Jeramy Ashlockjashlock@iastate.edu email >
Brent Pharesbphares@iastate.edu email >
Bridge Research Engineer
About the research
Due to quality control issues or soft toe conditions, the end bearing capacity of drilled shafts is often not mobilized before service load displacement limits are realized. Shaft capacity is therefore limited as it is developed primarily through mobilization of side-frictional resistance at relatively small displacements. It has been estimated that the end bearing component in-cohesionless soils can be as great as 20 times the resistance available due to side friction. To take advantage of such potential high end bearing capacities in soft-toe conditions, post-grouting of shaft tips is increasing in popularity. In the existing Broadway viaduct replacement project, initial load-testing has been completed on post-grouted drilled shafts that were constructed using the tube-sleeve (tube-a-manchette) approach. However, the increase in shaft capacity did not meet expectations, and questions remain as to the size and integrity of the grout bulb, and therefore the size of the contact area that should be used in analysis and design. It is our understanding that it has been suggested that the tube-a-manchette grouting technique be replaced with a flat-jacking approach, for which the distribution and contact area of the grout will be known. To date, there remain questions regarding the effectiveness of this technique. Current plans for replacing the Broadway viaduct also involve the replacement of the existing cellular abutments with slabs on LFCF material contained within mechanically stabilized earth (MSE) walls. Because this is the first such use of LFCF in a bridge project in Iowa, it will be highly beneficial to assess and document the performance and interaction of the fill with the surrounding walls and underlying foundation soils. The performance of the LFCF-MSE wall system will be studied by instrumenting the wall facade with tilt meters and the straps with strain gages. At each abutment, two soil settlement plates will be used to monitor the soil response at the base of the fill material and two will be installed immediately outside the MSE walls, for a total of eight settlement plates (to be installed by the DOT). Additionally, the pressure under the foamed concrete fill will be monitored using two soil pressure cells. The LFCF offers potential savings over the use of select fills, while post grouting of drilled shaft tips will likely provide savings in construction costs by decreasing the required shaft lengths for a given design load. Documentation and evaluation of the construction and performance of these materials and techniques will benefit future projects in which these technologies may be used.