1, selection points of foundation reinforcement methods
1. 1 Basic principles of foundation reinforcement method selection
In the reinforcement project, the choice of foundation reinforcement treatment scheme depends on the following aspects: (1) Functionality: it must meet the engineering purpose and requirements; (2) Practicality: The project scale, relevant parameters and technical indicators of this scheme are feasible at the current technical level; (3) Economy: Through technical and economic comparison and input-output analysis, the project cost is low and the project budget can bear it on the premise of meeting the functional requirements; After adoption, advanced technology should be adopted, the scheme should be optimized and the materials should be used reasonably. (4) Environment and safety: avoid environmental pollution or minimize project pollution; It can ensure the safety of dike structure and adjacent buildings, and ensure the safety of construction personnel.
1.2 foundation reinforcement method and selection
Selection of foundation reinforcement method: (1) First, analyze the feasibility of using natural foundation, and try to use natural foundation. (2) According to the requirements and conditions of building foundation, determine the scope and requirements of treatment. (3) The foundation treatment method must meet the design requirements of the dike for the foundation, mainly referring to the physical and mechanical indexes that should be achieved after the natural foundation treatment. (4) Considering the natural foundation conditions, treatment requirements, project cost and material sources, determine the appropriate foundation treatment method. (5) In principle, the foundation treatment method must be technically reliable, economical and reasonable, and meet the requirements of construction progress. Through analysis and comparison, one treatment method can be adopted, or a comprehensive treatment scheme consisting of two or more treatment methods can be adopted. (6) Pay attention to energy conservation and environmental protection, and avoid the pollution of ground treatment to surface water and groundwater, and the adverse effects of vibration and noise on the surrounding environment.
Comparison of several foundation reinforcement methods in 1.3
There are many methods of foundation reinforcement, but in dike reinforcement engineering, high-pressure jet grouting method, deep mixing method and vibroflotation method are more suitable, among which high-pressure jet grouting method and deep mixing method can not only be made into cutoff wall to deal with foundation leakage, but also be used for foundation reinforcement. Vibro-impact method, dynamic compaction method and drainage consolidation method can be applied to new or rebuilt dike projects. See table 1 for the characteristics and application scope of several foundation reinforcement methods.
Although several common foundation reinforcement methods can be used to reinforce embankment, they are often unreasonable in economy. According to the relevant requirements of dike construction specifications, the problem of dike body can be reconstructed manually, which can not only ensure the construction quality, but also make use of the local surplus labor force.
2. High pressure jet grouting and deep mixing reinforcement technology
The reinforcement technology of high-pressure jet grouting and deep mixing method is similar, and the main difference is that different feeding and mixing methods are adopted.
2. 1 high pressure jet grouting technology
The high-pressure jet method is to drill the hole to the designed depth with an engineering drilling rig, and then spray the solidified mud (usually cement slurry) into the surrounding soil through a special nozzle installed at the end of the drill pipe (spray bar) at the bottom of the hole with a high-pressure mud pump. At the same time, the drill pipe (spray bar) rotates and lifts at a certain speed, and the high-pressure jet destroys the soil structure in a certain range and forcibly mixes with the solidified slurry, and forms a consolidated body with certain properties and shapes in the soil after solidification.
The shape of the consolidated body is related to the moving direction of the jet. Generally, it can be divided into rotating jet, directional jet and oscillating jet. The jet grouting pile is mainly used to reinforce the foundation, improve the shear strength of the foundation and improve the deformation performance of the foundation soil, so that it will not be damaged or produce excessive deformation under the load of the superstructure. The fixed spray consolidation body is wall-shaped, and the oscillating spray forms a fan-shaped consolidation body with a large thickness. Fixed spray and swing spray are usually used for foundation seepage control to improve the hydraulic conditions of foundation soil and slope stability.
