1. Replacement method
(1) The replacement method is to remove the poor surface foundation soil, and then backfill with soil with better compaction properties for compaction or tamping to form a good bearing layer. This will change the bearing capacity characteristics of the foundation and improve its anti-deformation and stability capabilities.
Construction points: dig out the soil layer to be converted and pay attention to the stability of the pit edge; ensure the quality of the filler; the filler should be compacted in layers.
(2) The vibro-replacement method uses a special vibro-replacement machine to vibrate and flush under high-pressure water jets to form holes in the foundation, and then fill the holes with coarse aggregate such as crushed stone or pebbles in batches to form a pile body. The pile body and the original foundation soil form a composite foundation to achieve the purpose of increasing the foundation bearing capacity and reducing compressibility. Construction precautions: The bearing capacity and settlement of the crushed stone pile depend to a large extent on the lateral constraint of the original foundation soil on it. The weaker the constraint, the worse the effect of the crushed stone pile. Therefore, this method must be used with caution when used on soft clay foundations with very low strength.
(3) Ramming (squeezing) replacement method uses sinking pipes or ramming hammers to place pipes (hammers) into the soil, so that the soil is squeezed to the side, and gravel or sand and other fillers are placed in the pipe (or ramming pit). The pile body and the original foundation soil form a composite foundation. Due to squeezing and ramming, the soil is squeezed laterally, the ground rises, and the excess pore water pressure of the soil increases. When the excess pore water pressure dissipates, the soil strength also increases accordingly. Construction precautions: When the filler is sand and gravel with good permeability, it is a good vertical drainage channel.
2. Preloading method
(1) Loading preloading method Before building a building, a temporary loading method (sand, gravel, soil, other building materials, goods, etc.) is used to apply load to the foundation, giving a certain preloading period. After the foundation is pre-compressed to complete most of the settlement and the bearing capacity of the foundation is improved, the load is removed and the building is built. Construction process and key points: a. The preloading load should generally be equal to or greater than the design load; b. For large-area loading, a dump truck and a bulldozer can be used in combination, and the first level of loading on super-soft soil foundations can be done with light machinery or manual labor; c. The top width of the loading should be smaller than the bottom width of the building, and the bottom should be appropriately enlarged; d. The load acting on the foundation must not exceed the ultimate load of the foundation.
(2) Vacuum preloading method A sand cushion layer is laid on the surface of the soft clay foundation, covered with a geomembrane and sealed around. A vacuum pump is used to evacuate the sand cushion layer to form a negative pressure on the foundation under the membrane. As the air and water in the foundation are extracted, the foundation soil is consolidated. In order to accelerate consolidation, sand wells or plastic drainage boards can also be used, that is, sand wells or drainage boards can be drilled before laying the sand cushion layer and geomembrane to shorten the drainage distance. Construction points: first set up a vertical drainage system, the horizontally distributed filter pipes should be buried in strips or fishbone shapes, and the sealing membrane on the sand cushion layer should be 2-3 layers of polyvinyl chloride film, which should be laid simultaneously in sequence. When the area is large, it is advisable to preload in different areas; make observations on vacuum degree, ground settlement, deep settlement, horizontal displacement, etc.; after preloading, the sand trough and humus layer should be removed. Attention should be paid to the impact on the surrounding environment.
(3) Dewatering method Lowering the groundwater level can reduce the pore water pressure of the foundation and increase the self-weight stress of the overlying soil, so that the effective stress increases, thereby preloading the foundation. This is actually to achieve the purpose of preloading by lowering the groundwater level and relying on the self-weight of the foundation soil. Construction points: generally use light well points, jet well points or deep well points; when the soil layer is saturated clay, silt, silt and silty clay, it is advisable to combine with electrodes.
