Proc. Instn Civ. The Mersey Estuary Pollution Engrs Wat., Marit. &Energy, 1999, Alleviation Scheme: Liverpool 136, Dec., 171±183 interceptor sewers Paper 11851 Written discussion G. N. Olsen, CEng, FICE,M.F.Danbury,BSc, CEng, FICE, FGS and closes 31 March 2000 B. Leatherbarrow, BSc, CEng, MICE, MCIWEM & This paper describes the planning, design Geology and construction of the principal Liverpool 5. The solid geology of the east bank of the elements of the Mersey Estuary Pollution Mersey is composed of Sherwood sandstone. Alleviation Scheme. It excludes the This is overlain by glacial sand and gravel, primary wastewater treatment works at glacial till, marine and estuarine alluvium and, Sandon Dock which were separately ®nally, made ground, mostly associated with designed and managed by North West the construction of the dock system, but Water (NWW). The planning of the project including major former waste disposal sites, was originally undertaken by Liverpool Mersey Tunnel construction arisings, and the City Engineer as agent for NWW, and landscaping for the International Garden Festi- includes major tunnels, combined sewer val. over¯ows/storm storage chambers, outfall 6. The sandstone is typically reddish- penstock chambers, and a reactive real brown, but sometimes white, yellow or light time control system. The design and con- grey, varying in strength from zero to approxi- struction covers a period of major man- mately 50 MN/m2. The origin of the rockhead agement and technological change within surface is believed to be glacial erosion by the water industry. glaciers travelling from north-west to south- east in an ice-way following roughly the line of Keywords: environment; sewers & drains; the present Mersey estuary rather than by Graham N. Olsen, Principal/Project tunnels & tunnelling preglacial ¯uvial action. The rockhead surface Engineer, Bechtel is irregular and the line of the main interceptor Water, Warrington tunnel is crossed by a number of buried valleys Background and early planning causing the rock boundary to drop below the The history of the Liverpool drainage systems, crown of the tunnel and thus presenting mixed and the initial planning options for this project, face tunnelling conditions (Figs 1 and 2). 1 have been summarized in an associated paper. 7. The upper layers of the sandstone are The project addresses the former continuous frequently weathered, forming a transition zone discharge of raw sewage to the Mersey from a between competent rock and uncemented sand. large part of the Liverpool conurbation. The overlying material is glacial sand and 2. The minimum overall scheme objectives gravel and/or till, both of which contain were to boulders, many of igneous origin. The till is typically `sti' or `very sti' silty clay (`boulder MichaelF.Danbury, (a) ensure that the estuary water should at all clay'). formerly of Bechtel times contain a sucient level of oxygen to Water, Warrington 8. The Sherwood sandstone is a well-estab- obviate odour nuisance lished aquifer, and groundwater levels are (b) obviate the fouling of the estuary foreshore generally above tunnel level, having risen in and beaches by crude sewage or solids or recent years due to reduced abstraction. In fats from industrial euents.2 addition, there are perched water tables in the 3. In 1974, it was concluded that the objec- made ground above the glacial till. tives could be achieved with primary treat- ment,3 and with ecient stormwater over¯ows Site investigations passing forward modi®ed formula A ¯ows to 9. Some investigations were carried out for interceptor sewers.4 Liverpool in 1978/79 in connection with a Bruce Leatherbarrow, 4. In 1981, agreement was reached with the proposal to carry out tunnelling work in Oce Manager, Mersey Docks and Harbour Company (MDHCo) advance of inner ring road construction. In Halcrow Crouch, and with the planning authority that a treat- 1983, a further investigation was undertaken. Aberdeen mentworkssiteatSandonDockwouldbe 10. Stage-speci®c investigations were (All authors were acceptable. A 15-year programme was initiated, carried out progressively between 1985 and formerly of Liverpool with detailed design work for the tunnels, 1991 under the auspices of a geotechnical City Engineers storm sewage over¯ows and outfall penstock consultant. The main objectives were to estab- Department and chambers commencing in 1984. lish rockhead levels, rock strength properties, NWW Engineering) 171 OLSEN ET AL. groundwater conditions, and the nature of the approaches, it has proved possible to recalcu- Fig. 1. Liverpool and overlying glacial till material. Interpretive late the settings using formula A with current Sefton interceptor reports were produced advising on the nature of consumption ®gures. sewerÐlongitudinal the ground expected in the tunnelling works section and on the likely requirements for excavation support and de-watering for