Ductile Iron Piles provided a foundation system to cost-effectively support the foundation loads while also penetrating the heavily-obstructed fill encountered in one corner of the site.
Construction consisted of a new 8-story mixed-use development in the Assembly Square District. The 63,000 sq ft footprint included column loads on the order of 200 to 1,000 kips. Loads at mat foundations ranged from 1,900 to 4,300 kips for design.
Subsurface conditions across the site generally consisted of urban fill and organics/peat underlain by a very stiff marine clay crust transition to very soft marine clay underlain by glacial clay and sand, glacial till and finally bedrock. The southwestern portion of the site contained significant obstructions from a former structure in the fill including buried foundations, slabs, granite blocks and steel. The fill was up to 17 feet thick in this area followed by up to 52 feet of marine clay. The underlying glacial soils extended up to 9 feet before encountering bedrock at depths around 67 to 78 feet below grade.
Provide a foundation system to cost-effectively support the foundation loads while also penetrating the heavily-obstructed fill encountered in one corner of the site.
- Significant cost savings.
- Rapid installation with over 800 LF/day, saving over 2 months in pile duration.
- High capacity piles terminating on rock.
- Successful load testing to confirm design loads.
- Hybrid approach featuring pre-drilling to penetrate obstructions.
Design and Construction Solution
The geotechnical engineer recognized the potential to use Geopier® ground improvement to provide a cost-effective foundation system across the majority of the building footprint. However, the obstructed fill zone in the southwest building corner prevented the use of ground improvement. Initial alternatives considered included 1) removal and replacement followed by ground improvement or 2) micropiles. The removal and replacement option would require substantial shoring and result in expensive fill disposal costs. The micropile option would penetrate theobstructed fill but be slow and expensive to extend the piles to socket in bedrock for capacity.
With a recent successful Ductile Iron Pile project in neighboring downtown Boston, the geotechnical engineer realized the potential cost and schedule savings associated with the system over micropiles. Working closely with representatives of Helical Drilling, Inc. and DuroTerra, engineers developed a hybrid pre-drilling / Ductile Iron Pile solution to support foundations in the obstructed areas. The approach utilized a high-frequency Fraste|Eijkelkamp Mito 60 Sonic Drill rig to drill 12-inch diameter holes through the obstructed fill. Once drilled, the initial 5-meter long Ductile Iron Pile section was lowered into the hole and a second section was advanced. The annular space around the pile was then filled with sand. Following completion of all of the Sonic pre- drilling, the Ductile Iron Pile equipment would drive the piles through the remainder of the clay, glacial soils and set on competent rock.
Helical Drilling, Inc. installed a pre-production load test pile and four grouted DIP reaction piles using a Volvo 240 excavator and Tramac V1800 hammer. The load test pile featured a Series 170/9.0 Ductile Iron Pile section installed to achieve a set criteria of 1” of movement in 50 seconds to provide an 85 ton compression capacity. The test pile was advanced to rock at a depth of 78 feet. The test pile was instrumented with two telltales and three strain gauges to confirm load transfer with depth. The test was performed to 170 tons (200% of design load). Nearly elastic performance was noted in the pile with a deflection of 0.69 inch recorded at the design load of 85 tons and total deflection of 1.5 inches at 170 tons. The net deflection following rebound was only 0.15 inches. Results of strain gauge measurements showed approximately 130 kips dissipated along the 78-ft long pile shaft with over 200 kips transferred to the tip in end-bearing.
Production piles proceeded with the start of the Sonic drilling. Fill zones were highly variable with some areas featuring solid 15-ft sections of steel and concrete being removed. After about two weeks for completion of the predrilling work, a total of 61 Ductile Iron Piles were installed and grouted in only 5 working days – averaging over 800 feet per day. In comparison with the micropile option, the hybrid solution saved the project team between 2 and 3 months on the schedule and over $800,000.
Project Team Members
DIP Design/Build Partner: Helical Drilling, Inc.
Geotechnical Engineer: Haley & Aldrich, Inc.
General Contractor: Wood Partners
Structural Engineer: Veitas & Veitas Engineers