(4) Electroosmosis method: insert metal electrodes into the foundation and pass direct current. Under the action of the direct current electric field, water in the soil will flow from the anode to the cathode to form electroosmosis. Do not allow water to be replenished at the anode and use vacuum to pump water from the well point at the cathode, so that the groundwater level is lowered and the water content in the soil is reduced. As a result, the foundation is consolidated and compacted, and the strength is improved. The electroosmosis method can also be used in conjunction with preloading to accelerate the consolidation of saturated clay foundations.
3. Compaction and tamping method
1. The surface compaction method uses manual tamping, low-energy tamping machinery, rolling or vibration rolling machinery to compact the relatively loose surface soil. It can also compact the layered filling soil. When the water content of the surface soil is high or the water content of the filling soil layer is high, lime and cement can be laid in layers for compaction to strengthen the soil.
2. Heavy hammer tamping method Heavy hammer tamping is to use the large tamping energy generated by the free fall of the heavy hammer to compact the shallow foundation, so that a relatively uniform hard shell layer is formed on the surface, and a certain thickness of the bearing layer is obtained. Key points of construction: Before construction, test tamping should be carried out to determine relevant technical parameters, such as the weight of the tamping hammer, the bottom diameter and drop distance, the final sinking amount and the corresponding number of tamping times and the total sinking amount; the elevation of the bottom surface of the groove and pit before tamping should be higher than the design elevation; the moisture content of the foundation soil should be controlled within the optimal moisture content range during tamping; large-area tamping should be carried out in sequence; deep first and shallow later when the base elevation is different; during winter construction, when the soil is frozen, the frozen soil layer should be dug out or the soil layer should be melted by heating; after completion, the loosened topsoil should be removed in time or the floating soil should be tamped to the design elevation at a drop distance of nearly 1m.
3. Strong tamping is the abbreviation of strong tamping. A heavy hammer is dropped freely from a high place, exerting a high impact energy on the foundation, and repeatedly tamping the ground. The particle structure in the foundation soil is adjusted, and the soil becomes dense, which can greatly improve the foundation strength and reduce compressibility. The construction process is as follows: 1) Level the site; 2) Lay the graded gravel cushion layer; 3) Set up gravel piers by dynamic compaction; 4) Level and fill the graded gravel cushion layer; 5) Fully compact once; 6) Level and lay geotextile; 7) Backfill the weathered slag cushion layer and roll it eight times with a vibrating roller. Generally, before large-scale dynamic compaction, a typical test should be carried out on a site with an area of no more than 400m2 in order to obtain data and guide design and construction.
4. Compacting method
1. The vibrating compacting method uses the repeated horizontal vibration and lateral squeezing effect generated by a special vibrating device to gradually destroy the structure of the soil and rapidly increase the pore water pressure. Due to the structural destruction, soil particles may move to a low potential energy position, so that the soil changes from loose to dense.
Construction process: (1) Level the construction site and arrange the pile positions; (2) The construction vehicle is in place and the vibrator is aimed at the pile position; (3) Start the vibrator and let it slowly sink into the soil layer until it is 30 to 50 cm above the reinforcement depth, record the current value and time of the vibrator at each depth, and lift the vibrator to the hole mouth. Repeat the above steps 1 to 2 times to make the mud in the hole thinner. (4) Pour a batch of filler into the hole, sink the vibrator into the filler to compact it and expand the pile diameter. Repeat this step until the current at the depth reaches the specified compacting current, and record the amount of filler. (5) Lift the vibrator out of the hole and continue to construct the upper pile section until the entire pile body is vibrated, and then move the vibrator and equipment to another pile position. (6) During the pile making process, each section of the pile body should meet the requirements of compaction current, filling amount and vibration retention time. The basic parameters should be determined through on-site pile making tests. (7) A mud drainage ditch system should be set up in advance at the construction site to concentrate the mud and water generated during the pile making process into a sedimentation tank. The thick mud at the bottom of the tank can be dug out regularly and sent to a pre-arranged storage location. The relatively clear water at the top of the sedimentation tank can be reused. (8) Finally, the pile body with a thickness of 1 meter at the top of the pile should be dug out, or compacted and compacted by rolling, strong tamping (over-tamping), etc., and the cushion layer should be laid and compacted.
