Comprehensive Studies On Deep Soil Mixing And Lightweight Aggregates Applications To Mitigate Approach Slab Settlements

Bridge approach settlement is a common problem to the Transportation Departments nationwide. This uneven transition causes inconvenience to passengers and increases the cost of maintenance and repair of the distressed approach slabs. The Texas Department of Transportation spends millions of dollars annually to mitigate this problem across the state. The potential causes for this problem are numerous and purely site specific. Hence this problem may not have a unique solution. In this research, two methods were selected to mitigate the approach slab settlements. The first one is an improvement of ground foundation technique by using Deep Soil Mixing (DSM) columns to stabilize soft foundation soil. The second one is an improvement of fill material by using a lightweight fill material, Expanded Clay Shale (ECS), as a backfill in embankment construction. Therefore, this research was performed on two bridge sites - DSM site on IH30 and ECS site on SH360 located in the North and South of Arlington, respectively. In order to study effectiveness of the two mitigation methods, three tasks were carried out including, laboratory studies, instrumentation and field monitoring, and numerical analysis. Soil samples from two bridge sites from IH30 and SH360 were taken to perform laboratory studies at University of Texas at Arlington. The results reveal that foundation soils from both sites were classified as low plasticity clay. The laboratory investigations were also conducted to study the properties of DSM samples prepared in the laboratory and the ECS. The test results show that the foundation soil gained more strength by the DSM technique and the ECS exhibited its high internal friction property. Equipment installation was done on the DSM and ECS sites with various types of instrument including vertical inclinometer, horizontal inclinometer, sondex, and rod extensometers. The site investigation with the elevation surveys had been performed in every fortnight to monitor soil movements both in the horizontal and vertical directions. Results from the field observations from two bridge sites showed that both of the DSM and ECS techniques can be used to mitigate the settlement occurred in the embankment. In the DSM study, the settlement was reduced from 85 mm to 49 mm (measured in the control and test sections, respectively). On the ECS site, it is found that to construct embankment with the ECS could bring down the settlement from 85.3 mm occurred in the control section to 36.5 mm in the test section. The monitored soil movement data from the field studies were also used as a data validation in the numerical analysis to ensure that the numerical model would have a good prediction about soil displacements. After the results from the FEM closely matched with the field data, the models were used further to predict the long-term settlement. The analysis results show that in the long-term, the DSM and ECS methods can decrease the settlement in the embankment from 277 to 66 mm, and from 136 to 50 mm, respectively. Consequently, a parametric study was conducted to investigate parameters influencing on the amount of the settlement. The interesting parameters in both DSM and ECS studies include slope and height of the embankments, and area-ratio between DSM and foundation soil (only in the DSM study). The FEM results from both studies show that embankment height mainly affects the amount of the settlement, while the embankment slope does not affect much. Another factor can influence the settlement in the embankment is the area-ratio. From the DSM study, the area-ratio with a range from 0.5-0.7 exhibit greatly influence on the settlement. Finally, from a parametric study, the DSM and ECS design charts could be established in various heights, slopes and area-ratios to facilitate in ECS embankment and DSM columns design.