Earthwork is a fundamental stage in highway construction process that in- volves several activities and requires careful planning and various evalua- tions, including environmental and structural assessments. The main earth- work activities that we identified are depicted in Figure 5.1. They include digging, filling, recycling, material disposal in dump sites, temporary allo- cation of materials in depots, and material acquisition from quarries.
Figure 5.1: Activities flow chart.
earth, typically in the order of (tens of) millions of cubic meters by means of special plant machinery such as excavators and trucks. The filling activities consist of carrying across the construction site a certain amount of a certain material, to, e.g., fill earth cuts and trenches, or build bridges and artificial galleries.
A fundamental step in the digging process is the on-site classification, that is, earth materials are classified depending on their geological charac- teristics and treated consequently. Indeed, materials excavated within the site are often unacceptable for construction and must be recycled (through a recycling process to be described below) or disposed of. According to the field activity , three types of dug earth materials can be distinguished, depending on whether they can be reused for purposes of filling, recycled, or else wasted on a dump site. In this chapter we denote them by rawI (high-quality soil and gravel), rawII (soil suitable for filling but not for recy- cling), and rawWaste (waste product without the possibility of further use). Reusing those materials, instead of throwing them directly in dump sites, is environmentally relevant and can significantly reduce the cost of earthwork. RawI and rawII materials can be used directly for filling or, if necessary, stored in temporary depots and used later on in the construction process. Temporary depots are flat areas that are rented close to the construction site, where dug materials can be stored until they are needed for filling. RawI material can also be reconstituted into other products appropriate for filling, that is, asphalt and/or concrete, by means of a recycling process.
The inclusion of recycling activities in optimizing earthwork process is, as far as we know, a novelty of this chapter. Recycling is a two-stage process (see again Figure 5.1). In the first stage, separation, rawI material is trans- ported to the so-called separation plants, where it is broken and crushed into two types of recycled aggregates: recI and recII. In this stage some waste material, denoted by recWaste, is also produced. In the second stage, mixing, these recycled aggregates are carried to specialized plants where they are mixed with other materials to obtain asphalt or concrete. More precisely, in asphalt mixing plants recI is mixed with bitumen to produce as- phalt, and in concrete mixing plants recII is mixed with cement to produce concrete. The waste produced during separation, denoted by recWaste, as well as rawWaste and possible surplus of rawI, rawII, recI, and recII are dumped into dump sites. If necessary, raw material for producing asphalt and concrete (i.e., recI and recII), as well as other materials required for filling can be acquired from private quarries.
The earthwork process must take into account the fact that recycling and mixing plants, quarries, dump sites, and temporary depots have phys- ical capacity limitations on the total quantity of materials that they can process. Moreover, it must be considered that the aforementioned activities are bounded in time by the time windows defined by the MPS. In particu- lar, the overall planning horizon is divided into periods (weeks in our study
cases) and each activity time window is represented by a starting and an end- ing period. The optimization of the earthwork process requires to consider the transportation costs, as well as the costs incurred for storing materials in temporary depots, acquiring materials from quarries, and disposing of materials into dump sites.
To take into consideration all these issues, we modeled the earthwork activities by using a directed graph, described in detail below. Briefly, the highway building site is discretized in segments, and each segment is asso- ciated with a vertex of the graph. The graph is then extended by adding vertices that correspond to quarries, dump sites, temporary depots, and recycling plants. For modeling purposes we also add a vertex for each dig- ging and filling activity to be performed. A set of arcs is then included to represent the possible flows of materials from a vertex to another.
The arcs connecting the vertices are assigned a maximum flow capacity, that limits by above the sum of the material flows that can be carried over the arc in a given period, and can vary during the time. With respect to this point, we mention another interesting contribution of our approach, that is, the differentiation between private and public networks. The private network is in practice the highway being built, whereas the public network is the existing neighbor road infrastructure, that is forcedly affected by the construction process. Indeed, some public roads external to the building site must also be included into the graph to take into account different paths that can be used to reach dump sites and quarries, that are typically located far away from the site. These roads undergo the national transportation laws of the country where the construction is taking place, and can have very small flow capacities, due to the fact that earth transportation is typically performed by large and heavy trucks that have a large environmental and social impact on the surrounding areas.
In this framework, the goal of our optimization is to decide when and how moving, storing, recycling, buying, or wasting materials, while responding to digging and filling requests in the imposed time windows, respecting capacity conditions on nodes and arcs, and minimizing total costs.