In the design of soft soil foundation treatment, better soil layer should be used as the bearing layer; The interaction between superstructure and foundation should be considered; The building shape, load, structural type and geological conditions are comprehensively analyzed, and then the foundation treatment method is determined. See Table 4-3 for commonly used foundation treatment methods and principles in soft soil areas.
Table 4-3 Common Foundation Treatment Methods in Soft Soil Area
(A) Drainage consolidation method
According to the principle of effective stress, due to the action of consolidation pressure, pore water in soil is discharged, pore water pressure gradually dissipates, effective stress increases and soil strength increases. According to this principle, the drainage consolidation method has accumulated many successful and mature experiences for many years. The natural bearing capacity of soft soil foundation is very low, and the permeability coefficient is often less than10-7 cm/s. Even if the whole building is unstable, its settlement deformation will last for a long time, and the settlement may develop to affect the normal use of the building and even cause structural damage. For projects with a long construction period, the method of preloading in stages is often adopted to make the increase of foundation strength adapt to the increase of additional load, that is, under the action of a certain level of load, the foundation strength increases to a certain level before the next level of load is applied, which is often used to strengthen the foundation in earth dams, large oil tanks and other projects. Because the time required for consolidation and drainage of soil is proportional to the square of the length of seepage path, the time required for preloading and consolidation can be greatly shortened by setting connected drainage bodies in the foundation. Therefore, manhole drainage body is usually set in the foundation before preloading.
This reinforcement method has a long history and a large number of engineering examples. Compared with other reinforcement methods, its calculation theory and construction technology are relatively mature. It must be emphasized that the key of drainage reinforcement method is preloading load, and the magnitude and duration of preloading load are the main factors that determine the growth of foundation strength.
The design of drainage consolidation method includes loading system and drainage system. Only the drainage system has no loading system, pore water cannot be automatically removed without pressure difference, and the foundation cannot be reinforced. On the other hand, if there is only a loading system without a drainage system, the drainage distance cannot be shortened and the foundation strength cannot be improved during preloading. Therefore, in the design of drainage pressurization, drainage system and loading system must be considered together.
1. Drainage system
Setting drainage system is mainly to change the drainage boundary conditions of foundation, increase the drainage channels of pore water and shorten the drainage distance.
The drainage system consists of horizontal drainage body and vertical drainage body. The horizontal drainage body is generally a sand cushion, with a thickness of 30 ~ 100 cm as required. It is advisable to use medium-coarse sand with good gradation, and the permeability coefficient is about10-3 cm/s. Plastic drainage boards and permeable filter cloth can also be used. Vertical drainage body, commonly used manhole, bagged manhole and plastic drainage plate.
(1) manhole
The design of manhole includes selecting suitable manhole length, diameter and spacing, as well as the materials needed for manhole drainage system and the thickness of sand cushion.
The diameter and spacing of manholes mainly depend on the consolidation characteristics of cohesive soil and the requirements of preloading time. The commonly used well diameter in engineering is 20 ~ 30cm, and the commonly used well spacing is 6 ~ 9 times of the manhole diameter. The length of manhole depends on the thickness, load and construction period of soft soil layer. For stability control projects, such as embankments and bank slopes, the depth of manholes should be determined through stability analysis, and the length of manholes should be greater than the depth of the most dangerous sliding surface.
Manholes can be arranged in an equilateral triangle or square on the plane. When manholes are arranged in a square shape, the effective drainage range of manholes is square; When the manhole is arranged in an equilateral triangle, the effective drainage range of the manhole is a regular hexagon. In the actual consolidation calculation, Barron suggested that the influence range of each manhole should be generalized into an equivalent circle to solve it. The relationship between the diameter (de) of the equivalent circle and the spacing (L) of manholes is as follows:
When arranging equilateral triangles, the calculation formula is
Geological disaster prevention and control technology
In the formula, the symbol has the same meaning as before.
Square arrangement, the calculation formula is
Geological disaster prevention and control technology
In the formula, the symbol has the same meaning as before.
