In the past year flooding not only hit the headlines but caused considerable damage and upset to those in many of the UK’s low lying areas. One of these is Bristol City centre, an area recorded on the Ofwat register as susceptible to flooding, and the subject of one of Wessex Water’s 400 schemes in its five-year capital programme to relieve sewer flooding. The Bristol project has a number of interesting challenges related to the city centre infrastructure, access, topography and underlying geology.
Wessex Water, has its own engineering company, Wessex Engineering and Construction Services (WECS) who undertakes major capital works. WECS had experience in the area in the construction of the northern interceptor sewer that drains the north part of the city and was the main contractor for the works. Design work for the city centre flood alleviation was undertaken by Donaldson Associates and included route alignment, site investigation, and tunnel design. Specialist Engineering Services Ltd (SES), was appointed tunnelling contractor for the project who engaged Parsons Brinckerhoff for design of temporary works.
Richard Soloman, project manager for WECS gave an overview of the project, Damian McGirr lead for Donaldson Associates presented some of the issues in design and Mark Thomas Site Manager with WECS updated on construction progress.
The project options
The 57 properties included on the flood register included the Hippodrome theatre and a number of historic shops and houses. Bristol City centre is a mixture of Georgian and Victorian architecture linked by busy narrow streets. There are a number of in-fill buildings such as multistory car parks and an ICE rink. To the south of the city is the historic floating harbour while to the north is the University of Bristol. A number of possible solutions for flood alleviation were considered including distributed storage; upsizing existing sewers; construction of a major pumping station; the construction of a new combined sewer overflow; and the boring of a transfer tunnel. As Richard Soloman, WECS project manager commented, “the preferred option was not obvious but the result of careful balancing of demands and responsibilities”.
Distributed storage of floodwater (for later pumping) was eliminated because there was insufficient land within the city centre for the 4500m3 of storage on three sites required. Odour may also have been a problem with this option. The upsizing of existing sewers was also discounted on the interruption to road and services that enlarging would entail and also on capital cost considerations. Construction of a new combined sewer and pumping station was also unfavourable because of capital cost. This left the construction of a 800m gravity transfer tunnel to the northern interceptor north of the city centre. The selected solution had the advantages of a single site for a tunnel drive with straightforward land issues, it was sustainable (requiring no pumping), minimised likelihood of odour, virtually eliminated spills into the harbour and with capex of US$19M had the lowest whole life costs.
Soloman continued, “WECS was charged with finding and constructing the tunnel drive shaft within the city centre to connect the existing sewers to the new tunnel. A narrow 9m site was found between the Ice Rink and a multistory car park that with careful planning would do. Its main advantages were that it did not block access and had no residential neighbours. Its size was its major disadvantage” he added.
Once this decision had been taken Wessex Water engaged Donaldson Associates to carry out detailed design of the tunnel from the city centre to the northern interceptor. Overall the tunnel is 805m long and up to 70m below ground at its deepest point with the majority around 60m. Obtaining permissions was facilitated by the fact that for most of its length it is beneath one owner, the University of Bristol. McGirr also noted that “there were a number of historic buildings including Royal Fort with intricate plaster work.” There was also very sensitive equipment used for detecting earthquakes in the Bristol University Laboratories. In areas where cover was lower there was some concern that residents would be affected by 24hr tunnelling operations. A significant challenge was considered to be the connection into the existing shaft of the Northern Flood Relief sewer at the 63m deep 3.66m diameter shaft in Woodland Road.
Early works and geology
The first activity was to carry out site investigation along the route. Twenty boreholes were sunk to a maximum depth of 93m. These holes supported downhole geophysical measurements and the core recovered gave vital insight to the subsurface geology. The bedrock geology was a combination of quartzitic sandstones, mudstones, siltstones, limestones and minor conglomerates of the upper Carboniferous age. Above the tunnel, ground thicknesses varied from 10m to 80m. These rocks gently dip at about 20o along the tunnel line and as a consequence the tunnel line cuts through this succession which ranges from very hard and strong rocks such as the quartzitic sandstones (with strengths up to 487Mpa) to soft mudstones. Hydrologically the latter acted as barriers whilst the more permeable sandstones acted as conduits for ground water that entered the tunnel during construction. Another feature of the dipping geology was that during tunnelling the rock broke in a blocky fashion controlled by the bedding and jointing.
The geology according to McGirr became a major factor in determining the construction method, which in turn had a major impact on tunnel size. A minimum 1.5m diameter tunnel was required for the project design flow of 2000 litres per second. Consideration of health and safety for a tunnel drive of this length increased this to 1.8m minimum diameter, but 2m was preferred for operation activity. In fact, a much larger area of excavation 4m wide was required for construction machinery access. The permanent 2m diameter tunnel lining is still being finalised but it will need to withstand water pressures up to 6bar and have a design life of 60 years. Options being considered are: in situ structural lining with collapsible shutter, sacrificial shutter (weholite pipe) infilled in stages by foam concrete or using precast concrete pipe.
