Just south of Bismarck, the University of Mary stands atop a 175-foot-tall bluff overlooking Apple Creek and, farther out, the Missouri River. The campus’s first buildings, designed by celebrated midcentury-modern architect Marcel Breuer, frame the sweeping landscape through generous windows, angular outdoor structural elements, and cutouts in concrete walls.
The Sisters of the Annunciation who founded their monastery and school in the 1950s were keeping up the Benedictine tradition of building communities on inspiringly high ground, but there was little awareness at the time that slopes along North Dakota’s river valleys have their own tradition: succumbing to landslides, due to layers of weak clay and groundwater. The bluff supporting the campus, which Breuer referred to as the Jewel of the Prairie, would gradually begin deteriorating in a phenomenon known as “rotational slump.”
In 2015, the University of Mary, in cooperation with the North Dakota Department of Emergency Services and the Federal Emergency Management Agency, began working to address three landslides posing risks to the campus—not only the historic Breuer buildings, but several that had been added in the intervening decades, as well as a cemetery where the monastery’s nuns are laid to rest.
In late 2021, Barr began studying and designing stabilization measures for the largest landslide, which over the years had been advancing toward the cemetery. In a preliminary geotechnical assessment, we fed existing subsurface-drilling, groundwater-monitoring, and lidar data into GeoStudio’s SLOPE/W software to develop an initial model of the slope and evaluate options for stabilizing it. The model indicated that a combination of approaches would be necessary to halt the landslide.
Barr then conducted an in-depth geotechnical investigation that included new soil borings to collect a variety of samples as well as to install inclinometers and vibrating-wire piezometers that measured subsurface deformation and groundwater pore pressures, respectively. The soil samples were tested in a laboratory to characterize shear strength, permeability, and soil chemistry, and we incorporated the resulting data into a final seepage, stability, and deformation model created with the advanced numerical modeling program FLAC. The model also allowed us to account for the interaction between soil and structural stabilization elements known as auger-cast piles (11 vertical steel rods surrounded by a spiral steel cage set in cement grout).
The geotechnical investigation and analysis confirmed that two approaches were needed to stabilize the slope:
removing more than 70,000 cubic yards of earth from the top portion of the landslide
installing 65 piles extending as deep as 75 feet below the surface to anchor the soft, sliding soil mass to the stronger, more stable ground below
Over the summer and early fall of 2023, about half the earth excavated from the upper portion was shifted to the lower slope, which reduced the steepness of the original grade by nearly 65 percent, and the auger-cast piles were installed to inhibit future ground movement while raising the slope’s factor of safety by 25 percent.
Together, the measures will minimize further slumping at the bluff—preserving the unique architecture of the university buildings and monastery, as well as the cemetery honoring the nuns who established the community and taught and served at the school.