2. 1. 1 Strengthening mechanism
High-pressure jet method, such as three-tube high-pressure jet method, uses compressed air to wrap high-pressure jet water to impact and destroy the stirred soil, and at the same time uses low-pressure grouting pump to inject slurry, which is entrained by high-pressure water and gas jet and mixed with the stirred soil to form a consolidated body. The mechanism of pile, slab and wall formed by strengthening foundation can be explained by five functions: (1) The high-pressure jet cuts and destroys the soil, and the jet pressure impacts the soil in the form of pulses, resulting in the cavity in the soil structure destruction. (2) When the drill pipe rotates and lifts under the action of mixing and stirring, a gap is formed behind the jet. Under the action of jet pressure, soil particles are forced to move in the direction opposite to the direction of nozzle movement (that is, the direction with low resistance), and form a consolidated body after mixing with slurry. (3) The displacement triple tube high-pressure injection method is also called displacement method. When high-speed water jet cuts the soil, due to the introduction of compressed air, a part of the cut soil particles are discharged from the grouting hole, and the empty volume of the discharged soil particles is supplemented by the injected slurry. (4) Filling and infiltration consolidation. High-pressure slurry fills the gap between the washed undisturbed soil, and the separated water is consolidated, or it can penetrate into the sand layer with a certain thickness to form a consolidated body. (5) Compaction: In the process of cutting broken soil, high-pressure jet still has residual pressure on the edge of the broken zone, which can have a certain compaction effect on the soil layer, so that the compressive strength of the edge part of the high-pressure jet pile is higher than that of the central part.
2. 1.2 basic types
According to the injected medium and its pipeline, it can be divided into single-tube method, two-tube method and three-tube method.
(1) Single-pipe jet grouting method uses high-pressure slurry (20-30MPa) to jet cut the soil through a single pipe with a pile diameter of 40-50cm. Its reinforcement quality is good, the construction speed is fast and the cost is low, but the diameter of the added solid is small.
(2)- Pipeline rotary spraying method. On the basis of single pipe method, compressed air is added, and double channels and double grouting pipe are adopted. There is a coaxial double nozzle at the bottom of the pipeline. High-pressure slurry is ejected from the inner nozzle at a high speed at a pressure of about 20MPa, and compressed air of about 0.7MPa is ejected from the periphery of the jet. In the soil, a columnar consolidation body with a significantly increased diameter is formed, reaching 80 ~ 150 cm.
(3) Three-tube jet grouting method. Three grouting pipes are used to transport water, gas and mud respectively. The high-pressure water jet and the surrounding air jet cut the soil in a coaxial way. Adding a cylindrical air jet around the nozzle of high-pressure water jet for coaxial injection of water and air can reduce the friction between water jet and surrounding media, avoid premature atomization of water jet and enhance the cutting ability of water jet. When the nozzle rotates to spray and lift, a large negative pressure area is formed in the foundation, and at the same time, the pressed slurry is carried to fill the gap, so that a consolidated body with larger diameter and higher strength is formed in the foundation, which plays a role in strengthening the foundation.
2. 1.3 size
Cement is the basic material of jet grouting, and cement slurry can be divided into the following types; Ordinary slurry (1) generally adopts ordinary portland cement without adding any additives, and the water-cement ratio is generally 0.8:1~1.5:1,and the compressive strength (28ct) of the consolidated body can reach 1.0 ~ 20 MPa, which is suitable for. (2) The rapid setting and early strength type is suitable for projects with high groundwater level or requiring early bearing, and calcium chloride and triethanolamine should be added to the cement slurry. The compressive strength of cement-soil consolidated body mixed with calcium chloride is 65438±0.6 MPa, and it is 2.4MPa after mixing with 4% calcium chloride. (3) The average compressive strength of the high-strength jet consolidated body is above 20MPa. You can choose high-grade cement, or you can choose a compound formula composed of high-efficiency diffusion agent and inorganic salt.
When 2-4% sodium silicate is added into cement slurry, its impermeability is obviously improved. If the project aims at impermeability, it is best to use "soft materials". 10-50% bentonite (area ratio of cement weight) can be added to the cement slurry. Slag cement should not be used at this time. If there is only impermeability requirement and no frost resistance, pozzolanic cement can be used.
