The $1.75 billion I-15 Corridor Reconstruction (CORE) project is the largest highway project in Utah history. It encompasses a stretch of I-15, the major north-south commuter route between Salt Lake City and the Provo/Orem region in Utah County, one of the fastest-growing counties in the United States.
The Utah Department of Transportation's (UDOT) goal is to restore and rebuild 24 miles of highway, more than 60 bridges, 10 interchanges, and 130 mechanically stabilized earth (MSE) retention walls within a four-year schedule to be completed by the fall of 2014. UDOT selected Provo River Constructors (PRC), a design-build joint venture, to design and construct the project. PRC compressed this schedule to perform the work in three years with a completion date of Dec. 31, 2012. PRC addressed UDOT's other goals of maximizing scope and minimizing inconvenience to the public by extending the project south beyond the UDOT planned terminus at US 6 and I-15, and by keeping three lanes open in both directions during construction.
The PRC joint venture included Fluor Enterprises Inc., Ralph L. Wadsworth, Wadsworth Brothers, and Ames Construction. Major subcontractors include Fluor-HDR Global Design Consultants LLC (lead design) with Jacobs Engineering; Michael Baker Jr. Inc.; and Kleinfelder. The design-build joint venture reduced the design and construction schedule from longer than four and a half years to three years, thus eliminating one full construction season.
The CORE project's success is dependent on the design-build team finding creative ways to identify and resolve design and construction challenges in a seismically active geologic area.
Tight timeline/soft soils
The project's accelerated construction schedule and location within the alluvial deposits of Utah's Wasatch Valley pose several technical and management challenges with respect to structure foundations. I-15 between Salt Lake City and Spanish Fork sits within unstable lakebed deposits in the form of soft, saturated clay soils with loose, interbedded sand layers of the Utah Wasatch Valley. The transportation corridor also parallels the Wasatch Fault, which is less than 10 miles away. The Wasatch Fault is seismically active and capable of producing earthquakes with magnitudes greater than 7.0.
To meet the project schedule, the engineering team had to provide geotechnical and geologic seismic hazard recommendations within a strict deliverable schedule so that the civil and structural designers could provide their plans and specifications, and construction could begin four months after project go-ahead.
As with most highway projects, the site characterization and laboratory testing phases provided critical data for design and construction of the new or rehabilitated roads, bridges, and particularly, the required structural systems. However, few projects ever require the scale and scope that PRC implemented on the I-15 CORE. The field exploration effort included approximately 350 borings and cone penetration test (CPT) soundings, comprising more than 19,000 linear feet.
Field data was supplemented by numerous lab tests that were performed early during the geotechnical investigation, including moisture only, moisture density, plasticity index, sieve analyses, hydrometer analyses, consolidation, direct shear, consolidated undrained triaxial shear, and unconfined compression.
Consolidation testing allowed for accurate settlement estimates that were used for determining one- or two-stage MSE walls, impacts to structures adjacent to the I-15 corridor, and planned embankments. Both one- and two-stage MSE walls were utilized on the project and varied in height from about 5 to 30 feet. The two-stage MSE walls were incorporated into the project to allow for total and differential settlement to occur prior to placing the facing panel. Panel sizes ranged from 5 feet by 5 feet to 5 feet by 20 feet. The actual settlement was within 90 to 95 percent of the predicted settlement. The accuracy of the settlement data allowed PRC's MSE wall design team – a partnership between geotechnical, civil, and structural engineers, and the MSE wall designer – to minimize the number of two-stage MSE walls. Approximately 27 two-stage MSE walls were constructed for the project.
PRC has concerns about consolidation and long-term secondary settlements along the corridor, particularly along embankments. However, engineers found it difficult to obtain undisturbed samples of the clay soils, necessary for reliable analysis. Instead, geotechnical engineers relied heavily on in-situ testing (CPT soundings and vane shear). They also used stress history and normalized soil engineering parameters (SHANSEP) strength deformation properties to account for the increase in shear strength that occurs during the consolidation of the clay subgrade soils. After full analysis of the embankment geology, the team determined that prefabricated vertical (PV) drains and surcharge, and/or lightweight fill, would reduce the time (increase the rate) of primary settlement.
