Frost driven fracturing

Close to the Earth’s surface, the water that fills cracks and pore space in rocks and soil may experience freezing temperatures and cause fractures in the elements. The 'Frost driven fracturing' project is an experimental study in a simplified, model fracture with the goal to study the coupling between stress-build up and strain due to ice growth, water transport and fracture evolution.

Figure: Fracture propagation by ice wedging.

Figure: Fracture propagation by ice wedging.

About the research

Close to the Earth’s surface, the water that fills cracks and pore space in rocks and soil may experience freezing temperatures. The combined effects of fluid migration and freezing have profound effects on weathering and landscape evolution in cold climates. While it is well established that freezing conditions can cause fracturing of rocks, leading to rockfall, landslides and damage to structures, the physical mechanisms behind frost-driven fracturing are not well understood. Principally speaking, ice may affect fractures through

1) exerting stress as water expands during freezing;

2) ice segregation, where water flows towards the freezing and stresses are exerted due to the build-up of ice; and

3) hydrothermally caused shear strength reduction during melting.

In a complex field situation, fracturing may progress through a combination or sequence of these factors.

Recent field and experimental studies of real rocks indicate that ice segregation is an important process during the fracturing of intact bedrock. The thermodynamics of ice segregation and how this applies to ice lensing in porous soils has been extensively studied during recent years. However, it is not yet clear how ice segregation operates to propagate fractures in low porosity and low permeability rocks, or to what extent ice segregation is sufficient to initiate new fractures in hard, intact rocks. The key to this problem lies in understanding the dynamic ice-rock interface, where the transport and thermodynamic properties of the stressed ice surface and the nanoconfined water layer determine the evolution of the system.

The 'Frost driven fracturing' project is an experimental study in a simplified, model fracture with the goal to study the coupling between stress-build up and strain due to ice growth, water transport and fracture evolution. Glass will be used as a model for intact, brittle rock. A weak zone in the model will be made by sintering two glass blocks with roughened surfaces. The glass model will be placed in contact with a liquid water reservoir on one end, and cooled from the other end, thus establishing a temperature gradient with zero temperature somewhere inside the material. As water freezes from the top of the fracture, liquid water will presumably be pulled up from the unfrozen parts of the fracture and the liquid reservoir. Water flow and temperature fields will be monitored using particle tracking and fluorescence, while acoustic emissions will be recorded to monitor the breaking of micro-contacts in the weak zone.

Experiments will be compared with a recently developed/in progress numerical model for frost wedging of vertical fractures and theoretical models for premelting-driven interface flows will be used to interpret results.

Cooperation

Published Nov. 5, 2015 12:13 PM - Last modified Dec. 11, 2015 2:36 PM