2. 1.4 high-pressure jet grouting technology
The spraying range should be determined by field test. The consolidation range of high-pressure jet grouting is closely related to the type and compactness of soil, and the consolidation size formed by different jet types and jet methods is also different. The injection energy of fixed injection is concentrated and the injection range is large. The consolidation strength of jet grouting cohesive soil is 0.3 ~ 6.0 MPa, and the cohesionless soil consolidation strength is 4 ~ 15 MPa.
For anti-seepage engineering, fixed spray and swing spray are often used, and swing spray or rotary spray is often used when the particle size of the stratum is coarse. For the complex stratum with treatment depth greater than 20m, it is best to use double rows or three rows of holes to make the high-pressure jet grouting pile form a water plugging curtain. The hole spacing should be 1.73R(R is the consolidation radius of jet grouting), and the row spacing is the most economical. Generally, the hole spacing of fixed spray and oscillating spray is 1.2 ~ 2.5m, and the hole spacing of rotary spray is 0.8 ~1.2m. The anti-seepage effect of high-pressure jet grouting can generally reach 10? ~ 10? Cm/sec.
The pile spacing of high-pressure jet grouting pile should be determined according to the load of superstructure, bearing capacity of single pile and soil conditions. Generally, the spacing between piles is s = (3 ~ 4) d (d is the diameter of jet grouting pile), and the arrangement of piles can be rectangular or quincunx.
The purpose of high-pressure jet grouting drilling is to insert the grouting pipe into the predetermined soil layer and carry out jet operation from bottom to top. Recently, there is also a method of vibroflotation drilling and direct injection. Attention should be paid to the following matters when spraying: (1) When the grouting depth is large, it is easy to cause the consolidation body to be thicker on the top and thinner on the bottom, which will affect the bearing capacity or impermeability of the consolidation body, and measures such as increasing the pressure and flow rate or reducing the rotation and lifting speed should be taken to remedy it; (2) When it is found that the grouting quantity is insufficient and affects the engineering quality, the re-injection technology can be adopted; (3) When the grouting amount is more than 20% of the grouting amount, measures such as increasing the injection pressure, reducing the nozzle diameter, accelerating the lifting speed and rotating speed can be taken to recover the spilled slurry. (4) Adjust the injection pressure and grouting quantity according to the engineering needs, change the moving direction and speed of the nozzle, and control the shapes of the jet consolidated body, namely, disc shape, cylinder shape, large bottom shape, sugar paste reed shape, big hat shape and wall shape. (5) The slurry after sprinkler irrigation is divided into water, which may cause a cavity at the top of the consolidated body, which is not conducive to foundation reinforcement and seepage prevention. Therefore, measures such as static pressure grouting or adding expansive materials to the slurry can be taken to prevent it.
High-pressure pump is the key equipment in high-pressure jet grouting, and the pressure and flow rate are required to be adjustable within a certain range. The rated flow is 85 ~ l50l/min；; The rated pressure is 2o ~ 50mpa.
2. 1.5 responsibility inspection of high-pressure shotcrete
(1) Excavation inspection: After the slurry has a certain strength, it can be excavated to inspect the verticality, shape and quality of the consolidated body; (2) Borehole inspection: drill the core from the consolidated body and conduct indoor physical and mechanical properties test. Do water pressure or pumping test in the borehole to determine its impermeability; (3) Standard penetration test: The standard penetration test can be carried out in the middle of the jet grouting consolidated body.
(4) Load test: Static load test can be divided into vertical static load test and horizontal static load test. During the test, it is necessary to pour a concrete layer with a thickness of 0.2 ~ 0.3m at the stressed part. (5) Surrounding well test: add spray holes on one side of the slab wall to form a closed surrounding well with the slab wall, and conduct two tests of water pressure and pumping in the well, or observe the water level inside and outside the well, which is mostly used to check the anti-seepage effect.
