In many cases, contractors are being asked to provide a detailed slurry engineering program at the pre-bid phase to qualify for bidding the project. Even though this can be a very contentious subject for contractor’s ways and means, it is becoming a reality throughout the business. There are some hidden benefits to these pre qualifications, as it brings the ground conditions into focus and can sometimes expose ground conditions that can be problematic to the slurry separation plant and general slurry rheology.
Until recently, many projects utilizing slurry tunnel boring machines (TBMs) would rely upon the use of water alone as the primary carrier of the slurry. Water is definitely needed and is the most favorable under conditions where most of the formation is in high plasticity clay. When high plasticity clays are prevalent, slurries can benefit by adding chemicals that reduce the swelling of the clay particles and keeping these particles from degrading into sub-colloidal size particles.
These ultra-fine particles are very detrimental to the rheology of the slurry and can cause severe problems for the contractor as far as removing these particles from the slurry and effectively keeping the efficiency of the slurry circuit from degrading. If the slurry becomes too heavy and the plastic viscosity is not controlled, the slurry can easily degrade into a media that is extruding the material from the machine face and is not properly cleaning the cutting tools on the machine head and can impede the penetration rate due to secondary solids retention. If this is accomplished, the cutting tools are always making direct contact with the new material and will produce larger size cuttings that can help the separation equipment remove these materials more economically and keep the slurry rheology more favorable for faster mining rates.
Let’s look at some of the most encountered soil conditions that require slurry additives. When mining through river rock, large cobbles and large, loose gravel deposits, it can create a condition that could cause over excavation and plugging of the machine or slurry lines. Depending on the size of the material that is being excavated, bentonite at various viscosities can be very beneficial in stabilizing the material at the face as well as the surrounding formation.
The favorable rheology of the bentonite slurry will also help to move these larger size solids through the slurry lines and deliver them to the separation plant for removal. formation and the pipe. Let’s keep in mind that water has very erosive qualities if used alone with no buffer. If we add sand to that media, we now have an effective sand blast mix.
If the slurry is too thin, it will have the tendency to migrate out into the surrounding formation, loosening up the integrity of the sand conglomerate. The bentonite slurry will actually penetrate into the sand formation and plaster the sand with a wall cake that will keep your slurry from migrating and keep the sand from sloughing into the excavation. If you get a good penetration of bentonite slurry into the surrounding formation, this will also help to keep surrounding water from entering your excavation and washing out the bentonite lubrication that you are adding to keep jacking pressures low.
This condition has been encountered in several areas throughout the world in these problematic formations. In each case, adding bentonite to the slurry produced a substantial benefit to keep jacking loads low and advance rates stable. In extreme cases of fluid loss to the formation, it can sometimes be necessary to add a secondary additive to the slurry, such as a lost circulation material. Mica or sawdust are both good media and provide sealing capabilities and also add good lubrication qualities to keep the jacking loads low. Lost circulation materials are very economical and can have a very positive effect in preventing slurry loss into the formation.
In conclusion, if the proper slurry is utilized for the existing ground conditions, favorable results can be achieved at a very low cost that can save the contractor a significant amount of time and money on the completed project. There have been several projects completed in the microtunneling and foundation drilling markets that have had fantastic results by engineering slurry characteristics to address problematic soils prior to the launch of the project.
In the end, it is all about the successful completion of the project to the owner’s satisfaction and the bottom line profit afforded to the contractor. Slurry additives is one way to help achieve this scenario.