Ten foundation treatment methods (repost)
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This post was last edited by hesonchang214 on 2019-8-10 at 07:33. Ten methods of foundation treatment (repost). 1. Distinguishing between the concepts of foundation and subgrade. A building consists of three parts: the superstructure, the foundation, and the subgrade. The entire load of a building is borne by the underlying soil layers. The portion of the ground layer affected by the building is called the foundation. Therefore, the foundation refers to the layer of soil beneath the base surface that experiences stress and strain as it bears the loads of the building transmitted from the foundation. The substructure through which a building transfers its loads to the foundation is called the foundation itself. It is the enlarged portion of the building’s walls or columns that lies underground; it serves as the “feet” of the building. Its function is to bear the entire load of the superstructure and transfer it to the foundation. II. Classification of foundationsIII. Treatment methods for foundations
(1) Natural foundations
A natural foundation refers to a foundation that, in its natural state, can adequately bear all loads imposed on it, without the need for any artificial treatment. Natural foundation soils are divided into four main categories: rocks, gravelly soils, sands, and cohesive soils. (II) Artificial foundation: When the bearing capacity of a natural foundation is insufficient to bear the entire load transmitted by the foundation, the soil that has been artificially treated to serve as a foundation is referred to as an artificial foundation. The treatment methods include: replacement method, preloading method, dynamic compaction method, vibroflotation method, sand-gravel pile method, lime pile method, column hammer impact expansion pile method, soil compaction pile method, cement-soil mixing method (including deep mixing method; the powder jet mixing method and deep mixing method are referred to as the wet method and dry method respectively), high-pressure jet grouting method, single-fluid grouting method, alkali solution method, etc. 1. Replacement method: When the bearing layer beneath a building’s foundation is relatively weak and unable to meet the load-bearing requirements imposed by the superstructure, the soil replacement and cushion method is commonly used to treat such weak foundations. This involves excavating the soil layer to a certain depth beneath the foundation, followed by backfilling it with materials of higher strength, such as sand, gravel, or lime-treated soil, which are then compacted until dense. Practice has proven that soil replacement underlay can effectively address foundation problems of certain buildings with relatively low loads. Soil replacement underlays can be classified into sand underlays, crushed stone underlays, lime-soil underlays, etc., according to the backfilling materials used. The main functions of the cushion layer: 1) Improve the bearing capacity of the foundation ； 2) Reduce the amount of settlement ； 3) Accelerate the drainage and consolidation of weak soil layers ； 4) Prevent frost heave ； 5) Eliminate the swelling and shrinking effects of expansive soil. The replacement method is suitable for the treatment of shallow foundations, including silt, silty soil, loose plain fill, and miscellaneous fill. The replacement method is also applicable to the treatment of certain region-specific soils. For instance, in the Xi’an area, it can eliminate the collapsibility of loess. When used for mountainous terrain foundations, it can address issues such as inclined or fractured rock surfaces, uneven elevations, variations in soil hardness, and karst formations. In seasonal frozen ground areas, it can mitigate frost heave forces and prevent damage caused by frost heaving. http://img.civilcn.com/d/file/zhishi/sgjs/2019-07-19/f110549f9119b8184d09262aa075d9bb.png http://img.civilcn.com/d/file/zhishi/sgjs/2019-07-19/7782d8470b0ee72763d7f41c45c2e470.png 2. Dynamic compaction: This method involves dropping heavy weights ranging from several tons to dozens of tons from a height, repeatedly striking the ground in order to compact the foundation effectively. This powerful ramming force generates dynamic stresses and vibrations in the foundation; longitudinal and transverse waves are emitted from the ramming point and propagate deep into the foundation, resulting in varying degrees of consolidation in both the shallow and deep layers of the foundation. The dynamic compaction method is mainly used for sandy soils, unsaturated cohesive soils, and miscellaneous fill foundations. For unsaturated cohesive soil foundations, continuous tamping or intermittent tamping in multiple passes is generally employed ； And according to engineering requirements, field