Opened for traffic in 1940, the Pennsylvania Turnpike is one
of the older limited-access highways in the U.S. Some of its design
characteristics were patterned after the German Autobahn, which was built
during the 1930s. The most common attribute of both highways is their design
for driving vehicles "safely" to 100 mph. 

The original length of the turnpike was 160 miles stretching
between Harrisburg and Pittsburgh. This included seven tunnels that were built
through different sections of the Allegheny Mountains.

Today's turnpike is 515 miles long and includes a 360-mile
highway running east-west, a 110-mile northeastern extension and 45 miles of
various western extensions. The east-west route connects with other
limited-access highways at the New Jersey and Ohio borders; the northeast
extension connects to the east-west route and proceeds north to I-81 near
Scranton.

If a roadway's driving conditions are to be continually held
to high standards, a comprehensive ongoing maintenance program is necessary.
The Pennsylvania Turnpike Commission is well aware of this fact. As a toll
road, the commission must in particular maintain high roadway standards for its
paying customers, who will tolerate nothing less. 

Essentially, the Pennsylvania Turnpike is operated by a
private-public sector partnership and is perpetuated by receiving revenue from
collected tolls. The commission receives no tax revenue from local, state or
federal agencies. This past year the turnpike's income from tolls was $366
million plus $27.5 million received from plaza revenues and other sources.

As with any service-oriented enterprise, the commission must
offer a value-added highway service if it is to keep its customer-paying base.
There is an alternative highway for motorists. It is I-80 that runs parallel to
the east-west turnpike, albeit 45 to 50 miles to the north. This highway is, of
course, free to use. 

Over the next 10 years, the commission projects investing
$1.46 billion capital in 100 miles of the original-built roadway. Instead of continuing
to overlay the original concrete pavement with asphalt, the roadway will be
completely removed in favor of building a new one. Each contract let to a third
party for rebuilding a section of the roadway will encompass about 10 miles of
the 100-mile stretch to be reconstructed.

A touch of lime-pozzolan

A problem encountered on the first roadway section of the
project was a subgrade that would call for over-excavation because of its
inherent poor ground conditions. The subgrade included a large measure of clay,
making it unsuitable for building a new roadway with flexible pavement. While
the existing reinforced concrete (ridged) pavement was able to span most of the
subgrade deficiencies, this cannot be said of the Superpave specified for the
new highway. 

It was evident to the turnpike engineers that a
cost-effective method of stabilizing the subgrade was necessary. The
predominantly clayey ground conditions run unusually deep (4 to 12 ft) along
some sections of the highway's alignment. 

KCI Technologies Inc., Corporate Hunt Valley, Md., the
project manager and construction inspection company for the commission,
presented an acceptable solution to the problem. James Lockhart, P.E. and
project manager for KCI, proposed the incorporation of a lime-pozzolan mix
within a predetermined design thickness in the top layer of the soil. This, he
reasoned, would stabilize and strengthen the subgrade to an acceptable degree.

It is nothing new; stabilizing the top layer of unsuitable
soil in the subgrade with lime-pozzolan mixes has been successfully done on
other roadbuilding projects. It is new here, though--the commission has never
tried this method before.

As mentioned, a reinforced portland cement concrete (ridged)
pavement did not require the likes of soil stabilization. Because of its
rigidity and strength characteristics, a reinforced concrete slab can span the
physical-strength irregularities found in these clayey soils. However, since
Superpave is the pavement selected, a much improved soil condition is required
in the subgrade. By stabilizing the top layer of the subgrade, gross over-cuts
made to reach acceptable ground have been eliminated. Overcutting also would
necessitate importing large quantities of stone needed to backfill the
excavation.

Pozzolan has been used for construction purposes since the
Roman Empire where slag (pozzolan) from volcanoes was ground up. Today,
pozzolan comes from either natural sources (volcano slag) or is made by usually
combining fly ash with water-quenched boiler slag. Pozzolan is a popular
admixture for concrete mixes, and combined with lime and water it becomes a
good soil stabilizer. Whether the three ingredients are mixed solitary or mixed
directly into a soil, the same chemical action takes place thus giving the compound
cementitious properties. One of these properties is the gaining of superior
soil strengths. 

9% stablilized

This first contract let for the 100-mile project is
designated 76/85. It calls for rebuilding a 9-mile highway section, which
includes the two eastbound lanes and the two westbound lanes between the New
Stanton Interchange and the Donegal Interchange. All 9 miles of  the roadway's subgrade requires
stabilization by incorporating the lime-pozzolan.

There are five stages of work required to complete the
project. Stage 1 included paving the eastbound and westbound shoulders for
accommodating new traffic-flow patterns; further, there is the removal of the
median barrier and a reconstruction of the whole median area. By temporarily
paving the existing shoulders, there remains a two-lane roadway for two-way
traffic open during the course of the construction project. This means there is
an uninterrupted and efficient traffic-flow pattern.

