Before constructing a building, use temporary loading (sand, soil, other building materials, goods, etc.) to apply load to the foundation and give it 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 before the building is built.
Replacement Method
(1) Replacement method
It means excavating the poor foundation soil on the surface and then backfilling it with soil with good compaction properties for compaction or tamping to form a good bearing layer. Thereby changing the bearing capacity characteristics of the foundation and improving the resistance to deformation and stability.
Construction points: Excavate all the soil layers to be converted and pay attention to the stability of the pit edges; ensure the quality of the filler; the filler should be compacted in layers.
(2) Vibration displacement method
Special vibrating machines are used to vibrate and punch under high-pressure water jets to form holes in the foundation, and then fill the holes with coarse particles such as gravel or pebbles in batches to form piles. The pile body and the original foundation soil form a composite foundation to achieve the purpose of improving the bearing capacity of the foundation and reducing the compressibility.
Construction precautions: The bearing capacity and settlement of gravel piles largely depend on the lateral restraint of the original foundation soil. The weaker the restraint, the worse the effect of gravel piles. Therefore, this method is used for strength. Caution must be exercised when working with very low soft clay foundations.
(3) Ramping (squeezing) replacement method
The pipe (hammer) is placed into the soil using an immersed pipe or ramming hammer, so that the soil is squeezed sideways, and fillers such as gravel or sand are placed in the pipe (or ramming pit). The pile body and the original foundation soil form a composite foundation. Due to squeezing and tamping, the soil body is squeezed laterally, the ground rises, and the excess static pore water pressure of the soil body increases. When the excess static pore water pressure dissipates, the soil strength also increases accordingly.
Construction precautions: When the filler is sand and gravel with good water permeability, it is a good vertical drainage channel.
Preloading Method
(1) Stacking preloading method
Before constructing a building, use temporary loading (sand, soil, other building materials, goods, etc.) to apply load to the foundation and give it 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 before the building is built.
Construction technology and key points:
a. The preloading load should generally be equal to or greater than the design load;
b. Dump trucks and bulldozers can be used for combined operations for large-area loading, and light machinery or manual operations can be used for the first-level loading on ultra-soft soil foundations;
c. The width of the top surface of the stack should be smaller than the width of the bottom surface of the building, and the bottom surface should be appropriately enlarged;
d. The load acting on the foundation shall not exceed the ultimate load of the foundation.
(2) Vacuum preloading method
Lay a sand cushion on the surface of the soft clay foundation, cover it with a geomembrane, and seal it around it. Use a vacuum pump to evacuate the sand cushion layer to create a negative pressure in the foundation under the film. As air and water are extracted from the foundation, the foundation soil consolidates. To accelerate consolidation, the method of drilling sand wells or inserting plastic drainage boards can also be used. That is, before laying the sand cushion and geomembrane, sand wells or drainage boards can be drilled to shorten the drainage distance.
Construction points:
The vertical drainage system should be set up first. The horizontally distributed filter pipes should be buried in strip or fishbone shapes. The sealing film on the sand cushion 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; and make observations on vacuum degree, ground settlement, deep settlement, horizontal displacement, etc.; after preloading, the sand tank and humus soil layer should be removed. Attention should be paid to the impact on the surrounding environment.
(3) Precipitation method
Lowering the groundwater level can reduce the pore water pressure of the foundation, increase the self-weight stress of the overlying soil, increase the effective stress, and thereby preload the foundation. This achieves the purpose of preloading by lowering the groundwater level and relying on the weight of the foundation soil.
Construction points: Generally, light well points, jet well points, or deep well points are used; when the soil layer is saturated clay, silt, silt, and silty cohesive soil, it should be supplemented by electrodes.
(4) Electroosmosis method
Insert a metal electrode into the foundation and pass it through direct current. Under the action of the direct current electric field, the water in the soil will flow from the anode to the cathode to form electroosmosis. Instead of allowing water to be replenished at the anode, water is pumped from the good point of the cathode using a vacuum, thus lowering the groundwater level and reducing the water content in the soil. As a result, the foundation is consolidated and compacted, and its strength is increased. The electroosmosis method can also be used in conjunction with stacked preloading to accelerate the consolidation of saturated viscous soil foundations.
Compaction and Tamping Method
1. Surface compaction method
Use manual tamping, low-energy tamping machinery, and rolling or vibration rolling machinery to compact relatively loose surface soil. Layered fill soil can also be compacted. When the moisture content of the surface soil is high or the moisture content of the filled 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 compaction is to use the greater ramming 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 force-bearing layer is obtained.