Method selection
Selection of a construction method was made following on from submissions from contractors at tender. Three main areas of risk were assessed in comparing the options:
• project completion risks
• health & safety risks
• contractual/commercial risks
The potential noise and vibration of blasting was an issue. The removal of a TBM at the connection point to the Northern Flood Relief Sewer would have been challenging and costly with some health and safety risk. It was considered that the drill and blast method gave greater confidence that the tunnel would be completed within a reasonable timescale and budget. It offered greater flexibility in terms of dealing with the interbedded geology and in particular the extremely strong sandstone (almost 500MPa) to weak mudstone. Requiring more work within the tunnel, the drill and blast method had the greatest health and safety risks, and it was considered that these could be managed by the implementation and control of safe systems of work. There was also local experience of constructing tunnels by this method in similar ground conditions. In addition the drill and blast option was more cost efficient with a very similar programme to the TBM option.
The works
Design of the drive pit located on a narrow strip of land between an Ice Rink and Car Park was challenging with depth to rockhead varying from 1 to 7m. It was sized to accommodate a 4m span tunnel. A rectangular pit 15m x 6m serviced with a Gantry Crane was used.
Surface works began in the second quarter of 2007 with construction of the city centre surface works and the drive shaft by WECS. According to construction manager Mark Thomas, one of the constraints was the working hours: 24hr working Monday to Friday, with blasting window 07.00 to 23.00. Blast vibration was limited to 10mm/s2 ppv monitored at the closest property. This is above that that can be felt (approximately 1.5mm/sec2 ppv) but well below that where minor damage to buildings occurs (45mm/sec2 ppv).
The initial work says Thomas ‘was related to clearing the shaft area in the 9m gap between the car park and the ice rink. Because of the variation in depth to rockhead the pit was reinforced by 40 x 300mm diameter steel piles and with concrete waling beams. A 25 ton excavator was lowered into the shaft to carry out most of the excavation with a smaller 3 ton machine used for trimming. Shotcrete was used to finish the walls of the shaft.
Drill and blast cycle
The tunnelling cycle was drill, blast, re-entry, clean and support” he added. The first 60m of the tunnel is a rectangular section 4.5m x 3m with the remainder 3.5m x 3m. These sizes allowed the passing of the mucking out haulage and the jumbo drilling machine whilst avoiding the necessity to back-up the drill rig excessive distances for cleaning out the blast.
In the narrower section passing bays were excavated at suitable intervals. The tunnel is supported by steel sets.
Safety issues are very important maintained Thomas, “we monitor a wide range of gases in the tunnel including methane, NOx and carbon monoxide. Using explosives requires air flushing and we have a 900mm forced air ventilation duct”. The blasts are also monitored by camera to confirm correct sequencing of the charges and to keep vibration within the agreed parameters. Explosives for the blasting are stored in a secure site 20 miles away and delivered by two vehicles monitored remotely. Blasting has caused the minimum of disturbance to the city and its residents.
Progress to date was 117m, which is approximately a 6 week slippage on programme, however mitigation measures have been taken. Mucking methods were changed with the aim of bringing the project back on programme. Scheduled completion is April 2009, when risk of flooding in central Bristol will be removed for many properties.
Questions from the floor
Barry Couchman (London Bridge Associates) asked about the reasons for the large rectangular section of tunnel requiring large quantities of concrete. Mark Jones remarked that the size was determined by the size of the plant required to construct the tunnel and selected by the contractor. The rectangular section has naturally occurred due to the dip orientation parallel to the tunnel alignment.
Phillip Wall (Network Rail) asked what parameters are looked at in the weekly inspection of the supports. Mark Jones reported that the temporary works designer is inspecting weekly, site staff are monitoring as built section, and convergence of supports and rock quality is undertaken on a daily basis by the contractor’s staff.
Terry Crabb (LBA) asked about the structural function of the piles in the shaft at 3m centres with the freestanding rock and whether there are limits on dewatering. Damian McGirr agreed that rock is freestanding, but there is a need to make absolutely sure there is no settlement of the car park. The piles also support a beam supporting the gantry crane. There were no limits on dewatering, but despite the 60m water table, ingress of water has been limited because the mudstone acts as a dam limiting duration of inflows.
Andrew Smith (Joseph Gallagher) asked about appointment of the clients own construction division WECS. How is best value ensured and what contract is used? Richard Soloman responded. WECS costs are benchmarked and monitored and compared with other construction data to ensure value. SES has been engaged on an NEC cost reimbursable basis for the first 50m where expected rates of progress were uncertain, then on a target basis.
Duncan Wardrop (Lafarge Aggregates) asked about the vibration experienced and the maximum instantaneous charge for blasting. Mark Jones reported that 5.9m/s2 ppv was achieved today with a charge of 1kg/per/m3.
Rod Young (Barhale) inquired about cycle times achieved and shift cycle. Two 11hr shifts are worked with 9hr cycle times at present commented Mark Jones.
Helen Natrass (Sir Robert McAlpine) asked about noise limits in hours of darkness and how it affected the work. Mark Jones remarked that this hadn’t been a problem as there have been high night-time noise levels (78dB) from a night club in the locality.
Rapporteur: Michael Francis
Figure 2. Longitudinal geological section of the tunnel alignment Fig 2 – Longitudinal geological section of the tunnel alignment Figure 1. Plan map of the project area Fig 1 – Plan map of the project area The jumbo drill rig working at the face The jumbo drill rig working at the face The project shaft with much out in progress The project shaft with much out in progress
