Deep FoundationsContractors Inc. Installs SecantPile Cutoff Wallnear Toronto Waterfrontby Dave Bradshaw, P.Eng., Chief Estimator

Deep Foundations Contractors, Inc.

Overview

The Government’s recent initiative torevitalize Toronto’s waterfront has pre-sented several challenges to conven-tional drilling methods.

One waterfront project in particularpresented several unique geotechnical and construction con-straints. When George Brown College decided to build a newstate of the art campus in the heart of Toronto’s waterfront, theirdesign called for the excavation of four levels of undergroundparking at the edge of Lake Ontario. Funding was provided fromevery level of government, with the Provincial government con-tributing 61.5 million and the Federal government contribut-ing 30 million dollars. Federal funds were part of Canada’s twobillion dollar economic stimulus package and were contingenton project completion by October 2011. This schedule trans-lated into an aggressive construction timeline and a short con-struction window for the excavation support system installation.

ADSC Contractor Member firm, Deep Foundations Con-tractors Inc., was awarded the installation of a water-tight ex-cavation support system.

Our challenges included:

• Provision of a water-tight temporary cut-off wall to providea dry subgrade 50’ below the water table;• Design of an efficient and economical earth retention sys-

tem that could retain the large hydro-static pressures from LakeOntario; and• Completion of all drilling within an aggressive window

(400 piles in 60 days)

Why a Cut-off Wall

The project site sits over reclaimed land along Toronto’sshoreline. In the 1950’s the City of Toronto extended the south-ern boundary of the city by constructing a seawall in Lake On-tario and backfilling the area with fill from various construction

projects. The land was used as a port and industrial area for 30years. This type of land use resulted in some contaminated soiland any dewatering was considered costly to the overall project.In addition, dewatering was deemed ineffective due to the soilconditions; silty clay, with fill, organics and some sand layers.The zone of influence was too small per well-head and the de-watering contractor could not guarantee that the soil would bedry or stable enough for equipment to work on it.

Since dewatering was not viable, a temporary and water-tight

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Since dewatering was not viable, a temporaryand water-tight cut-off wall was required to fa-cilitate a dry excavation for the construction ofthe underground parking structure. A secantpile cut-off wall (SCW) was chosen to performthis function.

The Government’s recentinitiative to revitalizeToronto’s waterfront haspresented several challengesto conventional drillingmethods.

30 meters away from Lake Ontario.

cut-off wall was required to facilitate a dry excavation for theconstruction of the underground parking structure. A secantpile cut-off wall (SCW) was chosen to perform this function.

Method of Cut-off Wall Installation

Deep Foundations Contractors Inc. has been effectively con-structing SCW shoring systems along Toronto’s waterfront forover 25 years. The procedure for installing these shoring wallshas evolved in response to tightened city bylaws.

In the 1980’s and 1990’s, SCWs were installed using cranemounted vibratory hammers to install the casings, combinedwith conventional track-mounted drills for augering. In 2003 aCity of Toronto Bylaw was passed which virtually prohibited the

use of vibratory hammers for shoring work by private develop-ers. This prohibition on vibratory hammers was not due to thepotential for damage to neighbouring buildings, but to respondto the perception of damage by the general public. Basically, thevibrations are similar to the nuisance of noise. People can feel avery low level of vibrations and complain due to both loss oftime and perceived damage to homes. The effect was for the Cityto deny building permits if vibratory hammers were to be usedon the project. Therefore we needed a new method to installSCWs in the Toronto area.

In response, Deep Foundations Contractors Inc. began usingthe sectional casing method. This method uses hydraulic rotarydrill rigs to both advance and drill casings without the need forvibration. First, rotating the casing into the ground with a largeamount of torque, and then reaching over top of the casing withthe high drill rotary and removing the soil. As the George Brown

College project was situated directly adjacent to the TorontoHarbour, it was a perfect candidate for the sectional casingmethod.

Design of Secant Cutoff Wall

Initially the owner, George Brown College and the construc-tion manager, EllisDon Corpora-tion retained Isherwood Geo-structural Engineers* to designSCW shoring. This design wasthe basis for tendering purposes.Deep Foundations ContractorsInc., in consultation with Isher-wood, developed an alternativeSCW shoring proposal. DeepFoundations Contractors Inc.was eventually awarded theshoring contract based on this al-ternative design.The project site is situated ap-

proximately 30m (100ft) from anexisting and very sensitive sheetpile dock wall on reclaimed

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SECANT WALLS Contd.

