Deephams Sewage Treatment Works: 18.5m Internal Diameter Pumping Station Shaft

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1 Deephams Sewage Treatment Works: 18.5m Internal Diameter Pumping Station Shaft Stephen Woodrow 1, David Beadman 2, Oisín Gibson 3 1 AECOM, United Kingdom 2 Byrne Looby Partners, United Kingdom 3 J Murphy & Sons Ltd, United Kingdom ABSTRACT This project involves the improvement an existing Sewage Treatment Works in Enfield, London, which will serve a local population equivalent to 1 million in The project will enhance the existing treatment works, and includes the construction of a deep pumping station. The new pumping station requires an internal diameter of 18.5m and a depth of 21m to accommodate a low level sewer entry, two dry wells and associated controls and pumps. These will deliver dry weather flow of 1800l/s and storm flows of 3600l/s. This paper concentrates on the design and construction of the Pumping Station Shaft. The permanent shaft includes a 600mm thick in-situ concrete liner and a 3.5m deep base slab to resist flotation. The shaft requires a 24m deep dig to formation. Secant piles were used with added innovations and developments to facilitate the deep excavation in difficult ground conditions. This pushed existing technology to new boundaries, using innovations that can be adopted for other secant piled shaft designs in the future. 1.0 INTRODUCTION J Murphy & Sons Limited is Principal Contractor for the Deephams STW (Sewage Treatment Works) Inlet Improvement Scheme in Enfield (North London). The project is being carried out on behalf of Thames Water Utilities and will serve a 1 million population equivalent in The existing works is considered a strategic to the Thames Water Network and this improvements project aims to enhance screening facilities and expand storm storage capacity to make the nearby River Lee cleaner and healthier for aquatic life and improve the riverside environment. The project, being constructed between May 2010 and December 2012, aims to significantly enhance existing ageing plant through refurbishment and improvements to the inlet pumping stations, screenings plant, storm treatment and storage facilities with the addition of a new dedicated pumping station. The new pumping station requires an internal diameter of 18.5m and an invert depth of 21m to provide operational freeboard to the invert of a low level sewer, which is 15m below ground level. The new pumping station will include two dry wells to house the associated controls and pumps. These will deliver a dry weather flow of 1800l/s and storm flows of 3600l/s. To account for the permanent shaft design, which includes a 600mm thick insitu concrete internal lining and a 3.5m deep base plug to resist flotation forces, the temporary shaft requires a 24m deep dig to formation. The main parties associated with the design and construction of the shaft are as follows: Client Thames Water Principal Contractor J.Murphy & Sons

2 Designer AECOM Specialist Piling Subcontractor Murphy International Piling Specialist Temporary Works Designer for Secant Piles Byrne Looby & Partners The challenge put to the design team was to utilise existing or new construction technologies to provide a cost effective design for the shaft taking into account the critical nature of the programme. The shaft needed to be capable of providing the required 24m excavation in challenging ground conditions. 2.0 GROUND AND GROUND WATER CONDITIONS The ground conditions at the Deephams site are typical for the London area. Specifically at the site of the pumping station shaft, this consists of approximately 1m of made ground overlying a 5m band of River Terrace Gravels and approximately 12m of stiff overconsolidated London Clay. Below this is an approximately 8m thick layer of Lambeth Group and Upnor Beds, which consists of varying layers of clays silts and sands. This in turn overlays approximately 16m of densely packed Thanet Sands, below which is the Chalk formation. The shaft is constructed through the made ground, River Terrace deposits, London Clay and Lambeth Group, with the shaft base founded in the Lambeth Group close to the interface with the Thanet Sands. The site investigation data identifies two distinct ground water systems. Firstly, at the upper level, the River Terrace Deposits are water bearing. This is separated from the main ground water system in the Thanet Sands and Chalk below by the impermeable layer of London Clay. Piezometers identified a water table in the Thanet Sands and Chalk, with a hydrostatic level in the region of 12m to 18m below ground level. This is effectively 6m to 12m above the shaft formation level. The ground, and in particular the ground water conditions, provide a particular challenge to the shaft construction and the proposed shaft construction methodology had to account for these conditions. 3.0 CONSTRUCTION OPTIONS A number of possible options were considered for the shaft at the start of the design, including traditional construction with precast concrete shaft rings, diaphragm wall, hard/hard secant piles and hard/firm secant piles. Traditional construction would involve building the shaft using a precast concrete segmental shaft lining either constructed using underpinning methods or by jacking the shaft into the ground as a caisson. The under pinning option is only possible where the ground is free standing in the short term and the excavation can be maintained dry. This is unlikely at Deephams, where the shaft extends into the waterbearing Lambeth Group/Upnor Beds and Thanet Sand. Jacking a shaft into place as a caisson also has associated risks with regards to maintaining the shaft verticality and circular shape. A diaphragm wall option was considered in detail. A series of diaphragm wall panels forming the circular shaft, designed to act in ring compression, would enable the shaft to be excavated without any internal ring beams. However, the mobilisation cost of the diaphragm wall equipment including the bentonite handling systems meant this was an expensive option for such a relatively small wall perimeter. Also, the site working area required for the diaphragm wall construction compared to the other methods considered was significantly large, and as the site is relatively small and confined, the diaphragm wall option was not considered to be the preferred solution.

