The soil that is in the area where the Tuas ports are being build consists mainly of clay. This presents a problem, as clay is a very compressible soil, which means that the structures that are build on top of the clay could settle after or even during construction. Therefore the clay is being dredged and partly replaced with sand. Within the quay walls sand is used as a bottom layer, behind the caisson and as a backbone through the entire finger. The basin created by the sand is filled with clay to minimise the use of sand, which is topped of by surcharge (sand). When sand or clay have just been deposited, it is not yet ready for construction. The soil needs to be compacted first and after it has been compacted the excess water needs to be drained. This is done by vertical drainage in the case of the Tuas project.
To see what the effects of settlement are you can use our settlement calculator.
Calculations
The settlement of a soil layer is dependent on multiple factors. To calculate the settlement at a given time t with preloadingσ’ we will use the method of Koppejan, as prescribed in the book Soil Mechanics by A. Verruijt. The strain is then given by:
With Cp the primary consolidation coefficient, Cs the settlement consolidation coefficient, t0 the reference time, chosen as 1 day, sigma0´ the initial soil pressure at depth D and sigma0 the stress at depth D after loading.
And lastly the factor U is given as:
Which for our case will be :
With: t the time in days and Cv set at a value of 1.
To calculate the stress at depth D we will use a combination of the linear increase of stress due to the weight of the soil and the method of stress redistribution of Flamant for the added load on top of the soil. This yields the following equations for the stress:
With: p the imposed load, a the width over which the load is imposed and z the depth
To get the initial stress at the soil it should be noted that there was water above the soil before the caissons were placed. The 25 meters of water above the soil, where the caisson is now, imposed an additional load on the structure of about 250 kPa. We can now calculate the total initial pressure at a depth of 15 meters.
The same can be done for the stress at a depth of 15 meters when the caissons are placed with their associated loads. This is 745 kN for the filled caissons and 120 kN for the applied variable load.
The loads can also be visualised at certain depths, this is done for a range of depths and is displayed in the following figure.
Now that we know the initial and current loads, we can calculate the strain that is exerted on a depth of 15 meters below the ground level. For the values of and
we take the average values for sand of 100 and 200 respectively. This leads us to the following settlement function, depend only on the time t.
To get the settlement, we simply multiply by the thickness of the sand layer to achieve the desired result. This is graphed in the following graph.
In the Calculator you can play with the graph and values of the loads yourself. There even is an option for removing the variable load after a chosen amount of days to see what this does with the settlement, as the ground will ‘bounce’ back a little bit.
Vibro-techniques
To improve the ground on which is being build, a couple of methods can be used. The improvement of the ground is mostly done to meet specific bearing capacity and/or settling requirements. They do however have some other benefits. First of all it mitigates the liquiefaction potential of the soil, secondly it provides slope stabilization and lastly it prevents earthquake-induced lateral spreading. This last property is quite useful in the region around Singapore, since it is close to a fault line. The main reason ground improvement is mostly done however, is to improve the bearing capacity. One of the ways to do this, is to use a type of vibro-technique. There are four types of vibro-techniques used to improve the soil quality, which will be discussed here.
Wet top-feed
This method is the most classic construction method that has been mostly replaced onshore but remains the most useful system for offshore works. It is most often used for soft soils below the water table where larger column diameters are required.
The procedure is as follows: A layer of granular material is deposited over the seabed floor. The vibroflot, a vibrating needle with a horizontally-located, hydraulically driven eccenter in the head that causes a vibrating movement, is then lowered through the layer in the soil whilst the penetration is assisted by water flushing. Once the required depth is reached, the vibrator can be partially withdrawn to surge and flush out, this is done to increase the column diameter if needed. The water pressure is then reduced while the vibrator is kept in the ground. The granular material is than placed on the top at seabed level causing it to fall under gravity into the bore. The vibrator is then withdrawn slightly and pushed back into the base of the bore to construct the columns in a series of lifts.
Wet bottom-feed
The wet bottom-feed system combines the benefit of improving weak soils below the water table with all the added advantages of the original and well proven water-flush construction technique. The added ability is to surge for larger column diameter.
