• CDOT says westbound lanes of U.S. 36 are safe.
Chief engineer cites several reasons for assurance despite eastbound woes
Colorado transportation officials have assured the public that the westbound lanes of U.S. 36 through Westminster are structurally sound and safe — despite the utter collapse of the eastbound span, caused by soil shifting dramatically beneath it.
What makes them so sure the westbound side is safe?
The question is even more pointed since the westbound side is now pulling double duty, carrying an emergency configuration of two lanes in each direction while the eastbound side continues buckling and sinking.
A 300-foot stretch of the eastbound lanes and a retaining wall in the highway’s run-up to a railroad overpass began cracking then collapsing late last week just north of Church Ranch Boulevard in Westminster. The Colorado Department of Transportation has begun an investigation to figure out why.
The Denver Post spoke Friday to Josh Laipply, the chief engineer for CDOT, to get a better sense of the reasons behind CDOT’s assurances.
Laipply detailed several key differences:
Distance above the ground
On the east side, the retaining wall was roughly 30 feet tall, and it was atop a 15-foot embankment.
The west side’s wall is less than half as tall, and the embankment isn’t as steep, since the ground level is higher north of the highway.
As water-saturated clay weakened in the eastbound side’s embankment and behind the wall, there was significantly more soil above it, piling on weight.
“When we talk about slope failures, it’s that extra height that causes the extra strain,” Laipply said, adding: “The majority of weight on that is the soil — the cars (on the highway) are kind of de minimis in terms of the amount of soil pressure you have.”
Separate retaining walls
The independence of the two sides’ retaining walls provides additional safety, he said. Each was
built with industrialstrength straps that are embedded in the soil, going for 30 to 40 feet toward the median. Even as the eastbound side’s soil has shifted down the embankment, he said, the westbound’s wall structure has remained secure.
Initially, Laipply said, the failure on the eastbound side showed through cracks in the pavement and bulges in the wall, as the structure was overwhelmed by the force of the soil shifting.
Nearby wetlands
CDOT is still investigating whether the water that saturated the clay came from rainstorms in the wet spring, from groundwater or, likely, from the adjacent wetlands south of the highway.
“The westbound is inherently drier than the eastbound,” Laipply said. On the westbound side, “that clay layer is present, and may not be as wet. But it certainly doesn’t have the same amount of load” above it as on the eastbound side.
Of course, the eastbound side was supposed to be engineered to withstand wet and shifting soil, too. For now, though, CDOT is confident the westbound side won’t follow suit, since it hasn’t shown any of the troublesome early signs that were spotted on the eastbound side July 11, the day before CDOT closed the eastbound side and the deterioration accelerated.
But CDOT workers remain on the lookout just in case, Laipply said, performing detailed and recurring surveys with inclinometers in hand. (Haven’t heard of that tool? Think of incline, because those tools can be used to detect changes in slope and ground movement.)
“We’ve seen no cracks and we’ve seen no movement” on the westbound side, Laipply said. “If we do see movement, we’ll have plenty of time to say we’re concerned and pull people off.”