First, the replacement method

(1) replacement method

It is to dig out the foundation soil with poor surface, and then backfill the soil with good compaction characteristics for compaction or compaction to form a good bearing layer. Thereby changing the bearing characteristics of the foundation and improving the deformation resistance and stability.

Key points of construction: dig out the soil layer to be converted, and pay attention to the stability of the pit edge; Ensure the quality of the filler; Packing should be compacted in layers.

(2) vibroflotation replacement method

Using a special vibroflotation device, vibroflotation is carried out under the action of high-pressure water jet to form holes in the foundation, and then coarse particles such as gravel or pebbles are filled in the holes in batches to form piles. Pile and in-situ foundation soil form composite foundation, which can improve foundation bearing capacity and reduce compressibility.

Precautions in construction: the bearing capacity and settlement of gravel pile depend largely on the lateral constraint of in-situ foundation soil. The weaker the constraint, the worse the effect of gravel pile. Therefore, this method must be used cautiously in soft clay foundation with low strength.

(3) Compaction (extrusion) replacement method

Put the pipe (hammer) into the soil by sinking the pipe or rammer, so that the soil will be squeezed aside, and fill the pipe (or rammer pit) with gravel or sand. Pile and in-situ foundation soil form a composite foundation. Due to soil compaction, 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 strength of soil increases accordingly.

Precautions in construction: When the filler is sand and gravel with good water permeability, it is a good vertical drainage channel.

Second, the preloading method

(1) surcharge preloading method

Before the building is completed, the foundation is loaded by temporary stacking (sand, gravel, soil, other building materials, goods, etc.). ) and given a certain preloading period. After the foundation preloading completes most of the settlement and the bearing capacity of the foundation is improved, the load is unloaded and the building is built.

Construction technology and key points:

A, preloading load should generally be equal to or greater than the design load;

B, large-scale stacking can be combined with dump trucks and bulldozers, and the first-class stacking on ultra-soft soil foundation can be carried out by light machinery or manual operation;

C, the width of the top surface of the heap load should be less 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

Sand cushion is laid on the surface of soft clay foundation, covered with geotechnical film and closed around. The sand cushion is vacuumized by a vacuum pump, so that the foundation under the film forms negative pressure. As gas and water are pumped out of the foundation, the foundation soil is consolidated. In order to speed up the consolidation, it is also possible to use the method of drilling sand wells or inserting plastic drainage plates, that is, drilling sand wells or inserting drainage plates before laying sand cushion and geomembrane to shorten the drainage distance.

Key points of construction:

First, establish a vertical drainage system. The horizontally distributed filter tubes should be buried in strips or fishbones. The sealing film on the sand cushion should be 2-3 layers of PVC film, which should be laid simultaneously in sequence. When the area is large, partition preloading should be carried out; Observe vacuum degree, ground subsidence, deep subsidence and horizontal displacement; After preloading, the sand trough and humus layer should be removed. Pay attention 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 and increase the effective stress, thus 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.

Key points of construction: light well point, jet well point or deep well point are generally adopted; When the soil layer is saturated clay, silt, silt and muddy cohesive soil, it is appropriate to combine electrodes at this time.

(4) Electroosmosis method

Insert the metal electrode into the base and apply direct current. Under the action of DC electric field, water in soil will flow from anode to cathode, forming electroosmosis. Water is pumped from the well point of the cathode by vacuum instead of being replenished at the anode, thus lowering the groundwater level and reducing the water content in the soil. So that the foundation is consolidated and compacted, and the strength is improved. Electroosmosis can also be combined with surcharge preloading to accelerate the consolidation of saturated cohesive soil foundation.

III. Compaction and tamping methods

1, surface compaction method

Loose topsoil shall be compacted by manual compaction, low-energy compaction machinery, rolling or vibration rolling machinery. Layered fill can also be compacted. When the water content of topsoil or fill soil is high, lime and cement can be spread in layers for compaction, so that the soil can be reinforced.

2, heavy hammer compaction method

Heavy hammer tamping is to tamp the shallow foundation by using the large tamping energy generated by the free fall of heavy hammer, so that a relatively uniform hard shell layer is formed on its surface and a bearing layer with a certain thickness is obtained.