Construction points: Test tamping before construction to determine relevant technical parameters, such as the weight of the tamping hammer, bottom diameter, drop distance, final settlement amount, and a corresponding number of tamping strikes and total settlement amount; tamp down the elevation of the front trough and pit bottom It should be higher than the design elevation; the moisture content of the foundation soil during compaction should be controlled within the optimal moisture content range; large-area compaction should be done in sequence; when the base elevation is different, it should be deeper first and then shallower; during winter construction, when the soil has frozen, the frozen soil layer should be dug out or the soil layer should be melted through the heating method; after the completion, the loose topsoil should be removed in time or the floating soil should be compacted to the design elevation at a distance close to 1m.
3. Dynamic tamping is the abbreviation of strong tamping. A heavy hammer will be freely dropped from a high place, exerting a high impact energy on the foundation, and repeatedly hitting the ground. The particle structure of the foundation soil will be adjusted, and the soil will become dense, thus maximizing the strength of the foundation. and reduced compressibility.
Its construction process:
1) Level the site;
2) Pave-graded gravel cushion;
3) Replace gravel piers with dynamic compaction;
4) Level and fill with graded gravel cushion;
5) Tamp it thoroughly;
6) Level and spread geotextile;
7) Backfill the weathered slag cushion and roll it eight times with vibrating rollers.
Generally, before large-scale dynamic compaction of soil, a site with an area of no more than 400m2 should be selected to conduct typical tests to obtain data and guide design and construction.
Crowding Method
1. Vibration compaction method
The repetitive horizontal vibration and lateral squeezing effect produced by specialized vibrating equipment gradually destroys the structure of the soil and rapidly increases the pore water pressure. Due to structural damage, soil particles may move to low potential energy positions, causing the soil to change from loose to dense.
Construction technology:
(1) Level the construction site and arrange pile positions;
(2) The construction vehicle is in place and the vibrator is aligned with the pile position;
(3) Start the vibrator and slowly sink it into the soil layer until it is 30 to 50cm above the reinforcement depth. Record the current value and time of the vibrator passing through each depth, and lift the vibrator to the hole opening. Repeat the above steps 1 to 2 times to thin the mud in the hole.
(4) Pour a batch of filler into the hole, and sink the vibrator into the filler to vibrate and expand the pile diameter. Repeat this step until the depth current reaches the specified compaction current and record the amount of filler.
(5) Lift the vibrator out of the hole, continue construction of the upper pile section, and complete vibration construction of the entire pile body, then move the vibrator and machine tools to another pile position.
(6) During the pile-making process, each pile section should meet the requirements of compact 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 opened at the construction site in advance to centrally introduce the muddy water generated during the pile-making process into the sedimentation tank. The thick mud at the bottom of the tank can be dug out regularly and sent to a prearranged storage location. The relatively clear water in the upper part of the sedimentation tank can be reused.
(8) Finally, the lm-thick pile body at the top of the pile should be dug out, or compacted and compacted by rolling, strong tamping (over-compaction), etc., and the cushion layer should be laid and compacted.
2. Immersed tube sand and gravel piles (gravel piles, lime soil piles, OG piles, low-grade piles, etc.)
The immersed tube pile machine is used to hammer or vibrate the immersed tube in the foundation to form a hole or static pressure the immersed tube to form a hole, then feed the material in the tube, and lift (vibrating) the immersed tube while feeding to form a dense pile body, which forms a composite foundation with the original foundation.
3. Ramping gravel piles (stone piers)
The gravel (rock) is rammed into the foundation using heavy hammer tamping or strong tamping, and the tamping pit is gradually filled with gravel (rock) and rammed repeatedly to form a gravel pile or stone pier.
Mixing Method
1. High-pressure jet grouting method (high-pressure jet grouting method)
The cement slurry is ejected from the injection hole through the pipeline at high pressure, directly cutting and destroying the soil while mixing with the soil and partially replacing it. After solidification, it becomes a mixed pile (column) body, which together with the foundation forms a composite foundation. This method can also be used to form retaining structures or anti-seepage structures.
2. Deep stirring method
The deep mixing method is mainly used to consolidate 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 mix it with the soil to form a pile (column) of cement (lime) soil. Combined with the original foundation to form a composite 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 and the uniformity of mixing as well as the properties of the soil are the main factors that affect the properties of cement-soil piles (columns) and even the strength and compressibility of composite foundations.
Construction technology:
①Positioning
②Slurry preparation
③Sending pulp
④Drilling and mixing with shotcrete
⑤ Lift and mix gunite
⑥ Repeat drilling and gunite mixing
⑦ Repeat lifting and stirring
⑧When the drilling and lifting speed of the mixing shaft is 0.65-1.Om/min, the stirring should be repeated once.
