The Northeast Grid Reliability Project upgraded PSE&G’s power service in the northern part of New Jersey from 138 kV to 230 kV. A portion of the project traverses a protected brackish tidal wetland, the New Jersey Meadowlands.
QS Design Scope
QS was contracted to provide structure recommendations and conceptual foundation designs for 13 structures in the sensitive area. Recommendations had to factor in access difficulties and minimize overall disturbance to the surrounding environment. General soil conditions in this portion of the alignment consisted of a 10 to 20-foot-thick layer of over-consolidated clay/silt overlying a normally-consolidated clay/silt layer, ranging from approximately 45 feet to more than 200 feet in thickness.
The team studied the use of lattice steel towers and steel poles, as well as their impact on the temporary and permanent disturbance of the wetlands. Large structural loading demand from the steel poles, in conjunction with poor soil properties, led to difficulty in obtaining practical foundation options. The use of steel lattice towers was recommended to distribute the tower loading over a larger area, and to minimize demand on the foundations.
Multiple foundation options were analyzed, including drilled shafts, driven timber and precast concrete piles, helical piles, and micropiles. Design benefits and limitations were considered, along with constructability impacts associated with installation and testing of the foundation components. A geotechnical engineering report had not been completed at the time of the study, and historical geotechnical borings along with site-specific Cone Penetrometer Tests were used to develop appropriate geotechnical resistances for each foundation type.
Each foundation option was assigned relative cost and risk indices associated with constructability, and geotechnical and structural performance. The findings were presented to the owner to assist in the selection of feasible foundation systems that would meet the needs of the upgraded transmission line.