# Voxel-Based Digital Twin Framework for Earthwork Construction
## 2 Related Research
### Earthwork Operational Simulation and Monitoring  
While voxel-based modeling provides a robust foundation for representing earthwork environments with high spatial and semantic fidelity, the practical value of such detailed models is verified by their use in decision-making. Earthwork activities require different levels of planning and control due to their complexity and scale, from high-level process coordination to detailed real-time operational management. In this context, it is essential to classify them into different hierarchies. For instance, earthwork management is classified into two hierarchical levels, namely, activity level and operation level [34]. The activity level focuses on the overarching elements of earthwork activities (i.e., excavation, transportation, and deposition) and on improving and optimizing broad-scale operations. Operational-level earthwork management focuses on the detailed execution of specific activities and automated field operations. For example, excavation tasks emphasize granular information such as detailed volumetric representation, material properties, geotechnical and spatial characteristics, and equipment interactions with the environment. Automating operational-level tasks can significantly enhance the efficiency of earthwork. The application of simulation and monitoring techniques through the digital twin has the potential to greatly improve these operational-level tasks. Several simulation tools have been proposed for earthwork operations, serving diverse purposes such as simulating the earthwork process, training operators, evaluating alternatives, enhancing visualization, and providing a user interface that enables human-machine interaction [35]. Early simulation development has often emphasized photorealistic methods, providing high-quality visualization of the earthwork environment to support planning and decision making. For example, a virtual reality (VR) simulator for hydraulic excavators was developed to train equipment operators and improve the efficiency of standard earthwork operations [36]. Although effective, such systems often oversimplify the environmental details and model-equipment interactions because they use datasets that are static, such as the acquired OpenStreetMap, and interactions based on photorealism [37]. In this context, physics realistic approaches have been proposed, utilizing computational methods to simulate model-equipment interactions, such as bucket and material interactions. Computational methods such as the finite element method (FEM), material point method (MPM), and discrete element method (DEM) have been used to model soil deformation, efficiently simulating the interaction, enhancing equipment design, and providing alternatives [38; 39]. Physics realistic simulations are more accurate; however, their focus on micro-level details and high computational loads limits their scalability, particularly for large-scale earthwork operations. A simple and efficient method based on ray casting has been proposed, which uses rays from the edges of an excavator bucket to estimate the soil volume and weight, simulating earthwork operations and thereby providing an alternative method with sufficient realism [40]. Furthermore, volumetric modifications are estimated using surface meshes for training simulators, enabling interactive visualization and improving situational awareness for operators [41]. Moreover, a contextually realistic simulation environment was developed, providing an efficient platform for training [37]. Despite increasing popularity, several shortcomings exist. For instance, existing systems often use oversimplified data such as OpenStreetMap (OSM), which lacks the level of precision, subsurface details, and material information required. Although these simulators provide advantages, they are designed for specific environments and are not sufficiently flexible to adopt custom design data or incorporate earthwork information that changes from site to site.