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NCHRP Project 10-77 67 CHAPTER 7: BEST PRACTICES FOR AMG DESIGN MODEL DEVELOPMENT THE IMPORTANCE OF 3D MODELING When a project is constructed using AMG, construction machines compare the position of ground engaging tools against an electronic three-dimensional (3D) model that resides in a computer on the machine. Even when best practices are implemented, calibrating the construction equipment and performing QA/QC checks will not compensate for problems with the electronic 3D model. Therefore, these models are central to the success of AMG. Development of high-quality 3D models is a challenge, as the software must be highly capable and considerable training is required. Additionally, a wide range of experts from various disciplines (including surveying, route design, and hydraulics) must contribute to produce a usable model. Because of this required collaboration, good modeling demands communication, teamwork, and patience. Although the challenges are considerable, the payoff is great. Good 3D models do more than control the implements on construction machines. Coordination sessions in which team members view the model in 3D give greater understanding to the project within the context of its surroundings. Public input sessions can help a facilitator align stakeholder expectations by allowing them to view a âflythroughâ or virtualization of the model, complete with simulated traffic movements. Land acquisition negotiations can be less adversarial because the prospective seller will better understand the project impact. A contractor can review virtual construction projects and raise issues before equipment is mobilized and the expense of making a change is compounded later. Contractors can also use the model to compute partial payment quantities and monitor onsite equipment productivity. Finally, the contractor can become a member of the modeling team by converting the design model into an as-built model and possibly adding spatially referenced QC data. The as-built model can be used for operation, maintenance, and future construction projects. INITIAL PLANNING FOR MODELING Philosophy of Building a BIM-Type 3D Model With proper planning, 3D models can serve as a database repository for complete project information over the life of the entire project. Building information systems, as used in the vertical construction industry, are an example of this. The result is a 3D model that is a graphical user interface for the project, which can also be sorted and queried like a database (Burgess 2010; Bernstein 2012). This same philosophy could be brought to transportation projects. Under such a system, all project participants cooperate to build the model, adding relevant data and information in a way that is spatially associated with the 3D model. In early project stages, the surveyors could contribute by inputting control points, the preconstruction DTM, and positioning information for selected preconstruction features. Design modelers can then add route alignments and proposed surfaces that can be categorized by the types of material they represent. When construction materials are identified in the 3D models, their quantities can be automatically collected as the project is designed. That way, the ownerâs estimate of quantities can be quickly developed when the design is complete. At various points during the design phase, input from other stakeholders can be contributed by developing and sharing visually-rich 3D renderings and animations, which provide context to the design and proposed construction process. After the design model is completed, the constructor can document as- built elevations and indicate the location and results of QC tests. The ownerâs representative can contribute the location and results of quality assurance tests and indicate the date that progress payments were made for items of work, also noting if any retainage is withheld.
NCHRP Project 10-77 68 The model can then be turned over to the facilities operation group, which can use the model for asset management purposes, such as recording maintenance activities, facility conditions, and performance information. The model can also be used for recording accident events and as a basis for the next capital improvement in the future life cycle. Achieving such functionality requires many technological and cultural changes, which arenât likely to be realized in the near term. However, in the interim, improvements can be achieved by using the model, as previously described, as much as is currently possible. For example, rather than giving position information in an American Standard Code for Information Interchange (ASCII) electronic text file, surveyors can build (and often do) the preconstruction DTM and locate selected features (Burgess, 2010). Surveyors would have the opportunity to check their work by reviewing the 3D model, before it is turned over to the design team. On the construction side, rather than having the owner provide a 2D plan set, the 3D model might be turned over to the contractor, with the expectation that it will be detailed and enhanced to the point that it can exported for use in machine control and QC activities. The enhanced model could then be shared back to the owner to allow review and to facilitate quality assurance (QA) activities. All