The effect was for the City to deny buildingpermits if vibratory hammers were to be usedon the project. Therefore we needed a newmethod to install SCWs in the Toronto area.

Section of guide wall with drill rig in the background.

waterfront land. The subsurface conditions consist of hydraulicfills and sand deposits overlying weathered and sound shalebedrock interspersed with limestone layers. The existing gradeat approximately 77.0m geodetic (253 ft) is only 2.0m (6.5 ft)above the average lake level of 75.0m.

Our design consisted of 132 soldier piles and 267 filler piles.The soldier piles were reinforced with W690X125 (W27X84)wide flange beams spaced at 2.25m (7.4 ft) centres with twofiller piles in between (as shown in Typical Secant Pile Cutoffwall layout). The diameter of both soldier piles and fillers pileswas 1000mm(40in) with an average depth of 15m (50 ft), sock-eted into sound bedrock. The bedrock stratigraphy in the areaof this project was peculiar in that rock fissures extend deeperinto the formation than realized in other areas of the downtowncore; therefore, the filler pileswere also installed to the samedepth as the soldier piles to helpseal off any water comingthrough these rock fissures. Se-cant pile overlap of 250mm(10”) was implemented to en-sure adequate interlock withdepth. To support the walls, atotal of 248 rock anchors wereinstalled on 2 levels.

Quality of Work

Since the specifications re-quired a completely water-tightshoring system, the temporarycut-off wall had to have water-tight joints between the: secantpiles, tieback connections, and

the interface between the underside of pile androck.Any misalignment of secant piles or breaches in

the shoring wall would be detrimental given theclose proximity to Lake Ontario and the large hy-

drostatic pressures being exerted on the shoringsystem.

To ensure proper pile overlap and alignment, acontinuous guide wall was constructed prior to pileinstallation. Great care and accurate layout duringthe construction of the guide wall was the first stepin ensuring a quality SCW. Contour forms were

used to build the guide wall around the perimeter of the job site.The guide wall was approximately 600mm (2 ft) thick using re-inforced 20MPa (2900 psi) concrete. The temporary casingswere installed through the guide wall and checked with digitallevels after each section of casing was added. If the casingsstarted to wander off vertical, they would be brought back tovertical using the guide wall as a fulcrum pivot point.

Schedule, Schedule, Schedule …

The project schedule was aggressive with the shoring and ex-cavation work being conducted throughout the winter monthsto facilitate concrete work on the structure in the spring. Tales

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Any misalignment of secant piles orbreaches in the shoring wall wouldbe detrimental given the close prox-imity to Lake Ontario and the largehydrostatic pressures being exertedon the shoring system.

SECANT WALLS Contd.

Tieback drill rig on timber mats.

Bauer 40 pushing casings into soil.

of igloos and sub-zero temperatures might be used to describeother cities in Canada but not Toronto; construction does notstop (or even slow down) during the winter season. During thewinter months, the average pre-dawn temperature is -15C (5 F)and as the sun rises the temperature hovers just slightly belowfreezing. A snowfall of more than 10 cm (four inches) is not un-usual.

Two drill rigs, a Bauer BG40 and Bauer BG28*, were employedfor the soldier pile and filler pile installation. To advance theschedule and ensure on time completion, a night shift wasadded to the rotation. This night shift installed filler piles only,

thus avoiding any hoisting of steel reinforcingat night. During the night shift only one ma-chine operated, allowing for a maintenancecrew to work on the other equipment. The in-clusion of a maintenance shift at night mini-mized the amount of down time due toservicing of equipment, and maximized day-time operation.

