Heavy Haul MSE Walls Provide Industrial Site Infrastructure

Heavy Haul MSE Walls Provide Industrial Site Infrastructure


Aerial view of the heavy haul bridge under construction
Reinforced Earth Product in Field
Location: 
Potter Township, PA
Owner: 
Shell
Contractor: 
Trumbull Energy Services
Subcontractor: 
Mascaro Construction
Consultant: 
Jacobs Engineering Group

The Marcellus and Utica shale formations, which extend across southern New York, northern and western Pennsylvania, western Maryland, eastern Ohio and almost all of West Virginia, offer a plentiful supply of ethane, a natural gas liquid and a primary raw material in the production of plastics. To be usable, however, ethane’s large molecules must first be “cracked”, or broken apart, and its carbon and hydrogen atoms rearranged to form first ethylene, then polyethylene, which is delivered from the cracking process in the form of pellets.  Polyethylene pellets are the primary feedstock in the fabrication of products ranging from food packaging and containers, to automotive components, garden furniture and the absorbent beads in baby diapers, meaning there is substantial demand for this shale gas end product. 

On the banks of the Ohio River, about 30 miles northwest of Pittsburgh in Potter Township, PA and immediately adjacent to the interchange of I-376 and SR 18, Shell Chemicals is building an ethane cracker.  It’s the first large petrochemical facility built outside the Gulf Coast region in 20 years, and allows Shell to take advantage of not only the vast regional supply of ethane, but also the more than 70% of North American polyethylene customers who are within a 700-mile radius of Pittsburgh.  Meeting market demand for its planned annual production of 1.76 million tons of polyethylene pellets requires substantial upgrades to on-site transportation infrastructure to assure that Shell’s workers have unimpeded site access and that its product gets offsite and to market efficiently and timely.  These upgrades – along with the requisite construction cost-effectiveness – are being achieved in part using Reinforced Earth® MSE structures for portions of the nearly 400-acre site development.

Previously home to a zinc smelter, site environmental remediation and grading required significant hauling of excavated soil to prepare for cracker construction. In addition, the site is traversed by both SR 18 and a rail line, requiring several bridges, retaining walls, and even some traffic maintenance actions to accommodate ongoing rail operations (Fig. 1). Therefore, a “heavy haul” bridge was built to carry the West Haul Road over the relocated rail line and its sidings, as well as over SR 18 (Fig. 2). This road then connects on grade to SR 18’s new alignment, providing permanent access to the cracker plant site.  Elsewhere on the site another bridge was required to carry the East Loop Road, a primary internal roadway, over the railroad and highway. Additionally, a 2,000-foot long, up to 36 feet tall “Wall B” supports the East Loop Road where it runs parallel and close to the railroad before reaching the bridge. These structures were all designed to support heavy industrial vehicle loading.

Designer Jacobs Engineering Group (Houston, TX) selected Reinforced Earth MSE structures for Wall B and the four bridge abutments. Following standard Pennsylvania DOT (PennDOT) procedure, the bridges were designed with “mixed” MSE abutments – meaning the bridge seats are supported on piles which extend down through the MSE structures that form the abutment faces, wingwalls and approach walls. Working through general contractor Trumbull Energy Services (Pittsburgh), subcontractor Mascaro Construction (Pittsburgh) installed the structures, which consist of 5-foot by 10-foot precast concrete panels and galvanized steel reinforcing strips.

High Loads

MSE retaining walls and bridge abutments at an industrial site like this one often are required to carry much higher loads than would be the case along a typical highway, and Jacobs Engineering specified particularly high construction and operational loads for the West Haul Road bridge, the East Loop Road (on top of Wall B) and the East Loop Road bridge. Where a typical PennDOT highway MSE wall will be designed for 3 feet of live load surcharge, the walls on this site were designed for over 14 feet. These loads produced not only higher tensile stress in the steel reinforcing strips of these three structures, but also higher pullout forces on the reinforcing strips. Similar to increasing both the number of rebars and their development length in a reinforced concrete structure, these Reinforced Earth walls used more reinforcing strips per panel to carry the tensile load, as well as longer strips to provide more embedment length for resisting pullout.

Bridge Piles

At the abutments, piles were driven prior to wall erection, which is standard practice to prevent pile-driving damage to installed reinforcements. Corrugated metal pipe sleeves protect the piles from downdrag forces that may be caused by compaction and settlement (Fig. 3).   Of course, the sleeves also increase the effective diameter of the piles, which can add to the degree of skewing of reinforcing strips (Fig. 4).

Skewing Strips

Skewing strips up to 15° from perpendicular to the panels is consistent with normal design parameters, while skewing in excess of 15° requires situation-specific calculations.  Where large-angle skewing is necessary to be able to clear obstructions, reinforcement connection points on the panels may be shifted, or additional reinforcements may be added, to maintain an even distribution of reinforcements throughout the soil mass.

Crash Wall

Three of the four abutments – the north abutment on the West Haul Road bridge and both abutments on the East Loop Road bridge – required cast-in-place reinforced concrete crash walls (Fig. 5) due to the rail lines passing under the bridges.  A crash wall is intended to deflect a derailed train, much as a Jersey barrier does for errant vehicles on a highway, while also distributing impact forces to minimize the risk of rolling stock damaging the MSE wall face.  Crash wall rebar (Fig. 6) is tied to the panels using threaded anchors, tightened into ferrule loops embedded in the panel face approximately every 15 to 18 inches vertically and horizontally.

 

Two wire-faced TerraTrel® MSE walls were also required at the Shell site.  The original rail line location was just north of the planned West Haul Road bridge and directly in the path of that bridge’s approach embankment. To maintain rail traffic during bridge construction, Jacobs called for the north abutment wingwalls to temporarily terminate about 55 feet behind the abutment face.  One wire wall provided the needed temporary support to allow the abutment to be completed and the bridge deck installed over the relocated highway and railroad.  Once these relocated facilities were open to traffic, the remaining 148 feet of the Reinforced Earth wingwall was completed, including installation of a slip joint (Fig. 5) at the junction with the first-constructed portion of the wall, permanently burying the wire facing.

The remaining wire wall was constructed to establish necessary grade separation for a parking lot for Shell’s craft workers. Located in a sloped area west of the main cracker site, this wire wall balances cuts and fills. It is 34 feet high through most of its 550-foot mid-section, with long wing walls folded back roughly 90° to support worker driveways and the shuttle bus loop serving several parking areas.

Reinforced Earth MSE structures played a key role in the site development for the Shell ethylene cracker outside Pittsburgh. Requiring only a graded soil foundation and a simple, unreinforced concrete leveling pad under precast facing panels, MSE structures evenly distribute applied loads to the foundation soils and offer contractors a rapid and simple construction process. For Shell and other industrial owners, using Reinforced Earth mechanically stabilized earth structures is an important component of economical and timely site development.