Correlation between dredgeability and mechanical properties of rock

Due to high investments involved in rock dredging, a good prediction of the dredgeability is essential. The aim of this study is to investigate the correlation between rock mechanical properties and parameters describing the dredgeability. The site studied is an area of 3.5 km2 at Raʹs Laffan port in Qatar. The rock mass dredged consists of dolomitic limestone, calcarenite and caprock. Rock mechanical properties used in the study are the point load index (Is50), the unconfined compressive strength (UCS), the rock quality designation (RQD) and the fissure index (FI). Is50 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 between 5 and 13/m. The two production parameters investigated in this study are the volume excavation rate and the wear rate. The volume excavation rate or productivity is expressed as volume of rock material dredged per hour. The wear rate is expressed as the volume of rock material dredged per pick point used. elation is noted between the individual rock mechanical properties and the wear rate. However, a correlation between these individual properties and the volume excavation rate 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 rating system has been developed to incorporate three properties into one system. The three properties considered are the strength of the intact rock material (cf. UCS), the frequency of discontinuities (cf. FI) and the thickness of the various layers. Firstly, the strength of the intact rock is reduced as a function of the fissure index. Secondly, the 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 function of the strength and of the thickness of the package. The latter parameter is compared to the diameter of the cutterhead and of the suction pipe. For a complete borehole, the values for each layer or package of layers are added, resulting in the total rating. By making such a combination of the three rock mechanical properties, a correlation is observed with wear rate, as well as with the volume excavation rate.