How NTH handles the challenge of dealing with soft clay deposits as a foundation for structures
By Yazen Khasawneh, Ph.D., P.E.
In Michigan and the Midwest we are faced with the challenge of dealing with a relatively thick deposits of soft clays as a foundation for our structures. The stability and settlement of the soft clay layer under loading are the major concerns.
At NTH we have the expertise, knowledge, and tools to design safe and economical structures on soft clays. One example of such structures is highway and bridge abutments embankments on soft ground. The challenges of constructing embankment constructions on soft clay are settlement magnitude and rate, and the risk of undrained failure of the foundation soils. Settlement magnitude can be mitigated with ground improvement, and settlement rate can be accelerated by shortening the drainage path (eg. Using wick drains). The undrained failure is eschewed by carefully planned staged construction.
Staged construction is adding the load (total stress) in stages and allowing enough time for the evolution of the applied total stress to effective stress in the clay layers. The evolved effective stress will result in an increase of the undrained shear strength in clay and thus preventing undrained failure of the foundation soils.
Recently NTH professionals discussed an example project in which coupled hydro-mechanical finite element simulations were performed to optimize the design of the construction stages for the embankments along I-69 extension in southern Indiana. The height of the embankment along the extension varied from about 50 ft to 125 ft. The foundation soils consisted mainly of highly compressible fine grained soils. A hardening soil constitutive model was used to model the foundation soil layers, while the embankment soils and the bed rock were modeled as elastic-perfectly plastic with MC yield criteria.
The site was well instrumented during construction, which allowed for settlements monitoring and pore water pressure dissipation. The estimated settlement and pore water pressure response from the simulations reasonably agreed with the data collected from the instrumentation program. Various aspects of the project were also discussed with a focus on the modeling elements, constitutive modeling, and comparison with instrumentation data. The modeling was performed for five bridge abutment embankments. A successful implementation of the staged construction in the field allowed for successful project execution.
At NTH we have the expertise, knowledge, and tools to design safe and economical structures on soft clays. One example of such structures is highway and bridge abutments embankments on soft ground. The challenges of constructing embankment constructions on soft clay are settlement magnitude and rate, and the risk of undrained failure of the foundation soils. Settlement magnitude can be mitigated with ground improvement, and settlement rate can be accelerated by shortening the drainage path (eg. Using wick drains). The undrained failure is eschewed by carefully planned staged construction.
Staged construction is adding the load (total stress) in stages and allowing enough time for the evolution of the applied total stress to effective stress in the clay layers. The evolved effective stress will result in an increase of the undrained shear strength in clay and thus preventing undrained failure of the foundation soils.
Recently NTH professionals discussed an example project in which coupled hydro-mechanical finite element simulations were performed to optimize the design of the construction stages for the embankments along I-69 extension in southern Indiana. The height of the embankment along the extension varied from about 50 ft to 125 ft. The foundation soils consisted mainly of highly compressible fine grained soils. A hardening soil constitutive model was used to model the foundation soil layers, while the embankment soils and the bed rock were modeled as elastic-perfectly plastic with MC yield criteria.
The site was well instrumented during construction, which allowed for settlements monitoring and pore water pressure dissipation. The estimated settlement and pore water pressure response from the simulations reasonably agreed with the data collected from the instrumentation program. Various aspects of the project were also discussed with a focus on the modeling elements, constitutive modeling, and comparison with instrumentation data. The modeling was performed for five bridge abutment embankments. A successful implementation of the staged construction in the field allowed for successful project execution.
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