I. Physical Methods

1. Compaction Method

Mechanical compaction equipment is used to vibrate, tamp, or compact the foundation soil, thereby increasing its density and bearing capacity. This method is suitable for granular soil foundations.

2. Replacement Method (Replacement and Backfilling Method)

When the bearing stratum beneath a building foundation is relatively weak and cannot satisfy the foundation requirements imposed by the superstructure loads, the replacement soil cushion method is often used to treat the soft soil foundation. That is, the soil within a certain depth below the foundation is excavated and then backfilled with higher-strength sand, gravel, or lime-soil, and compacted to densification. By removing the weak soil layer and replacing it with stronger materials such as sand and gravel, the purpose of strengthening the foundation is achieved. This method is suitable for shallow foundation treatment, including muck, mucky soil, loose plain fill, and miscellaneous fill.

3. Preloading Method (Drainage Consolidation Preloading Method)

     The drainage consolidation preloading method utilizes the drainage consolidation characteristics of foundation soil. By applying preloading and adding various drainage conditions (drainage bodies such as sand drains and drainage cushions), it accelerates the consolidation of saturated soft clay as a soft soil foundation treatment method.

Preloading in excess of the building load is applied to the foundation to accelerate the expulsion of pore water from the soft soil layer and thereby complete soil consolidation in advance.

The drainage consolidation method is suitable for treating saturated and soft soil layers. According to the different drainage technical measures adopted, the drainage consolidation method can be divided into the following types: surcharge preloading, vacuum preloading, dewatering preloading, and electro-osmotic drainage.

Vibration or impact force is used to accelerate the consolidation process of the foundation soil. For example, the deep vibroflotation method can be used to treat deep loose soft soil layers.

A typical form of the dynamic consolidation method is the vibro-compaction and compaction method, which densifies the foundation soil through vibration or squeezing action, thereby improving bearing capacity and stability. Commonly used vibro-compaction and compaction methods include the vibratory roller method and the dynamic compaction method.

5. Soil Freezing Method

At lower temperatures or when required by the project, the soil temperature is lowered to freeze the water, thereby enhancing the strength and stability of the soil. It not only ensures stratum stability but also acts as a water barrier, allowing excavation of deep foundation pits. In some coal mine shaft projects in China, this method has been used for construction to a maximum depth of 500 m. In recent years, this method has been extended to other civil engineering projects.

Freezing method construction technology for cross passages in subway construction. As a mature construction method, the freezing method construction technology has been widely used internationally in urban construction and coal mine construction for more than 100 years. China has also used the freezing method construction technology for more than 40 years, mainly for coal mine shaft excavation, with a maximum freezing depth of 435 m and a maximum frozen alluvial soil thickness of 375 m. Since 1992, the freezing method has been widely used in subway construction in Shanghai, Beijing, Shenzhen, Nanjing, and other cities.

II. Chemical Methods

1. Grouting Method

The grouting method (grouting method) is a method of injecting certain settable grouts into cracks or pores of rock and soil foundations to improve their physical and mechanical properties. The purposes of grouting are seepage prevention, leakage plugging, reinforcement, and correction of building inclination. Grouting mechanisms include: filling grouting, permeation grouting, compaction grouting, andfracture grouting. Grouting materials include granular grouts and chemical grouts. Granular grouts are mainly cement grout, while chemical grouts include silicates (water glass) and polymer grouts.

2. Electro-Osmosis Method

The electro-osmosis method involves inserting metal electrodes into the soil and passing direct current under the action of an electric field.Water in the soilflows from the anode to the cathode, producing electro-osmosis, thereby reducing the moisture content of highly cohesive soil or lowering the groundwater level to improve soil properties as a reinforcement method.

3. Soil Chemical Stabilization Method

The chemical stabilization method involves injecting chemical grout or chemical agents into the foundation so that they react chemically with the foundation material, thereby improving the bearing capacity and deformation modulus of the foundation. This method is suitable for treating soft foundations and foundations with poor permeability. During construction, it is necessary to determine appropriate grouting hole positions and grouting parameters, inject the chemical grout or agents uniformly into the foundation, and ensure full reaction with the foundation material to achieve the reinforcement effect.

4. Soil Stabilization

Chemical substances such as clay or polymers are added to the foundation soil to improvethe engineering properties of the soil.