Refining the art: Neil Phillips looks at the progress made in the development of slurries and on-site slurry plants used for pipejacking projects
Oct 20, 2010
Over the past ten years the use of slurry tunnelling has increased in the pipejacking industry and, at the same time, developments have been made in surface separation plants and their operation. Many of these improvements have come from the oil and gas-drilling industry and are necessary due to the changes in waste-disposal legislation in the UK. For example, recent laws have been brought in to ban liquid waste going directly to landfill, while the price of off-site treatment and carriage have subsequently increased. This called for the pipejacking contractor to separate the slurry from the excavated material on site as effectively as possible, thus reducing the need to transport liquid and reduce the waste going to landfill. Competition for contracts and improvements to TBM excavation rates have also called for contractors to be able to maximise production to remain competitive. Several areas have improved over the years, including the composition of the slurry used, the use of flocculants, the separation plant itself and its more efficient operation.
In pipejacking, the main purpose of slurry is to act as a transport medium and equalise the face pressure from ground water. In recent years, slurry composition has moved away from traditional bentonite to either water, or water and gum (Xanthan or Guar gum). Some suspended solids will also accumulate in the slurry from excavated ground, but are not considered when designing the initial slurry mix.
Slurry consisting solely of water would be used on tunnel drives where there are only silt and clay particles in the ground, as the task of keeping the excavated material in suspension is not a problem. In these situations, keeping the slurry clean and as close as possible to pure water (specific gravity =1) is the main objective of the cleaning plant.
In sands, gravels and glacial tills, a gum may be added to the slurry to assist with carrying the solids. It can also help to prohibit the ingress of ground water, primarily due to the increased slurry viscosity.
A ‘filter cake’, which is commonly and wrongly referred to, is not given sufficient time to form during tunnelling, especially with smaller-diameter machines. In any case, ground support is also provided by the TBM. In recent years, current practice has only called for this extra carrying capacity in granular soils, particularly when gravels are present.
The main developments have been made in separation plants and their configuration. The cost of liquid waste disposal has resulted in the need for separation plants to be able to remove all excavated material from the slurry, even 1µm clay particles. Ideally, this should also be achievable at the same rate as excavation during each cycle of slurry, thus maintaining a constant slurry density. Current practice and developments in plant have allowed for a three-stage separation technique.
Industry standardisation has created plant that is hugely versatile and able to cope with any ground conditions, including minor alterations. The most common and versatile primary screen is a metal-linked clay-ball separator. This has a cut point of around 5mm, and does so without disturbing the material that is being removed. A large aperture shaker is also used as an alternative, but in mixed ground, clays and silts can cause blinding of the screen, allowing extra solids to enter into suspension.
Developments in the second stage of the separation process have resulted in plant that is easily adaptable once on site. This consists of a fine aperture shaker with a bank of hydrocyclones depositing their underflow on to this shaker. The hydrocyclones work on recycle from a small tank beneath the shaker, which is topped from the underflow from the secondary screen. This tank maintains a constant level by depositing only the excess hydrocyclone overflow into the main slurry tank.
The screen would vary from 250µm-1mm, with only the hydrocyclones’ overflow re-entering the main tank. The hydrocyclones should be producing a d50 (50% cut point) in the region of 25µm. Care needs to be taken due to the potential for blocking when using 100mm hydrocyclones. The bed on the shaker can then be maintained by adjusting the angle.
For the final stage of separation, decanting centrifuges have become a standard piece of plant. With the aid of a flocculant, these allow fine, suspended solids to be removed from the slurry. The centrifuge can be the most difficult piece of plant to specify in terms of required size and number. The specification depends on three factors: the size of the tunnel; ease of flocculation; and the amount of solids that go through the first two stages of separation. Currently, there is not a specific test to confirm the amount of solids that will enter suspension.
The understanding of flocculants and their importance in the system has increased in recent years. Several areas are important in mixing and dosage that can affect the success of the operation. When mixing flocculants, it is important for the mixture to age for a sufficient time before dosing into the slurry. The time will vary, depending on the flocculant, but a rough guide would be at least 20 minutes for a liquid flocculant and 45-60 minutes for a powder when they are mixed with water. This allows for efficient use of the flocculant and separation, but also prevents the chance of ‘unaged’ entering the main slurry tank and contaminating it.
Although the main slurry tank does not actually separate material, it can have a major effect on the performance of the TBM and centrifuge. This tank requires constant agitation to prevent material settling and to maintain homogeneous slurry. Current best practice calls for the tank to be agitated using high performance paddles that create turbulence within the tank, rather than just stirring. Air agitation has been shown to be ineffective, allowing material to settle. Air can also be sent to the TBM, making it more difficult to operate.
An important factor that can have a major contribution to the success of the plant is its configuration. This can affect the performance of the separate parts, but also the space used, which can be a major factor on an urban site. This is overcome by stacking plant in a three-tier system, which keeps the footprint down to 6.5m by 2.5m for a typical 1,200mm ID drive.
The operation of the plant can affect the success of the separation process, which should be carried out by a trained operative. Their sole task should be to run the plant, focusing the majority of their time on running the centrifuge. Major problems can occur if this is not operated correctly to fit the speed of TBM excavation, but also the ability to separate solids from the slurry. With developments in separation plant, it is now common practice to have an operative on site running the separation plant, even on small-diameter pipejacks.
When it comes to the operation of the centrifuge, the efficiency and quality of the slurry can be dramatically changed with poor operation. These problems can occur:
Over-flocculation, contaminating the slurry with flocculant
- resulting in particles becoming harder to remove from the slurry on the second pass through the centrifuge;- Failure to remove the finest particles
- producing of a fine, rich slurry with an increased mud weight and viscosity;
- Inefficient running;- not running the centrifuge to full capacity to keep up with production rates.
Density – mud balance;
Viscosity – marsh funnel;
Sand content – sand contents kit.
These should be carried out at the start and end on every pipe to check that the slurry separation process is keeping up with tunnel-excavation rates. If this is not the case, the separation operative can make changes to the running of the plant or potentially advise the TBM operator to stop tunnelling while the slurry is cleaned back to an appropriate level. It is not always appreciated that halting tunnelling to allow effective slurry cleaning can lead to an overall increase in tunnelling rates.
From the above information, it can be seen that with correct plant, set-up and operation, a slurry pipejack can work effectively within the bounds of the increasing environmental legislation that limits the disposal of slurry as a liquid waste. All of these improvements in the industry have come at a cost. It is important to note that without this extra cost a slurry tunnel could not be driven efficiently or to environmental standards. Also, the extra outlay can be shown to make significant cost savings when a whole project cost-analysis is carried out.
A research project is under way between Geotechnical Consulting Group, Leeds University and the Pipe Jacking Association (UK), with the aim of further developing slurry separation and its operation.
With increased understanding and industry improvements, the process of slurry tunnelling is moving away from its previous ‘black art’ reputation into a successful and logical means of tunnelling.
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