Project Summary/Scope: A new segment of US Highway 14 was constructed over highly organic clay near Nicollet, Minnesota, by using prefabricated vertical drains and surcharging in 2016. Overall, the subsurface investigation indicated that a 34 ft (10.4 m) thick layer of soft to very soft organic soil (swamp deposits), having undrained shear strength ranging from 250 to 700 psf (12 to 33.5 kPa), was underlain by inorganic stiff glacial till with some sand seams. During the design, several ground improvement methods were presented as feasible options to mitigate the estimated 3 ft (0.9 m) of settlement expected from the placement of 7 to 10 ft (2.1 to 3.0 m) of fill in this area. The preferred option consisted of prefabricated vertical drains (PVD), soil surcharges with extended waiting periods, and geotechnical instrumentation monitoring. This option was selected because it represented a good balance of low cost and acceptable geotechnical risk and fit well with the proposed construction schedule.
The PVD was designed with a 6 ft (1.8 m) triangular spacing for 80% of the embankment length with an estimated drain depth of 55 ft (16.7 m). The project PVD specifications called for a minimum equivalent circular drain diameter of 0.21 ft (0.06 m) and the drains should be installed with a protective mandrel with a maximum cross-sectional area of 12 in2 (6.45 cm2). Acceptable products included Colbonddrain CX1000, Amerdrain 407, Mebra-Drain MD-7407, or equivalent. The mandrel should be pushed into the ground in a smooth, uniform manner (static method). The remaining area was bookended with PVD designed with a 10 ft (3.0 m) triangular spacing to provide a transition zone between improved and unimproved soils. In addition to the drains, a 5 ft (1.5 m) soil surcharge was incorporated into the design to accelerate the settlement rate to fit within the overall construction schedule. With this potent combination of PVD and a surcharge fill, the proposed settlement period was reduced from 2 to 3 years to only 5 months.
To evaluate embankment stability and validate consolidation progress, a geotechnical instrumentation monitoring program was implemented to control the rate of fill placement and measure the magnitude and rate of settlement. After grading, the instrumentation was installed after the PVDs were installed and prior to fill placement. Following that, a separator geotextile layer was placed to full width and length of the embankment, then overlain with 2 ft (0.6 m) of drainage blanket consisting of <5% fines. Embankment and surcharge fill were placed in no more than 3 ft (0.9 m) lifts with a minimum waiting period of two weeks between lifts. Once the embankment and surcharge fill reached full height, a minimum waiting period of 5 months should be observed in conjunction with the monitoring results. The measured settlement results indicated distinct features of primary compression and post-surcharge secondary compression as fill was placed or removed.
Complementary Technologies Used: PVD, surcharging, instrumentation
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Performance Monitoring: Piezometric pressure, earth pressure, and settlement monitoring using remote, automated datalogging system.
Estimated Costs of Considered Options:
Surcharging only with extended waiting period: $0.3m
PVD and surcharging with extended waiting period and geotechnical monitoring program: $0.5m
Lightweight fill (geofoam): $0.8m
Column-supported embankment: $1.2m
Land bridge: $1.8m
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:
Lamb, RA, Chow, LC, Bentler JG (2018). “US Highway 14 embankment over soft soils - success with ground improvement and modern instrumentation,” Proc. of the Uni. of Minn. 66th Annual Geotech. Eng. Conf., February 23, St. Paul, MN.
Chow, LC, Bentler, JG, Lamb, RA (2019). “Primary and post-surcharge secondary settlements of a highway embankment constructed over highly organic soils: a case history,” Proc. of the 8th Intl. Conf. on Case. Hist. in Geotech. Eng., Geo-Congress 2019, March 21, Philadelphia, PA.
Date Case History Prepared: June 2024