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Underwater Blasting - Part 1

Underwater blasting is an extension of the traditional applications for explosives in civil works, normally used onshore. This technique deals with both drilling and blasting in underwater environments, in situations where the removal of rocks by purely mechanical means is not possible.

Normally it is applied to works such as deepening ports and canals, excavating trenches for laying cables for oil and gas pipelines, communication cables, demolition work, and excavation for IADC foundations.

An application example is the expansion works of the Panama Canal, one of the biggest engineering works of this century, where approximately 1 million cubic meters of rock were removed using this method (Couceiro & Lopez Cano, 2017).

How is it done?

Determination of the volumes to be excavated

Determining the volume of sediments and rock to be excavated using dredges is done using the bathymetry of the area delimited by the project (figure 01). The term bathymetry simply refers to the depth of the seabed concerning sea level (figure 02). Bathymetry is measured using “remote sensing” methods that investigate the seafloor indirectly, without making physical contact. Most estimation methods are based on the concept of using the time to infer distance. Specifically, the sensors emit a beam of sound, light, or radio waves and measure the round-trip time it takes for the beam to bounce off a surface and back to the sensor. The elapsed time is then related to the distance traveled by the beam and used to infer the bathymetry. The longer the time taken for the beam to return, the longer the distance traveled (Dierssen & E. Theberge Jr, 2014).

Figure 1. Bathimetry of Tefé and Itapema rocks, São Paulo - Brazil. Total of 33.362m3 of rock remove (Geourbe, 2011)

Figure 2. Bathymetric Method. (CPE Technology, 2019)

Drill pattern as a function of the dredging equipment

Due to the high costs normally involved in dredging operations, the drilling pattern must be defined in a such way that the fragmentation generated by blasting meets the size requirements for the type of dredging equipment been employed in the project.

The most common types of dredgers used, as well as their typical fragmentation requirements and recommended powder factor are indicated in table 01.

Table 1. Fragmentation and powder factors are required by different dredging types. (López Jimeno & Garcia Bermudez, 2017)

Once the operational characteristics of the dredging equipment available for the project are known, it is necessary to determine the optimal drilling patterns, which, together with the explosive selected for the project, will be able to promote the ideal fragmentation and maximum excavation rates during the project.

Predicting fragmentation

Predictive fragmentation models relate the characteristics of the rock, the explosive used, and geometric parameters of the blast to predict the outcome fragmentation. Different models can be used for this purpose. A comparison between the most relevant models can be seen in Ouchterlony and Sanchidrián (2019), where the authors review their development.

Thus, establishing an objective fragmentation suitable for the dredging equipment, the chosen model can be used to determine the ideal drill pattern to achieve this result.

Built-in applications, full available in the O-Pitblast® software allow the fragmentation curve of a blast to be automatically estimated during blasting plan design, giving the engineer the freedom to predict what the expected outcome will be.

Figure 3. Fragmentation prediction on O-Pitblast Software. (O-Pitblast)

It is always important to mention that, regardless of the choice of the predictive model used, it is of fundamental importance to keep the prediction results sharp and attached to the reality that this model must be calibrated.

Model calibration consists of acquiring and processing photos of the fragmented material. Normally, acquiring photos is done directly on the dredged material being transported by auxiliary barges (figure 4).

Figure 4. Backhoe dredger "OPTIMUS" and transport barge

A common practice is to analyze these photos with software designed for this purpose, such as the WipfragTM software, developed by the Wipware company. This software allows the analysis of the fragments and determination of the passing fragmentation curve, uniformity index, and specific passing fractions (X20, X50, X80, X90). An example of the type of analysis and the information generated by this software can be seen in Figure 5, where we have respectively a sample of the fragmented material, with its contours already properly delineated and the output information used for model calibration.

Figure 5. Automatic boundary delineation and fragmentation curve from Wipfrag Software.

The model calibration consists of the adjustment between the theoretical and real fragmentation curves, obtained through photographic analysis of fragmented material samples (Miranda, Leite, & Frank). This process can be done very simply and automatically through the O-Pitblast (O-Pitblast) software. Figure 6 shows how to obtain the fragmentation model calibration directly in this software.

Figure 6. Calibration window in O-Pitblast Software (Vinicius Miranda, Francisco Leite & Gean Frank)



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