In practical engineering, equilateral triangle layout is often used.
In the design of manhole, generally, the degree of consolidation is calculated by selecting the length, diameter and spacing, and then the settlement is calculated by using the obtained degree of consolidation. If it does not meet the requirements of the superstructure, it is necessary to re-select the size of manhole and recalculate it until it meets the requirements.
Water flushing method is often used in sand drain construction. This method uses a special nozzle to wash the hole with water and then fill the hole with sand. Sometimes, vibration sinking method is also used. This method is to make the casing sink to the design depth by vibration with a pipe shoe or a flap, fill with sand, and then pull out the casing while vibrating.
(2) Bagged manhole
According to the consolidation theory of sand drain, shortening the spacing is better than increasing the well diameter, and the principle of "thin and dense" should be adopted to arrange the vertical drainage body. At the same time, in soft soil with high water content, inspection wells are prone to necking, neck breaking, arching or dislocation. Therefore, bagged manholes with low material consumption, good continuity and simple construction are widely used. Bagged sand drain is to pack sand into woven bags with good water permeability, and the diameter is generally 7 ~ 12 cm. At present, the widely used bag-making material in China is polypropylene woven cloth.
Duct vibrating machinery is often used in the construction of bagged sand drain, and the construction procedures include positioning, sorting pile tips, sinking pipes, putting sand bags into ducts, drawing pipes and shifting.
(3) Plastic drain board
With the rapid development of plastic industry, the application of plastic drainage board has become quite common. The plastic drainage board is characterized by large single-hole cross section, smooth drainage, light weight and good durability.
The width of plastic drainage board is usually about 10cm, and the thickness is several millimeters, including core board and filter membrane. Core plate is a drainage channel, which has two types: groove type and porous type. The grooves are mostly polypropylene or polyethylene plastic core plates; Polyester nonwoven fabric is mostly used in porous, can be rolled into a tube, and is convenient to use, so it is widely used. The filter membrane is generally lined with polyester.
Plastic drainage board and manhole are vertical drainage bodies, and the reinforcement principle is the same, so the consolidation theory and design method of manhole foundation can be adopted.
The plastic drainage board is constructed by a special board inserting machine, and the drainage board is sent to the design depth by a conduit. There are vibration method, hammer method and hydraulic method for catheter sinking. There is a pile tip at the bottom of the conduit, which is used to prevent mud from entering the conduit when sinking the pipe, and to anchor the plastic drainage plate when pulling out the pipe to prevent the drainage plate from being taken out.
2. Pressurization system
Supercharging system refers to the preloading load applied to the foundation, which increases the consolidation pressure of the foundation soil and produces consolidation. Its material is solid (soil and stone, etc.). ), liquid (water, etc. ), vacuum negative pressure load, etc.
(1) pressurization method
Generally, the pressurization method is selected according to the purpose of preloading. If the purpose of preloading is to reduce the settlement of buildings, preloading should be adopted before the construction of buildings to make the foundation settle; If the main purpose of preloading is to increase the foundation strength, self-weight pressurization can be adopted, that is, to slow down the construction speed or increase the drainage rate of soil, so that the increase of foundation strength can adapt to the increase of building load.
(2) Design of supercharging system
The pressurization system of surcharge preloading method mainly refers to the scheme of surcharge preloading and the selection of surcharge materials. According to the different stacking materials, preloading can be divided into self-weight preloading, load preloading and water preloading. Bulk materials such as fillers, sand and gravel are usually used for stacking; Oil tanks usually preload the foundation by injecting water into the tank; For projects with stability as the control, such as dams, they will be loaded step by step in a controlled manner according to their own weight until the design elevation. In soft soil area, surcharge preloading must be carried out step by step, and the next load can only be added when the foundation strength under the current first load is increased enough to add the next load. According to the semi-empirical formula of Cape Town's ultimate load, the first allowable load can be preliminarily estimated.
