This paper describes the construction of an 8·2 m diameter by 37·5 m deep shaft. The shaft is part of the Crossrail works in central London, and is fonned of  precast  concrete  segments. The segmental lining is deemed to have a temporary function (design life 20 years), and will in due course be fitted with the pennanent (secondary) cast-in-situ  lining. The construction  of the shaft was unusual in that it was carried  out  from within the IO m deep triple-level basement of the newly constructed Moorhouse building with the top of the shaft at approximately +0·5 mOD (2 m below the B2 level slab) and the final formation level at -37 mOD. The shaft was constructed by ‘underpinning’, and at its closest point was within 2 m of the large-diameter bored base-grouted piles supporting the new 16-storey building above. Significant problems arose during construction of the shaft, but careful planning, with contingency measures to respond to all key foreseeable eventualities, proved critical to the success of the project.

The site stratigraphy consisted of  2·5-3·5 m of  made ground and Terrace Gravel over about 30 m of London Clay, underlain by about 18 m of Lambeth  Group  clays, silts, sands and gravels, Thanet Sand and chalk. The shaft was founded in the Upnor Formation, which forms the basal stratum of  the Lambeth Group. The Terrace Gravels are water bearing and form the upper aquifer in London. The Terrace Gravels  had been removed for the basement construction, so shaft sinking commenced below the B2 basement level directly into London Clay. The granular horizons in the Lambeth Group form an intermediate aquifer, with pore pressures in central London often found to be partially reduced by the underdrainage in the lower aquifer. The Upnor Formation at the base of the Lambeth Group is often found to be in hydraulic continuity with the Thanet Sand below. The Thanet Sand and chalk form the lower aquifer in London. Groundwater levels in  the  lower  aquifer have been substantially reduced by historical groundwater abstraction for water supply  over the  last  150 years. Following a reduction in abstraction, recovery in groundwater levels commenced about 50 years ago, but has recently been  arrested by resumed abstraction because  of  concerns  about  the impact of raised water levels on London’s infrastructure.

Groundwater level monitoring  and test  pumping were carried out in the area of the shaft  as part of  the site  investigation works. Water-bearing granular horizons were identified in the Lambeth Group, particularly at a level of -31 to -32 mOD and -35 to -38 mOD. Standing groundwater levels in these horizons were recorded at approximately -20 mOD (12 m excess head) and – 30 mOD (8 m excess head) respectively. Monitoring of groundwater levels in the Thanet Sand (the lower aquifer) indicated a standing groundwater level of -34 mOD, 4 m above the shaft formation level (Figure 2).

Table 1. Outline specification for dewatering system.

All activities associated with preparation for and construction of the shaft were programmed to be off the critical path of the main Moorhouse construction project. This allowed a high degree of flexibility over timing, but it meant that the physical constraints imposed by the site conditions often made access extremely difficult. The first preparatory work for shaft construction was installation of the dewatering system and instrumentation. When these works commenced, the Moorhouse superstructure was nearly complete, and a 14 t drilling rig had to be accommodated on the ground-floor slab at +13·42 mOD with only 7·5 m of headroom (which was achieved by temporarily omitting a mezzanine slab). Figure 3 shows schematically the set-up for drilling, and Figure 4 shows the reality of access difficulties on site.

Figure 3: Schematic showing dewatering and instrumentation installation on site.

7.1  Logistics

The permanent columns of the new Moorhouse superstructure imposed severe restrictions, particularly on the movement of excavated spoil across the ground-floor slab. Traffic congestion around the site was such that all vehicle movements associated with the project needed to be carefully controlled within a restricted period between 8·00 am and 6·00 pm Monday to Friday owing to the close proximity of residential accommodation, and enforced via a Control of Pollutions Act, 1974, Section 61 noise-restriction agreement. This proved to be less of a limitation on shaft construction activities than the fact that disposal of the excavated spoil could not continue beyond 4·00 pm because of the working hours of the tip. Excavation and segmental ring building was a continuous 24 h operation between 8·00 am on Monday morning and 6·00 pm on Saturday, and there was consequently  a need to be able to store a substantial volume of excavated material on site.