After the pile formation is completed, clean the soil clods and the shotcrete port wrapped on the mixing blade, and move the pile driver to another pile position for construction.
Reinforcement Method
(1) Geosynthetics
Geosynthetic material is a new type of geotechnical engineering material. It uses synthetic polymers, such as plastics, chemical fibers, synthetic rubber, etc. as raw materials, to make various types of products, which are placed inside the soil, on the surface, or between layers of soil to play a role in strengthening or protecting the soil. Geosynthetics can be divided into types such as geotextiles, geomembranes, special geosynthetics, and composite geosynthetics.
(2) Soil nail wall technology
Soil nails are generally set up by drilling, inserting bars, and grouting, but they are also formed by directly driving thicker steel bars, steel bars, and steel pipes. The soil nail is in contact with the surrounding soil along its entire length and relies on the bonding friction on the contact interface to form a composite soil with its surrounding soil. The soil nail is passively stressed under the condition of deformation of the soil. And the soil is mainly reinforced through its shear work. The soil nails generally form a certain angle with the plane, so they are called oblique reinforcements. Soil nails are suitable for foundation pit support and slope reinforcement of artificial fill soil, cohesive soil, and weakly cemented sand above the groundwater level or after precipitation.
(3) Reinforced soil
Reinforced soil embeds tie bars with strong tensile strength in the soil layer. The friction generated by the displacement of soil particles and the tie bars is used to make the soil and the reinforced material form a whole, reducing overall deformation and enhancing overall stability. Lajin is a horizontal strengthening body. Generally, strip-shaped, mesh-shaped, and filament-shaped materials with strong tensile strength, large friction coefficient, and corrosion resistance are used, such as galvanized steel sheets; aluminum alloys, synthetic materials, etc.
Grouting Method
Using pneumatic, hydraulic, or electrochemical principles, certain slurries that can be solidified are injected into the foundation medium or the gaps 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, leakage-blocking grouting, reinforcement grouting, and structural tilting grouting. According to the grouting method, it can be divided into compaction grouting, penetration grouting, splitting grouting, and electrochemical grouting. The grouting method is widely used in water conservancy, construction, roads and bridges, and various engineering fields.
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 alluvials of marine, lagoon, valley, lake and swamp, drowned valley, and delta phases. Mostly distributed along the coast, in the middle and lower reaches of rivers, or near lakes. Common weak cohesive soils are silt and silty soils. The physical and mechanical properties of soft soil include the following aspects:
(1) Physical properties: The clay content is high, the plasticity index Ip is generally greater than 17, and it is clay soil. Soft clay is mostly dark gray, dark green, smelly, contains organic matter, and has a high water content, generally greater than 40%, while silt may also have greater than 80%. The void ratio is generally 1.0-2.0, of which a void ratio of 1.0 to 1.5 is called silty clay, and a void ratio greater than 1.5 is called silt. Due to its high clay content, high water content, and large void ratio, 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. The undrained strength is usually only 5-30kPa. The basic value of the bearing capacity is very low, generally no more than 70kPa, and some even only 20kPa. Soft clay, especially silt, has high sensitivity. , which is also an important indicator that distinguishes it from ordinary clay.
Soft clay is very compressible. The compression coefficient is greater than 0.5MPa-1, up to 45MPa-1, and the compression index is about 0.35-0.75. Normally, the soft clay layer belongs to normally consolidated soil or slightly super-consolidated soil, but some soil layers, especially recently deposited soil layers may be under-consolidated soil.
The small permeability coefficient is another important feature of soft clay, generally between 10-5-10-8cm/s. If the permeability coefficient is small, the consolidation rate will be very slow, the effective stress will grow slowly, the settlement will be stable and slow, and the foundation strength will be reduced. Growth is also very slow. This characteristic is an important aspect that seriously restricts foundation treatment methods and treatment effects.
(3) Engineering characteristics: Soft clay foundation has a low bearing capacity and slow strength growth; it is easy to deform and is uneven after loading; it has a large deformation rate and a long stabilization time; it has the characteristics of low permeability, thixotropy, and large rheology. Commonly used foundation treatment methods include a preloading method, replacement method, stirring method, etc.
2. Miscellaneous fill soil
Miscellaneous fill soil mainly appears in some old residential areas and industrial and mining areas. It is garbage left or piled up by people’s living and production activities. These garbage soils are generally divided into three categories: construction waste soil, domestic waste soil, and industrial production waste soil. It is difficult to describe different types of garbage soil and garbage soil piled at different times with unified strength indicators, compression indicators, and permeability indicators.