Key points of construction: before construction, conduct tamping test to determine relevant technical parameters, such as the weight of tamping hammer, diameter of bottom surface and falling distance, final settlement, corresponding tamping times and total settlement; Before compaction, the elevation of tank bottom and pit bottom should be higher than the design elevation; When tamping, the water content of foundation soil should be controlled within the optimal water content range; Large-scale compaction should be carried out in sequence; When the basement elevation is different, it should be deep first and then shallow; In winter construction, when the soil has been frozen, the frozen soil layer should be dug out or heated to melt; After completion, the compacted topsoil shall be removed in time or the floating soil shall be compacted to the design elevation at the drop close to1m.

3. Dynamic compaction

Dynamic compaction is short for strong compaction. When the heavy hammer falls freely from a height, it exerts high impact energy on the foundation and tamps the ground repeatedly, so that the particle structure in the foundation soil is adjusted and the soil becomes dense, thus improving the foundation strength and reducing the compressibility to a great extent.

Its construction process flow:

1) Level the site;

2) Laying graded macadam cushion;

3) Set gravel piers for dynamic replacement;

4) Leveling and filling graded macadam cushion;

5) Full tamping again;

6) Leveling and laying geotextile;

7) Backfill the cushion layer of aeolian fossil slag, and roll it with vibratory roller for eight times.

Generally, before large-scale dynamic compaction, typical tests should be carried out in a site not exceeding 400m2 to obtain data and guide design and construction.

Fourth, the compaction method

1, vibro-compaction method

Repeated horizontal vibration and lateral extrusion produced by special vibroflotation gradually destroy the soil structure, which makes the pore water pressure increase rapidly. Due to structural damage, soil particles may move to the position with low potential energy, making the soil from loose to dense.

Construction technology:

(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 to slowly sink into the soil layer until it exceeds the reinforcement depth of 30-50 cm, and record the current value and time when the vibrator passes through each depth.

Lift the vibrator to the orifice. Repeat the above steps 1 ~ 2 times to dilute the mud in the hole.

(4) Pouring a batch of filler into the hole, and sinking the vibroflot into the filler to vibrate and enlarge the pile diameter. Repeat this step until the depth current reaches the specified dense current, and record the amount of filler.

(5) Lift the vibrator out of the orifice, continue the construction of the upper pile section until the whole pile body vibrates, and then move the vibrator and the machine to another pile position.

(6) In the process of pile formation, each section of pile body should meet the requirements of dense current, grouting quantity and vibration residence time, and the basic parameters should be determined through on-site pile formation test.

(7) The mud drainage system should be set in advance on the construction site to introduce the mud generated in the process of pile making into the sedimentation tank. The thick mud at the bottom of the tank can be dug out regularly and sent to the pre-arranged storage place, and the clear water at the upper part of the sedimentation tank can be reused.

(8) Finally, the pile with the thickness of 1m at the top of the pile should be excavated, or compacted by rolling and dynamic compaction (repeated compaction), and the cushion should be laid and compacted.

2. Immersed sand pile (gravel pile, lime-soil pile, OG pile, low-grade pile, etc.). )

After hammering, vibrating or static pressure sinking into the hole in the foundation, the pile is sent into the pipe, and the immersed pipe is lifted (vibrated) while being sent to form a dense pile body, which forms a composite foundation with the in-situ foundation.

3. Rammed stone pile (stone pier)

Gravel (block stone) is rammed into the foundation with a heavy hammer or dynamic compaction, and the rammed pit is gradually filled with gravel (block stone) and rammed repeatedly to form a gravel pile or block stone pier.

V. Mixing method

1, high pressure jet grouting method (high pressure jet grouting method)

Under high pressure, the cement slurry is sprayed from the spray hole through the pipeline, and it directly cuts and destroys the soil while mixing with the soil, thus playing a role of partial replacement. After solidification, it becomes a mixing pile (column) and forms a composite foundation with the foundation.

This method can also be used to form retaining structures or impervious structures.

2. Deep mixing method

Deep mixing method is mainly used to reinforce saturated soft clay. It takes cement slurry and cement (or lime powder) as the main curing agents, and uses a special deep mixer to send the curing agent into the foundation soil and forcibly stir it with the soil to form a pile of (column) cement (lime) soil, which forms a composite foundation with the in-situ foundation. The physical and mechanical properties of cement-soil piles (columns) depend on solids.

A series of physical and chemical reactions between chemicals and soil. The content of curing agent, mixing uniformity and soil properties are the main factors affecting the performance of cement-soil piles (columns) and even the strength and compressibility of composite foundation.

Construction technology:

① Positioning

② slurry preparation

③ slurry replenishment

(4) drilling and shotcreting mixing

⑤ Lifting, mixing and spraying concrete.

