Investigation of the mechanism of long term deformation of peat subgrade under embankment loading

• Author / Creator

  Acharya, Mohan Prasad

• Peat, the fragmented remains of vegetation is highly compressible foundation material. Road and railway embankments built on top of peat subgrades are associated with instability and long-term settlement. This thesis presents the results of the research work that included the field measurements, laboratory testing, and theoretical analysis to investigate the mechanism of development of pore pressure and long-term settlement in these peat subgrades. Pore pressure generation was studied based on the measurement of pore pressure, deformation, and temperature in peat subgrade at a field site over three years. The laboratory tests included an incubation of peat specimens at room temperature and measurement of pore pressure generation in four peat specimens while varying temperature between 2 to 20 degree Celsius. The results identified the decomposition of peat to form gas and the effect of gas bubbles in the development of pore pressure. The variation in pore pressure in peat subgrade during summer and winter months was found to be due to the thermal expansion and contraction of gas bubbles. To investigate the movement of the gas through peat, laboratory isotropic consolidation tests were conducted on five peat and six cellulose foam specimens. The result of the laboratory tests and the theoretical analysis indicated that the gas bubbles move through the pore constrictions towards the drainage boundary, where they restrict the flow of water, this results in an increase of pore pressure towards an upper limit or escape pressure, ultimately causing an expulsion of gas bubbles. This expulsion of bubbles results in a rapid drop in pore pressure as measured in the field site and volume change within the specimen. A correlation developed between the pore pressure drop and the corresponding volume change within the peat specimens was extrapolated to estimate the settlement of peat subgrade due to the gas expulsion. The mechanism of long-term settlement in peat was investigated by means of triaxial drained and undrained creep tests in nine peat specimens. The development of strain and strain rate under variable stress states are presented. The uniqueness of the stress, strain, and strain rate at different stress states was analysed and the creep isotaches representing this uniqueness is presented. Further, the applicability of creep equations developed for normally consolidated clays to define the development of creep in fibrous peat, are discussed. The result of this research identified that multiple mechanisms, including the creep deformation under constant embankment loading and the deformation due to the seasonal temperature driven rise in pore pressure, and expulsion of gases out of peat boundary are involved in the long term settlement of peaty foundation. The deformation is found to be predominantly volumetric with lateral deformation less than 25% of vertical deformation.

• Subjects / Keywords

  • Creep behaviour of peat
  • Settlement due to expulsion of gas bubbles
  • Thermal induced pore pressure in peat
  • Expulsion of gasbubbles from peat

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3HT2GK7B

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.