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In modern civil engineering, structural safety is directly related to the bearing capacity of the ground on which a structure is built. Population growth and urban expansion are forcing engineers to construct buildings even in areas that do not always have ideal geotechnical properties. Weak soils are formations that cannot safely support the loads of the structures built on them, exhibit excessive settlement, or pose a risk of liquefaction during an earthquake. Soil improvement methods play a vital role in constructing safe structures on such sites.
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Over 20 Years of Construction Experience
Weak soils typically consist of clayey, silty, or organic-rich areas with high groundwater levels, or loose, fine-grained alluvial deposits and uncontrolled artificial fill. According to engineering standards, these soils pose the following primary risks:
- Low Bearing Capacity: Sudden slippage and subsidence may occur in the ground under the weight of the structure.
- Excessive and Uneven Settlement: The uneven settlement of different parts of a structure causes cracks and collapse in the structural system.
- Risk of Liquefaction: This refers to the phenomenon in which loose, water-saturated sandy soils lose their solid properties and behave like a liquid when subjected to earthquake shaking.
Basic Soil Improvement Methods
Soil improvement processes aim to increase the soil’s density, shear strength, and bearing capacity, while optimizing its flexibility and permeability. These methods fall into four main categories: mechanical, hydraulic, physical/chemical, and inclusion methods.
Mechanical Methods
It is the process of reducing the void ratio of the soil and increasing its density
using mechanical energy.
- Dynamic Compaction: This involves compacting deep soil layers by dropping heavy blocks—typically weighing 10–40 metric tons—from a height of 10–30 meters onto the ground surface. It is highly effective in loose granular soils.
- Vibrocompaction: This process involves immersing one end of a vibrator into the ground using a water jet and using the resulting vibrations to pack the particles more tightly together.
Hydraulic Methods
These are methods aimed at completing settlement before construction by reducing pore water pressure in fine-grained and water-saturated soils.
- Preloading: Before construction begins on-site, soil fill equivalent to or greater than the structure’s load is placed to allow the ground to settle in advance.
- Vertical Drains and Prefabricated Vertical Drains (PVD): Used in conjunction with preloading. These are vertical sand or plastic strip drains installed in the ground at specific intervals to shorten the path of water flow.
- Vacuum Method: This involves accelerating the consolidation process by using vacuum pumps to extract air and water from beneath a waterproof membrane laid on the ground.
Physical and Chemical Methods
It is the process of strengthening the floor at the molecular level using chemical binders or heat treatment.
- Cement and Lime Stabilization: This process involves mixing lime or cement into the soil at specific ratios to permanently improve its mechanical properties, particularly to increase the strength of clayey and plastic soils.
- Jet Grouting: A process in which a cement slurry is injected into the ground at high pressure—typically 400 bar or higher—using a special drill bit, where it mixes with the soil to form cylindrical, concrete-like columns, known as jet grout columns.
- Permeation (Infiltration) Injection: Without altering the natural structure of the soil, voids are filled with low-viscosity chemical or cement grouts to make the soil impermeable and increase its strength.
Inclusion and Contribution Methods
A composite soil structure is obtained by embedding structural elements with higher strength into weak soil.
- Stone Columns: These are created by filling holes drilled into weak soil with clean gravel or crushed stone and compacting the material using vibration. They both increase the bearing capacity and act as vertical drains, thereby reducing the risk of liquefaction.
- Deep Mixing: This is a technique in which the blades of a mechanical mixer penetrate the ground to blend binding agents, cement/lime slurry, or powder directly with the soil in place.
Criteria Affecting the Choice of Method
There is no single correct method for every type of soil and every project. The selection of the most appropriate soil improvement project depends on the following parameters:
- Soil Type and Grain Size Distribution: While preloading and lime stabilization are preferred for clay soils, dynamic compaction or stone columns are more effective for sandy soils.
- Depth of Improvement: While simple compaction is sufficient for weak surface layers, jet grouting or deep mixing is required for deeper layers.
- Environmental Factors: Instead of methods such as dynamic compaction that generate vibration and noise in urban areas, injection systems that do not damage surrounding buildings are selected.
- Economic and Logistical Conditions: Equipment availability, material costs, and the project schedule directly influence the selection.
Weak soils pose a major threat to structures if not addressed with the proper engineering approaches. However, thanks to advancing geotechnical methods today, even the most unfavorable soils can be transformed into safe and economical construction sites. Soil improvement techniques, designed through proper laboratory testing, comprehensive site investigations, and expert engineering calculations, not only extend the lifespan of structures but also maximize their resistance to natural disasters such as earthquakes.