High-pressure jet grouting technology is mainly suitable for Quaternary alluvium, eluvial layer and artificial fill. For sandy soil, cohesive soil, loess and silt, reinforcement can be carried out. However, humus soil with too large gravel diameter, too much content and a lot of fibers has a slightly worse spraying quality, sometimes even worse than static pressure grouting.
High-pressure jet grouting method is not suitable for karst areas without fillers, frozen soil and foundations with serious corrosion to cement, and the velocity of groundwater is too high, so the sprayed slurry cannot condense around the grouting pipe.
2.2 Characteristics of high-pressure jet grouting
High-pressure jet grouting method has the advantages of low cost, fast construction speed, high strength of consolidated body and high reliability. Compared with ordinary grouting method, it has the following characteristics.
High-pressure jet grouting method is to use high-speed water flow to forcibly destroy the soil to form a consolidated body, and there is generally no grouting problem in the overburden; At the same time, because the high-speed jet is limited in the range of soil fragmentation, the slurry is not easy to run off, which can ensure the expected reinforcement range and control the shape of the added solid; It can be constructed in any part of the hole, and can also be sprayed at the bottom or middle of the hole. In addition, it can also be sprayed horizontally and obliquely. High-pressure jet grouting method usually uses cement slurry, which will not cause environmental and groundwater pollution and has good durability; The construction noise is low, and the single-pipe and two-pipe construction methods are relatively simple.
3. Deep mixing technology
Deep mixing method takes cement as curing agent, and through special deep mixing machinery, soft soil and cement (slurry or powder) J are stirred in situ in deep foundation, and cement and soft soil seeds produce a series of physical and chemical reactions, so that soft soil is hardened and modified. The strength of modified soft soil is much higher than natural strength, and its compressibility and permeability are much lower than natural soft soil.
3. 1 reinforcement mechanism
The basic principle of mechanical mixing reinforcement of soft soil and cement is based on the physical and chemical reaction process of cement-reinforced soil. The water content in soft soil decreases, the cohesive force between particles increases, and the strength and sufficient water stability of cement-soil increase. In cement-reinforced soil, due to the small amount of cement, it generally accounts for 10- 15% of the weight of the reinforced soil. The hydration reaction of cement is completely carried out around the active medium-soil, so the hardening speed is slow and the function is complicated.
3.2 Main characteristics of cement-soil
3.2. 1 physical properties
The bulk density of cement-soil is similar to that of natural soil, but its specific gravity is slightly larger than that of natural soil.
3.2.2 Unconfined compressive strength
The unconfined compressive strength of cement-soil is generally 30o ~ 400 kPa, which is several tens to hundreds times larger than that of natural soft soil. However, there are many factors that affect the unconfined compressive strength of cement soil, such as cement content, age, cement grade, soil water content and organic matter content, and additives.
In order to reduce the project cost, the measures of adding fly ash can be adopted. The strength of cement soil mixed with fly ash is generally higher than that of cement soil without fly ash. The strength of cement-soil with different cement mixing ratios is increased by 10% when adding the same amount of fly ash as cement, which shows that the deep mixing method with fly ash can not only consume industrial waste, but also improve the strength of cement-soil
3.3 Construction Technology
3.3. 1 reinforcement type
According to the current construction technology of deep mixing method, mixing piles can be arranged in column, wall and block types. Pile type is mainly used for foundation reinforcement of dikes, while wall type is used for seepage control reinforcement. The wall type is a reinforced wall type formed by overlapping adjacent mixing piles, which constitutes a cement-soil retaining wall. This kind of retaining wall has high impermeability and can form a good waterproof curtain.
construction technique
(1) wet construction. The main construction machinery is the deep mixer. The construction of deep mixing method can be mainly divided into several steps, such as positioning, ready-mixed sinking, preparing cement slurry, lifting shotcrete mixing, repeatedly stirring up and down, cleaning and so on.
(2) Dry construction. Dry method is to use cement powder, which is transported by air, sprayed through the gap generated by the rotation of mixing blades, and evenly distributed on the surface of the whole gap track with the rotation of mixing blades, and then mixed with in-situ foundation soil. Construction machinery mainly includes drilling rig, powder feeder, air compressor, mixing bit, etc.