project stakeholders benefit for several reasons: ⢠Time and accuracy are lost with each data transfer, so such a process eliminates the need for data transfers. ⢠To achieve project success, training creates a critical mass of users who (1) are familiar with the modeling software, and (2) collaborate on the project so they can provide mutual support in learning the software modeling application(s) and training inexperienced users. ⢠3D modeling programs tend to have a relatively large base of users; therefore, training programs tend to be well-developed and frequently updated with improved functionality and software code fixes. Software developers have taken steps to facilitate such integration by ensuring that their 3D modeling software can transfer files directly to the surveying and positioning hardware provided by all three of the major manufacturers: Leica Geosystems, Topcon Corp., and Trimble Navigation Limited. Overall Spatial Control An important part of the planning process is to decide how to provide overall spatial control for the project. It is often desirable to begin the project with the required level of control for final construction. Accuracy requirements for final construction are shown in Table 7-1. Table 7-1. Required Accuracy for Modeling (Taylor, 2010) Design Component Horizontal Vertical Roadway 0.04 ft 0.01 ft Ditch 0.1 ft 0.05 ft Note that, to be reasonably assured of achieving accuracy during construction, the model must be built to a greater level of accuracy than specified in the table. This is because some loss of accuracy will inevitably occur (in processes downstream from earlier model development) and the modeler must make reasonable allowances for these losses of accuracy. Geodetic or Cartesian Given casual thought, and global thinking, most people would assume that the zero sea level elevation for surveying measurement would follow a sphere. It follows a shape called a geoid, which
NCHRP Project 10-77 69 adjusts for changes in gravity on a global basis. However, modelers assume that their project is on a Cartesian coordinate system, where there is an x and y axis (eastings and northings), which describe a plane upon which the project is designed. In one mile, the plane departs in elevation from an assumed spherical surface by approximately 8 in. Therefore, it is obvious that a transformation solution will be required to map the Cartesian design coordinates to a set of spherical or geodetic coordinates for elevation. In these cases, geodetic surveying must be used to set the control points for the project and the model must be calibrated to those points before it is used for field activities during construction (See Appendix A). Another important consideration is whether to tie project control surveying to the appropriate state plane coordinate system. Such a coordinate system provides a method to assign northings and eastings that are consistent with those assigned to other projects within the coordinate system area. It would be possible to build a small project that has coordinates that emanate from an arbitrary (0,0), at the southwest corner of the project. However, in this case, the project model cannot be fit into a larger 3D model that might represent the entire road network for half a state since it uses a local, project specific coordinate system. If the model is to be retained for asset and operations management purposes, as well as future construction in the same area, the model should be tied to the state plane coordinate system that is in place in the area (See Appendix A). Models should be linked to the positioning coordinate system that is used for geographical information system (GIS) databases within the jurisdiction. Since asset management systems often use GIS databases, such coordination facilitates the use of the 3D model for asset management purposes after the construction project is built. Data Transfer (Inputs and Outputs) Another planning consideration for 3D modeling is deciding how and when data will be transferred into and out of the model (Taylor, 2010). The amount of data that needs to be transferred in and out of various software programs depends on several factors. If all team members, including construction personnel, build and modify the model cooperatively, the amount of data transfer in and out of the model will be greatly reduced. However, preconstruction topography and feature location data will have to be transferred from the surveying equipment into the model and the model will then have to be converted into a format that can be read by the construction machinery that implements AMG, as well as the surveying equipment that performs QA/QC checks. The results of the survey conducted for this project indicate that 75% of agency modelers obtain surveying information from their surveyors as 3D DTMs. These results also show that about half of the contractors who use AMG receive EED from the transportation agency. The other half are starting from scratch to build a DTM using only 2D plans. When a contractor receives only 2D drawings from the owner, extensive data preparation effort is required, as the contractor must essentially reproduce the 3D model that the designer developed on behalf of the transportation facility owner. This adds considerable effort to the required workflow (See Figure 7- 1).
