Volume measurements in dredging are very important as they are used a lot for payment of a contract. But what cubic meter are we talking about? Depending on where you measure the differences can be quite big.
Today we discuss the 5 reasons why a cubic meter is not a cubic meter, or in other words, how to explain the differences in volume, depending on where you measure.
Bulking accounts for the decrease in density of soil when it is disturbed. Soil on the seabed is compacted, most of the time. Once you start dredging it you mix the material with water. The density is reduced to allow the pumping of the mixture.
When the material is deposited it settles again, but almost never reaches the same density as it had on the seabed.
The quantity of solids, the grains of sand or the volume of rock, remains the same. But due to the disturbance of the soil it now takes up more volume.
The bulking therefore results in a higher volume of material after dredging than was removed from the seabed.
2. Reclamation losses
In the reclamation the material needs to settle. For coarse material this is not an issue, but for finer material this can take quite some time. It can happen on any reclamation site, but especially in an open reclamation the fines can flow out of the reclamation, together with the run of water.
Note that this loss of fines is not always a bad thing, as it does often improve the quality of the soil left in the reclamation.
Depending on your Particle Size Distribution (PSD) and the particulars of your reclamation this can be a significant percentage of the volume.
The result is you need to dredge more material than is required in the reclamation site.
Sedimentation in this case is the settling of material in the dredging area after you have dredged there. The result is that you can measure a certain volume onboard or on the reclamation, but when doing a hydrographic survey you find a lower volume, sometimes much lower.
This problem mainly occurs in areas where sedimentation already takes place or where there is a current.
How much this affects you depends on the payment details of your contract. The only way to “prove” the existence of sedimentation is to survey very shortly after dredging and then again a week or 2 later. If the removed volume decreased during those two weeks then sedimentation occurs. Though, where the material comes from can be another discussion.
4. Overflow losses
Overflow losses only apply to TSHD’s and can only explain a lower hopper volume when compared against material removed from the seabed.
A TSHD overflows the surplus of water from the dredging activity. There are some exceptions, but on most projects, this is customary.
As with a reclamation the fines take more time to settle and therefore often disappear together with the surplus water via the overflow. As these particles take a long time to settle they might not settle in the dredging area as they are taken away by currents.
The result is that you must dredge more material to fill the hopper. Especially in finer sand the average overflow losses can reach 30% or more.
As with the reclamation, this is not necessarily a bad thing, as the sand left in the hopper is coarser which is often considered to be of better quality.
5. Measurements errors
In anything that is being measured there is a measurement error.
Production sensors onboard rely on the situ density, which can be quite in-accurate.
Hand soundings, either in the hopper or behind the CSD, can be prone to human error.
Hydrographic survey volumes are influenced by the frequency used for the sounding.
Land surveys of the reclamation are typically done until the water line, while often a part of the reclamation material extends below water.
Any type of measurement can be in error, only in various degrees. Re-calibration and double measuring volumes can often alleviate or explain these errors.
Whenever you have a discussion or dispute the above reasons for a change in volume might help to point the direction where to look for the explanation.
To your long term great results,