Step 1. Preparation of Terms of Reference for Engineering Design

The implementing agency shall normally employ consultants to prepare the engineering design and supporting bidding documents, although design packages for smaller works may be prepared by in-house engineering staff. In either case TOR shall be prepared for the engineering design tasks and the following guidelines should be followed:

• The TOR shall clearly specify the objectives of the detailed engineering design. The scope of work normally includes elaboration of preliminary designs based on the actual field survey information, prescribed criteria and standards, further investigations, analysis of options and more realistic construction assumptions.
• A detailed design report shall be prepared containing drawings, specifications and the engineer’s cost estimate at a sufficient level of detail to allow the implementing agency may proceed with the procurement of works, goods and services for the implementation of the project.
• The TOR shall identify and describe surveys, investigations and studies to be completed for the alternative selected in the feasibility study.
• The TOR shall specify the engineer’s cost estimate to an engineering design level which should be accurate within ± 10% and based on realistic work norms, market prices, quotations and tendered unit rates.
• Any requirement for design optimisation for key project components should be clearly specified in the TOR.
• The governing design guidelines, standards and criteria should be clearly specified in the TOR.

Step 2. Appointment of Engineering Design Consultant

Refer to Part II Chapter 15 Appointment of Consultants.

Step 3. Field Survey and Investigations

Step 3.1 Site Selection

The project site will have been selected in the preliminary engineering stage but one of the basic steps in the detailed engineering design is to finalise the site location. This particularly true in the road and irrigation sectors. Considerations need to be taken of population to be served, use of existing roads for construction and operations/maintenance access, topography and sub-grade conditions, environmental factors, availability of materials etc. It would be rare that a single site satisfies all the factors involved. In most cases it will be necessary for the engineer to exercise his professional judgement and decide on the optimum site keeping in view the technical and other considerations.

Step 3.2 Surveys

This survey is normally carried out in three stages: (a) reconnaissance survey (b) preliminary survey and (c) final location survey.

Step 3.2.1 Reconnaissance Survey
This survey is based on available information and maps, and aerial photographs if these are available. Reconnaissance is normally done by a walk-over survey during the pre-feasibility project identification stage. While conducting this survey, assessment is made of various factors affecting the site selection such as: topography, soil conditions, availability of construction materials, water sources, climatic conditions, means of communication, construction feasibility, bridging requirements, population centres, costs etc.

Step 3.2.2 Preliminary Survey
This survey, done on the basis of the reconnaissance survey and follow-up appraisals, is made during the feasibility stage to gather data on alternative sites in greater detail.

Step 3.2.3 Final Location Survey
After the site has been selected based on evaluation criteria, this site must be translated into physical locations during the final location survey. The amount of survey points will depend on the type of works to be constructed and the topographical information available and needed.

Step 3.3 Guiding Principles of Site Location
The principles determining site selection will depend on the type of project and are described in more detail for each sector in Chapters 19 to 27. However, the following principles should apply universally:

• The site should offer the maximum economy in costs of construction, transport of materials and later operations and maintenance.
• The site should enable the completed facility to satisfy the intended level of service and all safety requirements.
• Every effort should be made to utilize existing facilities in order to minimize construction cost and land acquisition.
• The site should avoid or minimise negative impacts on social and cultural activities and the environment such as schools, hospitals, places of worship, archaeological and historical monuments, water courses and forested areas.
• The site should minimize interference with services like power transmission lines and water supply mains.
• Site selection should consider drainage, sub-grade conditions and slope stability.