NCHRP Project 10-77 70 Agency Prepares 2D Plan Drawings Agency Completes 3D Electronic Design Pavement Design Structural Design Site Conditions Surveying DOT/Government Contractor Agency Prepares 2D Plan Drawings 3D Model Provided 2D Drawings 2D Drawings Contractor Creates 3D Model for AMG Contractor Uses 3D Model for AMG Design Completed and Contracting Mechanism 3D M od el Road Requirements (size, location, ESALs, etc.) & Prioritization Figure 7-1. 2D vs. 3D Data Transfer Contractors reported (through their survey) a wide range of costs for DTM development: from $150 to $2,500 per lane mile; also, $750 per acre was mentioned. In some cases, contracting agencies provide contractors with electronic versions of 2D plans, which can save time in their model development effort. However, the amount of time required for the contractor to do the conversion using this resource can also vary considerably. In some cases, contour lines are intelligent objects that have assigned elevations and are continuous throughout the project. In other cases, the contour lines are merely linework, without intelligence. Sometimes microscopic gaps and overlaps can exist in the contour lines that will create difficulties in the DTM. If such conditions exist, the contractor must manually assign elevations to the contour lines and tediously repair gaps and overlaps using the modeling software application(s). If data preparation is required, a contractor has a choice of performing the work in-house or contracting it out. Contractors who choose to retain modeling work in-house have more control over the work; and, the person doing the work is likely to be more integrated into the contractorâs workflow. However, the contractor must help this person to maintain their expertise as the software and hardware applications change and improve. If the designated modeler has never performed data prep work before, the learning curve (gaining expertise to produce models for AMG implementation) is considerable. This is especially difficult if full-time roles cannot be devoted to data prep efforts (Burgess, 2010). Data preparation work can also be outsourced. Although the contractor loses some schedule control to an outside organization, the outsourcing employees usually specialize in data preparation work and do it full time; so, they have a higher level of familiarity and recent experience and are not subject to slow start ups as they climb the learning curve (Burgess, 2010). Contactors are more likely to outsource data preparation if they donât have enough modeling work to support a dedicated functional role. However, if a contractor can keep a person occupied with data preparation almost full-time, it will likely be more cost effective to keep the work in-house. Selection of Technology for Spatial Data Collection for Existing Terrain and Features Data for existing terrain and features can be collected using the same surveying techniques that are used for traditional 2D design, such as traditional optical surveying, robotic total stations, and photogrammetric methods. GPS rovers can also be used, especially to obtain existing topography. LIDAR is a mobile scanning laser system that is also becoming more popular, and it can be an effective method to obtain building height data, as built geometry for existing bridges, clearance heights (for power lines and bridges) and topography for landslide areas. Height data for buildings might be especially important if the
NCHRP Project 10-77 71 buildings are to be included in a 3D model to provide context for conceptual design reviews and public presentations. The 2010 survey results for this project indicated that the photogrammetric topographical collection method was the most prevalent for transportation agencies, followed by RTK GPS and total station surveying; as of the completion of this writing (2017), the order of prevalence may have changed Some jurisdictions have already collected information and stored it in GIS data bases. Data of possible interest to the 3D modeler include locations for features and DEMs. In some cases, this information may be used without conducting a separate data collection effort. However, checking the accuracy of this georeferencing to make sure it meets the needs of the project is important (Hixson 2010). One challenge for a quick change over to AMG construction is that sometimes survey crews work several months or even years in advance when they collect preconstruction data. If survey data was collected using a centerline and cross section approach it will not have sufficient density to make it suitable for AMG construction. The data that has been collected using methods that donât meet the needs for AMG construction have to be âflushedâ out of the system, before AMG can be used. It is difficult to tell whether preconstruction DTMs are developed with data that was properly collected. It is also possible to develop a DTM from a centerline and cross-section survey. Although it may appear to be suitable for a 3D model that will be used for AMG, difficulties may arise when construction begins, especially in the areas where new surface data meets the preconstruction surface (centerline and cross-sectioned) (See Appendix A). Consider Tradeoffs Tradeoffs between model accuracy and size need to be considered, especially if 3D models will be used for visualization when a high level of realism is required. High-accuracy models required for paving arenât usually required, for example, for visualization in a public meeting. However, if the model is intended for construction, high accuracy is required. Since visualization is often necessary early in the project to obtain stakeholder buy in, it is tempting to use lower-quality positioning information to expedite the development of the 3D model. However, such an action can result in regrets, because it is difficult to retrofit a model with good 3D positioning coordinates after itâs been started without them. It is usually better to start out with the level of accuracy needed for the entire project, even though a larger investment of effort is required up front. An alternative to this dilemma, if a 3D visualization is needed quickly, at the start of the project, is to develop a separate model using software that helps expedite the development of a model primarily for visualization, rather than for construction-level accuracy. As an example, the Iowa DOT develops 3D visualization models for context-sensitive bridges using SketchUp. A typical project requires only one week of a staff memberâs time to develop 3D electronic models for stakeholder presentations. One advantage of 3D modeling is that the design team and other experts within the agency can review the design in 3D at typical milestones in the design development process, such as 10%, 35%, and 90%. In some cases, the process is more effective if a visually-rich model provides context to the proposed project and its surroundings. Furthermore, the review process can be enhanced by making changes on the fly to the foundational elements of the design, such as alignment and grade of the primary route, and allowing the group to look at the results in 3D and come to agreement on the best alternative. For example, a 3D model that has AMG-level accuracy could be processed into a format that allows a review team to witness a virtual-reality simulation, where team members can decide where to âdriveâ within the model during the meeting. Based on the virtual drive-through, they may consider five alternative alignments for the main lines, change the model appropriately for each alternative, and then save the result. At the end of the session, an alternative is selected as the preferred design. It should be noted that if contextually rich visualizations will be an important part of an agencyâs modeling process, careful thought must be invested in the selection of hardware and software and in making plans for interoperability and replacement (Hixson, 2010).
