How can quality be seen as an integral part of a contract just as safety and other works, and how can it be placed as part of the contract document??

Intekhab,

Your question indicates one thing to me and that is the level of your organization's top management commitment to quality. If safety and other activities are usually embedded in your contracts then I assume that you are not far from the answer you are looking for; its only an organisational culture issue.

First to be able to have quality integrated your contracts your management must be able to see the bigger picture and not the immediate gains. To get quality into a contract consider the following:

• First have a quality program in place: (assuming your Organisation already has a strategic quality objectives and quality Policy backed by your cooperate vision and mission statement).
• From the above; a quality management system should be developed (in line with your cooperate objectives, policies and known international standards and guidelines supporting your line of business).
• From your top management's approved QMS and strategic objectives, Contract quality requirement (quality charters) and specifications to be made part of a contractual exhibit should be developed. Solicit  the help of a cross discipline team involving quality, engineering, supply chain, construction.... again depending on your line of business to ensure all aspects are covered.
• The above charter and specifications being approved by management will be maintained by quality dept. and communicated to contracting group each time they are packaging a new contract for insertion into the contract. 
• The quality representative involvement and the quality inputs must be included right from the PRE-FEED stage of the contracts before ITT, and the quality input must be part of the ITB (instruction to bidders).
• To achieve this you need your top management backing and sponsorship.
• This can only be possible if quality initiative and involvement is sort right from the project initiation and pre-feed stage.

Good Luck.