How to Use Preloading to Improve Soil Bearing Capacity?

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Preloading is an economical and effective soil improvement method that can reduce settlement and increase the bearing capacity of soft soil. In other words, preloading accelerates settlement of the soil before the construction of the structure. Commonly, the combination of preloading and vertical drains is used to make the method more effective.

The function of vertical drains is to shorten the duration of soil treatment. Preloading forces water out of the soil matrix, facilitating soil consolidation. Preloading with vertical drains permits the use of a cheaper shallow foundation. The soft soil is characterized by low strength and high compressibility and hence possesses low bearing capacity.

How to Use Preloading to Improve Soil Bearing Capacity?

1. Surcharging is the preloading of soft soil through the application of a temporary load to the ground. The stress due to exerted load should be at least equal or greater than the magnitude of expected design stresses.
2. Surcharge increases pore water pressure at the beginning, but the draining of water creates voids which is compressed by the temporary load.
3. The combination of preloading and vertical drains is used to make the method more effective.
4. Preloading with vertical drains 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.
5. Consolidation of compressible soil could take up to two years based on the thickness of the soft soil stratum.
6. However, the placement of vertical drains or artificial drainage paths into soft soil to make way for water towards the soil matrix substantially cuts down the soil improvement duration.
7. Vertical drains reduce the path of water out of the soil matrix. The water flows horizontally into the vertical drains and then drains it out vertically to the surface.
8. Vertical drains make preloading to fit within the construction schedule.
9. Preloading permits the use of a cheaper shallow foundation rather than a costly deep foundation, see Figure-2.
10. Temporary load is removed when 90% of expected settlement is achieved, and no pore water pressure remains in the soil.
11. The expected settlement can be estimated based on Asaoka  (1978) method.
12. Practically, the magnitude of the surcharge load and construction schedule control the duration for which the surcharge load needs to remain in its position.
13. Increasing the time of temporary overloading or increasing the size of overloading would reduce or nearly eliminate the soft soil's secondary settlement.
14. Vertical drains, shown in Figure-4, can be plastic cores encased by a geotextile. It should accept water from the soil around it and discharge it.
15. Hydraulic conductivity of soil around vertical drains governs how water goes into the vertical pipes and consequently decreases pore water pressure.
16. The depth of vertical drains lies within the pre-consolidation stress margin because the stress from the temporary load decreases with depth.
17. If there is a previous soil layer below the pre-consolidation stress margin, extend the vertical drains into the porous soil layer.
18. Distribute vertical drains across the entire footprint of the structure and a small distance beyond the building's footprint to avoid differential settlement that can increase consolidation duration.
19. It is possible to assume a homogeneous soil layer while vertical drains are designed.
20. For vertical drain design, evaluate soil properties (coefficient of consolidation for horizontal drainage and coefficient of permeability for horizontal seepage) at the maximum vertical effective stress that should be exerted on the soft soil in the field.
21. The drain-influence zone depends on the drain spacing. For square pattern layout, vertical drain spacing multiplied by 1.13 would give the drain-influence zone. For the triangular pattern layout, the drain influence zone is equal to spacing time 1.05.
22. Triangular pattern layout provides a more uniform consolidation between drain in comparison with square pattern layout. However, the square pattern layout has greater ease and control in the field, see Figure-3.

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