⑥ Repeat drilling and shotcrete mixing.

⑦ Repeated lifting and stirring

⑧ When the drilling lifting speed of the mixing shaft is 0.65- 1. Om/min, mixing should be repeated.

Pet-name ruby pile is completed, clean up the clods wrapped on the mixing blades and grouting holes, and move the pile driver to another pile location for construction.

Sixth, the reinforcement method

(1) geosynthetics

Geosynthetics is a new type of geotechnical engineering material. It uses synthetic polymers, such as plastics, chemical fibers, synthetic rubber, etc. As raw materials for manufacturing various types of products, these products are placed inside, on the surface or between layers of soil to reinforce or protect the soil. Geosynthetics can be divided into geotextiles, geomembranes, special geosynthetics and composite geosynthetics.

(2) Soil nailing wall technology

Soil nails are usually set by drilling, inserting steel bars and grouting, but they are also formed by directly hitting thicker steel bars, sections and steel pipes. Soil nails contact the surrounding soil along the whole length, and form composite soil with the surrounding soil through adhesive friction on the contact interface. Soil nails are passively stressed under the condition of soil deformation. Soil is reinforced mainly by its shearing action. Soil nails generally form a certain angle with the plane, so they are called diagonal bars. Soil nailing is suitable for foundation pit support and slope reinforcement of artificial fill, cohesive soil and weakly cemented sand above groundwater level or after precipitation.

(3) Reinforced soil

Reinforced soil is a kind of soil with strong tensile strength buried in the soil layer. By using the friction and tensile strength generated by the displacement of soil particles, the soil and reinforced materials form a whole, reducing the overall deformation and enhancing the overall stability. Brace is a kind of horizontal reinforcement. Generally, strip, mesh and filament materials with high tensile strength, high friction coefficient and corrosion resistance are used, such as galvanized steel plates; Aluminum alloy, synthetic material, etc.

Seven, grouting method

It uses the principle of air pressure, water pressure or electrochemistry to inject some solidified slurry into the foundation medium or the gap between the building and the foundation. Grouting slurry can be cement slurry, cement mortar, clay slurry, clay slurry, lime slurry and various chemical slurry materials such as polyurethane, lignin and silicate. According to the purpose of grouting, it can be divided into seepage control grouting, plugging grouting, reinforcement grouting and structural rectification grouting. According to grouting methods, it can be divided into compaction grouting, osmotic grouting, split grouting and electrochemical grouting. Grouting method is widely used in various engineering fields such as water conservancy, construction, roads and bridges.

Eight, common bad foundation soil and its characteristics

1. clay

Soft clay, also known as soft soil, is short for soft cohesive soil. It was formed in the late Quaternary and belongs to marine, lagoon, valley, limnetic, drowned valley and delta facies viscous sediments or river alluvial deposits. Most of them are distributed in coastal areas, middle and lower reaches of rivers or near lakes. Common soft cohesive soils are silt and mucky soil. Physical and mechanical properties of soft soil include the following aspects:

(1) physical properties

There are many clay particles, and the plasticity index Ip is generally greater than 17, which belongs to cohesive soil. Soft clay is mostly dark gray, dark green, smelly, contains organic matter, has a high water content, generally above 40%, and silt is also above 80%. The porosity ratio is generally 1.0-2.0, in which the porosity ratio of 1.0 ~ 1.5 is called silty clay, and the porosity ratio greater than 1.5 is called silt. Because of its high clay content, high water content and large void ratio, its mechanical properties have 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, indicating that the basic value of bearing capacity is very low, generally not exceeding 70kPa, and some even only 20kPa. Soft clay, especially silt, is highly sensitive, which is also an important indicator different from ordinary clay.

Soft clay has great compressibility. The compression coefficient is greater than 0.5MPa- 1, the maximum is 45MPa- 1, and the compression index is about 0.35-0.75. Usually, soft clay layer belongs to normal consolidated soil or slightly overconsolidated soil, but some soil layers, especially newly deposited soil layers, may belong to underconsolidated soil.

Low permeability coefficient is another important characteristic of soft clay, which is generally between10-5-10-8 cm/s. If the permeability coefficient is low, the consolidation rate is slow and the effective stress increases slowly, the settlement stability is slow and the foundation strength increases slowly. This feature 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 be deformed and uneven after installation; Large deformation rate and long stability time; It has the characteristics of low permeability, high thixotropy and rheology. The commonly used foundation treatment methods are preloading method, replacement method and mixing method.