tests are conducted to determine the number of compaction passes and the effective compaction depth. Existing experience shows that at a compaction energy of 100–200 ton-meters, an effective compaction depth of 3–6 meters can generally be achieved. Effects after dynamic compaction 3. Vibratory compaction (replacement) method: This method utilizes a vibratory compactor to vibrate and flush the ground under the action of high-pressure water, thereby densifying loose sandy foundations ； Alternatively, holes are drilled in cohesive soil foundations, and crushed stone is filled into these holes to form individual piles. These piles, together with the original soil, constitute a composite foundation. The reinforcement mechanisms of the vibroflotation method in sandy soil and cohesive soil are different. In sandy soil, the main mechanisms are vibratory compaction and vibratory liquefaction ； In cohesive soil, vibroflotation replacement is the main mechanism; the replaced piles and soil together form a composite foundation. The vibro-compaction method is suitable for densification and anti-liquefaction treatment of various types of liquefiable soils, as well as for the stabilization of foundations composed of gravelly soils, sands, silts, cohesive soils, man-made fill, and collapsible soils. The use of vibroflotation foundation treatment technology can enhance the bearing capacity of the foundation, reduce the settlement of buildings (structures) on it, improve the stability of earth and rock dams (levees) as well as the foundation, and eliminate foundation liquefaction. 3.1 Vibratory stone column method. The vibratory stone column method is a technique that utilizes locally vibrated stones in the ground to rapidly reinforce soft foundations. In recent years, it has also been widely used in the reinforcement and treatment of foundations for high-rise buildings. It has the advantages of technical reliability, simple equipment, easy-to-learn operation techniques, quick and straightforward construction, short construction time, no need for cement or steel, and a significant increase in the bearing capacity of the foundation after reinforcement. Suitable for medium and coarse sand, as well as some fine sand or silt foundations. 4. Drainage consolidation preloading method: The drainage consolidation preloading method is a treatment method for soft soil foundations that utilizes the drainage and consolidation characteristics of foundation soils. By applying a preload and providing various drainage conditions (such as sand drains and drainage mats), it aims to accelerate the consolidation process of saturated soft clay. The drainage consolidation method is suitable for treating saturated and weak soil layers. Depending on the various drainage techniques employed, this method can be classified into the following types: surcharge preloading method, vacuum preloading method, dewatering preloading method, and electro-osmotic drainage method. 4.1 Vacuum drainage consolidation preloading method: Vacuum preloading refers to sand drain vacuum preloading. That is, a sand cushion layer is laid on the clay layer, then sealed with a membrane. A vacuum pump is used to evacuate air from the sand cushion and sand wells, thereby lowering the groundwater level; simultaneously, under the effect of the groundwater level, the consolidation of the foundation is accelerated. That is, vacuum preloading involves reducing pore water pressure and increasing effective stress under constant total pressure, thereby causing soil compression and strength gain. 4.2 Surcharge Preloading Method: This involves temporarily piling up soil, stones, etc., on a construction site to apply a preload to the foundation. This causes the settlement of the foundation to occur in advance; furthermore, the consolidation of the foundation soil enhances its bearing capacity. Subsequently, the preload is removed and the building is constructed, thereby eliminating any uneven settlement of the building’s foundation. This method is known as the surcharge preloading method. Normally, the preload is equal to the building load; however, in some cases, to minimize complications arising from secondary consolidation, the preload may be made greater than the building load. Typically, the preload is approximately 1.3 times the building load. In special circumstances, its magnitude can be determined based on the specific requirements of the project. 