West County Connectors

West County Connectors


Ramp from NB I-405 to WB SR-22 Flies over I-405
Reinforced Earth Product in Field
Location: 
Orange County, CA
Owner: 
Orange County Trans. Authority / California Dept. of Transportation
Contractor: 
Atkinson Construction
Precaster: 
Pro-Cast Products

The Orange County Transportation Authority, in a joint venture with the California Department of Transportation (Caltrans), is nearing completion of the $277 million West County Connectors project.  Funded in part by $50 million from the Federal Highway Administration under the American Recovery and Reinvestment Act, this project is significantly improving traffic flow and safety through the SR-22/I-405/I-605 interchange connecting to Garden Grove, Westminster, Seal Beach, Los Alamitos, Long Beach and Rossmoor.  Eight traffic barrier-topped Reinforced Earth mechanically stabilized earth (MSE) retaining walls, in addition to providing safe, strong, long-lasting and economical support of roadways throughout the interchange, add to the beauty of the area by incorporating Southern California's seacoast scenery into the walls' architectural finish.

High occupancy vehicle (HOV) lanes are prevalent in Southern California, but slowdowns often occur where HOV lanes end and carpools must merge back into the mainline.  The West County Connectors project links the HOV/carpool lanes on the San Diego Freeway (I-405) with those on the Garden Grove Freeway (SR-22, Figure 1) and San Gabriel River Freeway (I-605), eliminating weaving from HOV lanes to exit ramps and allowing carpools to flow freely among the three freeways.  Specific improvements include constructing two new direct carpool connectors, adding a second carpool lane in each direction on I-405 (between SR-22 and I-605), reconstructing on- and off-ramps, building new retaining walls and sound walls, and adding/enhancing aesthetics and landscaping features.

Several of the Reinforced Earth walls on the project support flyover ramps.  Flyovers are typical in urban interchanges: narrow (1 or 2 lane) roadways that branch off the mainline, rise up and over adjacent roads, thread their way over, under or around other structures and roads, then land on a new mainline heading in a different direction.  These structures are quite easily and quickly constructed using back-to-back Reinforced Earth retaining walls.  This configuration, essentially an embankment with two vertical faces instead of two sloping faces (Figure 2), creates a narrow footprint and applies less total load to the foundation soil compared to the sloping-face embankment (and much less than cast-in-place concrete retaining walls).  Most work to construct back-to-back Reinforced Earth walls can be completed from inside the walls, a significant benefit on congested urban construction sites.  Reinforcing strip length is typically 70% of wall height, so a 1-lane ramp that rises to 35 or 40 ft. likely has its reinforcing strips meeting or overlapping in the center of the ramp fill.  Reinforced Earth design procedures treat each wall independently, except that both walls benefit from the lack of horizontal earth pressure that would normally be applied if the walls were retaining soil beyond the end of the reinforcements, as in a single (non-back-to-back) wall configuration. 

The project contractor, Atkinson Construction of Foothill Ranch, CA, erected the back-to-back walls simultaneously since they share the backfill between them.  Standard practice is to tilt facing panels in toward the backfill to prepare for the normal outward movement resulting from backfill compaction, leading to a plumb wall face.  The small outward soil movement is necessary to transfer load to the reinforcements, explaining why the reinforcing strips from back-to-back walls are not connected to each other.

One of the West County Connectors ramps crossed an area of settlement-prone soils.  To reduce the magnitude of the settlements (by further reducing the load on the foundation imposed by the back-to-back retained fill ramp structure), Caltrans specified the use of "lightweight fill (cellular concrete)", LFCC, instead of soil backfill, for the full width between the Reinforced Earth back-to-back walls.  With a specified unit weight of 30-42 pcf, the LFCC had only 25-33% the weight of ordinary granular backfill, achieving the desired load reduction.  Placed (pumped) in a near-fluid state, this lean concrete had a specified 28-day compressive strength of 60-120 psi.  Due to the fluid placement, compaction was not necessary, while wall plumbness could be maintained by bracing.  The challenge was positioning and holding the reinforcing strips while the LFCC was pumped in, and until it cured.

The fundamental spacing of Reinforced Earth reinforcing strips is in layers, spaced 30 in. on center both horizontally and vertically, and that spacing is evident in Figure 3 which shows the LFCC-filled back-to-back walls.  To position the strips, the contractor drove rows of vertical rebar stakes along lines parallel to the wall face and approximately 6 ft. apart. Horizontal crossbars were tied to the stakes to support the strips in an essentially horizontal position, just as if they were lying on the surface of compacted backfill.  Panel plumbness was maintained by a combination of external braces for bottom panels and internal ties anchored in the LFCC when external bracing was impractical (both bracing types seen in Figure 3).  Lift thickness of the concrete mixture was limited to 30 in. and, to avoid strips being in or near the cold joint between lifts of concrete, every layer of strips had to have a minimum of 6 in. of LFCC both above and below it.

The precaster for this project, Pro-Cast Products of Highland, CA, produced just over 85,000 sq. ft. of precast facing panels for the West County Connectors Reinforced Earth walls.  This was a complex task since walls incorporated several form liner treatments.  One was custom-designed to portray California's native sea lions swimming through beds of kelp (Figure 4).  Adjacent panels imitate slate using flakes of mica placed in the form liner prior to concrete placement, giving the concrete surface slate's characteristic sheen.  The fractured fins finish, used on three walls including the one visible in Figure 2, faces the neighborhoods, while the sea lions finish faces traffic as it speeds through the SR-22/I-405/I-605 interchange. 

Long service life, rapid and economical construction, creative architectural choices and the design and construction professionals of The Reinforced Earth Company made Reinforced Earth walls the right choice for this project.  The citizens of Orange County will benefit for many years to come.

Long service life, rapid and economical construction, creative architectural choices and the design and construction professionals of The Reinforced Earth Company made Reinforced Earth walls the right choice for this project.  The citizens of Orange County will benefit for many years to come.