2. Pipe-sinking gravel piles (gravel piles, lime soil piles, OG piles, low-grade piles, etc.) use pipe-sinking pile machines to hammer, vibrate, or statically pressurize pipes in the foundation to form holes, then put materials into the pipes, and lift (vibrate) the pipes while putting materials into them to form a dense pile body, which forms a composite foundation with the original foundation.
3. Rammed gravel piles (block stone piers) use heavy hammer tamping or strong tamping methods to tamp gravel (block stone) into the foundation, gradually fill gravel (block stone) into the tamping pit, and tamp repeatedly to form gravel piles or block stone piers.
5. Mixing method
1. High-pressure jet grouting method (high-pressure rotary jet method) uses high pressure to spray cement slurry from the injection hole through the pipeline, directly cutting and destroying the soil while mixing with the soil and playing a partial replacement role. After solidification, it becomes a mixed pile (column) body, which forms a composite foundation together with the foundation. This method can also be used to form a retaining structure or an anti-seepage structure.
2. Deep mixing method The deep mixing method is mainly used to reinforce saturated soft clay. It uses cement slurry and cement (or lime powder) as the main curing agent, and uses a special deep mixing machine to send the curing agent into the foundation soil and force it to mix with the soil to form a cement (lime) soil pile (column) body, which forms a composite foundation with the original foundation. The physical and mechanical properties of cement soil piles (columns) depend on a series of physical-chemical reactions between the curing agent and the soil. The amount of curing agent added, the mixing uniformity and the properties of the soil are the main factors affecting the properties of cement soil piles (columns) and even the strength and compressibility of the composite foundation. Construction process: ① Positioning ② Slurry preparation ③ Slurry delivery ④ Drilling and spraying ⑤ Lifting and mixing spraying ⑥ Repeated drilling and spraying ⑦ Repeated lifting and mixing ⑧ When the drilling and lifting speed of the mixing shaft is 0.65-1.0m/min, the mixing should be repeated once. ⑨ After the pile is completed, clean the soil blocks wrapped on the mixing blades and the spraying port, and move the pile driver to another pile position for construction.
6. Reinforcement method
(1) Geosynthetics Geosynthetics is a new type of geotechnical engineering material. It uses artificially synthesized polymers such as plastics, chemical fibers, synthetic rubber, etc. as raw materials to make various types of products, which are placed inside, on the surface or between layers of soil to strengthen or protect the soil. Geosynthetics can be divided into geotextiles, geomembranes, special geosynthetics and composite geosynthetics.
(2) Soil nail wall technology Soil nails are generally set by drilling, inserting bars, and grouting, but there are also soil nails formed by directly driving thicker steel bars, steel sections, and steel pipes. The soil nail is in contact with the surrounding soil along its entire length. Relying on the bond friction resistance on the contact interface, it forms a composite soil with the surrounding soil. The soil nail is passively subjected to force under the condition of soil deformation. The soil is reinforced mainly through its shearing work. The soil nail generally forms a certain angle with the plane, so it is called an oblique reinforcement. Soil nails are suitable for foundation pit support and slope reinforcement of artificial fill, clay soil, and weakly cemented sand above the groundwater level or after precipitation.
(3) Reinforced soil Reinforced soil is to bury strong tensile reinforcement in the soil layer, and use the friction generated by the displacement of soil particles and the reinforcement to form a whole with the soil and reinforcement materials, reduce overall deformation and enhance overall stability. Reinforcement is a horizontal reinforcement. Generally, strip, mesh, and filamentary materials with strong tensile strength, large friction coefficient and corrosion resistance are used, such as galvanized steel sheets; aluminum alloys, synthetic materials, etc.
