Memo on water main additions and retaining wall project, May

NYC 9/11 Public Portal Document

03/10/2024 00:40 212-669-4980 PAGE 09/25

The concrete sidewalk at the middle of the retaining wall has also settled and is uneven. See Photograph 7. This movement is the result of the retaining wall shifting which allows the soil behind the retaining wall to settle as the volume of soil redistributes itself.

The concrete sidewalk at the north end of the retaining wall has negligible settlement. Sec Photograph?.

3. One section of concrete curb along the west sidc of Cannon Place at the middle of the retaining wall has shifted west approximately 2". See Photograph S. This movement is the result of the retaining wall shifting and the volume of soil behind the retaining wall redistributing itself.

4. A section of the asphalt pavement adjacent to the shifted concrete curb is breaking up. Sec Photograph 9. This breaking up is the result of the retaining wall shifting and the volume of soil behind the retaining wall redistributing itself.

5. The Site of the proposed new Building is the side of a hill with a rock outcrop along Fort Independence Street.

A dry-laid rubble stone retaining wall is inherently unstable given that the stones are not mortared together to create a monolithic wall. A dry-laid rubble stone wall is also very susceptible to movement caused by freeze-thaw cycles as the water within the joints repeatedly expands with each freeze cycle and pushes the individual stones. A dry- laid rubble stone wall is also very susceptible to movement caused by vibration given that the stones arc not bonded together with mortar.

The dry-laid rubble stone wall on the wcst side of Cannon Place is continuously shifting as indicated by the conditions discussed above. Given that some type of rock cxcavatiion will be required to install the foundation of the proposed new Building the probability ofdamage: to the adjacent private property and the dry-laid rubble stone retaining wall caused by vibrations during construction is very high. In order to protect the Public and Private Property Owners from the loss that would occur should another section of this retaining wall fail the City of New York needs to implement the following safeguards.

A. Prior to construction beginning the location of the surface of the retaining wall needs to be surveyed at 10 intervals along the entire length of retaining wall.

B. Also prior to construction beginning alt existing Buildings adjacent to the retaining wall shall have a Pre-Construction survey performed to thoroughly document the existing conditions.

C. While the foundation of the proposed new Building is being constructed. including excavation, the location of the surface of the retaining wall needs to be monitored on a weekly basis. if the retaining wall begins to shift the construction shall be halted until the retaining wall is stabilized. When construction restarts the retaining wall monitoring shall recommence. This cycle shall be repeated as many times as required.

NYC-WTC 000159266