Preloading combined with vertical drains is a soil improvement technique that can effectively increase the bearing capacity of soft soil while reducing settlement. This method involves accelerating the settlement of the soil prior to construction of the structure. By using vertical drains in combination with preloading, the soil treatment duration can be shortened and the method can be made more effective.
The purpose of vertical drains in this method is to facilitate the water drainage from the soil matrix, thus aiding in soil consolidation. The use of preloading forces the water out of the soil, which is beneficial in this regard. The resulting improvement in the soil’s strength and compressibility makes it possible to use a cheaper shallow foundation for construction.
Soft soil typically has low bearing capacity due to its low strength and high compressibility. By using preloading combined with vertical drains, the soil can be improved to a degree that it is able to withstand the weight of the structure that is to be built on it.
Figure-1: Preloading without Vertical Drains and Preloading with Vertical Drains
How to Use Preloading to Improve Soil Bearing Capacity?
Surcharging is a technique used to strengthen soft soil through the application of a temporary load. The load must be sufficient to meet or exceed the expected design stresses, and it causes an increase in pore water pressure initially, but this drains away over time, leaving compressed voids. This technique is most effective when combined with vertical drains and is suitable for inorganic silts and clays with low to moderate permeability, decomposed peat, organic layers, dredge spoils, varved cohesive deposit, and clay and silty sand. Consolidation of compressible soil can take up to two years, but the use of vertical drains or artificial drainage paths can significantly reduce the improvement time.
Vertical drains reduce the path of water out of the soil matrix, allowing water to flow horizontally into the drains and then vertically to the surface. This makes it possible to fit preloading into the construction schedule and use a cheaper shallow foundation instead of a costly deep foundation. The temporary load is removed once 90% of the expected settlement has been achieved and there is no pore water pressure remaining in the soil. The expected settlement can be estimated using the Asaoka (1978) method, and the duration of the surcharge load is controlled by the load’s magnitude and the construction schedule.
Vertical drains can be made from plastic cores encased in geotextile and must accept water from the soil around them and discharge it. The hydraulic conductivity of the soil around the drains governs how water enters the pipes and decreases pore water pressure. The depth of the vertical drains lies within the pre-consolidation stress margin, and if there is a previous soil layer below this margin, the drains must be extended into the porous soil layer. To avoid differential settlement that can increase consolidation duration, vertical drains should be distributed across the entire footprint of the structure and a small distance beyond it.
The design of vertical drains assumes a homogeneous soil layer, and soil properties such as the coefficient of consolidation for horizontal drainage and the coefficient of permeability for horizontal seepage must be evaluated at the maximum vertical effective stress in the field. The drain-influence zone depends on the drain spacing and is 1.13 times the spacing for a square pattern layout and equal to the spacing times 1.05 for a triangular pattern layout. While the triangular pattern layout provides a more uniform consolidation between drains, the square pattern layout is easier to control in the field.
FAQs
What is preloading method?
Preloading is a technique that can enhance the bearing capacity of soft soil and decrease settlement, making it a cost-effective soil improvement method. This method involves accelerating the settlement of the soil before starting the construction of the structure. The use of preloading in conjunction with vertical drains is a popular approach that enhances the effectiveness of the method.
The process of preloading soil involves applying a temporary load to the ground surface, which compresses the soil and expels water and air. This, in turn, consolidates the soil and increases its density, resulting in increased strength and stiffness. The load is maintained for a predetermined period, allowing for the soil to settle and stabilize.
To improve the effectiveness of preloading, vertical drains can be used. These drains facilitate the dissipation of excess water and air from the soil, thus speeding up the consolidation process. Vertical drains are installed in the soil prior to preloading, and as the load is applied, water and air are drawn from the soil and expelled through the drains. This technique can accelerate the consolidation process by several times, leading to a reduction in the duration of the preloading process.
In conclusion, preloading is an effective method of soil improvement that can increase the bearing capacity of soft soil and reduce settlement. The use of preloading in conjunction with vertical drains can enhance the effectiveness of the technique, making it a popular approach in geotechnical engineering. This method can save time and money in construction projects and ensure the stability of the structure over its lifetime.
What is the function of vertical drains in preloading method?
Vertical drains serve the purpose of reducing the duration of soil treatment. These drains are employed in soil consolidation projects, which aim to improve the strength and stability of the soil. By implementing vertical drains, the consolidation process can be expedited as they allow for the dissipation of excess pore water pressure.
Excessive pore water pressure can cause soil to become unstable, which is why it is important to eliminate it during the consolidation process. Vertical drains create a pathway for the water to escape, which can reduce the time required for soil consolidation. This is beneficial as it helps to reduce project costs and improve the overall efficiency of the project.
In summary, vertical drains are designed to accelerate the soil consolidation process by eliminating excess pore water pressure. By doing so, they help to shorten the duration of soil treatment, which is advantageous for projects seeking to improve efficiency and reduce costs.
What are the types of soils that can be improved by preloading with vertical drains?
Preloading with vertical drains is a technique that can be employed in various types of soils, including inorganic silts and clays with low to moderate sensitivity, decomposed peat, organic layers, dredge spoils, varved cohesive deposits, and clay and silty sand. This method involves installing vertical drains in the soil to increase its permeability and facilitate the dissipation of pore water pressure.
The use of preloading with vertical drains is particularly useful in soils with low to moderate sensitivity, where the deformation caused by the applied load is expected to be small. This technique can also be effective in soils with organic matter or dredge spoils, which tend to have low shear strength and high compressibility. In these types of soils, preloading with vertical drains can help accelerate the consolidation process and reduce the settlement of the soil.
Furthermore, preloading with vertical drains is well-suited for varved cohesive deposits, which are composed of alternating layers of clay and silt. These deposits can be difficult to stabilize and can experience significant deformation under loading. By installing vertical drains, the pore water pressure in the soil can be reduced, allowing for quicker consolidation and improved stability. Additionally, this technique can be applied to clay and silty sand, where it can help accelerate the rate of consolidation and reduce the potential for settlement.
How long does consolidation of compressible soil take using preloading method?
According to the given information, the process of consolidation for compressible soil may require a considerable amount of time. This duration could extend up to two years, and it is dependent on the thickness of the soft soil stratum. The term “consolidation” refers to the process by which soil becomes denser and more compact under load. Compressible soil is known for its tendency to settle over time, which can lead to issues such as uneven settlement and structural damage. As a result, it is essential to ensure that the consolidation process is carried out correctly to minimize these risks. The duration of the consolidation process can be influenced by various factors, including the thickness of the soft soil stratum, the type of soil, and the magnitude of the load being applied. Therefore, it is crucial to take these factors into consideration when estimating the time required for consolidation.
What are the common layouts for installation of vertical drains?
Vertical drains can be installed in two main layouts: triangular and square. The triangular layout involves installing the drains in a triangular pattern, while the square layout involves installing them in a square pattern. These layouts are commonly used in the construction industry for various purposes.
The triangular layout involves placing the vertical drains in a triangular pattern. This layout is often used in areas with limited space or irregular shapes. The triangular pattern allows for a more efficient use of space, as the drains can be placed closer together without sacrificing drainage capacity.
On the other hand, the square layout involves placing the vertical drains in a square pattern. This layout is commonly used in larger areas or areas with a more regular shape. The square pattern allows for a more uniform distribution of the drains, which can lead to more effective drainage overall.
Both layouts have their advantages and disadvantages, and the choice of layout will depend on various factors such as the size and shape of the area to be drained, the type of soil, and the required drainage capacity. Ultimately, the goal of both layouts is to provide effective drainage and prevent problems such as soil settlement and instability.