7. Grouting method
Use air pressure, hydraulic pressure or electrochemical principles to inject certain solidifying slurries into the foundation medium or the gap between the building and the foundation. The grouting slurry can be cement slurry, cement mortar, clay cement slurry, clay slurry, lime slurry and various chemical slurries such as polyurethane, lignin, silicate, etc. According to the purpose of grouting, it can be divided into anti-seepage grouting, plugging grouting, reinforcement grouting and structural tilt correction grouting. According to the grouting method, it can be divided into compaction grouting, infiltration grouting, splitting grouting and electrochemical grouting. Grouting method has a wide range of applications in water conservancy, construction, roads and bridges and various engineering fields.
8. Common bad foundation soils and their characteristics
1. Soft clay Soft clay is also called soft soil, which is the abbreviation of weak clay soil. It was formed in the late Quaternary period and belongs to the viscous sediments or river alluvial deposits of marine phase, lagoon phase, river valley phase, lake phase, drowned valley phase, delta phase, etc. It is mostly distributed in coastal areas, middle and lower reaches of rivers or near lakes. Common weak clay soils are silt and silty soil. The physical and mechanical properties of soft soil include the following aspects: (1) Physical properties The clay content is high, and the plasticity index Ip is generally greater than 17, which is a clay soil. Soft clay is mostly dark gray, dark green, has a bad smell, contains organic matter, and has a high water content, generally greater than 40%, while silt can also be greater than 80%. The porosity ratio is generally 1.0-2.0, among which the porosity ratio of 1.0-1.5 is called silty clay, and the porosity ratio greater than 1.5 is called silt. Due to its high clay content, high water content and large porosity, its mechanical properties also show corresponding characteristics – low strength, high compressibility, low permeability and high sensitivity. (2) Mechanical properties The strength of soft clay is extremely low, and the undrained strength is usually only 5-30 kPa, which is manifested in a very low basic value of bearing capacity, generally not exceeding 70 kPa, and some are even only 20 kPa. Soft clay, especially silt, has a high sensitivity, which is also an important indicator that distinguishes it from general clay. Soft clay is very compressible. The compression coefficient is greater than 0.5 MPa-1, and can reach a maximum of 45 MPa-1. The compression index is about 0.35-0.75. Under normal circumstances, soft clay layers belong to normal consolidated soil or slightly overconsolidated soil, but some soil layers, especially recently deposited soil layers, may belong to underconsolidated soil. The very small permeability coefficient is another important feature of soft clay, which is generally between 10-5-10-8 cm/s. If the permeability coefficient is small, the consolidation rate is very slow, the effective stress increases slowly, and the settlement stability is slow, and the foundation strength increases very slowly. This characteristic is an important aspect that seriously restricts the foundation treatment method and treatment effect. (3) Engineering characteristics Soft clay foundation has low bearing capacity and slow strength growth; it is easy to deform and uneven after loading; the deformation rate is large and the stability time is long; it has the characteristics of low permeability, thixotropy and high rheology. Commonly used foundation treatment methods include preloading method, replacement method, mixing method, etc.
2. Miscellaneous fill Miscellaneous fill mainly appears in some old residential areas and industrial and mining areas. It is garbage soil left or piled up by people’s life and production activities. These garbage soils are generally divided into three categories: construction garbage soil, domestic garbage soil and industrial production garbage soil. Different types of garbage soil and garbage soil piled up at different times are difficult to describe with unified strength indicators, compression indicators and permeability indicators. The main characteristics of miscellaneous fill are unplanned accumulation, complex composition, different properties, uneven thickness and poor regularity. Therefore, the same site shows obvious differences in compressibility and strength, which is very easy to cause uneven settlement, and usually requires foundation treatment.
