Unearthing Artifacts Prior to the start of construction, archaeologists excavated an area within the railroad right-of-way that was once a part of the historic San Gabriel Mission, founded in 1771. Thousands of historic artifacts from California’s Mission period were found in the area, including pottery, arrowheads, remains of a historic mill, and human skulls and bones, which are believed to be Native American. Once construction commenced, an archaeologist and a representative of the local Native American tribe were required to be present during all excavation activities to examine the drill spoils for any artifacts. Temporary Track Support To support the temporary shoofly track, more than 5,600 lf (1,707 lm) of ground improvement work consisting of deep soil mixing (DSM) and the construction of two, 92 ft (28 m) deep drilled shaft piles that were 96 in (2.44 m) in diameter were performed using a Bauer BG 40 top drive drill rig. The DSM was an effective alternative to conventional sheet piling, eliminating potential vibration issues and providing much better support near the San Gabriel Mission and existing homes and businesses along the corridor. DSM allowed the project team to start excavation for the trench without having to wait to process sheets right next to the new shoofly track. Secant Pile Wall Construction The close proximity of the shoofly track on the west end of the project required the trench wall to be drilled in two phases. The first phase consisted of installing the south secant pile wall using a temporary shoring wall to the north to provide support for the shoofly track. The temporary shoring wall consisted of steel sheet piling and installation of 210 each, 3 and 4 strand tieback anchors. The second phase will consist of constructing the north trench wall after the train is lowered into the trench and the temporary shoofly track is removed. Once the temporary shoofly track was completed and train traffic diverted from the trench alignment, construction began on 15,600 lf (4,755 lm) of secant pile walls. The horizontal and vertical support components of the trench wall required 82 • DEEP FOUNDATIONS • MAR/APR 2017 3,700 primary 18 in (457 mm) diameter unreinforced secant piles and 3,700 secondary 36 in (914 mm), 48 in (1,219 mm) and 60 in (1,524 mm) diameter reinforced secant piles. The piles were installed with extremely tight tolerances of 1 in (25 mm) on location and 0.5% for plumbness. Installation was accomplished using various lo-drill and top-drive drill rigs, which required over 54,000 cu yds (41,286 cu m) of concrete and cement slurry. Malcolm installed 3,700 structural secant pi les wi th only two minor anomalies, both of which were repaired during trench excavation without delaying the project schedule. It was required that 2% of the tiebacks had to be performance tested to 200% of design load. The contract also required that two tiebacks of each type be extended creep tested to 200% of design load for a period of 10 hours. Stressing jacks were used with auto-grippers and power seating to expedite the testing procedure, and, on average, 30 tiebacks were tested a day with only two stressing jacks. Once the excavation exposed the inside face of the secant piles, the exposed sides of the piles were ground off to accommodate a 6 in (152 mm) cast-in-place fascia wall. A John Deere 350GLC excavator with an Antraquip grinding wheel attachment was used for this work. More than 94,000 lf (28,651 lm) of exposed pile face had to be ground off using extra care to not to damage the reinforcing steel contained within the piles. To prevent the grinding wheel from damaging the concrete pile cap, the top 2 ft (0.6 m) of each pile were chipped off by hand, using a man lift to hoist personnel up to the pile cap. Secant pile drilling operation next to historic San Gabriel Mission Walsh followed the piling installation with construction of a 4 ft (1.2 m) thick reinforced concrete cap beam running the entire length of secant pile wall on both sides of the trench. More than 2,600 tiebacks were installed through the cap beam to provide lateral support to the secant pile walls. The tiebacks consisted of 8 in (203 mm) diameter tiebacks with different strand configurations depending on loading requirements (i.e., 4, 5, 6, 7, 8 and 10 strand) double corrosion protected anchors averaging about 43 ft (13.1 m) in length, and were structurally connected to the cap beam. Testing of tieback anchors in cap beam
