Proposal To Tunnelize Interstate 95 Through Philadelphia

Were Ben Franklin to trek down Elfreth's Alley in 2024, as he did throughout the 18th century when it was called Cherry Street, he would be shocked to encounter a great brick wall blocking the Delaware River. From his pre-industrial perspective, he would be dumbfounded to learn this embankment carries an expressway running from Miami to Maine and contains a subway tunnel.

As Franklin came to grasp the geographic and population growth of Philadelphia during the 234 years since his death, not to mention of the nation′s railroads and interstate highways, he might at first be impressed. However - and even though colonial America was hardly a quality-of-life exemplar by modern standards - as he comprehended the scope of practical and environmental costs forced by the behemoth road upon the tens of thousands populating the neighborhoods along its 27-mile route through the city, he would become deeply disturbed.

As currently configured, Interstate 95 through Philadelphia is a major barrier. It divides neighborhoods, isolates the Delaware riverfront, and diminishes the pleasure, convenience, and safety of walking. It causes hundreds of thousands of motor vehicles per day to appear on neighborhood streets and is an immense source of noise and air pollution. Said harms – visceral and immediate – should spur at least as much alarm as fears of long-term climate change.

This article’s purpose is to describe one viable technical solution that would all but eliminate the quality-of-life costs that I-95 currently imposes on Philadelphia residents, while preserving and improving it as an interstate road.

Spanish engineers routinely “soterrar” or tunnelize highways and railways. What they refer to as “falso túnel” is in America known as cut-and-cover, which involves an open excavation and “bottom-up” tunnel construction. But in busy urban environments like I-95 through Philadelphia, the tunnel's permanent roof can be built first, allowing excavation to proceed under cover and the space above (street level) to be used for any purpose (i.e. “top-down” construction).

The reinforced concrete tunnel structure is made of three principal elements: walls, roof slab, and base slab. First, it is necessary to install three continuous screen walls (known in America as slurry walls or secant pile walls), followed by a roof slab to support street level. After finishing excavation underneath the roof slab, a reinforced concrete base slab is built at tunnel roadway level, and the structure is complete. (Enabling the subsequent build-out of drainage, waterproofing and finishes, electrical, ventilation, and pumping systems.)

Because the roof slab restrains movement of the screen wall during excavation, top-down, embedded wall construction usually precludes the need for temporary propping, and reduces the potential of inducing ground movements large enough to damage adjacent buildings. Furthermore, because time-consuming wall installation can occur while traffic still uses part of the viaduct (in viaduct sections) and because the street level space on top of the roof can be used temporarily for detour lanes, the top-down method minimizes the duration of lane outages and facilitates the possibility of always maintaining a minimum of two northbound and two southbound lanes.

Note that the slurry wall method is but one way to achieve a pre-installed embedded retaining structure (to become the tunnel wall). However, drilled concrete secant piles are used in the same way with top-down construction. Low-noise installation is an advantage of both technologies (in contrast to the noisy installation of driven piles). Since neither type guarantees watertightness, the completed tunnel relies on interior finish walls, waterproofing and pumping systems to stay dry.

A sequence is illustrated for a typical viaduct section around Wolf Street in South Philly (1). Maintaining five travel lanes on the existing viaducts, piling rigs first install the left and center walls (2). Then, the roof slab′s left half is constructed on grade, allowing the left tunnel to be excavated down to roadway level (3), where the base slab, pavement and finishes are completed, resulting in four new subterranean expressway lanes (4). After installing the right wall (4), the same process is repeated on the right side (5), I-95 is restored to full capacity, and the new riverfront land is put to use as a low-traffic boulevard and/or linear park (6).

The sequence would repeat in successive phases, each several miles long or between interchanges, requiring transition ramps to link new sections of tunnel with existing viaduct or at-grade highway. The temporary reduction in highway capacity necessary to tunnelize I-95 (i.e. lane outages) would therefore occur only within the current project zone, not simultaneously along all 27 miles.

