PRELIMINARY ENGINEERING

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1 Source: LAND DEVELOPMENT HANDBOOK PART IV.B REFINEMENT OF PREVIOUS ASSUMPTIONS C HAPTER 16 David A. Jordan, P.E. INTRODUCTION Preliminary engineering is an integral component of the third step in the land development design process: schematic design. As discussed earlier in this handbook, schematic design is preceded by feasibility and site analysis (step 1) and conceptual design (step 2). The last stage of the design process is final design. The focus of this chapter is on preliminary engineering associated with the development of the schematic design. Preliminary engineering is a refinement of the previous two steps in the design process. The purpose of this refinement is to create a deliverable, often in the form of a graphic, such as a site plan or rezoning development plan, but may include checklists and/or reports suitable for review by the developer/builder and local governing agencies. Generally, preliminary engineering is performed by civil engineers. However, the developer/builder, planner, architect, reviewers at the local governing agencies, as well as citizens may be contributors to this process. This stage of the design process includes an engineering review of the schematic design/preliminary site layout. During the earlier design stages, the development program was determined, pertinent information gathered, the site analyzed with all opportunities and constraints presented, and a preliminary site layout prepared. Now it is time to fine-tune the design by refining the previous assumptions. Although preliminary engineering reflects a level of design detail necessary for review and approval by local governing agencies, often a preliminary engineering deliverable is not always required by local governing agencies. However, the developer/builder himself typically requests this level of detail prior to incurring the cost associated with developing the final site plan. The deliverable documents, produced during preliminary engineering, represent a final check of the development program before the more detailed final engineering proceeds. This provides, not only the developer/builder and/or local governing review agencies, but also citizens, with a greater sense of comfort as to the feasibility and construction costs associated with the proposed development program. At this stage, the developer/builder may be the owner of the property or may be the contract purchaser with contingencies built into the contract, such as getting a rezoning or other application approved. Another contingency may be achieving a specific lot yield or obtaining the desired gross floor area of a building. It is important to understand that the development program is always subject to change, particularly if it is presented to citizens and/or public review agencies. Hence, the preliminary engineering phase of the design process is oftentimes an evolutionary process that can last for months or even years as the public process progresses. As an example, it is not uncommon for changes to the site grading and layout to occur as a result of citizen comments requesting additional open space and tree preservation areas. COMPONENTS OF A STUDY Although the level of detail required when performing a preliminary engineering study may vary, depending on the developer/builder s needs or the submission requirements of the local governing jurisdiction, certain items are always required. A comprehensive base map is an essential part of the preliminary engineering analysis. Ideally, this item includes field-run topographic survey information, the site boundary, 295

2 296 S CHEMATIC D ESIGN and the existing structures and features of the site. The base map should also include the site opportunities, such as buildable areas and natural site amenities, and site constraints, such as wetlands, floodplains, and environmental corridors. All of these items should be surveyed or at least tied down to the boundary survey. It is important that any demolition requirements be addressed as well with the base map. The focus areas for a comprehensive preliminary engineering study are identical to those that will be performed during final engineering; the difference is in the level of detail. Whereas final engineering documents are more polished and can be used for construction, the preliminary engineering documents are refined enough to ensure that problems do not arise if preparing the final documents for construction. The components of a preliminary engineering study are as follows: Suburban Street Design (and Site Layout) The horizontal configuration of roadways from the schematic design should be finalized and checked for geometric accuracy. Widths of proposed pavements should be shown based on expected traffic volumes. Any curbs, gutters, and sidewalks should also be depicted. Profiles of roadways should be prepared to verify vertical geometric accuracy, sight distance, and cut/fill requirements. The preliminary layout should be tied down to the site boundary, and lots and/or land divisions should be computed and checked for geometric accuracy. Building setbacks should be established and building envelopes represented. All on-site and off-site improvements should be shown, including easements necessary for utilities and ingressegress (access). (See Figure 16.1.) Storm Drainage Design The horizontal placement and sizing of the storm sewer pipe and structures should be done after the streets and general site layout have been designed. Typically, profiles of the storm sewer system are part of the final design. However, select profiling should occur during preliminary engineering to locate vertical conflicts with other utilities. Additionally, flow capacities of proposed and existing systems should be checked. Since the adequacy of the drainage outfalls is critical, as-built information of existing storm systems and field run cross-sections of existing drainage channels should obtained. Design of Stormwater Management Facilities Sizing the stormwater detention facilities is typically part of the preliminary engineering stage of the design process. In order to size the facilities, it is important to determine the specific water quality or pollution-removal requirements as well as the allowable stormwater release rates for the proposed development. The water quality requirements vary greatly from region to region within the country and even vary significantly at the regional level. Although the configurations and grading of such facilities are subject to change during the final engineering, it is important to determine if enough room has been set aside to achieve the required quality and release rate. Additionally, the spillway capacity of the stormwater management/water quality facilities should be determined. Oftentimes the spillway capacity, which is related to the hazard classification of the facility, has to be increased if the facility is found to be a hazard to downstream dwellings. Subsequently, a dam break analysis should be performed and hazard classification should be determined. If off-site easements are required downstream for a breach, these should be obtained as soon as possible. Floodplain Studies Existing floodplains that were approximated during the site analysis stage of design should be modeled using field-run topography. Additionally, these floodplains should be brought into the base map so that their expanse can be more accurately identified. Grading and Earthwork Grading is one of the most important steps of the preliminary engineering process. It determines the extent of the clearing limits of the project and determines if any requirements for off-site construction are needed. Additionally, earthwork analyses can be performed using the preliminary grading plan to check whether the proposed project balances or will require the import of fill or export of cut. The civil engineer should review soils mapping and/or available geotechnical reports during preliminary engineering. The investigation should include problem soils and rock formations. Wastewater Collection/Wastewater Treatment The horizontal placement of the sanitary sewer should be determined. Additionally, as-built information of existing sanitary sewer pipes should be obtained to verify capacities and vertical elevations for future tie-ins. Profiles of the sanitary sewer are normally part of final design. However, select profiling should occur during preliminary engineering to locate vertical conflicts with other utilities and determine whether there is adequate cover over pipes, such as at stream crossings and in roadways. Downstream treatment plants should be contacted to confirm that they have capacity for the proposed flows from the development. If public sanitary sewer is not available to the project, alternative methods for wastewater treatment will need to be explored. As an example, if septic systems are being used for sewage treatment, they should be preliminarily sized and configured. In order to do this, percolation tests in the area of these proposed fields will have to be performed to determine the rate at which the soil can absorb the effluent.

