present in the area. The soil profile encountered at the deepest shaft location indicated an approximately 9 ft (2.74 m) thick native fill layer atop glacio-marine clay to below BOE. The water table was determined to be present approximately 6 ft (1.83 m) below the existing grade, 39 ft (11.9 m) above bottom of excavation elevation (BOE) and at the top of the clay layer. Once again, the clay extended below the proposed BOE elevation, allowing the clay layer to be used as groundwater cut-off. All six of the circular secant pile shafts had to be impermeable due to their locations within contaminated soil and groundwater zones of the project. These shafts varied in inside diameters from 14 ft (4.27 m) to 27 ft (8.23 m) and depths from 22 ft (6.71 m) to 45 ft (13.72 m). The steel casings required for the secant piles were approximately 3.3 ft (1 m) diameter. The secants were installed by specialty the subcontractor, Hayward Baker of Providence, R.I. The quantity of secant piles at each shaft varied as a function of required shaft diameter (needed to drop in, extract, and maneuver the MTBM) as well as the minimum concrete wall thickness required to resist the overburden pressures at the base of the shaft. The number of secant piles installed for the shafts ranged from 22 to 38 with a typical center to center spacing of 2.6 ft ± (0.76 m) between primary and secondary secants. The minimum compres- sive strength for the secant piles was 2,500 psi (17.24 MPa) [750 psi allowable (5.17 MPa)] and was designed to cure and achieve minimum strength rapidly to expedite shaft construction and excavation, while considering proper bonding between primary and secondary secants. The geometric properties of circular secant pile shaft construction allowed GZA to consider that no internal bracing would be required. The theory was that the stresses incurred on the shaft walls due to soil overburden and construction surcharge would be transmitted and distributed along the circumference of the shaft due to arching effect. This hoop stress acting as a compression force on the secant piles would be resisted by the compressive strength of the concrete itself. 66 in (1.68 m) diameter MTBM cutting head (post-installation) DEEP FOUNDATIONS • MAR/APR 2013 • 75 The project requirements called for a minimum overlap of 9 in (22.9 cm) between primary and secondary secants and a minimum wall thickness of 25 in (63.5 cm) to resist the external lateral earth pressures, and this was critical in determining the layout of circular the secant piles. Lateral earth pressures of up to 4,500 psf (215.5 kPa) were calculated at the deepest shaft location, which resulted in a compressive hoop stress of approximately 278 psi (1.92 MPa) across the secant pile cross-section. GZA performed structural checks to ensure that the piles would remain in compression and never exceed the minimal allowable tensile capacity of the concrete, which could have resulted in cracking, loss of watertightness and possibly shaft failure. The quality control techniques demonstrated by Barletta and Hayward Baker during secant pile installation made it feasible to eliminate internal bracing at all of the circular secant pile shafts. By mobilizing the arching properties of the shaft, the hoop stresses incurred by the support of the adjacent soils were effectively transmitted around the perimeter of the shaft and back into the ground, where they were dissipated. This method of shaft construction provided cost savings to Barletta as a direct product of the elimination of internal bracing. The elimination allowed for a reduced shaft diameter, less construction material, and lower excavation quantities. Barletta thereby moderated costs of additional excavation, bracing (labor and materials), and soil T&D. An estimated one day was saved on the schedule for each level of bracing eliminated. These deep shafts would likely have required an estimated three levels of bracing each. Now, a typical excavation and bracing installation that would have taken four days would now only take one, resulting in an increased shaft construction efficiency of nearly 400%, which translated to significant actual cost and schedule savings. Conclusions The deep shaft design methods, designed by GZA and employed by Barletta, considered the site specific challenges, and identified innovative solutions to accomplish the project’s goals. The SOE design techniques used resulted in significant advantages in both schedule and cost savings. The proactive and flexible approach to deep shaft support, provided by GZA on the East Boston Branch Sewer Relief Project, employed state-of-the-art methods, and resulted in efficient, cost effective and constructible design solutions, while meeting the operational requirements of the construction team, and aided in minimizing impacts to the surrounding environment, and resulted in a successful project completion.