Excavation for each 1 m deep ring generated about 100 m3 of spoil (with bulking), and allowance was made for storing up to two rings’ worth on site. A muck store was formed on a ground-bearing area of the Bl slab. The store was filled by placing the muck skip on a tipping frame at ground-floor level (Figure 6). The muck skip was moved from the shaft area to the store area by means of a curved runway beam supported on temporary steel gantries: one over the shaft, and the other outside the superstructure building line but supported on the -basement structure. A 20 t excavator was used to empty the muck store during the day. Limited space and restrictions on slab loading meant that only two complete rings of shaft segments could be stored on site at any time; also, segment delivery had to be carefully coordinated with other deliveries and muck-away. Twenty-four-hour working imposed additional restrictions relating to noise in a residential area. This was overcome by the erection of an acoustic screen that was designed to prevent noise rising above ambient levels at night around the site. The acoustic screen reduced ventilation in the area of the shaft, and night shift work was carried out in uncomfortably hot conditions.

The shaft excavation was carried out using  a  6 t  track­ mounted excavator, which was lowered  into  the  shaft  using the gantry hoist and lifting slings. The excavated spoil was loaded into  a  5·5 m3  bottom-opening  circular  skip,  which was hoisted up the shaft and discharged  into  the store  using  the curved runway beam and tipping frame (Figure 7). Final trimming  of  the  ground  was  carried  out  using  hand-held clay spades to allow a gap  of  approximately  40 mm  behind the segments; the  gap  was  specified  not  to  exceed  100 mm at any location (Figure 8).

Only sufficient depth of ground at the formation was excavated in order to allow the building  of the segments. This was generally kept to a maximum of 200 mm below the segments, which provided sufficient space for the ring build. However, deeper excavation was required locally at the key segment to allow it to be lifted into position with the additional  space needed to accommodate the radial taper (Figure 9).

The segments were mounted on a building frame, and were moved into position using a track-mounted hoist located around the shaft perimeter. The segments were connected with spear-bolted cross-joints and tightened to the correct torque to compress the sealing gasket. The segments were fitted with EPDM (ethylene propylene diene M-class rubber) gaskets at the factory, with bitumen packers on  all  the segment joint faces. The ordinary segments were built first, followed by the taper joint segments; the ring was  then checked for level, and the position at each segment and thereafter adjusted as necessary.

Each ring of segments was bolted, using through-bolts, to the underside of the ring above. The joint between each ring had  a 4 mm polymer or PTFE packer fitted, along with alignment dowels, and each ring was also rotated relative to the last ring to stagger the cross-joints. Where necessary, additional polymer packing strips were inserted on the ring face (but not across the gasket)  to correct for any parts of the ring that  were out of level. Grouting was carried  out as soon as possible after the ring building was completed. A seal was formed to prevent grout loss from the back of the segments  by using air hose, cement bags and clay to ‘fluff up’ the joint. The ring was then grouted using an ordinary Portland cement grout, with a water/cement ratio of approximately 0·4, which was mixed in a vertical grout mixer on the surface and pumped down a grout hose and through grout holes in each segment. Grouting commenced in one location and was then continued systematically around the full ring. Each segment had a grout vent at the top, and grouting  continued  until grout was present  at the vent hole, which was then capped off. The grouting operation typically took 2-3  h, with grouting pressures kept below 1 bar (105  Pa) owing to the need to avoid leakage from beneath the bottom of the segments.

