1957+1991 I-476 (Pennsylvania Turnpike Extension) Lehigh Tunnel and New Austrian Tunnelling Method (NATM)

North Portal: (Satellite)

South Portal: (Satellite)

North Portal:

Street View, Jul 2024

South Portal:

Street View, Jul 2024

This 4,461' (1360m) tunnel goes under the Blue Mountain. [GribbleNation, this webpage has photos about adding the second tube.]

This source specifies 4,300' (1311m) for the 1957 northbound tube and 4,381' (1335m) for the 1991 southbound tube.

This is another view of the north portal to emphasize the retraining walls. They had to remove the side of the hill to make room for the new 1991 southbound tube.

Street View, Jul 2025

mcall
"Construction of the second tunnel in 1989."

Daniel Kauffman posted five photos with the comment:

Pennsylvania Turnpike's...Lehigh Tunnel, Lehigh and Carbon Counties.

A foggy ride into the mountains ⛰️ 

 The only road tunnel in Pennsylvania traversed by the Appalachian Trail is located under a major ridge of the Appalachian Mountains (Blue Mountain) that separates the Pocono Mountains Region and the Lehigh Valley Region. This tunnel consists of a North bound and a South bound tunnel, constructed at different times. My photographs are of the Northern tunnel, constructed in the 1950's and is a drill and blast, rectangular-shaped tube. This tunnel is lined with tiles that have become quite a driving distraction. Tens of thousands or about 20% of the tiles have become dislodged, some say due to maintenance policies, others mention seismic activities. The Southern constructed in the 1990's and is a New Austrian Tunneling Method, oval-shaped tube. This tunnel had a  bizarre occurrence in 2018, 70 year old truck driver Howard Sexton was mortality wounded by a 10 foot section of metal electrical conduit pipe that became dislodged because of corroded steel straps that fell and crashed through his cabs window. Howard was able to control his rig and pulled to the side a mile down the highway. Blue Mountain tunnel system....

Dave Costello: I always wear a set of amber or clear SHOOTING safety glasses going through the NB side. If you want to know why, just take a look at the ceiling at all of the missing tiles and then ask yourself what happens when one of them hit your windshield while you are doing 55+ mph. Pretty obvious what’s going to happen. You are going to get a face full of glass. I also firmly believe that the Pa Turnpike Commission doesn’t want you to know this. IYKYK

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Since there is a curve in the road and the oval portal is on the right side, this is the north portal.

Pennsylvania Things posted

BREAKING: Pennsylvania officials remind motorists that the Lehigh Tunnel is the only place where 70 mph traffic collectively agrees to slow down for absolutely no reason.

Drivers who were just fighting for their lives on the Turnpike suddenly enter the tunnel, become model citizens, grip the wheel at 10 and 2, and stare straight ahead like they’re taking their driver’s test again.

Then the second daylight appears, everyone immediately returns to felony speeds.

The Lehigh Tunnel remains undefeated.

I can't determine if this was the original north or south portal.

dr-sauer, this webpage has several photos about the construction work done between 1987 and 1991.

New Austrian Tunnelling Method (NATM)

The reason why the 1991 tube has an oval opening instead of the rectangular opening of the 1957 tube is that it used the New Austrian Tunnelling Method (NATM). NATM is still a drill-and-blast technique, but it uses techniques that reduce the cost of the support liner. First of all, only short segments are done at a time, and the design of the support liner is changed to reflect the rock that was bored through. The contract needs to be flexible to change the design as the rock conditions change. Secondly, the tunnel is removed in layers.

aates

After a segment is drilled and ballasted, a venilation system at the crown of the tube is extended to removed the harmful gases and dust. Then the debris generated by the blast, called muck, is removed.

aates
"The process of applying a thin layer of mortar on the excavated area of the tunnel is called face sealing concrete. This layer is imperative to prevent the loose material from falling. Depending on the geological conditions and stability of the exposed surface, a minimum of 30 mm to a maximum of 50 mm thickness of sealing shotcrete is typically applied."

The bore is allowed to deform to reduce stresses in the rock. But 3D instrumentation is used to measure if the deformation exceeds thresholds. If the deformation is too much, "timely interventions" are implemented. 

aates

Then a "lattice girder" is installed and a layer structural shotcrete is applied over the steelwork. The thickness of the layer is determined by an analysis of the deformation data.

aates

Finally, rock bolts are installed through the shotcrete into the surrounding rock.

aates
"The primary purpose of rock bolting is to “stitch” the rock mass and the shotcrete lining together. This integration helps control displacement and discontinuities, and improves the overall load-bearing capacity of the tunnel cross-section."

aates includes a table that compares Tunnel Boring Machine (TBM) with NATM. NATM can be better for shorter tunnels and/or tunnels with complex (variable) geology.