5. Compaction method: The reinforcement mechanism of the compaction method primarily involves driving pile pipes into the ground, which exerts a lateral compaction effect on the soil. Under this compaction action, soil particles move relative to one another; smaller particles fill the voids between larger particles. As a result, the particles become more closely packed, the volume of voids decreases, and the strength of the foundation soil is thereby enhanced. Therefore, the compaction method primarily aims to compact loose soil foundations, thereby improving the strength and deformation characteristics of the soil. 6. Deep mixing method: The deep mixing method is a chemical technique for strengthening foundations. It uses specialized machinery – various types of deep mixers – to forcibly mix the curing agent (cement slurry, cement powder, or lime powder, with certain additives added) with the soil in the ground over varying depths. Through a series of physical and chemical reactions that occur between the curing agent itself and the soil, the soil is hardened into a composite soil pile (mass) that possesses integrity, water stability, low permeability, and sufficient strength; or it forms a composite foundation together with the original soil, thereby increasing the bearing capacity of soft soil foundations and reducing soil deformation. 7. High-pressure jet grouting method: The high-pressure jet grouting method is a technique that makes use of high-pressure jets to inject chemical slurry, thereby breaking down the soil in the foundation and forcing the soil to mix with the chemical slurry to form a solid material with certain strength; it is used to treat weak foundations. The high-pressure jet grouting method is suitable for treating silt, silty soil, flowable, soft-plastic, or plastic cohesive soils. The high-pressure jet grouting method is suitable for both the seepage control of foundations or soil masses, where it forms a seepage barrier to prevent damage caused by seepage flow, soil erosion, or piping. It is widely used in the design of water-stop curtains for the retaining structures of Xi’an subway stations. Subways make extensive use of high-pressure jet grouting piles to construct water-stop curtains. 8. Cement-fly ash gravel piles (CFG): These piles are created by adding some stone chips, fly ash, and a small amount of cement to gravel piles; after mixing with water, they are formed using vibration-driven pile drivers or other pile-forming equipment, resulting in piles with a certain degree of bond strength. The piles and the soil between them form a composite foundation through a cushion layer. Cement-fly ash gravel piles along with cushion layers are a type of low-strength concrete pile; the composite foundation formed using these piles can significantly increase bearing capacity. Construction site of cement-fly ash-gravel piles. Before and after breaking the heads of CFG piles. 9. Cement mixing piles: Cement mixing piles utilize cement as the main curing agent; they represent an effective method for soft ground treatment. A mixing pile machine is used to inject cement into the soil and ensure thorough mixing. This process triggers a series of physical and chemical reactions between the cement and the soil, thereby hardening the soft soil and enhancing the strength of the foundation. After treatment, the soft soil foundation achieves significant reinforcement effects and can be put into use promptly. Suitable for handling silt, silty soil, peat soil, and silt loam. Cement mixing piles can be divided into wet and dry methods based on the state of material injection. The wet method relies mainly on cement slurry; it mixes well and is easy to remix, although the setting time of the cemented soil is relatively long ； The dry method primarily uses dry cement powder; the hardening time of cement-soil is relatively short, which can enhance the strength between piles. However, the mixing uniformity is poor, making it difficult to remix throughout the entire process. The foundation pit retaining structure 10 is formed using the cement mixing pile method. The sand and gravel compaction pile method refers to a technique in which gravel piles and sand piles are combined; these are considered coarse-grained soil piles. In this method, holes are created in soft foundations through vibration, impact, or water jetting, after which gravel or sand is forced into those holes, resulting in dense piles made of large-diameter gravel or sand. According to the pile formation process, they can be vibratory (wet) gravel piles and dry gravel piles. Crushed stone piles made using the vibration and water flushing pile-making process are known as vibroflotation crushed stone piles or wet-method crushed stone piles. Piles made using waterless compaction methods (such as dry vibration, vibro-compaction, hammering, etc.) are known as gravel piles. The construction principle of sand-gravel compaction piles. The various ground treatment methods described above are suitable for different regions and geological conditions; for instance, the drainage consolidation method is appropriate for coastal areas with soft soils, while the high-pressure jet grouting method is suitable for weak ground conditions.