SEATTLE -- The Alaskan Way Viaduct reopened Sunday night, about five days earlier than expected.

The Washington State Department of Transportation said Monday structural engineers looked at the viaduct and determined it's safe to open early, adding there were no adverse influences on the structure.

When the viaduct closed and Bertha began tunneling, there were about 60 rings -- the interior wall of the tunnel -- to install. Each ring is six and one half feet in width. Seattle Tunnel Partners (STP), a consortium of contractors which is building the tunnel project, estimated they needed to install about four per day.

After installing one on the first day and three on the second, production picked up, and crews were installing five or six per day. STP hopes to average six rings per day for the rest of the tunnel drive.

The viaduct reopened Sunday before Bertha reached the goal of 385 feet of tunneling. STP project manager Chris Dixon says he's confident it was the right decision.

"Soil conditions are better," said Dixon, who added that as Bertha goes deeper, she runs into more firm soil which is easier to control when tunneling. WSDOT and STP say there was practically no movement by the viaduct due to the tunneling.

There are six more rings to install before Bertha stops for maintenance. Crews will also get some rest after working around-the-clock on a two-shift schedule. Mining will resume next Monday.

Dixon said there were no safety incidents with the tunneling, which he called "remarkable" given the hectic work schedule.

Dixon predicts crews may eventually install nine or ten rings per day as Bertha goes deeper into the soil.

Regardless of how much longer it takes, commuters can rejoice that the traffic conditions have already improved after the viaduct reopening.