Segmental retaining wall flexibility provides solution for St. Louis commuter rail expansion.
BY DAN BENSMAN
MetroLink Cross County commuter rail extension, St. Louis
Aspen Consultants, Fenton, Mo.
To complete an 8-mile-long extension of a commuter rail line, designers chose Keystone Compac, a straight-face, highstrength segmental retaining wall system, to accommodate tight corners and difficult working conditions.
The ambitious, 8-mile-long Cross County extension of the St. Louis MetroLink commuter rail system has required massive earthmoving and extensive earth retention solutions. The project includes major reconstruction of an existing station and the addition of nine new stations. Additionally, numerous retaining walls are being used to support hillsides, rail bends, and walkways. But tight corners and difficult working conditions in some areas have complicated the design and construction process, which began in 2003 and is scheduled for completion in 2006.
The strength, cost-competitiveness, and installation speed of segmental retaining walls (SRWs) made them the right solution for this complex project. Project planners chose Keystone Straight Face Compac units manufactured by Kirchner Block & Brick, Inc., in Bridgeton, Mo. The Keystone Compac units were manufactured to meet the Missouri Department of Transportation’s specifications of 4,000 psi minimum compressive strength and less than 5 percent water absorption.
I was involved with more than 20 separate walls along the project that used Keystone,” said Ed Austin, P.E., a retaining wall engineer for Fenton, Mo.-based Aspen Consultants. SRWs lend themselves well to many different types of applications, from small walls for rail bed support to pedestrian walkways.” During the past 20 years, segmental retaining wall systems have been used to maximize buildable space at a relatively low cost compared with conventional retaining wall solutions. The systems are comprised of dry-stacked (mortarless) concrete masonry units with various schemes of mechanical interlock, relying primarily on the geometry and mass to resist overturning or sliding forces. The units typically are used in combination with horizontal layers of geosynthetic soil reinforcement, such as geogrids, which extend into the backfill to increase the effective width and weight of the gravity mass. The modular wall units are offered with numerous aesthetic options-various colors, shapes, textures, sizes, and geometrical presentations.
SRW units have some advantages compared with other earthretention options. Their precast concrete construction makes them durable and environmentally friendly, as well as less expensive and less difficult to install than natural or quarried stone wall alternatives.
Additionally, since on-site soil often can be used for backfill, the costs of importing fill, removing excavated materials, using heavy construction equipment, and extensive formwork frequently are minimized or eliminated. With the help of qualified engineers, SRWs can be used to build tall, near-vertical walls spanning hundreds of thousands of square feet.
Tackling tight corners and tough conditions
The geometry of two walls in the MetroLink project required construction of acutely angled corners. Special consideration was taken at these corners because of the lack of space for the standard reinforcement. In these cases, geogrids from opposing wall sides were overlapped but not connected to both sides at one time to avoid a restrained loading condition. My main concerns were from the contracting end-splitting the blocks correctly, bonding the blocks without too many small pieces, and installing the geogrids,” Austin said. From an engineering standpoint, it was difficult to determine the effects of the live loads and soil loads on the walls at the corners.” Soil-reinforced SRWs typically consist of geosynthetic materials (geogrids) connected to the retaining wall units and placed in horizontal layers in the compacted backfill. A sufficient number of geogrid layers must be specified to maintain internal stability and be of sufficient length to satisfy external stability considerations from the soil forces behind the wall. The calculated tension forces in these reinforcement layers must be less than the allowable design strength of the geosynthetic and also less than the allowable connection strength between the geosynthetic and the retaining wall unit.
In addition to the challenge of building uniquely designed walls, project wall installers faced difficult working conditions at several MetroLink job sites. Our biggest problem was with tricky and often difficult working conditions,” said Scott Rozier of Rosch Company, LLC, the retaining wall construction contractor. There were several instances when my crews needed to work high off the ground, jammed underneath the bridge decking or above active roadways. Although we always maintained safe working conditions, tough situations like that can take time and make tighdeadlines even tighter.” Design flexibility was especially useful in the extensive St. Louis MetroLink expansion project, which boasts more than 60,000 square feet of Keystone retaining walls. Walls not only were large and elaborate, but their design requirements changed over time. Additionally, Keystone SRWs work well with St. Louis’ climate-hot, wet summers and freezing conditions in the winter. The SRW units are manufactured of high-compressive strength, lowabsorption concrete that is resistant to spalling, abrasion, and freeze-thaw cycles. Unlike the movement cracks associated with some rigid retaining wall structures, the segmental system’s flexibility allows the units to move and adjust relative to one another without visible signs of distress.
Upon completion of the 8-mile MetroLink Cross County expansion, owner/developer Bi-State Development Agency expects to add 18,000 daily riders to the St. Louis commuter rail system. More information about the $700 million project is available online at www.crosscountymetro.org.
Dan Bensman is the communications specialist for Keystone Retaining Wall Systems, Inc. He can be contacted at firstname.lastname@example.org.