The main characteristics of miscellaneous fill soil are unplanned accumulation, complex composition, different properties, uneven thickness, and poor regularity. Therefore, the same site shows obvious differences in compressibility and strength, which can easily cause uneven settlement, and foundation treatment is usually required.
3. Fill soil
Fill soil is soil deposited by man-made hydraulic filling methods. In recent years, it has been mostly used for coastal beach development and floodplain land reclamation. The common flood dams (also called fill dams) in the northwest region are dams built with filled soil. The foundation formed by filled soil can be regarded as a kind of natural foundation, and its engineering properties mainly depend on the properties of the filled soil. A filled soil foundation generally has the following important characteristics.
(1) The particle sedimentation sorting is obvious. Near the mud inlet, coarse particles are deposited first, and away from the mud inlet, the deposited particles become finer; at the same time, there is obvious bedding in the depth direction.
(2) The water content of fill soil is relatively high, generally greater than the liquid limit, and is flowing. After the filling is stopped, the surface often becomes cracked after natural evaporation, and the water content is significantly reduced. However, the lower-filling soil is still flowing when the drainage conditions are poor. The finer the filling soil particles, the more obvious this phenomenon is.
(3) The initial strength of the filled soil foundation is very low and the compressibility is high. This is because the filled soil is in an under-consolidated state. The filled soil foundation gradually reaches the normal consolidation state as the standing time increases. Its engineering properties depend on particle composition, uniformity, drainage and consolidation conditions, and resting time after filling.
4. Saturated loose sand
Silt sand or fine sand foundations often have higher strength under static load. However, when vibration loads (earthquakes, mechanical vibrations, etc.) act, the saturated loose sand foundation may undergo liquefaction or massive earthquake deformation, or even lose bearing capacity. This is because the soil particles are loosely arranged and the position of the particles is dislocated under the action of external dynamics to achieve a new balance, which instantly generates high superstatic pore water pressure and the effective stress is rapidly reduced. The purpose of treating this type of foundation is to make it denser and eliminate the possibility of liquefaction under dynamic loads. Commonly used processing methods include extrusion, vibrating, etc.
5. Collapsible loess
Under the action of the self-weight stress of the overlying soil layer, or the combined action of self-weight stress and additional stress, soil that undergoes significant additional deformation due to structural damage after water immersion is called collapsible soil, which is a special soil. Some mixed-fill soils are also collapsible. Loess, which is widely distributed in parts of northeastern, northwest, central, and eastern China, is mostly collapsible. (The loess mentioned here generally refers to loess and loess-like soil. Collapsible loess is divided into self-weight collapsible loess and non-self-weight collapsible loess. Some old loess is not collapsible). When constructing projects on collapsible loess foundations, it is necessary to consider the possible harm to the project caused by additional settlement caused by subsidence of the foundation and select appropriate foundation treatment methods to avoid or eliminate subsidence of the foundation or damage caused by a small amount of subsidence. harm.
6. Expansive soil
The main mineral component of expansive soil is montmorillonite, which is highly hydrophilic. Its volume expands when it absorbs water and shrinks when it loses water. This kind of expansion and contraction deformation is often very large and can easily cause damage to buildings. Expansive soil has a wide distribution range in my country, such as Guangxi, Yunnan, Henan, Hubei, Sichuan, Shaanxi, Hebei, Anhui, Jiangsu, and other places. Expansive soil is a type of special soil. Commonly used 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. Contains organic soil and peat soil
When the soil contains different organic matter, different organic matter soils will be formed. When the organic matter content exceeds a certain content, peat soil will be formed. It has different engineering characteristics. The higher the organic matter content, the greater the impact on soil quality. The main performance is It has low strength, high compressibility, and has different effects on the incorporation of different engineering materials, which hurts direct engineering construction or foundation treatment.
8. Mountain foundation soil
The geological conditions of foundation soil in mountainous areas are relatively complex, mainly reflected in the unevenness of the foundation and site stability. Due to the influence of the natural environment and the generation 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, debris flows, and slope collapses. They can pose a direct or potential threat to buildings. When constructing buildings on mountainous foundations, special attention should be paid to site environmental factors and adverse geological phenomena, and the foundations should be treated when necessary.
9. Karst (karst)
In karst (karst) areas, there are often karst caves or earth caves, karst ditches, karst gaps, depressions, etc. The erosion or under-erosion of grunder-erosionses it to form and develop. 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 constructing the structure.
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