Keeping on top of the cracks, crumbles and structural stresses of 1,900 miles
of roadway in unincorporated King County is a year-round job. Just as the
King
County Road Services Division is capping off its warm-weather paving season, it
is already planning for next year.
It starts in the fall, when the division evaluates information gathered over
the past six months on roads that have not been recently repaved. Often the
problems on the surface of the roadway are connected to problems beneath the
asphalt. Much of the information comes from visual inspections, but the division
is using the latest testing technology to give staff a more "in-depth" look at
what lies beneath our roads.
The Roads Division engineering and maintenance staff are experts at dealing
with top-level problems, such as potholes, bumps, and fissures. But the job is
becoming more demanding as the county’s arterials grow older and traffic volumes
– especially heavy truck traffic – increase. And, there is a growing concern
about the condition of pavement that is more than 20 years old.
"Seventy-five percent of our county roads are in very good shape," said Jim
Eagan, a managing engineer in the Engineering Services Section. "But with that
other 25 percent, we are in some cases repairing roads in as little as four
years after an overlay due to structural deficiencies. That is not a great
return on our investment in asphalt."
Eagan said the division is looking for long-lived pavement treatments that
can stand up to heavy vehicle traffic. The arterials still need periodic
repaving during that 20-year timeframe, but they don’t need to be rebuilt from
the sub-grade up – which costs as much as $320,000 per mile.
Eagan’s unit is developing and implementing a
pavement-testing program to aid
in arterial structure design. This new testing will be incorporated into the
repaving program. The goal is to ensure a 20-year design life for all county
arterials.
Years ago, the only way to examine the structural capacity of a roadway was
to drive a loaded truck slowly over a road area where specially placed sensors
were installed to measure movement. The vertical movements were observed and
recorded manually. It was time-consuming, labor intensive and caused traffic
disruption.
Today, one of the modern testing tools staff rely on is a "falling weight
deflectometer". It is a machine that tests the vertical displacement - or the
"give" - in a roadway. If the pavement structure is too thin/weak or the
sub-grade is deteriorating and not solidly supporting the pavement structure,
heavy-vehicle traffic can cause cracking and potholes.
In the next few months, the division will be doing deflectometer testing on
14 miles countywide. What used to take several days, can be done in just half a
day with a two-person crew using this technology. Not only is it more accurate,
but it saves time and labor costs.
The division is also looking into the use of another technology -ground
penetrating radar to determine the thickness of the pavement structures, and
cavities/air pockets under the pavement structures.
The investment in arterial structural testing is expected to pay off in lower
maintenance and road construction costs. This year, the Roads Division has spent
$4.6 million on its pavement overlay program. By 2007, that amount is expected
to climb to $7.6 million. But, repaving a roadway is still much cheaper than
rebuilding it.
DOT Dash