CHAPTER 6. LESSONS LEARNED
This chapter presents a summary of the lessons learned from driven piles on the CA/T project. The conclusions presented below are based on the evaluation of field records, project specifications, and pile load test data compiled from the project files. Five contracts were evaluated, including three located in East Boston/Logan Airport, one located in downtown Boston, and one located in Charlestown. Significant findings are summarized below:
- The dominant pile type used on the CA/T project was a 41-cm square PPC pile. Based on the contractor's bid estimates, the PPC piles were also the most economical pile type.
- Pile heave in excess of the 1.3-cm criteria was identified on one cut-and-cover tunnel structure requiring 445 restrike events for the 576 piles used in the structure. The heave occurred even though preaugering of the marine clay layer was performed. Pile heave issues were not identified at other structures where the pile spacing was greater than about 1.8 m.
- Installation of displacement piles in contract C07D1 caused excessive movement of an adjacent structure. Despite the use of wick drains and partial preaugering, vertical displacement continued up to 8.8 cm. The wick drains were not effective in rapidly dissipating excess pore pressures from pile driving.
- The heave issues observed in contract C07D1 prompted the use of preaugering on subsequent contracts. Preaugering was performed over a portion, generally 30 to 70 percent, of the final pile embedment depth.
- Pile capacities evaluated using dynamic methods were conservative in hard driving conditions (i.e., penetration resistance greater than 10 blows per 2.5 cm) where the soil resistance may not be fully mobilized.
- Quake values from CAPWAP analyses ranged from 0.25 to 1.19 cm, with an average value of 0.64 cm. These values are higher than the values typically used in wave equation analyses; however, they are within the range of published values.
- Comparison of CAPWAP data evaluated at the end of initial driving and during restrike shows that the capacity of the piles increased over time by at least 20 percent from an increase in shaft resistance.
- Only 1 out of 15 piles tested in a static load test was brought to failure according to Davisson's criteria, because the specifications did not specifically require that the pile be brought to failure.
- Three of the 15 piles did not successfully demonstrate that 100 percent of the design load was transferred to the bearing soils. Two piles did not meet the criteria because of high shaft friction, and the third did not meet the criteria because of a malfunctioning bottom telltale.
- Comparison of dynamic and static load test capacities was only possible on one pile (IPW), which reached the Davisson's failure criteria in the static load test. CAPWAP and static capacities were in good agreement for this pile.