TUD025 Dynamics in Saturated Rock Cutting

Samenvatting project

In de loop der jaren is de trend dat de grootste baggerschepen alsmaar groter worden. Voor het uitgraven van bijv. havens en vaargeulen in gesteente wordt gebruik gemaakt van een zogenaamde snijkopzuiger. Het gehele ontwerp van het schip wordt geoptimaliseerd om het schip in staat te stellen de grond zo effectief mogelijk te kunnen ontgraven. Een van de grootste uitdagingen daarbij is de interactie tussen de snij kop en de bodem. Deze interactie is bepalend voor de sterkte van het schip, wat de eisen zijn aan de aandrijving en motoren. Middels dit onderzoek verwachten we beter in staat te zijn om de interactie tussen het schip en de zeebodem beter te kunnen voorspellen en daarmee het ontwerp van de snijkopzuigers robuuster en efficiënter te kunnen maken.

Doel van het project

The objective in this research is to theoretically and experimentally determine how inertia and system stiffness influence the dynamic rock cutting process. The aim is to develop a physics-based chisel penetration criterion extension for the quasi-static rock cutting models in order to improve the model predictions and determine when excavation using chisels can no longer be executed i.e. from cutting-mode into scraping-mode.

The main research question in this research is:

“What is the contribution of inertia and system stiffness on the chisel penetration in saturated rock?”

The sub-questions to be answered are:

  • What are typical strain-rates during saturated rock cutting with a sharp chisel?
  • What is the effect of a non-rigid body motion on the rock cutting process?
  • How does the chisel geometry affect the quasi-static rock cutting process?
  • How does the chisel geometry affect the strain-rates during saturated rock cutting?
  • How does chisel wear affect drive-line dynamics and vice versa?
  • Case study

Motivatie

In the dredging industry, the use of mechanical excavation to dredge various soil types is common practice. A great example of mechanical excavation is a cutter head used by a Cutter Suction Dredger (CSD). Although used for all soil types, the CSD is mostly known for its capability to excavate/cut rock. Typically, rock strengths up to an Unconfined Compressive Strength (UCS) of ~50 MPa can be dredged by mechanical excavation (PIANC, 2016). When stronger rock is encountered, drilling and blasting is used to pre-fracture the rock mass.

Nowadays, drilling and blasting of strong rock (50 – 100 MPa) is often considered to be environmentally unacceptable, driving the exploration of mechanical excavation equipment performance in strong rock conditions. This leads to a trend where rock excavation equipment is becoming more and more powerful (Helmons & van Rhee, 2019). A recent example where strong, intact rock was dredged is the Port of Leixões project that was executed by the IHC built CSD Spartacus. The encountered conditions challenged even the most powerful CSD ever built, resulting in heavy vibrations, large amounts of wear and tear and often low productions.

To achieve operational efficiency and reduce operational risk in case of a CSD dredging rock, accurately estimating the excavation or cutting forces is of utmost importance. This starts with the determination of rock properties such as the UCS and occasionally the Brazilian Tensile Strength (BTS). By drilling boreholes at the project site and using the borehole sample material in a hydraulic compression tester, the quasi-static rock properties are determined. The magnitudes of these properties are then used in quasi-static rock cutting force models to estimate the required installed power of the CSD to be deployed. These rock cutting models are often validated based on rigid body cutting experiments at constant velocities where no dynamics are involved.

Innovativiteit

Excavation or cutting of a (intact) saturated rock matrix with a cast-steel chisel covers two fields of engineering; mechanical engineering to describe the chisel’s motion, force, inertia and drive-line stiffness and soil mechanics to describe the reaction/behaviour of the rock matrix subjected to external forces. As shown in literature, the strain-rate induced by the chisel in the rock matrix can have a large effect on the apparent strength of the matrix. Therefore, the determination of typical DiRC strain-rates during the saturated rock cutting process with a sharp chisel (no wear) is considered to be a starting point.

As the rock cutting process changes over time due to chisel wear, it is important to understand the implications on the failure behaviour. This allows for the (theoretical) determination of the effect on the strain-rates. Due to wear, the total required force to penetrate the rock matrix increases. The magnitude of this increase has to be determined to quantify the effect on the drive-line response.

Currently available in the Royal IHC laboratory is a Linear Cutting Setup (LCS). The LCS is designed such that the cutting forces of a single chisel can be measured in three (3) Degrees of Freedom (DOF) with large precision. This excludes the contribution of inertia and system stiffness (rigid body motion) as also often seen in literature, but does allow for quantification of cutting forces using worn chisels and might allow for experimental determination of strain-rates at low cutting velocities (𝑣𝑐 ≤ 0.25 𝑚/𝑠). A new experimental setup will have to be designed and built in order to determine the contribution of a non-rigid body motion the chisel penetration/rock cutting process. For this design, the aid of a mechanical engineer is required for detailed design of the new experimental setup.

The experimental data obtained from the new experimental setup will be used to develop a physics-based chisel penetration criterion for the current rock cutting models.

Valorisatie

Looptijd project

Startdatum: 01/09/2024

Eind datum: 31/08/2029