2012 Issue

69 GEOTECHNICAL | continued on page 70 Thin-walled Shelby tube samples were obtained at depth intervals of about ten feet for laboratory testing. Soil com- pression characteristics obtained from one dimensional consolidation testing were used to estimate the consolidation settlement expected to occur as a result of embankment construction. Consolida- tion analyses estimated approximately 32 inches of primary settlement would occur beneath the new embankments, with an additional five inches expected to occur within ten years of construction. The time required to complete 95 percent of the primary consolidation was estimated to be about one year. Due to the anticipated settlement, two- stage walls were recommended for MSE walls within the Jordan River Basin. Two- stage MSE walls are initially constructed with flexible facing which can tolerate the expected deformations, with permanent concrete facing panels added after the majority of settlement is complete. The geotechnical design process evalu- ated multiple options to decrease the time required to complete the settlement. The project team elected to use prefabricated vertical drains spaced at approximately six- foot intervals to decrease the estimated pri- mary settlement time to about four months. A surcharge loadwas recommended to help reduce the long-term secondary consolida- tion settlements. The geotechnical report also recommended that the area between the frontage roads be filled to the future subgrade level within the JordanRiver Basin, inorder to reduce the settlement of the com- pleted frontage roads during constructionof future phases of the project. Global stability analyses determined that the proposed embankments may be unsta- ble for the short-term undrained strength conditions of the cohesive foundation soils within the Jordan River Basin. The design team developed an instrumenta- tion program to measure settlements, monitor short-term stability, and ensure that embankment placement did not occur at an unsafe rate. Instrumentation included seven inclinometers, five settle- ment measuring devices, and five vibrating wire piezometers located along each of the frontage road alignments. Stability analyses also indicated that the Jordan River Basin MSE walls would have acceptable factors of safety for the long term strength conditions, except at the Jordan River bridge abutments. The geo- technical report recommended deeper embedment of the MSE wall foundations and installation of high-strength geotex- tile as means of increasing the factors of safety at these abutments. The contract documents for the project provided the owner the ability to tempo- rarily suspend embankment fill placement if the measured deflections or groundwa- ter pressures were considered excessive by the engineer. The documents also al- lowed up to four months for embankment settlement to occur following completion of surcharge placement. Embankment fill placement began in April 2010. Fill placement was temporarily sus- pended for approximately two weeks at the maximum section of the westbound frontage road when instruments indicated excessive deflection rates. Once the de- flection rate subsided to acceptable limits, embankment construction continued, culminating with surcharge placement in September 2010. After approximately ten weeks under the full surcharge load, mea- sured settlements in themaximum fill areas were in the order of 27 to 35 inches. Bridge Foundations The 2100 North frontage roads will cross the Jordan River via two parallel steel plate girder bridges, each spanning ap- Jordan River bridge site with MSE Wall — November 2010.