3 A hard/hard secant pile shaft would enable excavation without ring beams to well below half of the shaft depth. However the practical verticality tolerances (typically 1 in 200) coupled with the maximum excavation depth resulted in a very deep cut into the primary piles to achieve overlap at formation level within the shaft. The deep cut and the depth of the secant piles is beyond the usual construction constraints for a hard/hard secant pile. To overcome this problem, Murphy International Piling proposed to adopt the Irish practice of using very low strength primary pile concrete, as a hard/firm secant pile solution. Typical concrete cube strength for primary piles in the UK is 15-20N/mm 2 with a corresponding typical depth limit of 17-20m. The need to secant down to below the 24m excavation depth is considerably beyond this limit. Irish practice is to use lower cube strength primary pile concrete, 5-10N/mm 2. Setting each secant pile around the perimeter of a circle results in an unbalanced cut for the secondary piles with a larger cut on the inside face than on the outside face, leading to a tendency for the secondary piles to drift outside the circle and out of tolerance. To reduce the risk of this occurring, each secondary pile was set out on a straight line between the adjacent primary piles ensuring a balanced cut into the primary piles. The circle was formed with a series of such straight lines, with the secondary piles set out on a slightly smaller radius. Figure 1 Diagram showing preferred hard/firm secant pile option for shaft construction. 4.0 DEWATERING The dewatering requirements to provide a relatively dry and stable excavation to 24m below ground level provided its own challenges. In addition to the 18.5m diameter shaft there were adjacent 6m and 4m diameter shafts with excavation depths to 20m and 16m respectively. These also required dewatering. The team needed to consider the varying strata containing water including a perched water table, along with the anticipated pressure and recharge from the underlying chalk layer. Two local abstraction wells for both private industry and supply to a local reservoir provided an additional challenge along with reducing anticipated settlement to local ground and third party property adjacent to the site. The contract was further constrained by a requirement to limit abstraction rates to 200l/s as this was considered an acceptable maximum rate for disposal on site to the treatment works and nearby river. Initially a proposal to utilise deep chalk wells depressurising the entire site from water recharge offered by this layer was considered. However, the expected abstraction rates and effects of wide ranging settlements eliminated this option as a solution. In place of a deep well system the team opted for a shallower 35m deep system which depressurised the dense Thanet sands close to each shaft and also to dewater directly from the tricky Lambeth

4 Group/Upnor Beds that recharges from local aquifers. To assist with draw down from directly beneath the centre of the 18.5m diameter shaft the team opted to install the primary piles to a toe level 4m higher than the adjacent secondary piles. This allowed vertical windows between the piles through the Thanet Sand, thereby draining more effectively over the shaft footprint beneath the proposed shaft invert level. This also had the added bonus of reducing the primary pile depth and resulted in material and time saving to the contract. Base stability was also a primary consideration and elimination/reduction of risk of an unstable formation was paramount to the shaft success. Monitoring wells and monitoring pile installation allowed the team to become confident in the final solution of 35m deep wells at 2m centres around the perimeter of the main shafts. Yield rates from the shallow system were in the region of 0.2 to 0.5l/s per well, considerably reducing the abstraction rates allowing comfortable discharge rates on site. Given the low abstraction rates and considerably small zones of influence in the very dense sands the effects on settlement proved to be minimal. 5.0 TEMPORARY WORKS A reinforced concrete guide wall was cast at ground level, carefully set out to a 5mm tolerance to minimise the positional tolerance at the top of the shaft. The internal face of the guide wall was set out 130mm outside the theoretical internal face of the secant pile shaft to allow for verticality tolerances whilst drilling each pile and to ensure that the minimum required internal diameter was achieved at the top of the base slab. The outside face of the guide wall was set at 1205mm diameter from the internal face to allow the 1180mm diameter casing to pass through the guide wall with sufficient tolerance. A circular shaft ideally works in ring compression to avoid the need for any temporary propping. The degree of interlock achieved by a secant pile circular shaft theoretically reduces with depth as the positional tolerance due to verticality increases. At the same time the horizontal pressures due to soil and water pressures increase. The