The design content of vacuum preloading pressurization system mainly includes: the vacuum degree in the sealing film, the average consolidation degree needed to strengthen the soil layer and the construction technology design. Generally, it is required that the vacuum degree in the sealing film should be kept above 650mmHg stably and evenly distributed. The average consolidation degree of soil layer within the depth of shaft should be greater than 90%. In architectural design, the division of reinforcement area is a very important link. Practice shows that the area of each reinforcement part should be large rather than small. At present, the area of a single vacuum preloading area in China has reached 3× 104 m2. However, due to the limitation of construction capacity or site conditions, it is necessary to divide the site into several reinforcement areas and reinforce them in stages. The following factors should be considered when dividing the area: ① Ensure that each building is located in a reinforced area, and the building sideline is far away from the effective sideline of the reinforced area. (2) should be based on meeting the requirements of the construction period, the general reinforcement area is 6000 ~10000m2; (3) The distance between reinforcement areas should be as small as possible, or closed ditches should be adopted.
(2) Well point dewatering preloading
The effect of preloading can be achieved by reducing the groundwater level of soft soil layer and increasing the self-weight pressure of soil. Applicable to the following situations.
1) When there is no preloading load, it can be considered. Because it is characterized by using self-weight preloading.
2) When the groundwater level is close to the surface, it is more suitable for soft soil with high water content and low plasticity. When there is still a certain thickness of soft soil below the precipitation range, preloading and consolidation are needed.
3) Because the soil will not be destroyed during preloading, there is no need to control the loading rate.
If this method is combined with sand drain, the reinforcement effect of deep soil will be better.
Because of the low permeability coefficient of soft clay, small and dense well point system must be adopted, and sometimes electro-osmosis is needed to accelerate seepage and prevent siltation, but the cost is high and it is rarely used in China. Well-point dewatering preloading is a technique for stabilizing slopes or for underground engineering construction after well-point drainage. Generally, it is mostly used for medium and low plastic clay. This method is not applicable when the permeability coefficient of soil layer is lower than1.0×10-6 m/s.
(3) Deep mixing pile
Deep mixing pile is a composite foundation composed of lime slurry or cement slurry mixed with soft soil to strengthen the foundation. 1975, Japan successfully developed mixing construction machinery, which was used in water construction and later widely used in Japan. In the early 1980s, SJB- 1 deep mixer was developed in China, and it was used in reinforcement engineering practice in Nanjing, Tianjin, Lianyungang and Fuzhou, and achieved satisfactory results.
The construction machinery consists of motor set, mixing series, slurry supply equipment and crane (Figure 4- 14). The construction procedure is as follows:
1) Lift the mixer to the predetermined hole position, start the motor and adjust it to normal speed.
Fig. 4- 14SJB- 1 deep mixer (unit: mm)
2) spudding, that is, cutting into soft soil when the mixing head slowly sinks to the expected depth.
3) Prepare cement slurry or lime slurry, press it into the hole through the slurry conveying pipe, and gradually inject and stir it under the push of the mixer.
4) The mixing head sinks again, is lifted once, and is stirred once again, so that grouting is stopped, and the reinforcing agent and the soft soil are evenly mixed to form a pile.
The curing agent can be lime or cement. Cement slurry is often used, with a general cement content of about 10% and a water cement ratio of 0.45 ~ 0.50, and a small amount of gypsum, triethanolamine and water reducing agent are added.
Another feature of deep mixing piles is that they can be used to reinforce deep layers. In Japan, it is usually used to strengthen the soft layer at the bottom of the bay, and also used to build underground continuous walls. The mixing depth abroad can reach more than 25m, but in China, due to the limitation of construction machinery, the maximum depth is generally around 10m, which leads to many mixing piles becoming friction piles because they cannot penetrate soft soil. Therefore, the result of single pile test depends not only on the strength of the pile, but also on the friction around the pile to a greater extent. The mechanical properties of soil around the pile and the length of the pile become extremely important factors.
In addition, for the treatment of soft soil foundation, jet grouting pile reinforcement and anchoring technology are often used, and relevant specifications can be referred to when using it.