State hopes soil study holds key to better roads

A road crew works on the subgrade alongside Interstate 25 south of Albuquerque. A new study by the New Mexico Department of Transportation hopes to provide better recommendations to road construction crews as they stabilize the soil beneath New Mexico’s roads. (PHOTO: Gabe Presley, KRQE News 13)

SANTA FE (KRQE) — For years, New Mexico’s highway department has been pouring concrete, asphalt and money into an expensive hole it shouldn’t have had to fill.

The soil beneath the state’s highways can be notoriously unstable. Given enough water or radical temperature changes, it can create cracks, bubbles and potholes.

That erratic behavior means road contractors who encounter unexpected soil conditions often have to wait more than a week for the New Mexico Department of Transportation to decide how to stabilize the ground before they can put pavement across it.

But the NMDOT has not made consistent decisions. And if department engineers recommend the wrong treatment for unstable soil, maintenance crews will waste valuable time and limited funding to fix a road that should have been able to go years without significant repair.

In the state’s words, engineers have had to “reinvent the wheel” every time a construction crew encounters less-than-favorable soil conditions. Now, the NMDOT is spending hundreds of thousands of dollars to create a soil stabilization guide officials believe will save millions of dollars in unnecessary cost overruns and premature repairs.

It is difficult for the state to quantify exactly how much New Mexico has spent fixing roads that failed because of underlying soil conditions. A project to repair 20 miles of highway, for example, might contain just a few hundred yards of problems caused by pockets of unstable soil — engineers refer to it as the “subgrade” — beneath the road surface.

New Mexico State University and a private engineering company are conducting the study. Officials say it will result in interactive guidelines that give department engineers a better understanding of how the solutions they recommend for unstable soil will perform.

It should also give quick, consistent answers to road construction crews. The department thinks the study could turn a week-long wait for answers into a day or two.

“I think what you’re going to see is the project will get completed on time,” said NMDOT operations engineer Ernie Archuleta. He’s also more optimistic that projects will finish on budget.

Unanticipated soil conditions often lead to costly change orders, Archuleta said. The department estimates it could save up to 5 percent of a project’s budget by properly solving soil problems during construction. On a $10 million project, that’s a potential up-front savings of half-a-million dollars. The pavement should also last longer, multiplying the savings.

A truck rolls southbound along Interstate 25 near San Felipe Pueblo. NMDOT says the uneven roadway is an example of what can happen when the underlying soil hasn’t been properly stabilized.
A truck rolls southbound along Interstate 25 near San Felipe Pueblo. NMDOT says the uneven roadway is an example of what can happen when the underlying soil hasn’t been properly stabilized.

New Mexico’s soil isn’t uniformly a nightmare for road construction, but pockets of poor base material such as gypsum are frequent enough to cause swelling and buckling of roadways. The southbound lanes of Interstate 25 between San Felipe Pueblo and Algodones are a good example, Archuleta said, of what happens when poor soil reacts with water, weight and temperature to fail. The road rolls up and down for about 100 yards along that stretch of interstate and, though it’s been repaired in recent years, it will have to be fixed again.

It’s typical of roads built over soil that isn’t properly stabilized. “You may have a pocket or a void underneath there, and that’ll just start to expand,” Archuleta said.

Road crews use both mechanical and chemical stabilization to fix such problem spots. Mechanical methods can be simple compaction with a heavy roller, or they can involve “geogrid” structures that provide artificial stability. Chemical stabilization often means a material such as lime or cement is added to the existing soil to make it capable of handling traffic on the road that will eventually be built on it.

NMDOT said it doesn’t have an expected delivery date for the full report, though Archuleta estimated it is 70 percent complete. When it’s finished, it will be paired with a new statewide map of subgrade soils.

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