NCHRP Project 10-77 72 GENERAL PROCEDURE FOR DEVELOPING MODELS The actual development of a 3D design model is a multi-step process and the process is summarized Figure 6-2, based on information from Taylor (2010). The steps can vary depending on the agency and the modeling software that is utilized. It can be expected that workflow will change as software applications are improved and modified. Some of the steps described below may be merged with other steps, or become unnecessary, as software capabilities increase. Also, some steps may require several iterations as alternatives are explored and adjustments are made. Figure 7-2. Flowchart for 3D Model Development Procedure
NCHRP Project 10-77 73 Reference CAD Standards At the beginning of the modeling process, reference should be made to the computer aided design standards for the agency. Layering protocols are usually included in such design standards. Layers allow designers to separate various design elements, so they can be dealt with independently, and so they can be turned âonâ and âoff,â when the model is viewed. Such separation avoids visual clutter and allows team members to focus on their assignment, while being aware of the larger context of the project and the activities of teammates. Burgess (2010) listed the following as typical CAD layer titles ⢠Finished surface ⢠Drainage ⢠Earthwork (Corridor) ⢠Alignment ⢠Parcel Map ⢠Original surface In some cases, it can be desirable to separately track each layer of material in a pavement system. When this is done, separate quantities can be accumulated for each material and a contractor can use the electronic surface to provide grade control for each material without having to offset from the finished elevation. If such a scheme is used, the layering can also include: ⢠Each subgrade and base layer ⢠Undercuts Standard layer protocol facilitates the shared use of the model. (See Appendix A). Add Horizontal and Vertical Alignments A review of the DTM is completed and strategies are developed to add horizontal and vertical alignments. In some cases, multiple alternative alignments are proposed and analyzed with the goal of selecting the best one for the project. Create Typical Sections from Templates Typical sections and templates provide a cross sectional view of the alignment that is perpendicular to the centerline. In this step, they are selected and applied to the route alignments. The location of these elements when viewed in plan is called a pattern line. While vertical and horizontal curves are rendered as actual curves on the main survey lines, they are rendered as chords on the proposed surface between pattern lines. Therefore, the density of pattern lines is an important consideration for ensuring sufficient accuracy. Pattern lines are usually laid out at regular intervals and at important locations, such as the beginnings and ends of curves, superelevations, intersections, and widening and narrowing transitions. Guidelines for pattern line= spacing on designs intended for AMG use are provided in Table 7-2. Note that these guidelines may need to be modified depending on the location of the model. Further densification may be necessary near intersections and other non-typical parts of the alignment. Inflection points on neighboring templates should be connected to break lines to ensure that the inflection is rendered appropriately in the 3D model. A formula driven method is also available to determine the minimum data density required (or maximum line segment length) to accurately portray the curves (both horizontal curves and equal-tangent vertical curves) within the surface model. Three approaches are introduced in Vonderohe and Hollisterâs paper (2013). One approach is âprioriâ which calculates maximum data density required based on design speed. The second approach is using operator in a design software application so that the data density varies along the segment depending on each individual curveâs parameters. The third approach is pure
NCHRP Project 10-77 74 parametric approach which is open to changes for the future AASHTO recommended values of âmaximum allowable side friction factor, driver reaction time, braking deceleration, driver eye height, object height, headlight height, and inclination angle of top of headlight beamâ (Vonderohe and Hollister 2013). The equations under each approach are given for calculating maximum line segment length of three types of curves (horizontal curves, crest vertical curves, and sag vertical curves). Table 7-2. Recommended Pattern Line Spacing (Taylor, 2010) Pattern Line Spacing Horizontal Curve Radius Vertical K Value 10 ft ⥠300 ft and tangents ⥠13 and tangents 5 ft 75 ft < 300 ft 6 < 13 2 ft < 75 ft < 6 Generate Design Surface Model After the templates have been associated with the various route profile lines, the modeling software can generate a design surface model. Check Design Surface The design team should now check the design surface for errors by visualizing it in 3D. If adjustments are necessary, considerable time can be saved when using advanced software with intelligent objects. For example, if an adjustment must be made to the profile grade, intelligent cross-sections will move with the profile grade and adjust themselves to fit into the new circumstance. This prevents the modeler from having to go back and adjust each cross-section separately (Burgess, 2010). Merge Design Surface and Preconstruction Surface When the design and existing surfaces are merged, the limits of earthmoving can be determined, as well as proposed virtual slope stake locations. In reviewing this aspect of the model, attention should be paid to the location of break lines at the transitions from cut to fill. Conduct