Averting a washout on Lincoln Drive

This was the nightmare that David J. Perri, an engineer for the city's Streets Department, did not want to read about in the newspaper.

It went like this:

In the middle of the night, heavy rains drench the region.

The Monoshone Creek, which runs into Wissahickon Creek along Lincoln Drive, washes out.

A 15-foot stone retaining wall collapses into the Monoshone, taking with it part of Lincoln Drive. Not seeing the crater in the road, an unsuspecting driver plunges into the creek bed.

Commuters can rest easy.

Lincoln Drive isn't going anywhere. But the possibility of that worst-case scenario playing out was a little too close for comfort for Perri, the city's chief transportation engineer.

Blame last year's unusually wet weather.

In August and September, Philadelphia recorded 29.6 inches of rain - that's 70 percent of the annual rainfall in only two months.

After Tropical Storm Lee in September, city engineers made a disturbing discovery along Monoshone Creek: A century-old stone retaining wall had dropped more than five inches.

Not only that, but it was starting to rotate away from land, and rushing water had carved out gaps underneath the wall.

"The wall was beginning to collapse," Perri said.

The Streets Department knew it had to act fast - but just how fast didn't become clear until engineers started to prepare the wall for repairs in November.

They discovered that Lincoln Drive was not sitting on rock.

It was sitting on soil.

That meant that if the retaining wall had collapsed, it would have taken the road with it, Perri said.

"We needed to move fast," he said.

When it was built more than 100 years ago, Lincoln Drive was a picturesque, if not tortuous, route for horse-drawn buggies into the city.

Now, the Lincoln Drive speedway handles 40,000 cars a day. Cars heading southbound come within five feet of the retaining wall and creek bed.

A shutdown of Lincoln Drive for repairs would have had a serious economic impact, with an estimated $7 million in lost productivity, Perri said.

As it was, the roadway did not have to be shut down.

JPC Group Inc. was contracted by the city for $1.2 million to repair 300 feet of damaged wall - roughly the length of a football field.

Work began in February and ended early this month. The creek was dammed and water diverted to allow crews to install 38 tiebacks: plates attached to rods, 50 to 75 feet long, that were anchored in bedrock.

Perri said the base of the wall was stabilized and covered with a layer of concrete and sand, injected to fill voids.

When the creek was drained during construction, engineers discovered another problem: The floods had carved voids under the bike path as well.

"We would have lost the trail at some point too," Perri said.

Today, giant tiebacks that look like the bolts in Frankenstein's neck stick out every few feet. Perri said the city plans eventually to improve the look of the concrete facade to make it blend in better with surroundings.

The repaired section of the creek is just south of historic RittenhouseTown in Fairmount Park.

Perri, who has worked for the city for 31 years, feels as if the city has averted a disaster.

"This," he said, "was the worst we've ever seen along the Monoshone."

David J. Perri,

an engineer

with the city's Streets Department, explains the emergency repair work on the retaining wall on Lincoln Drive. See the video at

www.philly.com/lincolndriveEndText