The provision for variations in construction projects contracts has become the convention to provide the much-needed flexibility during the course of construction. Yet variations often lead to conflicts and disputes due to the different perceptions of the project stakeholders. One of the most contentious issues is in interpreting the rules to value the varied works. The need for a robust mechanism for the valuation of variations has been articulated in many publications. In light of this, this research aimed to develop such a mechanism and so eliminate or at least reduce conflicts and disputes in construction projects, particularly on civil engineering works. Initial investigation via a small-scale survey, corroborated the occurrence and significance of variation events in construction projects in the UK, whilst further literature study confirmed the valuation of variations as one of the main sources of conflicts and disputes. Subsequent study developed a negotiation model, and frameworks for valuing variations in excavation works and concrete works to support the development of a mechanism for the valuation of variations on civil engineering works. The development of such a mechanism involved knowledge acquisition from experts/practitioners in the field. Primary data was collected through a questionnaire survey of quantity surveyors in the UK, resulting in the development of a best practice model for preparing quotations, negotiation model, and decision-making model. The best practice model aimed to provide a guideline to prepare quotations with higher likelihood of being accepted. The negotiation model aimed to enable the project stakeholders to better understand the negotiation process and so lead towards an amicable agreement. The decision-making model aimed to provide a decision-making tool for the project stakeholders to reach a more objective decision in valuing variations. Following internal validation of the findings, the models were incorporated into a Knowledge Based System (KBS) to preserve the captured knowledge and to enable effective dissemination. The KBS was developed using Java Server Pages (JSP) technology and also MySQL. The development of the KBS applied a user-centred approach in which user orientation, user acceptance, and user involvement of the system were paramount. Follow-up interviews were conducted with a smaller group of experts/practitioners to demonstrate and externally validate the KBS. Findings from these interviews led to improvements to the KBS, and ultimately demonstrated the robustness and validity of the system. The KBS can be used by project stakeholders (i. e. contractors, engineers/consultants, and employers) to reduce or at least minimise conflicts and disputes in the valuation of variations, particularly on civil engineering works. The KBS developed in this research could be expanded to encompass various other types of works (other than excavation works) and procurement routes (other than traditional procurement using the ICE 7th Measurement Version, 1999). Additionally, there is scope to develop interfaces to link the KBS to various other systems such as CAD, estimating tools and project management scheduling in order to deliver a fully integrated package in valuing variations on construction projects.