Necessary Manual Design in Complicated Areas Some manual design may be necessary at complicated locations, such as intersections, special ditches, and culvert entrances and exits. The efforts required for such designs will depend on the intelligence of the modeling objects, as well as the complexity of the situation. As software becomes more capable, less effort will be required. The amount of manual design may depend partly on agency policy about where designing ends and detailing begins. Communicating to the contractor about which areas are not fully modeled is desirable, because it will allow the contractor to plan for the necessity and cost of detailing these areas (Taylor, 2010). Perform Constructability Review A final check for errors and constructability should finally be conducted. The results of the survey for this research project indicated that such efforts in checking are a worthwhile investment; a clear majority of the agency respondents report that such checks expose design errors before construction. In addition, extra staff time should be planned for this activity, as survey respondents reported that 3D design reviews take longer than 2D design reviews.
NCHRP Project 10-77 75 Generate Final Files After the constructability review is complete and adjustments have been made, a process will have to be executed so that the model is designated as the final design model. In some cases, this may require the application of the digital signature of the designer of record. However, in many cases, a process will be executed to extract 2D plan, profile, cross section and detail views, so the intent of the model can be represented by a traditional 2D plan set. In such a case, the designer of record can sign the 2D plan set, either in hard copy or electronically. 3D MODEL SPECIAL CONSIDERATIONS Three dimensional designs of transportation facilities for AMG require special considerations. An upfront understanding of the required degree of accuracy and precision is required for an efficient process Too little accuracy renders the model useless for AMG and will cause a contractor to have to repeat the modeling effort originally undertaken by the designers or to give up on AMG and go back to traditional construction methods. In general, in comparison to traditional construction, more pattern lines will be required to support AMG, because the machinery will require exact instructions on how to grade at the tops of cut, toes of slopes, cut to fill transitions, and any changes in the routeâs curvature, superelevation, or width. When AMG is in use, the operator will not be able to âeye inâ the alignment of such features, as they could when the job was laid out with traditional stakes. In some cases, a designer may not completely detail complicated areas with the expectation that the contractor will consult standard drawings or other sources to provide the necessary remaining detail. If this is done, such areas should be clearly designated. As software applications become more capable, this practice is expected to diminish. Modeling techniques that facilitate AMG can also facilitate the design and resurfacing, restoring and rehabilitation (3R) projects. New data collection techniques allow designers to measure the elevation and alignment of surfaces that are to be rehabilitated with greater efficiency and accuracy compared to past systems. With this information, the design for the new surface could be developed to provide a predetermined elevation and alignment that will fit the existing surfaces and improve the ride quality. Then accurate estimates for material needs could be developed a priori, if changes do not occur to the original surface between the times that the data is collected and when construction occurs. When the model is being transferred to the contractor for final detailing during the data preparation process, the original designer normally remains the engineer of record if any design changes should be requested by the contractor and executed by the designer. Often, a set of 2D plans is extracted from the 3D model and serve as the record document. As a courtesy, the agency can grant the contractor access to the 3D model, so it can be detailed within the intent of the design and converted into machine control files. Usually a waiver is signed by the contractor to acknowledge that the 3D model is provided for information only and is not the legal record of the design. Other jurisdictions have developed schemes to seal and save a record electronic copy of the 3D model that constitutes the design that was supervised by a professional engineer. Comparisons can be made between the record copy and the subsequent detailed models to detect what was part of the original design and what detail and formatting was added for machine control use. In many cases, contractors have had to develop complete 3D models from paper plans, because the issues could not be resolved. As experience is being gained, protocols for transferring an electronic 3D model to the contractor for detailing have been developed that have resolved many of these issues. The survey that was conducted for this project provided some insight into current practices. Of agency procurement respondents that exchange EED with contractors, about half reported that primary responsibility for creation of the DTM is with the agency and the other half indicated the responsibility is with the contractor. However, a clear majority of the agency procurement respondents reported that the ownerâs warranty for constructible plans is for 2D âstampedâ drawings only. After the design âintentâ has been communicated, contractors often must develop or create further details to build a project. Those details are communicated to the owner and designer in the form of shop
