Cushion method is to dig out part or all of the collapsible loess under the foundation, and then tamp it with plain soil or lime soil layer by layer to make cushion, so as to eliminate part or all of the collapsible amount of the foundation, reduce the compressive deformation of the foundation and improve the bearing capacity of the foundation, which can be divided into local cushion and integral cushion. When only the collapsible loess under the basement 1~3m needs to be removed, local or whole soil cushion should be used for treatment. When it is required to improve the bearing capacity of cushion soil or enhance the water stability at the same time, local or whole lime-soil cushion should be used for treatment.
The design of cushion mainly includes the thickness and width of cushion, the compaction coefficient after compaction and the determination of design value of bearing capacity. The principle of cushion design is not only to meet the requirements of buildings for foundation deformation and stability, but also to meet the requirements of economy and rationality. At the same time, the following aspects should also be considered:
1. The width of the local soil cushion is less than the width of the bottom edge of the foundation. After foundation treatment, surface water and pipeline leakage may still penetrate into the untreated collapsible soil layer from the cushion layer, resulting in collapse. Therefore, the local soil cushion is not considered to be waterproof, and the foundation is likely to be soaked in water, which requires anti-seepage. It is not allowed to treat the foundation with local soil cushion.
2. The plane treatment range of the whole cushion layer, each side exceeds the width of the outer edge of the building external wall foundation and should not be less than the thickness of the cushion layer, that is, it should not be less than 2m.
3. In the self-weight collapsible loess site where the groundwater level is impossible to rise, it is more suitable to use all-soil cushion to treat the foundation for buildings with high possibility of foundation wetting or strict waterproof requirements, while the overall collapse of the foundation has not been eliminated. However, in the self-weight collapsible loess site where the groundwater level is likely to rise, the possibility of collapse of the untreated collapsible soil layer below should be considered after the groundwater level rises.
Second, the heavy hammer surface compaction and dynamic compaction
Heavy hammer surface compaction is suitable for treating collapsible loess foundation with saturation less than 60%. Generally, a weight of 2.5~3.0t, with a drop distance of 4.0~4.5m, can eliminate the collapsibility of 1.2 ~ 1.8m loess layer below the basement. In the range of compacted layer, the physical and mechanical properties of soil are significantly improved, the average dry density is significantly increased, the compressibility is reduced, the collapsibility is eliminated, the water permeability is weakened and the bearing capacity is improved. Non-gravity collapsible loess foundation has a large initial collapse pressure. Using heavy hammer to treat part of collapsible loess layer can reduce or even eliminate the collapsible deformation of loess foundation. Therefore, the advantages of heavy hammer compaction in non-gravity collapsible loess site are obvious.
Generally speaking, the mechanism of dynamic consolidation of foundation is to use a heavy hammer with a certain weight to impact the vibrating foundation within a certain distance, so as to increase the degree of compaction, improve the vibration liquefaction conditions of soil and eliminate the collapsibility of collapsible loess. The process of dynamic compaction is to instantly exert huge impact energy on foundation soil, causing a series of physical changes of soil, such as the destruction of soil structure or drainage consolidation, compaction, thixotropic recovery and so on. As a result, the foundation strength is improved and the pores are compacted in a certain range.
Single-point dynamic compaction is to exert a comprehensive effect on the foundation through repeated huge impact energy and accompanying compression waves, shear waves and Rayleigh waves, so that the soil can be loaded instantly, and the loaded tension and pressure can be used alternately, so that the original contact form between soil particles can be changed rapidly, resulting in displacement, and the process of soil compression-compaction can be completed. Although the cohesion of reinforced soil is reduced due to damage or disturbance, its original cohesion is greatly improved with the increase of soil density. Single-point dynamic compaction is shown in figure 1, and a compaction core is formed under the rammer, which is approximately parabolic. The maximum thickness of the compacted core is close to the radius of the rammer, the soil is in the form of a thousand layers of cakes, and the dry density is greater than 1.85g/cm3.
Third, the compaction pile method
Compaction pile method is suitable for treating collapsible loess foundation above groundwater level. During construction, pile holes shall be arranged and drilled on the foundation surface according to the design scheme, and then the prepared plain soil (silty clay or silty soil) or lime soil shall be filled into the pile holes in layers according to the optimal water content, and compacted (rammed) in layers to the design elevation. Through the lateral extrusion in the process of hole-forming or pile compaction, the soil between piles can be compacted, thus forming a composite foundation. It is worth noting that coarse sand, stone or other permeable materials shall not be used to fill the pile hole.
