Project Summary/Scope:
The new I-90 Dresbach Bridge over the Mississippi River is a short distance north of the existing bridge. The east (Wisconsin) approach embankment required a significant widening of the maximum 35 feet (11 m) high embankment onto the adjacent marshy floodplain. During the preliminary design phase, geotechnical investigation was limited to the toe of the existing embankment. Based on that, a base design for staged construction was developed, and total settlements were estimated between 0.7 and 2.2 feet (213 mm to 671 mm) at the end of construction. Following a supplemental geotechnical investigation, re-evaluation of the design showed that the required embankment stability and settlement performance (i.e., long-term stability factor of safety of 1.5 and post-construction settlement of less than 3 inches or 75 mm) could be achieved with the excavation and replacement (a.k.a., muck-excavation and backfilling) method, which the contractor determined was more economical and elected to use instead of the originally proposed staged construction with prefabricated vertical drains and geosynthetics. The subsurface profile along the muck-excavation and backfilling portions generally consists of 12 to 13 feet (4 m) of soft organic clay, underlain by interbedded soft to medium stiff silty clay and medium dense sand layers.
The muck-excavation and backfilling process was performed under a partially submerged condition using conventional construction equipment such as backhoe or dragline excavator. The excavation was backfilled with MnDOT Select Granular Borrow Modified Materials, having less than 10% passing No. 200 sieve and a maximum “#40/#10” ratio of 65%. Because dewatering was not feasible, the backfill was pushed into the excavation such that the fill mass imparted a sideways and downward “scouring” action to advance the fill along the excavation bottom. The “scouring” action pushed remnant pieces of unsuitable materials ahead of the backfilling process where a backhoe could periodically remove the unsuitable materials. One month after muck-excavation and backfilling, additional piezocone soundings were conducted to validate the success of the muck-excavation process, i.e., conditions before-and-after and depths of excavation and replacement. Additional soundings showed that upper 13 feet of the organic clay was replaced with backfill materials with cone resistance ranging from 5,000 to 1,000 psi (34 to 7 MPa), from ground surface to the replacement depth. This range of cone resistance indicates that the upper soft organic clay was replaced with sandy backfill. The remaining embankment fill to attain final placement required fill be placed in loose lifts not exceeding 8 to 10 inches (0.2 m to 0.3 m) in thickness and compacted to a minimum of 95% of the Standard Proctor Density. To ensure that long-term settlement was mitigated, a 5-foot-thick (1.5 m) surcharge fill was placed to consolidate the in-place fill.
A geotechnical monitoring program was in place to monitor the embankment stability continuously during construction due to the presence of the underlying soft to medium stiff silty clay layers. The deformation ratio (ΔYmax/ΔS), which is the maximum incremental lateral displacement over incremental settlement (Tavenas et al. 1979, Tavenas & Leroueil, 1980), was derived from horizontal and vertical Shape Accel Arrays (SAA) and used as an indicator to evaluate the embankment performance. During the backfilling activities, it was observed that ΔYmax/ΔS approached 1.4, which indicates the immediate settlement of the new backfill and that the underlying clay exhibited an undrained response, but the ratio quickly reduced to 0.5, close to the value reported in Ladd (1991) as a stable foundation. After the removal of surcharge, the lateral deformation was nearly zero and there was about 0.3 inch (8 mm) of rebound in vertical deformation before settlement stabilized at 8.9 inches (226 mm).
This case study demonstrates the application of muck-excavation and backfilling for an interstate highway project. The method is feasibly time-saving and economical, but it is limited to subsurface conditions where excavation depth is practical for conventional construction equipment. The validation tests after backfilling are important for confirming the success of soil correction.
Figure: Schematic cross section of embankment and instrumentation
Alternate Technologies: Perforated vertical drains, geosynthetic reinforcements, and surcharge. These technologies were deemed more costly compared to muck-excavation and backfilling.
Performance Monitoring: Piezometric pressure, earth pressure, settlement, and lateral deformation monitoring using remote, automated datalogging system.
Cost Information: Cost information is not available.
Case History Author/Submitter:
Liang Chern Chow, Project Manager, lchow@haleyaldrich.com
Rich Lamb, Foundations Engineer, rich.lamb@state.mn.us, (651) 334-3563
Project Technical Paper: Chow, LC and Bentler, JG (2016). “Design and instrumentation of a widened interstate embankment constructed over soft floodplain soils,” Proc. of the Uni. of Minn. 64th Annual Geotech. Eng. Conf., March 4, St. Paul, MN.