7.2  Predicted and Measured Group Movements

Details of the design associated with the shaft construction are given in Morrison et al.,1 and the calculated and measured deformations at inclinometer IGl are shown in Figure 10 for four intermediate dig stages and the final conditions with the base of the shaft constructed. Significant differences exist between the measured and predicted displacements associated with IGl, but this is not unexpected. The soil model used in the analyses1 was moderately conservative, although not to the point where the results of the analyses could have suggested a different method of construction. In addition, the analyses ignored the stiffening effect of the 1·B m diameter piles of the Moorhouse building, which would almost certainly have affected the displacement oflGl in particular, since this inclinometer was positioned between the pile and the shaft.

The material exposed to the water was stiff, fine-grained, overconsolidated clay, and was relatively impermeable. Water could not be expected to penetrate very far into the excavated surfaces, especially given the duration of exposure, and as a result significant softening did not occur. When the shaft was entered, following removal of the water, the softened material was found to extend to a depth of about 100 mm. It would seem reasonable to assume that much of this material was left­ over excavation arisings that had not been removed from the shaft. Inclinometer tube profiles showed that the maximum inclinometer movement was 2 mm. This movement is thought to be genuine but, when viewed in the context of previous and subsequent displacements, was not particularly significant or excessive.

7.3  Shaft Flood

Excavation and construction of  rings  proceeded  as  planned until the excavation reached ring 8, whereupon  the  ring was built and grouted at the end  of  a Friday  night shift,  and the shaft was left until the following Monday morning. In the early hours of Sunday morning a coupling on the site water main failed, causing extensive flooding  to the basement  of Moorhouse, including the partially constructed shaft. The water from the main filled the shaft to  a  depth  of  about  2 m, implying about 100 m3 • Excavation of the shaft was at  a relatively early stage, in a substantial  layer  of  stiff  London Clay, and the formation was at about -8·5 m0D. Pumping to remove the water by means of a high-lift submersible pump commenced at about 4·00 pm on the  Monday,  and  this operation was complete by mid-evening, a few hours later. The water therefore remained in the shaft for a maximum period of about 36 h. Excavation for ring 9 was  completed  by  the morning of the Wednesday, at which point the ring was built.

The material exposed to the water was stiff, fine-grained, overconsolidated clay, and was relatively impermeable. Water could not be expected to penetrate very far into the excavated surfaces, especially given the duration of exposure, and as a result significant softening did not occur. When the shaft was entered, following removal of the water, the softened material was found to extend to a depth of about 100 mm. It would seem reasonable to assume that much of this material was left­ over excavation arisings that had not been removed from the shaft. Inclinometer tube profiles showed that the maximum inclinometer movement was 2 mm. This movement is thought to be genuine but, when viewed in the context of previous and subsequent displacements, was not particularly significant or excessive.

7.4  Secondary dewatering in the Upper Lambeth Group

Throughout construction of the shaft, the ground conditions and dewatering performance and output were monitored and recorded by the  shift engineer as excavation proceeded. Although excavation through London Clay was straightforward, probing ahead of the excavation, from within the shaft, was carried out when the shaft was approximately 3 m above the upper_ granular horizon in the Lambeth Group (between -31 m0D and -32 mOD} and the hole was checked for water (Figure 11). This was because, although the dewatering system functioned well, the water levels monitored in the Upper Lambeth Group (notably between -30·7 m0D and -32 m0D} remained approximately 1 m above the design level of 0 m excess head of water (Figure 12).

As the excavation approached  the level  at which  two of the three piezometers were indicating about 1 m of water, the issue manufactured with provision for drilling the inclined wells for secondary dewatering. These could be installed at any level but, owing to the offset nature of the joints in alternate rings (to maximise the stiffness of the shaft), the positions of the individual wells were governed not only by the ring elevation but also by its orientation. These restrictions were, in tum, exacerbated by a third logistical problem in that there was only sufficient space on site to store two complete  rings of segments. This meant that a special ring of segments would have to be installed almost immediately if it was brought to site. The time taken to install the secondary dewatering wells would in itself result in increased movements around the shaft.

