Cassio Ishii | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.

This is an ALES MSc Final Exam Seminar by Cassio Ishii.  This seminar is open to the general public to attend.

Thesis Topic: Estimating the mechanical contribution of Salix and Balsam Poplar cuttings in soil reinforcement for Bioengineering projects in Alberta

MSc with Dr. Axel Anderson

Seminar Abstract:

After the floods in 2013, the effort towards reducing the impact on the environment and improving the desirable effects of vegetation, especially the Riparian Zone, is an increasing trend in most cities in Alberta. Projects with fast-growing plants with natural or synthetic support materials for soil reinforcement and erosion reduction, known as Soil Bioengineering, are becoming common as an alternative to only conventional engineering to treat streambank failures, banks where pipelines cross water bodies, and road rehabilitation among others. Moreover, rehabilitation of riparian vegetation with those techniques has increased in the last decades in response to concerns over water quality, fisheries, wildlife, flooding damages and aesthetics. However, despite the policies promoting greener solutions, the lack of technical information on plants with soil stabilization means engineers are resistant to recommend the use of Bioengineering. Therefore, the move from the use of traditionally “hard” elements to “greener solutions” as a functional material in streambank soil stabilization has been challenging for decision makers. This lack of more precise technical information about the effect of the plant roots on the soil strength and the associated uncertainties related to plants maintains the preference for the conventional approaches in larger projects.

The main focus of this study was the strength that roots from primary species of Alberta, such as Willows (Salix spp) and Balsam Poplar (Populus balsamifera ), can provide to soil reinforcement in bioengineering works, and how they can improve their success. However, we added an incidental section about the survival rate of cuttings as we evidenced a significantly high mortality rate at the visited sites. So, the first part of this study reported the survival and attributes of Bioengineering sites in Alberta. After up to two growing seasons in treated sites, cuttings presented survival rates lower than 50% in the visited sites for Willows, and 30% for Balsam Poplars. The causes of high mortality were alleged to be related to the incompatibility between the species needs and site resources.

In the other part of the study, we built a large-scale shear box to validate models calibrated with data from the mentioned species, and we assessed some aspects such as root architecture, often called biological uncertainties under engineering perspectives. The results indicated that root systems from tested species affect surficial erosion, mostly at 30 cm deep even though they were planted deeper than that, as recommended by practitioners. Therefore, the mechanical contribution to the soil stabilization of mature Willows (Salix ssp) and Balsam Poplars (Populus balsamifera ) assessed were not more than surficial protection, increasing the soil cohesion underground up to around 30 cm deep. They added, including the soil cohesion, from 14.23 kPa to 21.51 kPa calculated with Fiber-Bundle Model (FBM) or from 16.92 kPa to 40.07 kPa with Wu and Waldron Model (WWM) for the mature plants although the large-scale shear test suggested an underestimation from 41% to 75% for FBM and from 29% to 64% for WWM, implying other forces acting against the shear force not accounted by the models.