3. Fill soil Fill soil is soil deposited by hydraulic filling. In recent years, it has been widely used in coastal tidal flat development and floodplain reclamation. The water-falling dam (also called fill dam) commonly seen in the northwest region is a dam built with fill soil. The foundation formed by fill soil can be regarded as a kind of natural foundation. Its engineering properties mainly depend on the properties of the fill soil. Fill soil foundation generally has the following important characteristics. (1) The particle sedimentation is obviously sorted. Near the mud inlet, coarse particles are deposited first. Away from the mud inlet, the deposited particles become finer. At the same time, there is obvious stratification in the depth direction. (2) The water content of fill soil is relatively high, generally greater than the liquid limit, and it is in a flowing state. After the filling is stopped, the surface often becomes cracked after natural evaporation, and the water content is significantly reduced. However, the lower fill soil is still in a flowing state when the drainage conditions are poor. The finer the fill soil particles, the more obvious this phenomenon is. (3) The early strength of the fill soil foundation is very low and the compressibility is relatively high. This is because the fill soil is in an underconsolidated state. The backfill foundation gradually reaches a normal consolidation state as the static time increases. Its engineering properties depend on the particle composition, uniformity, drainage consolidation conditions and the static time after backfilling.
4. Saturated loose sandy soil silt sand or fine sand foundation often has high strength under static load. However, when vibration load (earthquake, mechanical vibration, etc.) acts, saturated loose sandy soil foundation may liquefy or undergo a large amount of vibration deformation, or even lose its bearing capacity. This is because the soil particles are loosely arranged and the position of the particles is dislocated under the action of external dynamic force to achieve a new balance, which instantly generates a higher excess pore water pressure and the effective stress decreases rapidly. The purpose of treating this foundation is to make it more compact and eliminate the possibility of liquefaction under dynamic load. Common treatment methods include extrusion method, vibroflotation method, etc.
5. Collapsible loess The soil that undergoes significant additional deformation due to the structural destruction of the soil after immersion under the self-weight stress of the overlying soil layer, or under the combined action of self-weight stress and additional stress, is called collapsible soil, which belongs to special soil. Some miscellaneous fill soils are also collapsible. Loess widely distributed in Northeast my country, Northwest China, Central China and parts of East China are mostly collapsible. (The loess mentioned here refers to loess and loess-like soil. Collapsible loess is divided into self-weight collapsible loess and non-self-weight collapsible loess, and some old loess is not collapsible). When carrying out engineering construction on collapsible loess foundations, it is necessary to consider the possible harm to the project caused by additional settlement caused by foundation collapse, and choose appropriate foundation treatment methods to avoid or eliminate the collapse of the foundation or the harm caused by a small amount of collapse.
6. Expansive soil The mineral component of expansive soil is mainly montmorillonite, which has strong hydrophilicity. It expands in volume when absorbing water and shrinks in volume when losing water. This expansion and contraction deformation is often very large and can easily cause damage to buildings. Expansive soil is widely distributed in my country, such as Guangxi, Yunnan, Henan, Hubei, Sichuan, Shaanxi, Hebei, Anhui, Jiangsu and other places, with different distributions. Expansive soil is a special type of soil. Common foundation treatment methods include soil replacement, soil improvement, pre-soaking, and engineering measures to prevent changes in the moisture content of the foundation soil.
7. Organic soil and peat soil When the soil contains different organic matter, different organic soils will be formed. When the organic matter content exceeds a certain content, peat soil will be formed. It has different engineering properties. The higher the organic matter content, the greater the impact on the soil quality, which is mainly manifested in low strength and high compressibility. It also has different effects on the incorporation of different engineering materials, which has an adverse effect on direct engineering construction or foundation treatment.
8. Mountain foundation soil The geological conditions of mountain foundation soil are relatively complex, mainly manifested in the unevenness of the foundation and the stability of the site. Due to the influence of the natural environment and the formation conditions of the foundation soil, there may be large boulders in the site, and the site environment may also have adverse geological phenomena such as landslides, mudslides, and slope collapses. They will pose a direct or potential threat to buildings. When constructing buildings on mountain foundations, special attention should be paid to site environmental factors and adverse geological phenomena, and the foundation should be treated when necessary.
9. Karst In karst areas, there are often caves or earth caves, karst gullies, karst crevices, depressions, etc. They are formed and developed by the erosion or subsidence of groundwater. They have a great impact on structures and are prone to uneven deformation, collapse and subsidence of the foundation. Therefore, necessary treatment must be carried out before building structures.