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7 Literature Review This review focuses on previously published academic literature on DTMs and their use. However, little documented knowledge exists in this domain, especially related to the use of DTMs in construction. Thus, the review begins with an introduction to DTMs, the technology used to create DTMs, the use of 3D models in construction, and similar products in the building construction sector. A higher-level overview of construction use of 3D models in the U.S. highway construction industry can be found in the background section of the Introduction. 2.1 Digital Terrain Models Because many engineering problems require accurate representations of the Earthâs surface, solutions have been developed to efficiently transform terrain data into modelsâsuch as DTMsâ that can be analyzed by computers. DTMs are continuous representations of the ground surface and are generated using âa large number of selected points with known XYZ coordinates in an arbitrary coordinate fieldâ (6). These points are used to produce a 3D model that can be analyzed with the help of computer algorithms. Generally, these models capture landform characteristics (e.g., elevation, slope), terrain features (including hydrographic and transportation networks), and natural resources. 2.2 Reality Capture Technologies for Creating DTMs DTMs are constructed using data acquired via remote sensing technologies such as LiDAR, 3D laser scanning, and georeferenced point clouds with high-resolution imagery. Gant and Boivin summarized the data acquisition process for generating DTMs, as shown in Figure 3 (7). Using remote sensing technology carries several benefits, including time and cost savings, less rework, increased productivity, enhanced bidding quality, and fewer safety incidents (8). It is estimated that more than half of state DOTs were using some type of LiDAR technology back in 2013 (3). Common LiDAR data collection technologies include (1) airborne or aerial LiDAR, which uses airplanes or drones, GPS devices, and inertial measurement units (IMUs); (2) terrestrial mobile systems or mobile laser scanning (MLS), which uses moving vehicles, multiple 3D scanners, positioning hardware, cameras, data acquisition systems, and computer monitors for shoulder-to-shoulder highway corridor mapping; and (3) static laser scanning, which uses scanners and cameras mounted on tripods to survey highway structures like bridges and tunnels. Most researchers have found that these latest technologies outperform traditional surveying methods. A study comparing MLS technology to traditional surveying on an Iowa DOT inter- change project confirmed the accuracy, safety, and efficiency benefits of MLS (9). Additionally, C H A P T E R 2
8 Practices for Construction-Ready Digital Terrain Models the Alabama DOT investigated potential benefits of the use of MLS, including increased safety, time savings, and increased level of detail, accuracy, and scalability (10). Another study, performed in conjunction with the North Carolina DOT, outlined guidance for how DOTs can determine whether LiDAR can be practically used by transportation agencies; it specifically examined different aspects and performance measures for effectively deploying LiDAR equip- ment or taking advantage of contracted services (11). The Utah DOT found that using LiDAR data with supplemental surveying methods resulted in cost savings of 24% and time savings of 22% as compared to using traditional surveying methods alone (12). Researchers surveyed 50 DOTs, six transportation agencies, and 14 MLS service providers on the adoption of MLS (13). They found that a main reason DOTs have not used MLS more widely is that they want to see more evidence of its benefits validated through costâbenefit studies (13). A costâbenefit analysis of Washington DOT and Caltrans projects that used MLS showed that the agencies enjoyed millions of dollars in savings as well as intangible benefits related to the environment, people, and traffic (14). Another study provided guidelines for using MLS in transportation applications; incorporating tasks from project planning, design, and construction to operations and maintenance; and addressing data collection methods, formatting and management, storage requirements, QA, and translation and formatting of derived products (15). 2.3 3D Models in Highway Construction State DOTs have increasingly turned to 3D models in their construction projects. These models can help automate highway construction and are essential for stakeholder communica- tion and coordination on large transportation projects, on which multiple design, construction, and consultant teams must work together (16). The shift from two-dimensional (2D) plans to 3D models was mainly driven by contractors using AMG and having to reengineer 3D models from 2D plans, which is a burdensome and time-consuming task (Figure 4) (17). Recognizing the importance of 3D models in all phases of a highway project, including planning, design, maintenance, and operations, contractors are now using 3D models for bid preparation (e.g., more accurate earthwork quantities), clash detection, and field inspection (8). During the project design phase, shifting from 2D to 3D models has brought numerous challenges, such as an increase in cost and time to generate designs; a lack of standards, which results in incompatibility issues due to different modeling technologies; problems related to data management privacy and errors; the need for training expertise (especially on software); lack of guidelines and specifications for 3D designs (such as error tolerance and level of Figure 3. Point cloud processing pipeline [Reproduced from Gant and Boivin (7)].
