several 6 star suites while retaining the hotel clientele. The hotel will also benefit from an upgrade to their facilities with the addition of two swimming pools, restaurants and spa allowing it to continue to compete as one of the world’s leading hotels.
The success of the project has relied on the ingenuity of the entire team with an open dialogue between everyone from the client and structural engineers to the miners and carpenters, refining the construction process at every stage with a number of bespoke designs. The client’s vision promoted an infectious enthusiasm among the team to deliver a five-star solution for a 5 star hotel.
Author: Michelle Mackey, project engineer for McGee Group Limited at Claridges Hotel