Evaluating the Behaviour of Rock Discontinuities in Direct Shear Under Constant Normal Load and Constant Normal Stiffness Boundary Conditions Using Additive Manufacturing Technology

• Author / Creator

  Salami, Marzieh

• The inherent heterogeneity of natural rock and the variance in the surface roughness of discontinuities limit the repeatability of rock joint sampling and testing. One way to overcome
  this challenge is by using additive manufacturing (AM) techniques. AM, also referred to as 3D printing, has shown promise in producing complex geometries rapidly, however its use in geosciences is preliminary. Specifically, the rock prototypes fabricated with sand-powder material have shown low compressive strength. Through this research program the feasibility of using 3D printing to replicate rocks and to investigate the shear behaviour of rock discontinuities was
  explored. The settings that effected the print properties were studied and higher compressive strength of rock analogues were achieved. Single asperity joints and standard roughness profiles were fabricated with a sand 3D printer and tested in direct shear under constant normal load (CNL) boundary conditions. It was concluded that it is feasible to use 3D printing to produce rock replicas and joint analogues for direct shear testing.
  Discontinuities affect the mechanical and hydraulic properties of rock mass and thus understanding their behaviour better informs design schemes in geotechnical engineering
  projects. Surface roughness is one of the complex features of rock joints that causes dilation and contributes to its shear resistance and therefore needs to be properly assessed. Furthermore, boundary conditions influence the resistance of rock joints to shear. While previous studies have typically focused on constant normal load (CNL) conditions where there is no constraint on joint dilation such as rock slopes, however these conditions do not appropriately capture the behaviour
  of discontinuities in rock blocks in tunnels, reinforced rock slopes and underground CO2 sequestration sites. In these situations, constant normal stiffness (CNS) boundary conditions are prevalent. Thus, the influence of fracture geometry on the shear behaviour of joints in both
  boundary conditions needs to be investigated further.
  Using 3D printing, the geometrical components that contribute to fracture surface anisotropy and therefore effect shear strength and deformation of discontinuities were studied. This was done with a focus on two methods that are frequently adopted to quantify surface roughness, i.e., statistical parameters and the joint roughness coefficient (JRC). First, a set of 7 triangular profiles differed from one-another in shape, height and directionality of joint asperities
  were selected. Then Barton’s standard joint profiles with JRC=6-8 and JRC=18-20 were divided into 5 and 10 segments and rearranged to create 9 new geometry configurations. All the joints were printed and tested in direct shear under CNL and CNS conditions. For triangular joints, it was demonstrated that based solely on one statistical parameter, a clear correlation between that parameter and peak shear strength didn’t exist. It was concluded that even though statistical
  parameters are good indicators for defining the anisotropy in roughness of the profiles, they are not adequate. Other factors such as contact area and edge effects that results in local stress
  concentrations should be considered. For the new configured JRC profiles, it was revealed that when the profile with JRC=18-20 was rearranged, the new profiles had more significant difference in terms of geometric properties compared with the original profile and thus their shear response, especially in CNS conditions differed from the original joint. However, for profiles with JRC=6-8,
  the shear response of the new joint configurations was close to the original joint. Consequently, the empirical method of JRC may be adequate for determining the joint roughness of smoother joints, however for rougher joints, other factors such as distribution of stress on the asperities are essential and should be considered.
  Lastly, through discrete element method (DEM) modelling, synthetic rock specimens were generated. Three 3D printed joint profiles with standard JRC values were simulated numerically and tested in direct shear under various applied normal stresses. The simulations captured the trend of the shear behaviour of laboratory tests, however there were some discrepancies that were attributed to the calibration of the intact material, direct shear setup, and implementation of
  smooth-joints contacts.

• Subjects / Keywords

  • direct shear
  • additive manufacturing
  • constant normal stiffness

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-cda2-eb36

• 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.