Construction and Survey Accuracies for the execution of rockworks

Best practices from the "Maasvlakte 2" Port Expansion Project

Construction and Survey Accuracies

Various aspects concerned with the construction and surveying of dredging and stone dumping works are 
explained in this guideline. Recent experience during 
various projects has demonstrated that disagreements 
concerning the quality of the work completed can 
readily arise between clients and contractors during 
the handover of hydraulic structures. These problems 
mainly concern the monitoring of dumped stone works. In light of the problems it seemed to be desirable that clients, contractors and specification writers should have the opportunity to gain more knowledge about the details of this subject in a relatively simple way. This guideline attempts to meet that need.


After presenting an overview of rock dumping 
equipment and work methods, the term construction 
accuracy is first explained in Chapter 2. Terms such as 
the natural roughness and the added roughness of a bed are described in a general sense. This is followed by separate sections for both dredging and dumped stone works which examine the magnitude of the construction accuracy that may be realistically expected in more detail. In these sections, various sources of inaccuracies are examined, including theoretical considerations as well as realistic values to expect in practice. The quantitative information presented is intended only to give the reader an impression of the order of magnitude of the possible irregularities. Deviations from these values in practice are certainly not inconceivable.


The state of affairs in the surveying of dredging and dumped stone works is described extensively in Chapter 3. Since many problems can arise during monitoring of dumped stone works considerable attention is given to this. Various important aspects are first explained in general terms,  including the definition of ‘the level’ of a bed, (including the relationships between the level of the tops of a bed, the mean measured level and the level that corresponds with the total weight dumped in an area), the extent of the penetration of a survey signal into a bed, the possible settling and compaction of the underlying layers and the physical penetration of the stones from a dumped area into these underlying layers. Along with the description of the sources of survey inaccuracies for dredging, there is also a closer examination of the errors associated with the horizontal positioning and the quality of the equipment of the surveying system as a whole. These errors can have an important role in monitoring dredging works.


During the monitoring of rockworks the most important sources of possible errors are associated with 
penetration of a survey signal into the rock layer and the penetration of dumped rock into the underlying layers. These subjects have been placed centrally in the treatment of survey inaccuracies during the monitoring of dumped stone layers. From practical information from the project in the Waalbocht near St. Andries in The Netherlands [1], information collected of the dock trial in Dock VI of the Verolme company [10] and the recent information of the results of the pit trials of the construction of the 2nd Maasvlakte extension of the port of Rotterdam [17] an overview is given of the major errors that can arise in this respect.


In the present guideline the specific values listed were applied in the construction of the Maasvlakte 2
project. For other projects the applicability of these values should be considered in view of the function of the specific structure: the rock gradings to apply, the surveying system to use and the physical aspects such as water depth, slope steepness and soil properties etc. Chapter 4 is concerned with the possible consequences of the various errors that may arise. During surveying with echo-sounding systems, particularly the estimation of the depth for navigation above the tops of a dumped
stone area and the estimation of a dumped stone area and the estimation of the real thickness of the layer of dumped stone applied under water significant problems can arise. Practical examples of evaluations of rock works are also included. In addition, recommendations are made in an effort to keep these problems under control in practice. Relatively speaking, a great deal of attention is given to the frameworks of contracts and the survey control connected with the execution of dumped stone works.



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