We are developing a ground-based radio interferometric phased array for radio detection of high energy neutrinos, in an effort to lower the energy threshold of radio detection experiments while increasing the effective volume at high energies. The radio detection technique looks for Askaryan emission from neutrinos interacting in large volumes of glacial ice. The principle behind the phased array technique is coherent summing of the broadband, impulsive Askaryan signal from multiple antenna channels, increasing the signal-to-noise ratio for triggering on weak signals. We first discuss simulations and validation measurements related to the phased array technique, including results from a preliminary Monte Carlo simulation, a demonstration of beamforming and measurements of thermal noise correlation in an anechoic chamber, and results from a trigger simulation. We then discuss the design and development of the first ground-based interferometric phased array trigger system, a 16-channel system that has been built and will be deployed as part of one Askaryan Radio Array (ARA) station in December 2017 at the South Pole.