limiting depth at which the shaft was able to theoretically work in ring compression was calculated at 12m, balancing a realistic secant pile overlap with the low strength of the primary pile concrete. For excavation below this depth, some additional support was needed in the form of concrete ring beams. Three temporary concrete ring beams were required to achieve the full excavation depth of 24m. The first ring beam was cast between 11m and 12m depth and two further concrete ring beams were provided at 16m and 20m depth. Figure 2 Waling beam arrangement incorporated into the permanent internal lining. The secant piles were designed to span vertically between the concrete ring beams to support the additional soil and water pressures. The resistance to the horizontal pressures was designed using a combination of the theoretical available ring compression from the secant pile overlap and the internal ring beams. At the top of the shaft, a number of pipes passed through the secant piles, interrupting the ring compression over the top 6m depth of the shaft. The secant piles were designed to span vertically in cantilever over this depth, supported by the ring compression in a zone

5 immediately below the pipes. The secant piles immediately below the level of the pipes were designed to support the additional ring compression from the cantilever piles above. The secant piles formed the temporary support with the permanent support provided by an internal concrete lining wall. This was cast from the bottom of the shaft using a single face internal shutter and required a high quality formed finish. Rather than using steel temporary waling beams, part of the permanent lining wall was utilised as the temporary ring beam. There was a concern about the ability to construct the required quality finish to the internal lining wall during the excavation stage. This concern was overcome by reducing the thickness of the concrete ring beam by 125mm such that the inner face could be cast later during the construction of the lining wall, ensuring a high quality finish to the internal face in the permanent case. The weight of the secant piles was mobilised in the permanent works to resist flotation. In order for the primary secant piles to be sufficiently durable to form part of the permanent works, there was a minimum specified concrete compressive strength of 10N/mm 2 (cube strength). The concrete mix had to balance the demands of lower strength against chemical resistance. Therefore, the mix design incorporated a new and innovative blend of cement that uses 90% ground granulated blast furnace slag (GGBFS) replacement and admixtures to ensure a maximum compressive strength of 10N/mm 2 at fourteen days, while increasing the cement content to enable the chemical classification to be met. The resulting concrete mix for the primary piles was carefully controlled to ensure that this minimum long term strength was achieved without gaining too much strength in the short term and restricting the ability to drill the secondary piles through the primary piles. 6.0 PERMANENT WORKS The design of the permanent works was carried out in accordance with the appropriate British Standards and Eurocodes. Where appropriate, the permanent works design was carried out to incorporate the temporary works designs for the shaft construction. The main elements of the permanent works design included the shaft base slab, internal shaft lining and internal dividing walls forming the wet well / dry well arrangement. Flotation was a key consideration for the short term and long term stability of the shaft. For the long term condition, the design of the shaft considered a worst case water level at ground level to take into account any potential increase in the future ground water system, as well as any short term flooding of the site, which is located adjacent to the nearby Lee Reservoir. In the temporary case, the design calculations considered the various stages of the construction process in order to determine the required maximum ground water level in order to minimise the draw down required by the dewatering system, thus providing cost and time savings to the project. Figure 3 Construction of Shaft Internal Lining. The assessment of restraint against flotation included for the weight of the secant piles, internal lining, well walls and base slab. In addition to the dead weight of the shaft works, further restraint was taken from the frictional effects between the shaft construction and the surrounding ground. Flotation was the principal consideration in determining the required thickness of the 3.5m thick base slab. The base slab itself was connected into the shaft lining