NCHRP Project 10-77 76 drawings. The shop drawings are examined by the agency or the designer and returned, noting any exceptions taken by the reviewer. When the contractor has finished constructing the facility, as-built drawings are provided showing exactly how the contractor fulfilled the design intent. Corollary submissions could be expected of contractors who during the data preparation phase, have developed a detailed version of the 3D model. By reviewing the contractorâs model, agency personnel would gain knowledge about what is important for contractors to have for 3D models and provide feedback about possible deviations from design intent involving the existing and proposed surfaces. In some cases, the changes that the contractor proposes or inconsistencies noticed could require revision of catch points and tie-ins. The contractorâs model would have further usefulness in facilitating inspections. In any case, it would be helpful for the agency to have a copy of the 3D model that the contractor personnel are using to facilitate QA activities. However, the survey responses for this investigation from agency procurement personnel indicated a clear majority of field inspectors do not have access to the DTMs. As of this writing, a standard nationwide protocol for handling such interactions has not been developed. However, the survey for this project also showed that more than half of the responding contractors currently share their as-built version of the model back to the agency. One of the coauthors knows of at least one contractor who electronically compares the contracting agencyâs model with their model and notifies the contracting agency of any inconsistencies that the contractor cannot resolve. As mentioned before, the model developed by or on behalf of the agency needs to be checked during the design process. Compared to non-AMG modeling, additional time should be allowed for QA/QC 3D model checks when AMG construction is contemplated. More checks will have to be made between pattern lines to ensure that the chorded approximation between pattern lines does not diverge too much from the curvilinear expectation. Attention should be paid to possible discontinuities in the surfaces. Often, such discontinuities are obvious, because they show up as tall spikes or deep holes in the visualized 3D model. However, care should be taken during the review process, because the signs that such an issue might exist may be subtler than that which was just mentioned. QA/QC for 3D modeling that will be used for AMG is another area for which national standards do not exist; and, it would be desirable to develop such standards. For an agency, if implementation of AMG requires a switch from 2D to 3D modeling, considerable adjustment of workflow and expectations will be necessary. However, considerable benefit will follow in terms of fewer design errors, due to better visualization, better stakeholder understanding of the project, greater efficiency, and less cost of construction, rework, and/or redesigns. Vonderohe et al. (2010) provides an example of how Wisconsin DOT planned to switch from 2D to 3D modeling; this description might be helpful to agencies who are planning such a change. Some questions that will eventually have to be answered are as follows: At what level of development should designer and stakeholder reviews of 3D models take place? Should one review be at 35% design development? If 35% is the right level of development, what are the characteristics of a model that is at 35% design development, so that designers and managers can recognize that the time for the review has arrived? (Manore et al., 2010). CONTRACTOR USE OF 3D MODELING According to the survey for this project, on D-B-B projects, if a contractor receives an electronic 3D model from the agency, the most likely time for this to occur is at, or close to, the preconstruction meeting. However, based on comments received from the survey, it appears that in the future, most agencies are planning to provide the model at the pre-bid stage of projects. When 3D models are shared with contractors by agencies, a clear majority share the models that were used to develop the contract documents on an âas-isâ basis. Usually no enhancements for AMG are provided. The most common data formats for this exchange are .dtm, followed by .tin, followed by .ttm (in that order, descending). If only parts of the model are shared, the route alignment data is most often provided. After a contractor receives a 3D model from an agency or designer, the effort that is required to
NCHRP Project 10-77 77 perform data preparation tasks and to create machine guidance files from the model will vary greatly depending on what data is provided and the contractorâs resources. In general, most contractors start the process by reviewing the 3D model and finding out which layers are relevant to their needs. Usually a contractor will want to have DTMs of the original and proposed surfaces. If a DTM is available for each layer of the proposed surface, such as top of subgrade, top of base, and others, the contractor may select layers for those surfaces also. Also, a linework file that shows the configuration of the transportation facility that is being built will be helpful. The DTMs will be used to define the proposed surface for the contractorâs modeling effort. The difference between the existing and proposed surfaces provides an estimate of volume. The linework file will be used to provide context on computer displays for the machine operators and other personnel, regarding existing and proposed facilities. For example, it might be most efficient for a machine to operate