Our approach is proposed in the framework of the design of a Corporate Memory (CM) for the company CIRTIL . This company aims to save and capitalize its knowledge and its know-how concerning the production activities related to technical incidents on database servers and applications. In this paper, we present the S3 model which permits both to model knowledge and to index documents. We also show how semantic indexing of fragments document can be improved by mean of the domain ontology. We show finally the interest of our model with the implementation of a prototype.