MJE Drilling (MJ) was contracted to install drilled shafts at the Moxie Freedom Substation in Northeast Pennsylvania. The foundations would support six H-Frame structures and one 180-foot monopole, all with potentially significant embedment into bedrock. Original designs included drilled shafts with diameters ranging from 9 to 12 feet, and rock embedments ranging from 11 to 28 feet. Designs for the largest structure consisted of a 12-foot diameter, 34-foot deep shaft with a minimum shaft embedment into rock of 28 feet.
Available geotechnical data indicated significant variability in depth to bedrock. MJ engaged QS to review geotechnical data and structure loading, and determine if an alternate design could be utilized should shallow rock be encountered at any of the shaft locations.
Qs Scope & Solution
Rock socketed drilled shafts pose unique challenges for designers and contractors alike, and this topic has become a national focus of the Deep Foundation Institute’s Drilled Shaft Committee. Traditional design approaches for drilled shafts that encounter a shallow rock contact surface include deeper and/or larger diameter shafts. On this project, QS developed alternative designs which incorporated detailed geotechnical assessments and constructability reviews derived from continuous interaction with MJ. The final value engineered designs, and those ultimately approved by the facility owner, resulted in an approximate 40% reduction in shaft/concrete volume and a 60% reduction in total rock socket depth. The drilled shaft supporting the 180-foot monopole specifically, was redesigned as an 11-foot diameter, 25-foot long shaft with a minimum shaft embedment into rock of 7 feet.