3 F IGURE Example of a preliminary site plan. Water Distribution/Water Treatment The horizontal configuration of the water supply system should be determined. Water pressures in the immediate vicinity of the proposed development should be checked, as well as the fire flow requirements. If the public water supply system is not accessible, then borings for well water must be performed or other methods of water supply to the project will need to be examined. Erosion and Sediment Control A general erosion and sediment control program should be determined at this stage of the design program to ensure compliance with local code requirements. Some jurisdictions will review and approve rough grading/erosion and sediment control plans so the developer/builder can get onsite and start construction while preliminary and/or final engineering efforts are still under way. Cost Estimating Preliminary cost estimates of the proposed improvements, including the fees associated with the development program, can be determined at this stage of the project now that the preliminary sizing of the above-mentioned items is complete. Such costs can be approximated using nationally published reference manuals or by using the local governing agency pricing guidelines. Oftentimes the developer/ builder has a database of cost guidelines it wishes to utilize or the designer uses previously designed and built projects for unit price costs. Miscellaneous Items Checklists and reports are often prepared during the site analysis and site selection stage of the design process. It is good practice to update and/or prepare new checklists and reports as part of the preliminary engineering process. An example of a preliminary engineering checklist is provided in Figure During the preliminary engineering stage of the design process, it may become apparent that waivers and/or modifications to local design standards and zoning ordinances may be required. Obtaining waivers or modifications may be critical in meeting the objectives of the proposed devel-

4 298 S CHEMATIC D ESIGN F IGURE Example of a preliminary checklist.

5 F IGURE (Continued) Example of a preliminary checklist.

6 300 S CHEMATIC D ESIGN F IGURE (Continued) Example of a preliminary checklist. opment program, to facilitate the eventual construction efforts, or to speed up the design review process. Unique or nonstandard construction details should be preliminarily prepared and analyzed during this stage of the design process. Gas, electric, cable, and telephone line availability should be checked if not already investigated during the feasibility stage of the development program. VALUE ENGINEERING PRIOR TO FINAL DESIGN An important part of preliminary engineering is determining the most economical approach to design and construction without significantly altering the design program. The process of economizing the design program is termed value engineering and should be accomplished before final engineering. The main purpose of this effort is to identify cost-saving opportunities to the developer/builder. It may or may not include all members of the project team, such as the architect, planner, public agency reviewer, attorney, construction manager, and developer/builder. It may become apparent, for instance, after the preliminary grading study and earthwork calculations are performed, that the site elevation should be raised or lowered to balance the earth materials. Reconfiguring lots may reduce street lengths and utility requirements. Perhaps regrading or adding retaining walls might preserve more open space and trees. Preliminary construction cost estimates and review fee estimates may or may not affect the design program or the design and construction schedules. Value engineering should be accomplished prior to final design because changes that occur after final engineering has commenced are costly and create schedule problems. Delays to plan approval and to obtaining permits for construction are almost certain if major design changes are required during final engineering to save costs. ADDITIONAL READING Brown, Thomas L., Site Engineering for Developers and Builders, National Association of Home Builders. Washington, D.C., Colley, B.C., Practical Manual of Land Development, McGraw-Hill, New York, De Chiara, Joseph, and Lee E. Koppelman, Site Planning Standards, McGraw-Hill, New York, Galaty, Fillmore W., Wellington J. Allaway, and Robert C. Kyle, Modern Real Estate Practice, Real Estate Education Company, Chicago, National Association of Home Builders (NAHB), Land Development, NAHB, Washington, D.C., National Association of Home Builders (NAHB), Land Development Manual, NAHB, Washington, D.C., Northern Virginia Builders Association, The Basics of Land Acquisition, Northern Fairfax, Va, Northern Virginia Building Industry Association and Virginia Bankers Association, Guidelines for Environmental Site Assessment, Fairfax, Va., Urban Land Institute (ULI), Residential Development Handbook, ULI, Washington, D.C., 1978.