Soldier Pile / Filler Pile Installation

Sectional casings of various lengths were ad-vanced through the loose saturated soil andsocketed into bedrock. The drilling spoil wasremoved from the casings using augers anddrilling buckets. The drilling bucket or augerwas never advanced below the bottom of thecasing to prevent any loss of ground and tominimize the amount of water within thedrilled shaft. To seal the pile excavation fromthe inflow of water, the casings needed to beadvanced through fractured rock and into the

sound shale. At some locations the pile excavation had to be ad-vanced below the design elevations. This method sealed off fis-sures in the rock at greater depths to allow placement ofconcrete in dry conditions. The pile concrete design strengthwas 8MPa minimum (1200 psi). To ensure adequate flowability,a mix design was adapted for a slump range of 150mm-200mm(6”-8”). During casing extraction a positive concrete pressurehead of minimum 3m (10 ft) above water table was maintainedto prevent any possible necking of fresh concrete and pile con-tamination by adjacent soil.

Rock Anchors

In an effort to minimize the number of tieback holespenetrating the secant wall below the water level, theshoring system was designed with the upper level ofrock anchors situated above the lake level at approxi-mately 75.0m. This eliminated the potential for waterleakage. The lower level of rock anchors were installedat elevation 68.4m (225 ft), approximately 7.0m (22 ft)below lake level. Grout bags were attached to the rockanchor tendons to seal the tieback hole in the secantwall. The rock anchors were installed using 150mm diam-

eter (6”) casings, to lengths of 22m (72 ft) at the upperlevel and 12.5m (40 ft) at the lower level. The bondzone was in the sound shale bedrock. Klemm* andHutte tieback drill rigs, equipped with dual head drives,installed the anchors with a duplex procedure. The tieback machine working platform was 2m (5.5ft)

below grade; however, the soil at this elevation was tooloose and too wet to support any heavy equipment. A

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SECANT WALLS Contd.

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Excavating into wet soil. Lower backhoe on bedrock, middle backhoe onmats.

Completed secant pile wall.

series of timber mats were constructed andtieback rigs and crews worked from theseplatforms during drilling.

The anchor capacity was 1350KN (300kips) which equates to a rock / grout bondstress of 700kPa (100 psi). Pre-productioninsitu load tests were performed on tworock anchors and tested to 200% to verifythe design bond capacity. All other anchorswere proof tested to 133% of design load.

Monitoring

ADSC Technical Affiliate Member, MonirPrecision Monitoring Inc.* from Oakville,Ontario, was contracted directly by GeorgeBrown College to provide monitoring forthis project.

During the installation of soldier piles, in-clinometers were attached to the steelbeams at 10 locations evenly spacedthroughout the site and initialized prior toexcavation. During the excavation phase,target monitors were also installed on eachpile top to monitor any movement at the surface. Weekly mon-itoring of the pile targets and inclinometers was conducted dur-

ing the excavation phase. The inclinometers typically indicatedout-of-site movement ranging from 10mm to 20mm at the topof piles and into site movements of 5mm to 10mm at the secondtieback level. The monitoring readings were as anticipated byIsherwood’s design engineers and the shoring system was con-sidered to have performed well.

Potential Problem

During the final stages ofexcavation to subgrade, El-lisDon became concernedwith the large amount ofwater present. The fear wasthat all the work done todate to ensure a water-tightjobsite was pointless be-cause the fissures in thebedrock were allowingwater to travel around theSCW. The suggested pathof the water could create

base heave of the rock surface and flooding of the jobsite.Deep Foundations Contractors Inc. mobilized an airtrack drill

and installed several relief holes vertically into the rock. Theseholes depressurized the trapped water present in the rock. Oncethe trapped water was removed, the subgrade became a drywork area proving a watertight system had been successfully in-stalled.

Conclusion

At the outset, EllisDon’s construction team emphasized a fewcritical elements for a successful project. Adequate shoring de-sign for the conditions, completing the project on schedule, andproviding a dry excavation on completion were a few. The suc-cessful completion of this project provided all of the above.

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Project Team

Project Name: George Brown College Waterfront Campus

Owner: George Brown College

Construction Manager: EllisDon Corporation, Mississauga, Ontario

Geotechnical Engineer: Trow Associates Inc. Brampton, Ontario

Geostructural Engineer: Isherwood Geostructural Engineers*, Mississauga, Ontario

Shoring Contractor: Deep Foundations Contractors Inc.*, Gormley, Ontario

*Indicates ADSC Members

ADSC Technical Affiliate Member, Monir Pre-cision Monitoring Inc.* from Oakville, Ontario,was contracted directly by George Brown Col-lege to provide monitoring for this project.

SECANT WALLS Contd.

Plan view, excavation complete.