【Haichuan Construction Technology】Detailed Explanation of Foundation Treatment Methods and Test Requirements
Thread Content
The foundation is the basis of a building, and its quality directly affects the safety and durability of the entire building/structure. When the bearing capacity of natural soil layers such as rock, gravelly soil, sand, silt, and clay is insufficient to support the loads of buildings, or when their deformation and stability do not meet the required standards, in addition to conventional pile foundations, the natural foundation can also be artificially treated. The purpose of foundation treatment is to improve the engineering properties of the foundation soil so that it can meet the building’s requirements regarding foundation stability and deformation. This includes improving the deformation characteristics and permeability of the foundation soil, enhancing its shear strength and resistance to liquefaction, as well as eliminating other adverse effects. There are numerous methods for foundation treatment, which can be classified into the following categories based on their principles. 1. The replacement cushion method involves removing the weak or uneven soil layers in the upper surface of the ground, filling it with hard materials with larger particle sizes, and compacting them to form a cushion layer. Depending on the fill material, they can be classified into lime soil foundations, sand and gravel foundations, fly ash foundations, and geosynthetic material foundations. (1) The lime soil foundation is a treatment method in which the soft soil within a certain range below the base surface is removed, and then lime soil with a volume ratio of 2:8 or 3:7, namely 28 lime soil or 37 lime soil, is backfilled in layers and tamped under optimal moisture content. (2) For sand and sand-gravel foundations, a compacted layer of sand or sand-gravel is used to replace a certain thickness of soft soil beneath the foundation, in order to increase the bearing capacity of the foundation, reduce settlement, and accelerate the drainage and consolidation of the soft soil. (3) Fly ash foundation refers to the use of fly ash as a backfill material for foundation backfilling. (4) Geosynthetic material foundation: Geosynthetic reinforcement layers are used to improve the integrity and bearing capacity of the foundation.
2. Compacting and tamping the foundation (1) Foundation compaction using hammers: The weight of the hammer ranges from 1.5 to 3 tons, and the dropping height is between 2.45 and 4.5 meters; the hammer strikes the surface of the foundation soil, with a reinforcement depth of generally 1.2 meters. This method is suitable for treating slightly moist clay soils, sand soils, and collapsible loess. (2) Dynamic compaction foundation: The hammer weight ranges from 8 to 40 tons, and the drop height is between 6 and 30 meters. The massive impact energy (500–800 kJ) is used to compact the soil, with a reinforcement depth of over 10 meters; it is suitable for gravelly soil, sandy soil, clay soil, etc. (3) Dynamic compaction replacement method: A heavy hammer is lifted to a high height and then dropped freely to create a compaction pit; the hard materials such as sand, gravel, and steel slag filled into this pit are continuously compacted to form a dense mass.
3. Composite foundation. A composite foundation in which vertical reinforcement elements (piles) are installed in the foundation soil, and these piles together with the soil between them bear the loads. Common types include Cement-Fly Ash-Gravel Piles (CFG piles), vibroflotation piles/sand-gravel pile composite foundations, and lime-soil/soil compaction pile composite foundations. (1) Cement-fly ash gravel piles (CFG piles) refer to piles with high bond strength formed by mixing cement, fly ash, gravel, and water, which together with the soil between the piles and the cushion layer constitute a composite foundation. (2) Vibro-compacted pile/sand and gravel pile composite foundation: Holes are created by methods such as vibrating pipe sinking, and sand and gravel materials are filled in to form piles, which serve to compact and displace the soil. (3) Lime-soil/soil compaction pile composite foundation: Through the hole-forming and compaction process, lime-soil or plain soil is compressed into the holes and compacted to eliminate collapsibility and enhance bearing capacity.
4. Grouted foundation (chemical reinforcement method). Chemical solutions or adhesives are injected into the soil to bind it together, thereby improving the strength of the foundation, reducing settlement, or preventing seepage. The main methods include high-pressure jet grouting (jet grouting piles) and deep mixing method (cement-soil mixing piles), among others. 5. Preloading of foundations, also known as the vertical drainage consolidation method, involves applying loads or vacuum preloading to the foundation so as to consolidate the soil in the foundation and reduce post-construction settlement. Suitable for deep saturated soft clays.
Foundation testing: After foundation treatment is completed, it is necessary to test the bearing capacity of the foundation. Foundation testing is a crucial step in ensuring the quality and safety of construction projects, and it must be carried out in strict accordance with relevant standards and specifications. The main regulatory standards currently in use include the \"Code for Acceptance of Construction Quality of Building Foundation Engineering\" (GB50202), the \"Technical Code for Building Foundation Treatment\" (JGJ79), and the \"Technical Code for Inspection of Building Pile Foundations\" (JGJ106). The main inspection items, inspection frequency, inspection parameters, and testing methods are shown in the table below.
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