6 by high strength macalloy steel dowel bars drilled and grouted into the hard secondary secant piles to provide the required shear restraint. The dowels were not connected into the firm secondary piles due to the weaker concrete strength. Once completed, the base slab would act as a base for constructing the cast insitu internal lining. In order to ensure a water tight shaft, and allowing for water ingress between the interlocking secant piles, the internal lining was designed for full hydrostatic pressure in the permanent case. The minimum required structural thickness required for the internal lining was 600mm. However, in order to incorporate the construction tolerances for the secant pile walls and to achieve the minimum required 18.5m internal shaft diameter for the shaft, the setting out of the shaft and design tolerances were such that the internal lining could in practice be thicker if required, depending on the practicalities during construction. The internal dividing walls, which form the wet well / dry well arrangement within the shaft were designed incorporating reinforcement connecting into the internal shaft lining to provide a rigid continuous connection. The internal walls were designed as 600mm to 650mm thick. 7.0 CONSTRUCTION DETAILS 7.1 Rig and Guide Wall During design it was decided by Murphy International Piling that a Bauer BG40 Rig was required to bore the piles. The rig is capable of providing 40Tm torque, which was able to handle the very dense Thanet sands in addition to the 10N/mm 2 strength required from the adjacent primary piles. The Deephams project benefitted from a brand new rig, which proved an exceptional addition to the project plant for its duration on site. The first item to be constructed was the Guide wall. The team opted to use a large 1.205m diameter polystyrene void former accounting for a pair of one primary and one secondary pile with the overlap cut out of each pair. Figure 4 Guide Wall Construction with Polystyrene void former. 7.2 Piling Following casting of the guide wall a piling mat was constructed inside the shaft ring. This was mainly due to site constraints and meant the rig and piling operations were kept with the confines of the shaft. A ballast ramp was constructed to enable the rig entry to the centre of the shaft to complete construction of the seventy-six 1.2m diameter piles. These were constructed with a 320mm interlock (allowing for construction tolerances). Due to the tight project programme, Murphy wanted to cut the original estimated programme by five weeks in order to provide a ten day curing window for the first waling beam during the Christmas shutdown (2010). This meant the team required a minimum of five piles to be cast every week compared with the initial programme of three to four piles a week. In order to facilitate this accelerated programme, consideration was given to procuring a second rig. However, given the site physical constraints this proved impossible. Another means of accelerating the program which was adopted included procuring a second set of pile casings allowing continuous boring to be undertaken such that while one pile was being poured another pile was being bored.

7 The piling sequence required, as standard, to advance a section of casing by 5m and bore the inside of the casing removing spoil. This was then repeated until 1m above the bottom of the London Clay. The process of advancing the casing and boring the contents continued until the required 25m or 29m bores were completed. The team then lowered the piling cage into place in the case of a secondary pile followed by concrete pouring using a tremmie tube to the bottom of the bore. Concreting was completed on a continuous basis with the casing being extracted in stages of 5m sections following concreting past the associated elevation. Once the concreting had reached the top of the bore the last section of casing was removed and the pile finally topped up with concrete and left to cure. This sequence was carried out in an arrangement that saw the first eight primary piles cast then the secondary pile between the first two primary piles being bored and cast. This sequence of a primary followed by a secondary pile continued around the circumference of the shaft. Figure 5 Piling Rig and Casings. For the low level piles at the piped sections of the shaft the piles were cast to the desired lower level plus 1m and allowed to cure with the remaining top section of bore filled with gravels which were removed during the shaft excavation leaving the required low level sections without the need to breakout large sections of piled wall. The reinforcement required for the secondary piles demanded a thorough review from a constructability aspect. There were six different pile cages required to account for the pipe cut outs at different levels and the stronger cantilevering piles adjacent to the low level cut outs. The team also intended procuring a GFRP section in two pile cages to account for the low level tunnel entry into the shaft facilitating the future Tottenham Low level Sewer. This was subsequently removed accounting for tunnelling preferences and the team reverted to a standard steel reinforced cage for the tunnel section. The secondary pile cages measured 29m in length and required to be fully fixed together prior to installation into the bore. This was completed using two prefabricated sections constructed off site and lapped together on site. The team adopted 100mm x 10mm thick steel plate