Measuring slurry density without radiation 18 Jan 2018

During underground excavation, slurry is used to transport soil to the surface or maintain stability of the face, and the correct management and measurement of the slurry system is vital for safety and efficiency. Drawing on experience from the dredging industry, Netherlands based Royal IHC has developed a density meter to measure slurry using harmless radio waves instead of radioactive material as source. This new technology can be used as standard, requires no special permits and is completely safe.

Fig 1. Slurry system in operation

Fig 2. Antennae signals record data

The measurement of slurry density during tunneling is important to ensure that the correct amount of soil is being excavated in relation to the advance rate, as a too high soil content could indicate overexcavation and lead to settlements on the surface. It is also important to monitor the density and quality of the recirculated slurry, as some soil particles will not be separated in the separation plant (Fig 1).

Slurry density is currently measured using a radioactive source and receiver and the level of deflection of the radiation is an indicator of the number of particles between the two. This method is used in pipe diameters from 300mm up to 1000mm plus and is well-proven in dredging and tunnelling, but has a number of disadvantages; handling of a radioactive source requires special permits, transportation of the equipment must be done with special precautions, only certified personnel can work with the equipment, and those personnel must be present when equipment is installed on site.

The new Royal IHC density meter uses radio waves to sense the volume concentration of solids present in the carrier fluid. The section of the slurry pipeline where the measurement is to be taken is fitted with a pair of antennas – transmitting (Tx) and receiving (Rx) attached to transparent polyurethane windows (Fig 2) and the measurement is taken by exciting an electromagnetic field in between so that a microprocessor can calculate the amount of solids present in the pipe.

The data is then sent to the signal conditioner, a robust marine grade instrument that calculates the density of the slurry and collects the data for analysis.

Outer and inner views of the pipe

The new system is much safer for users, being free from ionizing radiation, and having a steel pipeline that provides screening from any electromagnetic waves.

In field tests it performed as well as the radioactive meter, and in laboratory tests the only limiting factor appeared to be the salinity of the fluid, which will not be a problem during tunneling if the bentonite slurry fully penetrates the soil and replaces groundwater in the TBM mixing chamber.

The new meter is also easy to operate and will be a welcome replacement to contractors in the management of their tunnelling slurry systems.