the major struc- Combined sewer overflows (CSOs) tures. Predictions for tunnel settlement were 15. The CSOs are designed to separate out also produced. any visible solids and ¯oating material that would otherwise pass to the river during storms. Fig. 4 shows an isometric view of a Overall design considerations typical rectangular CSO and outfall penstock 11. The site investigations were comple- chamber arrangement, with a control gate and mented by an overall investigation into the calibrated vortex connection to the tunnel. sewer routes, including historical dock, tunnel 16. In 1975, a literature survey was under- and wartime records. taken concerning the design of storm over- ¯ows.8 This survey, together with subsequent Tunnels research, led to the conclusion that a new 12. The investigations highlighted several approach should be adopted for the design of problem areas, including old dock temporary CSOs for the Mersey Estuary Pollution Allevia- back-drain systems used during the deepening tion Scheme (MEPAS) project, where the inter- of docks by underpinning,5,6 old ventilation ceptor sewers would in some cases be at a headings above the underground railway (one considerable depth relative to certain outfalls. constructed with a Beaumont-English machine7 Additional head losses caused by the construc- (Fig. 3)), and the constructional details of tion of the CSOs were also to be minimized so Stanley Dock Passage. as not to increase the risk of ¯ooding. The 13. An early decision was made to preclude CSOs are designed to pass the set ¯ow to the the use of compressed air (unless its use was interceptor via a calibrated vortex controlling essential) when groundwater levels were found the CSO gate. Once the setting is reached, the at or about 10 m above the proposed tunnel CSO starts to ®ll, and the hydraulically level. There was the possibility of the back- powered gate opening reduces under the drain connections providing a leakage path increasing head. between tunnel level strata and both dry and 17. Modelling work was undertaken at wet docks, and major cost and health implica- Liverpool University, leading to the develop- tions if pressures above 1 bar were needed. ment of two types of no-weir CSO (circular 14. The interceptor sewer tunnel route is shaft and box shaped). The modelling was shown in Fig. 1, plan, and Fig. 2, section. The undertaken using the Sharp±Kirkbride stilling ¯ows to be conveyed were based on over¯ow pond as a standard against which eciencies settings using modi®ed formula A4 and water were judged.9 The resulting designs were con- and population projections to 2002. As that date siderably more ecient than previous designs 172 MERSEY ESTUARY POLLUTION ALLEVIATION SCHEME at removing and detaining both solids and ¯oating material, and also had sucient capa- city to meet the calculated ®rst ¯ush volumes.10,11 18. The deep circular CSO is designed for con®ned sites with a depth to the interceptor favouring construction by underpinning with precast segments. The hydraulic design princi- ples are similar to the rectangular type, but it has an inclined section to the main bae, together with a second closely spaced bae to create a long path for re-entrained ¯oating material. A second ¯oating material dead zone improves performance even further (Figs 5(a) and (b)). 19. Ecient CSOs obviate the need for storm screens, reducing the additional head losses on the outfalls. 20. Modelling work was undertaken by Liverpool John Moores University to develop the method of controlling the pass-forward ¯ows from the CSOs; in some cases the head within the deeper chambers ¯uctuates by as much as 10 m depending upon storm and tidal conditions. It was found that vortex drops could be calibrated with an almost linear relationship between depth and ¯ow, but that a stilling chamber was required immediately downstream of the gate to be controlled in order that hydraulic jumps did not occur in the vortex approach.12 (See } 35 and 36.) 21. CSO sizes range up to 12 m wide 6 15 m deep 6 25 m long. Referring to Figs 4, 5(a) and (b), dry weather ¯ow leaves both CSO designs via a normally open control gate, and proceeds through open channel ¯ow within a culvert to the vortex connection to the tunnel. During storms, ¯ow rates approaching the set pass- forward ¯ow are detected at the vortex, and the Fig. 2. MEPAS location, route, stages and values control gate begins to close to limit the ¯ow. This results in a rise in the level within the CSO, and the detention of ¯oating material at the rear of the rectangular type of CSO, or in the ®rst and second zones of the circular type. Material with a speci®c gravity greater than 1 moves preferentially to the controlled outlet rather than the outfall (with varying eciency depending upon the rate of storm ¯ow). As ¯ows subside at the end of the storm, ¯oating material trapped in the chamber is passed to the controlled outlet.