links welded to the cage at the lapping sections to give a more robust lapped connection to facilitate lifting vertically before being lowered into the pile bore. 7.3 Excavation and permanent works construction Following completion of the piling and sufficient time for curing the last piles the construction team commenced excavation. The main deadline was to cast the first waling beam before the Christmas break thus allowing curing of the waling beam to take place during the Christmas shut down. The waling beams were cast using a circular shutter system acting in ring compression thus requiring minimal shuttering props. As the team opted to cast a section of the permanent lining wall as the quasi permanent waling beam the rear reinforcement was fixed into the back of the waling beams with reinforcement couplers on the bottom of the bars to allow the permanent lining wall reinforcement to be tied in. With successful completion before Christmas of this waling beam the team advanced the excavation to the remaining waling beam levels casting and curing the remaining two waling beams in the New Year. Unfortunately during this excavation there was excess water in the Thanet sand which was subsequently found to be recharging through a poorly decommissioned exploratory borehole. This meant the team needed to install four further

8 abstraction wells within the shaft which affected the speed of excavation from the halfway point. The team finally complete the excavation to formation and quickly set about fixing the reinforcement and dowels for the base plug. The team opted to pour the base plug in one deep pour (3.5m) requiring two duty pumping rigs at surface and supply from four nearby concrete batching plant a total pour of 1200m 3 was completed in twelve hours. A detailed survey of each pile was carried out and it was found the 80% of the piles were installed effectively vertical, while the remaining 20% had small deviations in verticality well within tolerance and in an outward direction meaning that the 130mm set out line was not breached. This allowed the construction of an 18.74m internal diameter shaft instead of the initial 18.5m. Following the base plug pour the same shutter used for the waling beams was rebuilt and used to complete the seven 3m lifts for the permanent 600mm thick concrete lining. Murphy adopted an innovative access system of mast climbers which facilitate full access around the circumference of the shaft allowing ease of fixing and pouring the lining wall with minimum internal temporary supports or walkways. The final stage of the major works in the shaft structure was to complete a set of internal walls thus creating a central 12m x 5.5m wet well in the centre of the shaft. This was complete in three lifts using traditional shuttering methods. 8.0 CONCLUSIONS In summary, the design and construction of the 24m deep Pumping Station Shaft excavation for the Deephams STW Inlet Improvement Scheme has utilised secant pile technology. This is one of the deepest secant piled shafts constructed, and has been built in challenging ground conditions. In order to complete the shaft construction successfully, the project team utilised a number of innovations. These included: Use of a 1m deep concrete guide wall, laid to 5mm accuracy The guide wall set the primary and secondary piles on two radii, with the secondary piles 65mm inside that of the primary piles to minimise construction tolerances on the verticality of the pile installation. Use of lower strength concrete (10N/mm 2 ) for primary secant piles, making it easier to drill the secondary piles. This concrete mix for the primary piles utilised a blend of cement with GGBS replacements and admixtures to a 10N/mm 2 maximum strength at 14 days, while increasing the cement content to resist the high sulphate content and chemical classification of the surrounding ground. Installation of 4.2t, 29m long pile rebar cages. Steel plate links welded to the cage were used to give a more robust lapped connection for lifting and installation. With a final dig of 24m and a high water table, a Thanet Sand dewatering system was utilised as an effective alternative to the traditional deep chalk well dewatering system. To further enhance the dewatering capabilities of this system, the length of the primary piles were shortened by 4m compared to the harder secondary piles to allow the dewatering system to be more effective in drawing the water down from within the footprint of the secant piled shaft below formation level. Instead of installing conventional temporary steel waling supports; cast insitu concrete waling beams were used for the temporary condition. These were then incorporated into the permanent internal lining design. The Deephams project team have worked closely together to identify innovation in the design and construction methods to ensure the successful completion of the pumping station shaft. These innovations can be replicated across the industry where appropriate to provide added value and risk mitigation for other shafts constructed using secant piling techniques.