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Excavation/Structural Support


Sunflower, AR: Jet Grouting For Ground Improvement, Bridge Over Whiskey Springs - AZ St Hwy 87

jet-excavation-structural-support-sunflower-az

Jet grouting was required for improvement of the foundation soils of an MSE retaining wall and wing wall for a highway bridge abutment. The deformation of the MSE wall was caused by the load imposed by the MSE wall backfill on the poor foundation soils (colluvium). The movement of the wing wall likely resulted from compression of the loosely placed backfill soils beneath the wing wall foundation. Jet grouting was selected as the most feasible method for mitigation to stop further deformation of the MSE wall and wing wall.

Prior to starting full-scale production, a jet grout test program was performed in the field to determine the grout parameters and overall effectiveness of the jet grout technique. As part of the program, a total of seven vertical and three inclined columns (15, 45 and 75 degrees) were constructed using varying jet grout parameters including jet diameter, injection pressure, grout ratio, lift rate and rotation rate. Jet grouting was performed using a single-phase system. The final jet parameters included using two 3.0 mm jets, an injection pressure of 5,000 psi, lift rate of 1 foot/min, rotation rate of 12 rpm and a grout ratio of 1.5:1 (W:C). An average effective column diameter of 3 feet was determined by exposing the top five feet of each column during the test program.

Limited site access required the jet grout column layout to be designed using vertical and angled columns (as much as 5 degrees from horizontal) to provide treatment underneath the MSE wall and wing wall. Extensive site grading was performed to provide access for the jet grouting equipment. Soil conditions (i.e. naturally occurring cobbles and boulders) required an air track drill rig, which utilizes rotary percussion drilling techniques, to pre-drill holes to aid the construction of the jet grout columns drill rig and penetrate the naturally occurring cobbles and boulders.

Three hundred eighty (380) jet grout columns were constructed to an average depth of 30 feet. The treatment zone limits under the MSE wall included a soil volume of 5,000 CY and extended from the base of the MSE wall to a depth of 1 foot into the Tertiary sand layer (30 foot average depth). The treatment zone limits under the wing wall, as defined by the project specifications, included a soil volume of 1,000 CY and extended from the existing ground surface to the top of the original grade prior to backfill placement (35 foot average depth).

Verification of the jet grouting program was performed by collecting wet grab samples consisting of the return spoils and, to a lesser amount, by excavation and core drilling. All of the wet grab samples met or exceeded the minimum project requirement of an unconfined compressive strength of 150 psi at 28 days. Further verification of the jet grouting program was performed by exposing several column locations. For this particular project, the ability to recover adequate core samples for the purpose of grout verification proved extremely difficult due to the soil conditions.



 

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