Stage 2 specified the complete removal and partial
reconstruction of the eastbound lanes. Stage 3 involved the final
reconstruction of the eastbound lanes and the partial reconstruction of the
median strip. Stage 4 activities, which are currently under way, are similar to
Stage 2 except it involves the westbound lanes. Finally, Stage 5 is the
completing of the median construction by installing precast concrete barrier
sections.

The general contractor for this project is the Angelo
Iafrate Construction Co., New Stadium, Pa. According to Jim Lockhart, the
project got off to a slow start but now the work is going smoothly. He expects
it to be completed by late summer or early autumn of this year. 

Rolling over

The lime-pozzolan mix was incorporated (roto-tilled) into the
top 12 in. of the soil at a rate of 6 to 12% by weight of the
lime-pozzolan/soil mix. A Caterpillar SS 250 B soil stabilizer was used for
incorporating into the soil the prespread lime-pozzolan and the sprinkled
water. Water was applied at the rate of 8 to 20%, depending on the soil's
initial moisture content.

Using Bomag vibratory rollers in the static mode, the
lime-pozzolan/soil mix was compacted to 93% of original soil density. Roger
Christenson, an equipment manager for Iafrate, said he tried various compactors
at the beginning of the project and was disappointed with all of them.
"Because of various problems we were having with the rollers, I decided to
see what roller our heavy-equipment serviceman on the project would recommend.
A field serviceman's recommendation on equipment usually turns out to be a good
one. Most of the problems we were experiencing with the rollers were
unreliability and too much time required to fix and maintain them. The
serviceman recommended Bomag vibratory rollers so I began renting them. These
rollers solved our roller problems. We are getting good production with the
Bomag rollers and they have been reliable. Our operators say these rollers are
less fatiguing to operate when compared with the other makes tried on this project."

The model roller chosen was a Bomag BW 211D-3. This is a
single-drum, hydrostatic-travel type featuring two selectable vibration
frequency modes with dual selectable amplitudes. The operating weight is 23,000
lb and it has a nominal rolling width of 84 in. For compacting the subgrade,
the Bomag compactors were operated in the static mode, as no specific density
was specified.

Taking time off

After allowing the pozzolan/lime/soil mixture to cure for
three days, a 7-in. lift of 2-A stone was laid over as the sub-base. An
Ingersoll-Rand ABG tracked paver Titan 423 was used for laying much of the
stone. Other methods also were used but proved not to be as efficient. Unique
in its design, this versatile machine can lay with precision many paving materials
including asphalt paving, cement-treated paving mixes, graded aggregates,
roller compacted concrete and sand or gravel. On this project, its usage was
confined to laying the stone sub-base.

There were several reasons for selecting the ABG Titan
paver, according to Pat Riley, project manager for Iafrate. One was for getting
a sub-base with a precise thickness that would be smooth and thus require a
minimum time in fine grading it with a motor grader. This precision sub-base is
the foundation for enabling the contractor to pave it to meeting the tightly
written smooth-ride specifications. John Ozimok, P.E., construction manager
with the Pennsylvania Turnpike, said the specifications for a smooth ride are
quite demanding. It called for no more than 28 in. deviation per mile. "We
require that this new roadway has a high-quality ride. That is one reason we
are stabilizing the subgrade in the first place. We also want a low maintenance
roadway with superior longevity, so we specified Superpave. We are rebuilding
this highway to eliminate high maintenance costs brought about from adding new
asphalt overlays every four years," he explained.

Superpave was selected over concrete paving for good reason,
according to Ozimok. He pointed out there has been a minimum of traffic-flow
interruptions during the paving process. This would not be true during paving
with concrete, he said. Further, Superpave over its useful lifetime is easily
maintained and eventually, when required, it can be overlaid late at night and
very early in the morning when the traffic flow is at a minimum.

Both Iafrate's Dave Lepage, operations manager, and Charlie
Liebel, project superintendent, said the Titan paver performed well with
quality results.

An important function of this paver is it partially compacts
the loose stone (or paving) as it is laying it. This reduces the number of
passes required for a compactor to bring the loose stone to specified density.
Compaction with the ABG Titan paver brought the stone to 94%. Starting at that
density, the Bomag compactors compacted the stone to 99%.

Production was very good, with 2,400 tons of stone laid with
the ABG Titan paver. It only required the Bomag compactors to make four passes
over the single 7-in. lift. Of the four passes, two were made static and two
were made in the vibratory mode. 

So far this soil stabilization program is a success, albeit
there was a learning curve involved for the owner and contractor. In some
sections of the highway, the ground conditions were so soft that portland cement
was incorporated and the thickness design of the soil-stabilized layer was
increased from 12 in. to 15 in. 

Since the soil stabilization program has been so successful
on this first project, the turnpike decided to forge ahead on the second
project using the same method. That project recently has gotten under way with
another contractor. It will be a 10-mile section  east of the first project.