MSE pile-supported embankment with MIRAGRID 8X

Overview

The I-15 CORE Project, a corridor expansion project of Interstate 15 in Provo, Utah, is the largest roadway construction project in the state. At the time of construction, it was the largest roadway construction project in the U.S and cost $1.4 billion.

Geneva Road is a major frontage road west of the I-15 Freeway. Kiewit Construction was awarded the contract to widen the 4 mi section of roadway on this UDOT design build project. The team engineer, Terracon Consultants, faced design challenges with grade separation over three high-use railroad tracks for Union Pacific Railroad (UPRR) and the new Utah FrontRunner commuter rail. Specifically, challenges included soft foundation soils with limited right-a-way boundaries and the risk of high potential seismic loads.

Challenge

To support the roadway embankment on the soft soils, a 30 ft high Mechanically Stabilized Earth (MSE) wall was designed using multiple layers of geosynthetics. The design used conventional timber piles integrated with MIRAGRID® 8XT geogrid reinforcement in the soil embankment and MSE wall technology. To reduce construction costs, timber piles were not used under the entire embankment; instead, only three rows of timber piles were designed four feet on center below the MSE wall face, and multiple layers of geogrid were used to distribute the embankment load to the pile foundation.

Over 2,000 timber piles were driven to a depth of 35 ft. Two layers of MIRAGRID 8XT biaxial geogrid were installed and backfilled above the wooden capped piles. The embankment construction commenced with MSE walls constructed over the timber pile foundation. Up to 4 layers of MIRAGRID 8XT geogrid was installed inside the MSE walls. The MIRAGID 8XT geogrid was placed in 3/8 in minus stone and sand backfill and spaced in 2 ft vertical lifts. Being a uniaxial geogrid (high strength in the roll direction), the MIRAGRID 8XT geogrid was placed 3 ft. back from the MSE wall facing panels and unrolled across the entire embankment.

Solution

A unique design of strategically placed timber piles used in conjunction with multiple layers of MIRAGRID 8XT geogrid resulted in an efficient embankment design on soft soils. The design allowed for single stage construction of the bridge embankment, without typical surcharge delays, to meet the project schedule.