Challenge

August 2011 - Rehabilitation of a sanitary sewer system was proposed in the state capital city of Frankfort, Kentucky. The project included the construction of approximately 500 feet of new sewer line adjacent to an existing, completely exposed/elevated sewer line. The planned height of the proposed sewer line ranged from approximately four to 17 feet above grade. The original line was supported on poured concrete columns formed against or next to stone retaining walls along the bank of the Kentucky River. The atypical nature of the project presented many specific challenges for implementing a solution to support the new sewer. The steep embankments and limited working space between the retaining walls and the river limited equipment access to most areas of the project. Geotechnical information was also limited as explorations could not be completed with standard machine-mounted equipment.

Solution

Various sewer support options were considered, but helical piles were eventually selected given the challenges noted above. Helical test probes were completed to gather relevant soil information. Shale and limestone bedrock was encountered at varying elevations along with rubble fill, limestone “floaters” and tree roots in the upper part of the profile. Buckling of the piles was considered due to the exposed lengths and the potential for scour and impact loading from floating debris during flood conditions. The support system design included 57 Model 349 (3.5-inch OD by 0.300-inch wall) round shaft helical piles with single 12” V-style helix plates on the lead section. Specified installation criteria included a pile embedment depth of 20 feet to bedrock or a minimum installation torque of 12,000 ft-lb. Construction benches were cut into the hillside where possible to allow pile installation with a drive head mounted to a mini-excavator. Where construction benches could not safely be made, the drive head was mounted to a Sky Trak lift vehicle with extendable boom. The Sky Trak accessed various areas of the sewer alignment from a barge anchored along the bank of the river. Steel angles were welded to the piles and connected to the stone retaining wall to brace the piles and reduce unsupported lengths. Piles were fitted with custom saddle brackets to contain the new sewer pipe. Pile lengths ranged from 11 to 38 feet with depths of 7 to 21 feet below grade. The piles were hot-dip galvanized for corrosion protection. Where welding was required, the zinc coating was first ground off and then a spray-on zinc coating reapplied. Despite the difficult working conditions, the piles were installed in 14 working days.

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Project Summary

Architect/Structural Engineer: Sherman-Carter-Barnhart, P.S.C.

General Contractor: Cleary Construction, Flo-Line Contracting

Certified Pile Installer: Indiana Foundation Service

Products Installed: (57) Foundation Supportworks® Model 349 Helical Piles with Custom “Saddle” Brackets, 12” V-Style Helix Plate on Lead Section, Pile Lengths of 11 to 38 feet