Boston Building Uses a Little Of Everything to Gain Foothold By William J. Angelo, McGraw-Hill Construction | ENR
The soil is poor, primarily fill and clay, and needs to be cut down about 18 to 26 ft for the new foundation. Doing so would undermine adjacent existing foundations so NEFCO first drilled steel soldier piles about 55 ft deep and encased all toes in concrete. It then placed wooden lagging against the streets and part of the garage. NEFCO next dug nine 3 to 5-ft-wide underpinning pits 10 to 12 ft below the building and filled them with concrete to shore the southwest corner.
To stabilize the rest of the southern wall, workers drilled a 70-ft-long, 30-ft-deep secant pile wall consisting of 27, 3-ft-dia piles. Every other pile contains an I-beam for structural support. Lean-mix concrete shafts are drilled between them to integrate all shafts into a wall. The secant wall and underpinning pits will be separated from the new building’s foundation by a shear break.