CORRELATION BETWEEN DREDGEABILITY AND MECHANICAL-PROPERTIES OF ROCK

Due to high investments involved in rock dredging, a good prediction o f the dredgeability is essential. The aim of this study is to investig ate the correlation between rock mechanical properties and parameters describing the dredgeability. The site studied is an area of 3.5 km(2) at Ra's Laffan port in Qatar. The rock mass dredged consists of dolom itic limestone, calcarenite and caprock. Rock mechanical properties us ed in the study are the point load index (Is(50)), the unconfirmed com pressive strength (UCS), the rock quality designation (RQD) and the fi ssure index (FI). Is(50) varies between 0.5 and 6.5 MPa, UCS between 5 and 70 MPa, RQD between 0 and 70% and the average FI-per borehole bet ween 5 and 13/m. The two production parameters investigated in this st udy are the volume excavation rate and the wear rate. The volume excav ation rate or productivity is expressed as volume of rock material dre dged per hour. The wear rate is expressed as the volume of rock materi al dredged per pick point used. A correlation is noted between the ind ividual rock mechanical properties and the wear rate. However, a corre lation between these individual properties and the volume excavation r ate is not observed. The main reason for this is without doubt that a process as complex as dredging is not influenced by one property only, but by a combination of several properties. Therefore, a rock mass ra ting system has been developed to incorporate three properties into on e system. The three properties considered are the strength of the inta ct rock material (cf. UCS), the frequency of discontinuities (cf FI) a nd the thickness of the various layers. Firstly, the strength of the i ntact rock is reduced as a function of the fissure index. Secondly, th e newly determined strength is divided in three classes (<10, 10-20, > 20 MPa). A rating value is given to each package of successive layer(s ) belonging to one class. The arbitrarily chosen rating values are a f unction of the strength and of the thickness of the package. The latte r parameter is compared to the diameter of the cutterhead and of the s uction pipe. For a complete borehole, the values for each layer or pac kage of layers are added, resulting in the total rating. By making suc h a combination of the three rock mechanical properties, a correlation is observed with wear rate, as well as with the volume excavation rat e. (C) 1997 Elsevier Science B.V.