Additionally, bridge, sign, and high-mast foundations will require the use of driven pipe piles or drilled shafts. Based on the field and laboratory tests, geotechnical engineers offered a number of recommendations to reduce or minimize soil reinforcements and foundations. For instance, the bridge foundations comprised 12- or 16-inch-diameter, closed-end pipe piles that were to be driven into a sand/gravel bearing layer. These pile lengths will vary from approximately 50 to 150 feet below ground surface. Pile driving analysis (PDA) testing, performed in accordance with AASHTO requirements, helped verify shaft and tip capacity estimates.
The embankment geology analysis allowed the design team to:
- refine the requirements for settlement durations and PV drain requirements;
- reduce the amount of PV drains and surcharge duration needed to meet settlement requirements;
- consider using lightweight aggregates to reduce the amount of PV drains and surcharge;
- reduce the replacement ratio and number of stone columns required to mitigate for liquefaction and lateral spread; and
- minimize pile depth to only what was required for the settlement and/or axial capacity, and then verify depth during construction so piles were not over driven.
Ready to construct
Within four months of project go-ahead, the design and geotechnical team had completed released-for-construction packages using technical design reports that described the design and construction requirements for individual bridges and MSE walls. The team prepared individual bridge foundation reports along with early release foundation only structural drawings in order to expedite bridge construction, and construction could commence while the remainder of the bridge design was being completed.
Geotechnical deliverables included design recommendations presented in plans that were included in the structural drawing package and reports. Backup geotechnical information included boring logs, laboratory test reports, a discussion of design issues and methodologies, and calculations.
During construction, design engineers are working alongside contractors to address and develop remedial solutions as issues arise. In one instance, the team evaluated dynamic pile test results with respect to the verifiable pile capacity shown on the drawings. They also assessed the actual pile tip elevations with respect to the assumed pile tip elevations shown on the plans. Engineers also evaluated field data with respect to soft subgrade, cut slopes, and pre- and post-ground improvement borings with respect to the adequacy of the ground improvement.
To counteract potential and probable seismic activity, engineers recommended stone columns and jet grouting to mitigate the potential for liquefaction and lateral spreading at several bridge locations. This ground improvement would limit lateral spreading and help achieve allowable safety factors. The engineers utilized the plasticity index and hydrometer tests in assessing the liquefaction potential of the onsite soils and the lateral and vertical extents. The engineers were required to drill one boring for every 2,200 square feet (for both pre- and post-ground improvement conditions) within the ground improvement areas. The pre-ground improvement data demonstrated that the liquefiable layer was not continuous and that lateral spreading would not be an issue, eliminating the need for ground improvement at one of the bridge locations.
To verify that embankment, MSE wall, and surcharge construction did not induce slope stability or settlement issues, engineers installed and monitored a variety of instruments that included the following:
- 150 inclinometers,
- 220 settlement manometers,
- 25 vibrating wire settlement plates,
- 15 vibrating wire piezometers, and
- 15 magnet extensometers.
The monitoring system helps control fill placement to maintain slope and bearing stabilities. These monitors also help manage the surcharge removal as ground improvements reach 98 percent of the primary settlement.
Throughout the construction, PRC facilitates a comprehensive and detailed quality management plan that requires that every submittal be reviewed by the independent quality assurance firm and UDOT.
PRC continues to maintain the tight construction schedule. The project is on schedule for completion in December 2012. The design-build team is about 40 percent complete with the I-15 CORE construction. Construction began in April 2010. To date, 29 of the total 63 bridges are underway, 637,000 square yards of concrete pavement have been placed (90 lane-miles), and work on all 10 interchanges is complete. Crews plan to place 1.2 million square yards of concrete before the end of the year, double the total that has been placed on the project so far.
James M. Schmidt, P.E., P.Eng., D.GE., is the vice president and senior principal geotechnical engineer for Kleinfelder. He can be contacted at email@example.com.