Therefore the decision to use secondary dewatering and the timing of its installation was critical.

Shaft excavation and construction continued until trial pits could be excavated from about 2 m above the water-bearing layer. Three trial pits at third points around the perimeter of the shaft remained dry following excavation. On this basis all parties agreed to proceed with excavation through the water­ bearing layer. Upon excavation the silty sand appeared relatively dry, but during the course of the ring build, about 2-3 h, small amounts of material continually fell from the sides all around the shaft (Figure 14). At this point the piezometer reading had dropped and recorded a small suction. When the ring build was complete, the average space behind the segments was about 400 mm, gauged from the quantity of grout that was used to fill the void. However, the slumping of material that took place during the ring build, and continued until grouting was complete,  means that it was difficult to know the real extent of the overbreak, as it could not be seen. Excavation continued for subsequent rings, and the piezometer reading recovered to show 1 m head (Figure 12). The additional movement resulting from excavation through this layer became apparent from the inclinometers near to the shaft. A maximum displacement of 7 mm was recorded on inclinometer IG2 (Figure 15). This movement represented nearly 500/o of the total movement for the entire shaft construction.

Excavation for the base plug of the shaft entailed undermining  the bottom ring of segments by 2 m in a layer of clayey coarse gravel to fix a heavily reinforced cage. This excavation was nearly twice the depth of the ring excavations, and greater movements could therefore be expected. Additionally, the excavation was to remain open for much longer while the base plug was constructed. This layer was the Upnor Fonnation, which had been targeted by the ejectors with the aim of eliminating the excess head of about 5 m. All of the piezometers around the shaft indicated that the head had been successfully drawn down, and the excavated faces were gunited to maintain stability during the two weeks needed to fix steel and pour the base. Dewatering continued until the base plug achieved design strength, and monitoring continued for a further month. Inclinometer readings showed additional movement during this period, with the most movement being recorded at IG2 (Figure 16), where a warning trigger level was reached. However, considering the time taken to complete the construction of the base plug, this was not excessive, and the overall movement remained well inside the action threshold.

7.5  Grouting to Restress the Ground

Any need for grouting to restress the ground around  the shaft was dependent on the results of the ground monitoring. If the acceptable inclinometer movement  limits were exceeded  then the grouting was deemed necessary to ‘restress’  the ground around the shaft and adjacent piles. It should be noted that grouting was not intended as remedial work to the new Moorhouse development  per se, but as a measure that would help to safeguard Moorhouse in the event of future tunnel and station box construction associated with Crossrail. If  required, the grouting would be carried out after the shaft was complete and the base concreted. Several of  the  shaft  rings  had additional large grout holes, which could  be used  to  allow radial drilling and subsequent grouting to be carried out. In the event, the maximum movement in relation  to  a  trigger  level was about 15 mm on inclinometer IG2, which exceeded the warning trigger movement of 14 mm but did not exceed the action trigger movement of 21 mm.

Figure 14:  Silty sand slumping from excavated face during ring build in water-bearing layer in Upper Lambeth Group.

An 8·2 m diameter by 36 m deep shaft has been constructed within the basement of a new 16-storey building. The shaft has been constructed through about 20 m of London Clay and then through the soils of the Lambeth Group, where two water­ bearing layers were encountered . At its closest point the shaft was within 2 m of the new, heavily loaded piled foundations of Moorhouse. The proximity of the existing foundations and the influence of the proposed developments associated with Crossrail posed significant problems to the design and construction team with respect to limiting the impact of the shaft’s construction on Moorhouse. A sophisticated dewatering system was devised and installed to enable excavation and construction through the Lambeth Group, and also to enable the shaft to be excavated to within 2 m of the underlying Thanet Sand. Elaborate precautions were taken to ensure that construction solutions were in place to deal with foreseeable events, and a detailed monitoring exercise was carried out to help control ground movements and thereafter demonstrate that ground movements did not exceed acceptable limits.