parallel with a curb line. In that case the operator can find the curb line in the machineâs computer monitor and then follow it by operating the machine so that the virtual machine in the monitor runs parallel to the curb line. After the proper layers are selected, the contractor will likely delete all other layers to limit computer memory requirements and reduce the possibility of visual confusion or working on the wrong layer. At this point, the contractor will examine the model in 3Dâto size up the project and look for discontinuities. After this step, the contractor will add detail and make corrections where necessary. Then, the file will be converted into machine control format and distributed to the machinery and QA/QC positioning devices. The most time-consuming circumstance is when only plan, profile, and cross-sectional views of the facility are available. In this case, the contractor will have to develop the DTM completely from scratch; however, unlike the designer, the contractor will not be required to generate alternatives and select from the best one. Instead, the contractor can focus on reproducing the selected design. Time can be saved if electronic versions of the alignment, profile, and cross-sections are available. These base components can then be assembled into a new model. The results of the survey for this project indicate that half of the 3D models that contractors use are developed from âscratchâ from 2D plans. However, in discussions with contractors and consulting modelers who are engaged by contractors, the authors have noted that some modelers prefer to start the construction model from scratch and later compare the construction model to the design model to detect errors. It was asserted that an experienced modeler could develop a model from scratch for a modest additional cost in comparison to enhancing a designer developed model. The advantage of being able to compare the designer developed model with the constructor developed model to detect errors was considered to justify the additional cost. For their purposes in 3D modeling, contractors often use the 3D modeling software that is provided by the manufacturer of their positioning hardware. Although this software may not be as capable and flexible as the software that the designers use, it has the advantages of being quicker to learn (by having less capability, there are fewer menu options to learn), lower cost, and coming with technical support available from an entity that is very motivated to satisfy the contractor (to sell more hardware). One of the export options is to generate a machine control file. Executing that option is the final modeling step for the contractor before construction starts. When 3D models for AMG are in use, plan changes can be a considerable challenge. If the record set of plans is a paper version, agency personnel could issue a plan change and the contractor would be responsible for making necessary revisions to their model. Even, if the agency does make revisions of its version of the 3D model, the contractor would have to make changes to its version, because it is likely that the contractor has made several enhancements to the model that was developed by the agency. From the survey for this project, about half of the respondents indicated that the agency took the lead in modeling the changes half of the time and that the contractor took the lead the other half of the time. Anytime changes to the model are necessary, the contractor will have an issue with version control. This can be especially challenging if the project is in a remote location and physical data storage devices (such as memory sticks or secure digital memory cards) must be placed in the machines. Positioning hardware manufacturers have developed wireless data exchange capabilities between construction machines and central servers that can considerably ease the effort of version control.
NCHRP Project 10-77 78 Within construction contracting organizations, the creation, development, and maintenance of the models are tasked equally between estimator functional roles, dedicated modeling specialists, and outsourced consultants. After contractor personnel have developed a 3D model, they can use it for their own purposes. The survey for this project indicates that 72% of contractors use 3D models for estimating quantities and/or developing the means and methods for earthwork construction tasks. In addition to the above uses, contractors who use 3D models and AMG have better control over elevations and material placement. This lessens the need to plan for intentional overruns to ensure that minimum thickness tolerances are met. The result is a savings of 3 to 6% in material volume (www.transportation.org quoted by Burges, 2010). Also, the 3D model can be the basis for as-built plans. Data collection can be facilitated by operating construction equipment and other vehicles that have positioning equipment over final grades. Thus, the contractor QC personnel can easily obtain positions and elevations of as-built features. SUMMARY OF DATA TRANSFER METHODS A review of this process makes it clear that data must be transferred from one entity to another several times. The results of the workshop that was conducted for Phase I of this project indicated that data transfer was the greatest challenge and biggest opportunity for AMG. According to the survey that was conducted that was conducted in 2010 for this project, there was no predominant file type being used by AMG users (See Figure 7-3). The. dgn and. tin file types accounted for more than half the activity in the design model and contract document creation processes. For mapping the original terrain model, .dtm files were more predominant. For contractors, the AMG process could use any of the eight possible formats, with none being predominant. The LandXML format was developed to be agnostic regarding choices of software and hardware platforms and thus enhance interoperability; it was used less than 10% of the time, except in contract document creation. The diversity of file formats demonstrated here represented a challenge in developing robust file transfer protocols that are required for efficient use of AMG. Figure 7-3. File Types of EED Exchanged Across AMG Functions Interestingly, at the date of the survey (2010) hardware and software vendors seemed to have a
