Support for elevated rail lines

July 2009 » PROJECT CASE STUDY
Soil reinforcement cost-effectively controls settlement of soft and compressible soils beneath an earth structure.
Wayne Duryee, P.E.
Precast concrete T-Walls and mechanically stabilized earth walls provide grade separation for rail traffic in Wichita’s Center Corridor.

The merger of the Union Pacific and Southern Pacific Railroads in 1996 was expected to increase rail traffic significantly along the Central Corridor route through Wichita, Kan. Options considered to accommodate increased rail traffic and alleviate disruption of street traffic included construction of a rail bypass. But, the most feasible solution consisted of improving the existing train routes by elevating the tracks and enhancing rail traffic flow through Wichita’s active downtown corridor. Street traffic flow was also enhanced with the elevated rail lines passing above cross streets with grade-separation structures.

Project
Central Corridor railroad grade separation, Wichita, Kan.

Participants
Dondlinger and Sons (general contractor), Wichita, Kan. Peterson Contractors, Inc. (RAP installer), Reinback, Iowa HNTB (project engineer), Kansas, Mo.

Product application
Design-build Rammed Aggregate Pier (RAP) soil reinforcement provides support for Central Corridor railroad grade-separation retaining walls.

The primary Central Corridor project extends for three miles through industrial and residential areas with tight right-of-way limits, thereby requiring the use of retaining walls to support the elevated tracks. Three existing bridges were modified or replaced, three new bridges were added, and five street closures were included in the design. A total project budget of $98 million was established for the project.

Elevating the tracks while maintaining existing rail traffic required four phases of construction that included installing more than 19,000 linear feet of precast concrete retaining wall (T-Wall) as high as 25 feet above grade. Additionally, phased wall construction required approximately 9,000 feet of temporary, mechanically stabilized earth (MSE) retaining walls, also as high as 25 feet.

Design bearing pressures on the foundation soils beneath the retaining walls are as high as 6,000 pounds per square foot.

However, soil conditions along the wall alignment challenged the geotechnical engineer and wall designer. The Central Corridor site is underlain by highly variable sand and clay fill overlying irregular thicknesses of compressible silt and clay soils as deep as approximately 27 feet. These unsuitable bearing materials are underlain by medium-dense to dense sands, underlain by shale bedrock. The groundwater level is approximately 13 feet below grade.

Construction of the grade separation retaining walls on top of the uncontrolled fill and compressible silt and clay soils could result in excessive total settlements of as much as 6.5 inches. Excessive differential settlements also were expected because of the variability in the soil profile. The high bearing stresses imposed from the retaining walls and rail loads exceeded the unreinforced soil bearing capacity. Furthermore, long-term consolidation was anticipated within these compressible materials, which could cause long-term maintenance issues for the railway.

Design criteria required limiting total settlements to 2 inches and minimum factors of safety of 1.5 against global instability and 3.0 against bearing capacity failure. Construction activities also were required to remain within the existing confined railroad right-of-way, with minimal impact on adjacent structures.

Project engineers HNTB evaluated three alternatives to support the heavy loads over the unsuitable bearing materials (see Table 1). All three alternatives were deemed to provide acceptable performance for the project, but cost, schedule, and constructability were concerns.

Because of the disadvantages of the deep foundation and massive over-excavation and replacement solutions, the design team opted for the Rammed Aggregate Pier (RAP) design-build approach as a ground improvement solution for the project. Figure 1 depicts a typical cross section of the railroad grade-separation retaining wall showing the RAP elements extending through the uncontrolled fill and compressible silt and clay layers.

Figure 1: Typical cross section of the railroad gradeseparation retaining wall shows the RAP elements extending through the uncontrolled fill and compressible silt and clay layers.

RAP soil reinforcing elements were installed in a grid pattern prior to construction of the retaining structures to reinforce and stiffen compressible foundation soils, to reduce the magnitude and duration of settlement, and to improve bearing capacity, global stability, and the composite shear strength of the foundation soils.

A total of 7,700 RAP elements with a diameter of 30 inches were installed in multiple phases to facilitate phased wall and track construction. RAP depths ranged from about 6 feet to 30 feet, as needed to penetrate the uncontrolled fill and compressible silt and clay layers. Layers of high-quality aggregate (road base course) were placed into each RAP cavity in thin lifts of about 1-foot compacted thickness, and each layer was compacted with a specially designed, high-energy beveled tamper foot using vertical ramming energy, resulting in high levels of strength and stiffness (see Figure 2). The ramming process increases the stiffness of the surrounding soil and the lateral stress within the matrix soil surrounding the stiffer RAP elements.

Figure 2: Three-step installation procedure of RAP elements.

The RAP elements were concentrated at a higher area replacement ratio (tighter spacing) beneath the wall face and within the reinforced backfill zones for total and differential settlement control, bearing capacity, and global stability. Behind the main reinforced backfill, a lower area replacement ratio (wider spacing) was used to satisfy the settlement control criteria set forth in the project specifications.

The RAP soil reinforcement system design was verified through quality control, vibration monitoring, field modulus tests, and settlement monitoring. A verification program was implemented to provide full-time quality control during RAP installations, as well as monitor performance of the ground-improvement system during and following wall construction. Early during construction, vibration monitoring adjacent to RAP installations proved that the peak particle velocities measured at the ground surface were well within the project requirements and would have no negative effects on neighboring structures.

Full-scale modulus load testing was completed on three, non-production RAP elements to verify design stiffness assumptions. Modulus test results revealed that RAP stiffness modulus values were two to three times greater than the stiffness modulus value assumed for design. Vertical wall settlements were continually monitored during construction throughout the project alignment at minimum spacing intervals of 150 feet to verify that design and performance criteria were met. Measured settlements along the face of the precast concrete T-Wall following completion of the west wall and embankment fill revealed total settlement magnitudes generally ranging from 0.75 inch to 1.5 inches — less than the project settlement criterion of 2 inches.

Peterson Contractors, Inc (PCI), licensed installer, completed RAP installation in two phases: Phase I was completed in less than four months and phase II was completed in three months. Two crews performed the installations in 133 working days, averaging 60 elements per day. Installation of RAP elements near the tracks did not interrupt train traffic, which averaged about 40 trains per day. PCI finished Phase I two weeks ahead of schedule and accelerated the overall construction schedule by one month.

Installations of RAP elements near railroad tracks did not interrupt train traffic.

The RAP soil reinforcement solution performed as intended and met or exceeded the project performance requirements. It proved to be a cost-effective solution for reinforcing and controlling settlement of the soft and compressible soils encountered at the Wichita Corridor project.

Wayne Duryee, P.E., with HNTB in Kansas City, Mo., can be contacted at wduryee@hntb.com. Aaron Gaul, P.E., with Geopier Foundation Company — Midwest, can be contacted at geopier@kc.rr.com. Jorge R. Parra, Ph.D., P.E., with Geopier Foundation Company, can be contacted at jparra@geopier.com.


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