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Groundwater control around a large basement M.C. Ervin and J.R. Morgan Abstract

Groundwater control around a large basement M.C. Ervin and J.R. Morgan Abstract: Melbourne’s Crown Casino was constructed on a site bordering the Yarra River and underlain by problem soils of the Coode Island Silt Formation. The development needed to provide two levels of basement car park over the entire site. An innovative approach to groundwater control around the excavation was required to avoid depressurisation of adjoining soils, leading to settlements. Analysis showed a conventional bentonite cut-off wall would still allow de- pressurisation by lateral flow through the Coode Island Silt during the construction period. The high cost and construc- tion difficulty of a very low permeability wall mitigated against it. An hydraulic wall was proposed in conjunction with a conventional cut-off wall. This comprised a curtain of wick drains surrounding the cut-off wall and charged with wa- ter. Control of seepage through an underlying aquifer by a cut-off wall was considered, but a more cost-effective method using recharge by wells was adopted when shown necessary. Monitoring of groundwater pressures around the site showed that the maximum change in water pressure was less than 1 m head, the design criterion. Part way through construction, recharge was initiated when monitoring of the deep aquifer showed pressure reduction attributed to verti- cal leakage through a basalt tongue. Key words: excavation, basements, groundwater, clays, settlement, monitoring. Résumé : Le Crown Casino de Melbourne a été construit sur un site en bordure de la rivière Yarra et repose sur des sols problématiques de la formation Coode Island Silt. Le développement nécessitait la construction de deux niveaux de sous-sol sous l’ensemble du site. Une approche innovatrice pour le contrôle de l’eau souterraine autour de l’excavation était requise pour éviter la dépressurisation des sols adjacents résultant en des tassements. Un analyse a montré qu’un rideau de bentonite conventionnel permettrait tout de même une dépressurisation par écoulement latéral à travers le silt Coode Island durant la période de construction. Le coût élevé et les difficultés de construction d’un mur de très faible perméabilité plaidait contre cette solution. Un mur hydraulique a été proposé conjointement avec un mur rideau conventionnel. Cette solution comprenait un rideau de drains minces foncés entourant le mur rideau et chargés d’eau. Le contrôle de l’infiltration à travers un aquifère sous-jacent a été considéré, mais une méthode plus économique consistant en une recharge hydraulique par des puits a été adoptée lorsque nécessaire. La mesure des pressions d’eau souterraine autour du site a montré que la variation maximum de la pression d’eau a été inférieure à 1 m de tête d’eau, le critère de conception. En cours de construction, la recharge hydraulique a été initiée lorsque les mesures dans l’aquifère profond indiquaient une réduction de pression attribuée à une perte d’eau à travers une languette de basalt. Mots clés : excavation, sous-sols, souterrain, argiles, tassement, mesures. [Traduit par la Rédaction] Ervin and Morgan 740 Introduction The Crown Casino in Melbourne was constructed on the south bank of the Yarra River, a prominent site in South Melbourne adjacent to the Central Business District. Previ- ous developments on the site had been limited to low-level structures because the poor ground conditions made high-rise construction costly. During the 1980s the State Government gained control of the site and established the Victorian Casino Central Author- ity to facilitate development of Victoria’s only casino. The licence for this casino was awarded to Crown Limited for a major development incorporating a four-level podium build- ing comprising the casino and ancillary facilities, a retail section, a cinema complex, a 40 storey hotel structure, and a two-basement car park. This paper describes the groundwater aspects of the tech- nical problems involved in the design and construction of the basement. The excavation extended about 4.5 m below the groundwater table in soils which from previous experience were known to be susceptible to settlement when the groundwater table was lowered. From a knowledge of the ground conditions, the principal routes for leakage into the site were identified. For one of these, an innovative method of isolating the site from its surroundings by use of an “hy- draulic wall” was developed. This, in conjunction with mon- itoring of groundwater levels outside the site and installation of groundwater recharge wells, allowed the work to be com- pleted without noticeable effect on the surroundings. The proposed casino basement was to be the first major excavation in the Coode Island Silt since construction of the nearby Victorian Arts Centre. During that project, settlement occurred in a significant area of South Melbourne due to de- pressurisation of the Moray Street Gravels aquifer and con- sequential consolidation of the Coode Island Silt. This took place despite attempts to manage this potential problem, and Can. Geotech. J. 38: 732–740 (2001) © 2001 NRC Canada 732 DOI: 10.1139/cgj-38-4-732 Received June 13, 2000. Accepted January 30, 2001. Published on the NRC Research Press Web site at http://cgj.nrc.ca on August 7, 2001. M.C. Ervin1 and J.R. Morgan. Golder Associates Pty Ltd., 50 Burwood Road, Hawthorn, Victoria 3122, Australia. 