Because it projected  into the Piscataqua River channel, Henderson's Point created a navigational hazard for warships visiting the navy yard. Consequently, a cofferdam was constructed, behind which  of rock and  of soil were excavated.
The construction of a cofferdam at Henderson's Point, which projected into the Piscataqua River channel, created a navigational hazard for warships visiting the navy yard. This led to the excavation of rock and soil behind the cofferdam.