NCHRP Project 10-77 79 different view of data transfer according to the survey results for this project. About half of them responded that their products were capable of data exchange via LandXML and they ranked that methodology as the most important. LandXML was reported as one of the most prevalent import/export file formats by the software and hardware vendors, along with .dwg, .dgn, and .dxf file formats. The vendors expressed that their software import/export capabilities were equally driven by owner and contractor needs, requirements, and demands. DESIGN MODEL DEVELOPMENT BEST PRACTICES SUMMARY ⢠Success in 3D site modeling is central to the success of AMG. ⢠Changing from 2D to 3D modeling comes with many important challenges in a variety of aspects, including training, workflow alteration, and clarifications or adjustments in professional practice. Two years of training and staged implementation may be required to completely switch from 2D to 3D design (Hixson, 2010; Vonderohe et al., 2010). A design team would have to complete three 3D projects before it reaches the returns of its accustomed level of productivity (using 2D methods). Early reports indicated that productivity would never exceed that of 2D modeling (Hixson, 2010). However, the product provided by the design team would be vastly improved (Hixson, 2010). Later experience in Wisconsin and Iowa indicate that productivity is eventually increased using 3D Modeling (FHWA 2013) ⢠Changing from 2D to 3D modeling comes with important benefits, including better communication and error checking with internal and external stakeholders, less effort re- entering data from one phase of design and construction to the next, greater efficiency in construction, and site efficiency benefits, such as allowing the contractor to use the 3D model for cost estimating, development of means and methods, and for productivity tracking. Other incidental benefits include using the 3D model framework for as-built, QA and QC records, and after the project is completed, using it for maintenance and operational information storage, such as locations and dates of maintenance activities and crash-incident analysis. ⢠Decisions regarding how data is transferred from one part of the AMG process to another are important. Having all stakeholders cooperatively build and modify the 3D model would provide a seamless method of data transfer. However, such a practice raises important questions regarding design responsibility and the division between activities that must be performed under the supervision of a licensed individual and those that can be performed without such supervision. For example, if more than one licensed individual supervises the modeling effort, communication will be required to clarify which person has responsibility in each specific area. ⢠For a contractor, the most important layers of a 3D model are the existing and proposed surfaces and line work for the proposed facility. Considerable contractor effort can be saved by giving them access to the 3D models developed by designers. However, legal and professional issues often restrict contractors from gaining such access. Contractors often detail areas that are hard to model. Changes that occur to the model after construction begins can be a challenge to incorporate into the construction version of the 3D model. ⢠At the time this survey was conducted (2010) for this investigation, a predominant format for data transfer has not emerged. Greater understanding between designers and constructors regarding constructor needs with regarding to file transfer would be desirable. AREAS FOR FURTHER STUDY There is a need to develop standards for all parts of the modeling process to ensure that all participants in the design and construction of transportation facilities can efficiently deliver a product with confidence, knowing that it will suit the needs of subsequent users. The standards should include the following:
NCHRP Project 10-77 80 ⢠Overall site control - Geodetic - State plane coordinate referencing - Local control ⢠Separate standards for various surfaces and elements that require various levels of accuracy (such as earthwork finishing versus paving versus storm sewer construction) ⢠Horizontal and vertical accuracy ⢠Measurement and calculation densities - Data collection gird sizes - Use of break lines - Pattern line densities ⢠Completeness of design modeling effort - Definition of designing versus detailing - Methods for delineating parts of the model that the contractor will complete ⢠When reviews are required - During design - During machine control model preparation - Protocol for executing reviews ⢠QA/QC - Timing, type, and density of measurement - Protocol for review and documentation ⢠Design changes and corrections during construction - Designer versus contractor responsibility - Review process ⢠Construction closeout - Accuracy and density of as-built measurements - Modeling standards for documenting as-built locations - Inclusion of QA/QC data ⢠Turnover of model for archiving and use for maintenance and operation