The construction technology mainly includes: (1) column alignment, (2) running-(3) completion of drilling, (4) lifting for dust removal, and (5) lifting for pile formation.
3.4 Scope of application
Deep mixing method is most suitable for strengthening saturated soft clay from various sources, and is often used to strengthen silt, muddy soil, clay, loam and other geology. Multi-head small-diameter piles are used, and the pile depth can reach 30m and the wall depth can reach 18m.
In the dike reinforcement project, the deep mixing pile is suitable for treating the landslide section on the soft foundation dike. At the same time, it can also form an impervious wall to achieve better impervious effect.
3.5 Main advantages of deep mixing method
3.5. 1 Good reinforcement effect, flexible reinforcement method and wide application.
Deep mixing method can adopt different reinforcement types, different pile lengths and replacement rates to meet the reinforcement purposes of different soil conditions and different load requirements. It is suitable for projects with long and narrow rivers, complex geological conditions and high settlement requirements. Using mixing pile retaining wall as river bank slope support can not only ensure slope stability, but also have seepage control function.
3.5.2 Fast construction speed
Generally speaking, the efficiency of each deep mixer to build a mixing pile cutoff wall is 13.2m? /Taiwan Province, time.
3.5.3 The original soft soil can be fully utilized, and there is no problem of soil abandonment.
Deep mixing method is an in-situ reinforcement technology, which can make full use of undisturbed soil and there is no problem of soil abandonment.
4, the main application engineering introduction
4. 1 Foundation Treatment Project of Xu Jingang Subsidence Section in Fengtai County
In the flood season of 2005, the Jingangwei burst due to high water retaining level, unstable foundation and sliding dike foundation. After geological investigation, it is found that there is a silt layer under the embankment foundation in this section. The thickness of silt layer is greater than L 1m, and the bearing capacity of foundation is small. In view of the thick silt layer in this section, it is not suitable to dig out and change soil, and the foundation at the platform on the water side of the dam back is designed to be strengthened. Design pile spacing 1.0m, driving distance 1.2m, pile diameter 0.5m, pile bottom entering bearing stratum greater than 1m, design bearing capacity of composite foundation 120kpa. After the implementation of the project in 2OO6, in case of the flood in 2007, the foundation of the embankment section with similar geological conditions adjacent to this section slipped, resulting in the dam breach, and the dam reinforcement section remained as strong as ever.
4.2 Foundation Reinforcement Project of Xudahuxu Collapse Section in Fengtai County
The collapsed section of Xudahuwei is located on the old river, with a silt layer of 6-7m below the dike and a dike length of 120m. During 20 12 earthwork reinforcement, due to the height of the embankment, the deep foundation slipped and the embankment sank nearly 3 meters. Because there are 35KV high-voltage transmission lines above the dam in this section, it is unsafe to use cement jet grouting pile for construction machinery, so high-pressure cement jet grouting pile is used. The designed pile spacing is 1.5m, and the row spacing is1.5m. The plum blossom layout is adopted, with a pile diameter of 0.8m, a pile tip depth of 2m and a pile length of 8.5m. The designed bearing capacity of composite foundation is 130Kpa. After the pile foundation is reinforced, the dam is filled with earthwork, which is running well at present.
5. Conclusion
The main purpose of embankment foundation reinforcement is to improve the bearing capacity and impermeability of foundation. Generally, it is suitable for geological conditions with thick soft layer on the foundation. If the soft layer is thin, it is more economical to dig silt for soil replacement. The materials of dry jet grouting pile and rotary jet grouting pile should generally use ordinary portland cement, and composite portland cement can also be used for seepage control requirements. When the water content of silt layer and soft foundation soil is large, the effect of dry jet grouting pile is better than that of rotary jet grouting pile. The jet grouting pile is easy to break due to the large amount of slurry leakage and leakage, and the grouting pressure is not easy to master.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd