During a tunnel construction project, engineers used explosives to excavate partway into a mountain. But due to time constraints and environmental concerns, they later introduced a tunnel boring machine (TBM) to excavate the rest of the tunnel. The length y of the completed tunnel (in feet) is based on the number of days x since the TBM started working. The length can be calculated using the equation y = 25x + 225. What does the y-intercept represent in this scenario?