Owing to the numerous sections of poor rock, interspersed throughout the
work with stretches of sound rock, the progress of the excavation was
very irregular, especially in 32d Street. The rate of excavation in good
ground is shown in Table 1. In the sections of bad ground, the
operations of excavation, timbering, and lining were often carried on
alternately, and it is impracticable to include them in the table.