Literature Review 9 detail); issues with model validation; and contractual issues, especially at the level of 2D versus 3D model deliverables (8). Nonetheless, the benefits of 3D models outweigh their drawbacks. These benefits include cost and time savings and increased productivity. The report also noted that various DOTs have forged ahead with 3D modeling and have aggressively moved to resolve the challenges mentioned (8). For instance, the Wisconsin DOT began developing 3D design models for large projects, justifying the costs by noting that staff members would gain experi- ence they could then apply on smaller projects. The agency has also conducted review sessions on 3D models with designers, consultants, construction professionals, and industry personnel for model validations. Elsewhere, the Iowa DOT has dedicated information technology (IT) staff during the design phase to support 3D design efforts; the Oregon DOT has established guidelines for determining the increased tolerances and level of detail required for 3D design (by surveyors, designers, and project managers) and also allocates extra time for 3D modeling in project schedules and reviews and performs quality checks and assurances on digital files. Agencies handle training individually because it presents a challenge that requires significant organizational and cultural changes. DOTs have invested in pilot projects to demonstrate the advantages and benefits of 3D design, partnered with consultants and software developers, and formed leadership teams to guide transitions (8). An FHWA report on the use of 3D digital design data in highway construction found that leveraging 3D data creates opportunities and challenges with respect to risk allocation, enter- prise data management, workforce development, and industry-related matters (18). Looking at six projects, the study found that all construction parties benefited from 3D drawings, with resident engineers and inspectors being the most affected. Given appropriate training, resident engineers viewed 3D drawings as a safer and more efficient method for real-time verification and an easier way to measure payment quantities from post-construction drawings. Inspectors could also document inspections in a more accurate and transparent manner. 2.4 Building Information Modeling for Infrastructure As DTMs have gained momentum in the highway sector, the building sector has further advanced 3D modeling through building information modeling (BIM). BIM has upended con- struction industry paradigms through the shift from 2D-based drawing information systems to 3D object-based information systems. BIM can also model beyond three dimensions, with 4D (time), 5D (cost), and 6D (as-built operations) modeling as options. It establishes a shared knowledge resource for information on a facility, forming a reliable basis for decisions during its life cycle, from inception to commission and beyond. For instance, a recent study divided BIM Designers create 3D engineered models Drafter creates 2D plans for contract Bidder/Contractors: Translate and interpret 2D plans Re-engineer 2D plans into a 3D model for building and AMG â â Construction Administration uses 3D model for inspection, administration, and QC Figure 4. Comparison of 2D and 3D design workflows [Reproduced from Arena (17)].
10 Practices for Construction-Ready Digital Terrain Models applications into three project phases: pre-construction, construction, and post-construction (19). During pre-construction, BIM serves as an early collaboration platform for sharing infor- mation and using 3D models for estimating takeoffs. During construction, BIM can be used to keep track of schedules, cash flows, and work progress in real time to reduce work and overruns. Additionally, BIM enables the creation of digital as-builts, which can be used further on in the project life cycle. Widely acknowledged as a successful innovation in the construction industry, BIM has been studied by construction researchers to identify its benefits. In 2012, McGraw-Hill Construc- tion surveyed construction professionals to identify the short- and long-term benefits of using BIM (20). These benefits are listed in Figure 5. The study also highlighted the business benefits of major construction project stakeholders, especially the impact of BIM on architects, engi- neers, contractors, and owners. An international survey verified previous findings of BIM benefits in efficiency and process, performance and knowledge, sustainable building, technical aspects, finances, and legal-related matters (21). Most of the survey respondents felt that BIM enhances overall project quality as well as productivity and efficiency. With the growing success of BIM in the vertical construction industry, BIM for infrastruc- ture (BIMfI) is also finding success in Europe and Asia as an asset life cycle management meth- odology. Consequently, BIMfI has garnered increased interest in the United States and within DOTs (22). FHWA defines BIMfI as âa collaborative work method for structuring, managing, and using data and information about transportation assets throughout their life cycleâ (23). AASHTO and FHWA are working with buildingSMART International to establish open standards for the exchange of data. buildingSMART International is the body overseeing the development of Industry Foundation Classes (IFC) standards, which are platform-neutral, open standards for the exchange of building and construction industry data. AASHTOâs Joint Technical Committee on Electronic Engineering Standards has formally adopted the use of IFC. This partnership aims to develop the IFC Bridge Design to Construction Information Exchange to develop standards for bridge information modeling and related data exchange protocols (24). DTMs are a key element of BIMfI because they serve as the baseline 3D envi- ronment for developing the BIM model (25). Figure 5. Short- and long-term benefits of BIM (20).