Executive Standards:

"Code for Design of Foundation Reinforcement of Existing Buildings" JGJ123-2016.

Design Steps:

1. Select a reinforcement scheme, combine with the current structural condition, consider the interaction of the three, and preliminarily screen schemes for reinforcing the foundation, reinforcing the base, or combining reinforcement of the superstructure stiffness with reinforcement of the foundation and base. 2. Based on the expected effect, construction difficulty, material source and transportation conditions, construction safety, impact on adjacent buildings and the environment, equipment conditions, construction period, and cost, select the optimal scheme.

Reinforcement Methods:

Foundation Reinforcement:

Anchor Static Pressure Pile Method

Root Pile Method

Pit Static Pressure Pile Method

Lime Pile Method

Grouting Reinforcement Method

Base Reinforcement:

Base Supplementary Grouting Reinforcement Method (crack repair)

Method of Enlarging Base Area

Method of Deepening Foundation

Foundation Deviation Correction

Unloading Method

Brief Description of Various Foundation Reinforcement Methods

1. Anchor Static Pressure Pile Method

a. Suitable for reinforcement of foundation soils such as silt, mucky soil, cohesive soil, silty soil, and artificial fill.

b. The anchor static pressure pile uses the uplift resistance of anchor rods to statically press precast piles or steel pipes into the soil, improving the original foundation bearing capacity through pile bearing capacity. When the original pile cap bearing capacity is insufficient, it should be reinforced, or cantilever beams or brackets can be set as the pile cap for pressing piles.

c. Steel materials can be used; for reinforced concrete piles, square piles with side lengths of 200~300mm are preferred; each pile section should be 1.0~2.5m long.

2. Root Pile Method

a. Suitable for repair and story addition of existing buildings, restoration of ancient buildings, and reinforcement projects such as subway crossings on foundation soils such as silt, mucky soil, cohesive soil, silty soil, sandy soil, gravelly soil, and artificial fill.

b. The diameter of root piles should be 150~300mm, and the pile length should not exceed 30m. The layout can be straight piles or inclined piles in a grid structure.

3. Pit Static Pressure Pile Method

a. Suitable for foundation soils such as silt, mucky soil, cohesive soil, silty soil, and artificial fill, and where the groundwater level is low.

b. The pile body should be open steel pipes with a diameter of 150~300mm or reinforced concrete square piles with side lengths of 150~250mm.

4. Lime Pile Method

a. Suitable for reinforcement of foundations and surrounding soil below groundwater level in cohesive soil, silty soil, loose silty fine sand, silt, mucky soil, miscellaneous fill, or saturated loess.

b. Composed of quicklime and fly ash (volcanic ash or other admixtures). The calcium oxide content of quicklime should not be less than 70%, the powder content should not exceed 10%, the water content should not exceed 5%, and the maximum block size should not exceed 50mm. Common mix ratios are 1:1, 1:1.5, or 1:2. Cement, sand, and stone chips can also be added to improve pile strength.

c. Reinforcement effects of the lime pile method:

1. Hole formation and compaction—in miscellaneous fill with more coarse particles, the compaction effect is good; in cohesive soil with a small permeability coefficient, the compaction effect is poor.

2. Water absorption—1kg of pure calcium oxide absorbs 0.32kg of water; lime piles absorb 65%-70% of water.

3. Expansion and compaction—water absorption causes expansion; under a pressure of 50-100kPa, the expansion amount is 20%~30%.

4. Heat generation and dehydration—1kg of calcium oxide generates 280 calories; the pile body temperature is 200~300℃.

5. Ion exchange—sodium ions in soft soil exchange with calcium ions in lime, improving the properties of the soil between piles and forming a hard layer with high strength on the pile surface.

6. Replacement—soft soil is replaced by higher-strength lime piles, increasing the bearing capacity of the composite foundation. The improvement range is related to pile strength and replacement rate.

5. Grouting Reinforcement Method

a. Suitable for foundation reinforcement in sandy soil, silty soil, cohesive soil, and artificial fill.

b. Divided into permeation grouting, fracture grouting, and compaction grouting.

c. Permeation grouting: The grout displaces free water and gas in the soil, fills fissures or voids, and cements the surrounding soil to form a relatively dense solidified body, thereby improving the compressive strength and impermeability of the soil layer. Permeation grouting does not cause large volume changes in the soil. When the stratum is Quaternary strata such as sand layers, pebble layers, gravelly soil (permeability coefficient greater than 10-4cm/s), permeation grouting should be used.

Fracture grouting: Under relatively high pressure, grout is injected into strata with small porosity. Under high pressure, the grout fractures and flows along the structural planes of the stratum, forming vein-like and network-like distributions in the stratum. Irregular vein-like and network-like consolidated bodies and the soil compacted by grout pressure form a composite stratum, thereby providing certain bearing capacity and water-stopping ability. When the stratum is cohesive soil, with large burial depth and small permeability coefficient (less than 10-5cm/s), fracture grouting should be used.

Compaction grouting: Using grout with a certain consistency or rapid-setting type, pressure is applied to produce a compaction effect on the soil, thereby improving the physical and mechanical properties of the soil. The consolidated body generally appears as spherical or block-like distributions in the soil.

d. The spacing of grouting holes is 1.0-2.0m. For fracture grouting, the pressure in sand should be 0.2-0.5MPa; in clay, 0.2-0.3MPa. For compaction grouting, when using cement mortar grout, the slump should be 25-75mm, and the pressure 1-7MPa; the upper limit is taken for small slump.

Brief Description of Various Base Reinforcement Methods

1. Method of Enlarging Base Area

a. Suitable for base reinforcement when the foundation bearing capacity or base area dimensions of existing buildings do not meet design requirements.

b. During design, asymmetric widening can be used as needed. Attention should be paid to the strength of the joint surface between old and new foundations. Methods such as roughening, treating the interface, applying interface agents, or adding mechanical connection strength can be used.

c. For the original rigid foundation, when widening, it is necessary to consider whether the rigid angle still meets requirements; otherwise, both widening and heightening should be considered.

Reinforced concrete widening for the original rubble rigid foundation.

The original flexible foundation is heightened and changed to a rigid foundation.

Reinforced concrete jacket with widened base plate.

2. Method of Deepening Foundation

a. Suitable for cases where the shallow layer of the existing building foundation has a good coating as a bearing layer and the groundwater level is low.

b.

d—For basements, if a box foundation or raft foundation is used, it is calculated from the outdoor ground level; if independent or strip foundations are used, it should be calculated from the basement indoor ground elevation.

3. Tension-Compression Pile Cantilever Beam Unloading Method

a. The unloading method is suitable for base reinforcement where the original foundation bearing capacity is insufficient and the method of enlarging the base section is not suitable (one side cannot be constructed), and the improvement range is relatively large.

4. Foundation Deviation Correction

a. Suitable for existing buildings whose overall tilt exceeds the allowable values of current national standards and affects normal and safe use.

b. Reinforcement methods to correct uneven foundation settlement using methods such as forced settlement deviation correction, jacking (deviation correction), or soil solidification on the lower side.

5. High-Pressure Jet Grouting Method

a. Suitable for foundation treatment in silt, mucky soil, cohesive soil, silty soil, loess, sandy soil, artificial fill, and gravelly soil.

b. Before the grout hardens, the foundation within the effective jet range is disturbed and its strength decreases, which may easily cause additional deformation. Therefore, when treating existing buildings or constructing adjacent to existing buildings, additional settlement during construction should be prevented.

c. Methods such as controlling construction speed, sequence, and accelerating grout setting time are usually used to prevent or reduce additional deformation.