2. Miscellaneous filling

Miscellaneous fill mainly appears in some old residential areas and industrial and mining areas, and it is the garbage soil left over 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. It is difficult to describe different types of garbage soil and garbage soil piled at different times with unified strength index, compression index and permeability index.

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 easily leads to uneven settlement and usually requires foundation treatment.

3. Rinse and fill

Flushing fill is artificially deposited by hydraulic flushing. In recent years, it has been mostly used for coastal beach development and river beach reclamation. The common falling dam (also called dam filling) in northwest China is a dam made of filled soil. The foundation formed by filling soil can be regarded as a natural foundation, and its engineering properties mainly depend on the properties of filling soil. Generally speaking, filled soil foundation has the following important characteristics.

The sorting property of (1) particle deposition is obvious. Near the mud inlet, coarse particles are deposited earlier, while 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 the filled soil is high, generally greater than the liquid limit, and it is in a flowing state. After stopping filling, the surface often cracks after natural evaporation, and the water content is obviously reduced, but the lower filling is still in a flowing state when the drainage condition is poor, and the finer the filling particles are, the more obvious this phenomenon is.

(3) The early strength of the filled soil foundation is very low and its compressibility is very high, which is caused by the underconsolidation of the filled soil. With the increase of standing time, the filled foundation gradually reaches the normal consolidation state. Its engineering properties depend on particle composition, uniformity, drainage consolidation conditions and standing time after filling.

4. Saturated loose sand

Silt or fine sand foundation often has high strength under static load. However, when the vibration load (earthquake, mechanical vibration, etc. ), the saturated loose sand foundation may be liquefied or a large number of seismic subsidence deformation, or even lose its bearing capacity. This is because the loose arrangement and dislocation of soil particles reach a new balance under the action of external force, which instantly produces higher excess pore water pressure and the effective stress drops rapidly. The purpose of treating this foundation for several months is to make it more dense and eliminate the possibility of liquefaction under dynamic load. Commonly used treatment methods include extrusion method, vibroflotation method and so on.

5. Collapsible loess

Under the action of self-weight stress of the overlying soil layer, or under the action of self-weight stress and additional stress * * *, the soil that produces significant additional deformation due to the structural damage of the soil after soaking is called collapsible soil, which belongs to special soil. Some miscellaneous fills also have collapsibility. Loess widely distributed in northeast, northwest, central and east China has collapsibility. (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 has no collapsibility. In the construction of collapsible loess foundation, we must consider the possible harm of additional settlement caused by foundation collapse to the project, and choose appropriate foundation treatment methods to avoid or eliminate the harm caused by foundation collapse or a small amount of collapse.

6. Expansive soil

The mineral composition of expansive soil is montmorillonite, which has strong hydrophilicity, and its volume expands when it absorbs water, but shrinks when it loses water. This kind of expansion and contraction deformation is very large, which is easy to cause damage to buildings. Expansive soil is widely distributed in China, such as Guangxi, Yunnan, Henan, Hubei, Sichuan, Shaanxi, Hebei, Anhui, Jiangsu and other places. Expansive soil is a special kind of soil, and the common foundation treatment methods include soil replacement, soil improvement, pre-soaking and engineering measures to prevent the change of water content in foundation soil.

7. Organic soil and peat soil

When the soil contains different organic matter, it will form different organic matter soil, and when the organic matter content exceeds a certain content, it will form peat soil with different engineering characteristics. The higher the content of organic matter, the greater the impact on soil quality, mainly manifested in low strength and high compressibility, which has different effects on the mixing of different engineering materials and has adverse effects on direct engineering construction or foundation treatment.

8. Mountain foundation soil

The conditions of foundation soil in mountainous areas are complex, mainly in two aspects: uneven foundation and site stability. Due to the influence of natural environment and foundation soil formation conditions, there may be large boulders in the site, and there may also be adverse geological phenomena such as landslide, debris flow and slope collapse in the site environment. They will pose a direct or potential threat to buildings. When building buildings in mountainous areas, special attention should be paid to site environmental factors and adverse geological phenomena, and the foundation should be treated if necessary.

9. Karst (karst)

In karst (karst) areas, there are often caves or earth caves, gullies, cracks, depressions and so on. The formation and development of groundwater is caused by its erosion or subsurface erosion, which has great influence on the structure and is prone to uneven deformation, collapse and foundation collapse. Therefore, before building the structure, necessary treatment must be carried out.