1Corresponding author (e-mail: mervin@golder.com.au). litigation initiated by affected property owners resulted. Therefore control of groundwater was of paramount impor- tance for the casino project. Subsurface conditions The casino site is located in the Yarra River Delta and is predominantly underlain by Quaternary-age sediments. These sediments evolved at the head of Port Philip Bay dur- ing phases of sedimentation and erosion associated with sea level changes. The delta sediments rest on Silurian-age siltstone and sandstones. In the northeastern part of the site basalts of the Newer Volcanics are also present. Figure 1 shows a plan of the site, and Fig. 2 is an approxi- mate long section which indicates the main strata. The delta sediments at the site comprise the following: (1) Highly compressible silty clay of soft to firm consis- tency—These clays are known as Coode Island Silt, a widespread, Holocene-age deposit in the Yarra Delta, long recognised as a difficult material to deal with be- cause of its low strength and high compressibility. These weak clays extend to about 20 m depth at the site. (2) Stiff to very stiff silty clay known as Fishermens Bend Silt—This sediment varies in thickness at the site from 2 to 6 m. (3) Moderately dense to very dense sands and gravels known as the Moray Street Gravels—Extending throughout the Yarra Delta, these sediments are a known aquifer of low salinity. Because of their high hydraulic conductivity they tend to have a major influence on groundwater behaviour in the area. The engineering properties of the Coode Island Silt and Fishermens Bend Silt have been discussed by Ervin (1992), and the hydraulic properties of the Moray Street Gravels are given in Lane et al. (1992). The basalt in the northeastern part of the site underlays the Coode Island Silt at depths of from 3 to 15 m and ex- tends to a planar base at about 19 m depth. The basalt is © 2001 NRC Canada Ervin and Morgan 733 Fig. 1. Site plan. Fig. 2. Geological long section. moderately to slightly weathered and of high to very high strength. It is extensively fractured and jointed. The Silurian siltstones and sandstones which form the basement rock in the area occur at depths of around 30–35 m, being deepest in the southwest. They are generally steeply dipping and range from extremely weathered and low strength at the rock surface to slightly weathered and medium to high strength at depth. Groundwater investigations A study of the geological profile and its regional setting identified the two permeable strata which would control seepage into the excavation. These were the Coode Island Silt, which showed frequent horizontal sandy lenses in its upper levels, and the Moray Street Gravels. The latter de- posit extends for a considerable distance beyond the site, and underlies Coode Island Silt and other deposits. Because of its high permeability, changes in pressure within the Moray Street Gravels can be transmitted well beyond the source of those changes. In addition to the investigations aimed at providing design data for the other geotechnical requirements, specific investi- gations were performed to provide data on aspects relating to the groundwater problem. These investigations comprised the following: (1) Groundwater levels were monitored at different depths in the strata by separate standpipes sealed into observa- tion bores. (2) Piezocone and electric friction cone penetrometer tests were performed at the groundwater study locations prior to installation of bores to assess the presence and extent of sand layers within the Coode Island Silt and Fishermens Bend Silt. (3) Rising-head tests were carried out in four boreholes drilled in the Coode Island Silt and in two drilled in the basalt. (4) Short-term (24 h) pump tests were carried out in two pump wells, surrounded by observation wells in the Coode Island Silt. (5) Laboratory measurements of vertical hydraulic conduc- tivity were made on six undisturbed samples of silty clays from the Coode Island Silt. These samples were taken from boreholes drilled on the site. The tests were performed as constant-head triaxial permeability uploads/Ingenierie_Lourd/ groundwater-control-around-a-large-basement.pdf