The procedure is as follows. The vibroflot is penetrated to the required depth which, once reached, will have water jetting to flush away any loose material and clear a cavity of around one meter wide. As the water flush clears the cavity the hopper gate is opened to deliver a charge of stone that exits from the tip of the vibroflot. The vibroflot is then re-inserted to compact the stone tightly into the seabed, before being lifted slowly back to the surface.
Marine vibrocompaction
Vibrocompaction is needed to prevent liquefaction, a process where shock waves and oscillations can cause loose sands to act like a liquid. The compaction improves the soil density to compact the loose sands and granular particles. The method is highly effective for land reclamation projects and mitigating the risk of liquefaction in areas of the world subject to seismic activity, therefore making it perfect for the use in Singapore.
The procedure is as follows. Whilst the motor of the vibrator is running, water is discharged from the vibrator nose when the vibrator is slowly lowered to the required design depth. The soil near the vibrator will be saturated and liquefies locally and temporarily due to the vibrations. This makes it possible for the soil particles to rearrange themselves because they are free from stress. They can therefore be rearranged into a more dense state, making the bearing capacity higher and it reduces the risk of liquefaction. It should be noted though, that this technique is increasingly difficult as the percentage of clay and silt increases. The fine material should not be higher than 10 to 12%.
Dredged trench method
When a very large bearing capacity is needed, the instalment of stone columns within the dredged ground is also possible in addition to densification of the dredged material. This method works quite similar to the wet-top feed method.
A trench is dredged from the seabed to remove all the soft soil and sand. This trench is then filled with crushed rock. After the trench is filled, a layer of overbuild is made to prevent material from falling into the trench. After this the material will be densified with the vibroflot in a similar fashion as is done with the wet-top feed method. This provides stone columns in the already dredged underground for the increased bearing capacity and settling performance.
It should also be noted that use of stone columns is more preferable when the particle size becomes smaller. Vibro compaction is not suitable for particles smaller than around 0.1mm whilst the stone columns could be used best starting from this particle size.
Vertical Drainage
Soil, whether it is sand, clay or silt, will slowly settle when a constant force is applied to it. This process has timespans that range from months in the case of sand, to decennia in the case of clay. These timespans pose a problem when construction has to be done in limited time, especially when the soil is mainly made of clay. The soil in the Tuas project has this problem. The top layer consists of combinations of weak types of clay, like marine and estuarine, before the stable soil like sand is reached.
The settling of soil can be described in three phases, immediate settlement, consolidation and creep. The immediate settlement takes place right after a load has been placed, of which a large part is the plastic deformation. This settlement would therefore disappear as soon as the load is taken away. The ‘real’ consolidation settlement happens over time. Water is squeezed out of the soil, leaving the cavities open for compression. The finer the particles are, the slower the water flows out of the soil. This explains why the consolidation process of clay is much longer than that of sand. The final part is called the creep. Once the consolidation is done, and thus all the water has left the soil, the soil particles will take on the full compression. This will push the particles closer together making the soil more dense and increasing the settlement. Creep happens more drastically in soils with more organic material. [2]
To reduce the settling time of soils vertical drains are used. The purpose of vertical drainage is to increase the transport of water in the soil and thereby decreasing the time that is needed for the consolidation process. Vertical drains take advantage of the shorter horizontal than vertical hydraulic conductivity of the soil deposits. They are installed vertically in the ground at a fixed distance apart from each other. This way the water only travels half the distance between the vertical drains.
In a study that is done for the construction of Singapore Changi airport, a site very close to the Tuas terminal, it was shown that the combination of PVD’s with preloading are the most effective method for improving soft clay under the land reclamation [1]. The study was done as follows: first the soil was preloaded with stress, however this provided only an average degree of consolidation rather than a homogeneous improvement throughout the entire soil layer. In addition, there is no acceleration process when only preloading is done. Therefore a combination of preloading and vertical draining was proposed and tested. The theoretical values were tested against the measured values of the project test-site, which were a very close match.
Other Construction Aspects
Trailing Suction Hopper Dredgers
One of the vessels used to reclaim the land for the construction of the port. The Queen of The Netherlands is Boskalis´ biggest Trailing Suction Hopper Dredger. For more details click here.
Backhoe Dredgers
Magnor is one of the backhoe dredgers that is being used at the port construction. For more details click here.
Caissons
The caissons are one of many aspects which make this construction process unique. The caissons are made in a huge factory that has been set up on the project site. For more details click here.
Tuas Terminals 