Development of a semi-empirical three-dimensional model for predicting the peak cutting forces in rock cutting

Abstract

On a global scale, the present-day demand of coastal protections, port expansions and construction of artificial islands, rises. To be able to accommodate for all these environmental changes, complex dredging operations have to be performed. Depending on the type of soil and in-situ conditions, a suitable excavation and transportation method has to be chosen. From an economic point of view it is desired to make this process as efficient as possible. The only way in doing this, is to understand the physics of the rock cutting process and be able to predict the behavior of the rock formation during cutting.

This research focusses on the physics that come paired with the excavation of rock deposits under atmospheric conditions. The removal of rock results from the interaction between the cutting tool and intact rock, destructing the internal structure of the rock formation. A rock may either fail in a brittle tensile, brittle shear or ductile manner. Over the past decades various calculation models have been developed to predict the cutting forces in rock cutting. Most known prediction models assume one failure mechanism to be dominant, using a specific cutting tool geometry. For the sake of this research a sharp pickpoint is used. The elaborated, existing calculation models within this research using a sharp pickpoint as a cutting tool, all consider the rock cutting process to be a two-dimensional problem. To investigate if this hypothesis about the rock cutting process is correct, experimental research is conducted. A distinction is made between two different rock types, both varying in strength (e.g. sandstone and artificial rock).

Experimental research revealed that the measured forces greatly underestimate the results of the existing prediction models. The author states that this inaccuracy partially arises from the fact that, in practice, the excavation of rock is actually a three-dimensional problem, since visual observations clearly showed that chips broke out sideways. Based on the experimental measurements, two new calculation models were set up, incorporating this three-dimensional effect. The first model implemented fracture mechanics theory, focussing on the required local crack tip stresses to initiate a tensile crack. Due to several unknown empirical parameters, the tensile-dominated fracture model gave unsatisfactory results, underestimating the measured cutting forces. The second model was set up by focussing on the actual physics that were observed during cutting. Based on visual observations and individual chip investigation, three different failure mechanisms can be substantiated that occur during cutting: crushing, major shear failure and tensile failure.

By elaborating on the developed prediction model, it appears that the forces due to crushing dominate the total force spectrum. For a crushed zone to develop, penetration into the material is required. The author discovered that rock's resistance to obtain a certain penetration, needs to be accounted for when calculating the stresses within the crushed zone. By achieving the desired penetration, a large vertical force arises, indicating that frictional effects also need to be considered. This means that forces due to crushing are dependent on the hardness of the rock and frictional resistance between the cutting tool and rock's top interface. Based on production measurements and cutting groove analysis, the crushed zone dimensions were able to be determined, whereupon an expression for the pickpoint's indentation area was found. A surprising discovery was made during the analyzing phase of the force data. Regardless of the pre-set cutting configuration, a basic force level was observed during cutting, which is quite well-approximated by the crush force component, including frictional effects. Despite the formation of a crushed zone, chips failed in a cataclysmic manner. Individual chip investigation revealed that a certain fraction of shear and tensile failure is present along the failure plane. It appeared that the contribution due to tensile failure becomes more significant as the penetration depth of the pickpoint increases. This can be explained by the fact that a tensile crack has the ability to grow and propagate to the rock's top interface. The results of this research show that predictability of the forces increased significantly, compared to the existing two-dimensional calculation models.