Though illustrated at a representative viaduct section, tunnelizing I-95 would be complex at a few spots, one challenging example being a simultaneous interface with the Market-Frankford line′s subway-elevated transition, foundations of the Ben Franklin bridge Philadelphia anchorage and approach viaduct, and the -676 interchange. Interchanges with I-76 (Walt Whitman Bridge), the Betsy Ross and Commodore Barry bridges are further examples.

Observe the relative convenience, both financial and political, of burying an existing highway, for which virtually zero right-of-way acquisition is necessary, let alone the massive displacement of residents and expropriation of private land wrought decades ago during the original construction of I-95. ?

But wouldn’t the lane outages required to tunnelize I-95 cause intolerable congestion during the years of construction?

Despite the sudden loss of all eight lanes of I-95, the 2023 bridge collapse due to a truck fire on Cottman Avenue in North Philadelphia, which did temporarily rupture truck shipping, failed to produce “carmageddon”, according to an INRIX traffic analysis published weeks later. Here′s why the surrounding area wasn’t snarled for 12 days before I-95’s reopening with a temporary bridge: as people became aware of the outage, they made other plans. Throughput was zero for almost two weeks and life went on. (The same is reported to have occurred following the 1973 collapse of a portion of Manhattan’s West Side Elevated Highway, which no longer exists.)

Given the outcome of this real world “worst-case” capacity scenario (i.e. zero throughput), one may conclude, for the reasons listed below, that the lane outages necessary to tunnelize I-95 need not necessarily affect shipping at all, and would quickly become history for private car users as stretches of new tunnel enter in service.

• planned long in advance.
• always maintain a minimum of two lanes in each direction in project area.
• other traffic management, public awareness, etc.

A boringly obvious objection will be the tens of billions of dollars in direct construction costs that tunnelizing 27 miles of urban freeway would require. This kind of objection is, in the richest society in world history, a value judgement. Were enough people convinced on merit of tunnelizing I-95 (i.e. of nixing at its source the next couple centuries worth of environmental suffering for tens of thousands of Philadelphians), and prioritized it over, say, giving away money and “free” weapons to fuel dangerous wars globally, some portion of our several hundred billion dollar unauditable military budget (which doesn’t include $187 billion in additional gifts to foreign countries since 2022), for example, could be democratically redirected to the effort. Alternatively, if the citizenry demanded a freeze on highway expansion projects nationwide in favor of first prioritizing the tunnelization (or other transformational mitigation) of problematic urban highways, it would happen. We’re not there yet.

Keep in mind, furthermore, that US construction projects are significantly more expensive than similar projects in peer nations. The American construction industry and public agencies should take on the challenge of building with a cost efficiency approaching that of heavy civil works in countries like Spain, for example, which builds urban rail for roughly 1/3 of the per-mile cost of the same in US cities (ref Jacobin Magazine / Eno Center for Transportation).

Perhaps Elon Musk is only partially misguided in proposing tunnelized roads for Los Angeles. Tunnelizing I-5, I-10, I-105, I-110, I-405, I-605 and I-710 a la espa?ola would not only modernize LA’s urban freeways, but would also create the most extensive system of urban linear parks on the planet, among other advantages.

Although they won’t be read by the author, this article may evoke comments claiming that tunnelizing all of I-95 in Philadelphia is impossible, for this and that reason. The fact is, however, that it is possible, and it can be done with top-down construction. Armed with this knowledge, support of or opposition to the idea is, therefore, a reflection of one’s values, not of any technical or financial reality.

Josh Shapiro himself, the governor of Pennsylvania, was quoted as saying following the 2023 collapse of an overpass, “Pennsylvania can do big things; we can get stuff done.” Ben Franklin would certainly agree.

?Relevant side notes:

• Currently under construction is a project to cap another quarter mile of I-95 in Society Hill. Great, we′re 1.9% of the way there.
• There are tentative plans to demolish some well-used baseball fields in South Philadelphia in order to “expand” I-95.
• An I-95 tunnelization project was actually executed in central Boston 30 years ago.

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