Lime-soil compaction pile and soil pile foundation are generally suitable for collapsible loess, artificial loess and artificial fill with water content 14% ~ 22% above the groundwater level, and the treatment depth can reach 5 ~10m. Lime-soil compaction pile is to form a pile hole in the soil by hammering or vibrating sinking pipe, and then fill the pile hole in layers with fillers such as plain soil or lime soil. In the process of pore-forming and tamping packing, all the soil originally located in the pile hole is squeezed into the surrounding soil. Through this compaction process, the collapsibility of soil layer is completely changed and its bearing capacity is improved. Its main mechanism is divided into two parts:
(1) Mechanical piling, horizontal compaction of soil layer, improvement of physical and mechanical properties of soil.
When the soil is squeezed to form a hole, the original soil in the pile hole is forced to extrude laterally, so that the soil around the pile is squeezed, disturbed and remolded, so that the void ratio of the soil around the pile is reduced and the gas in the soil overflows, thereby increasing the compactness of the soil, reducing the compressibility of the soil and improving the bearing capacity of the soil. The compaction range of soil decreases from the edge of the pile hole to the periphery, and the dry density of soil near the hole wall can approach or exceed the maximum dry density, that is to say, the compaction coefficient can approach or exceed 1.0, and its compaction influence radius is usually 1.5 ~ 2d (d is the diameter of compaction pile), and gradually outward, the dry density gradually decreases until the natural dry density of soil, which proves that the pipe is immersed.
(2) Lime-soil piles and compacted soil between piles together form a composite foundation.
When the upper load is transmitted through it, because they can adapt to each other's deformation, the stress can be effectively and uniformly diffused, and the foundation stress spreads quickly, and the additional stress has been greatly attenuated below the reinforcement depth, so there is no need for a solid underlying layer.
The pile diameter should be 300~450mm, which can be determined according to the selected pore-forming equipment or method;
The pile spacing can be 2.0~2.5 times of the pile diameter;
Above the elevation of pile top, 2: 8 lime soil with a thickness of 300~500mm shall be set, and its compaction coefficient shall not be less than 0.95;
Characteristic value of bearing capacity of lime-soil compaction pile and soil compaction pile composite foundation: According to Technical Specification for Building Foundation Treatment (JGJ79-2002), it should be determined by field load test of single pile or multi-pile composite foundation. When there is no test data, the preliminary design can be determined according to local experience, but the characteristic value of bearing capacity of lime-soil compaction pile composite foundation should not exceed 2 times before treatment, and the characteristic value of bearing capacity of soil compaction pile composite foundation should not be greater than 1.4 times and 180kpa before treatment.
Static load test can be used to determine the bearing capacity of single pile and soil between piles, and can also be used to determine the bearing capacity of single pile composite foundation or multi-pile composite foundation. When no-load test is adopted, the bearing capacity of soil between piles can be determined by static preliminary study.
It is not necessarily feasible to determine the bearing capacity of piles through static preliminary research, especially for lime-soil filled piles, but dynamic sounding can be used.
The treated composite foundation shall be subjected to load test according to the requirements of Appendix A of Technical Specification for Building Foundation Treatment (JGJ79-2202).
For high-rise buildings or important construction projects, the bearing capacity eigenvalue and deformation modulus of the treated composite foundation should be determined by load test as far as possible, which is not only safe and reliable, but also not limited by the bearing capacity eigenvalue in the code, which broadens the application scope of soil compaction piles and lime-soil compaction pile foundations.
When the buried depth of the foundation is more than 0.5m, the characteristic value of the treated foundation bearing capacity can be calculated according to relevant specifications, and the depth correction coefficient is 1.0, and the width is not corrected, that is, Fa=Fak+0+ 1.0*γm *(d-0.5).
Engineering data show that the characteristic value of bearing capacity of lime-soil compaction pile foundation has exceeded 400kpa, which broadens the application scope of lime-soil pile.
With the expansion of the application scope of lime-soil piles, some methods do not have compaction effect on the soil between piles, and the soil used is not limited to loess and fill. In this case, a theoretical calculation method is needed, and the formula for calculating the bearing capacity of composite foundation can be established according to its action mechanism:
( 1)、Fspk =(k 1 * Fpk * Ap+K2 * Fsk * As)/A
Where: fspk-characteristic value of bearing capacity of composite foundation (kpa)
FPK-Characteristic value of bearing capacity of soil pile or lime-soil pile (kpa)
Fsk——Characteristic value of bearing capacity of natural soil foundation (kpa)
A-effective reinforcement area (m2), A=Ap+As.
AP—— Cross-sectional area of soil pile or lime-soil pile (m2)
As—— Compressive area of soil between piles (m2)
K1-coefficient related to different pile diameters and different soil qualities of soil piles or lime-soil piles. For general cohesive soil and miscellaneous fill with void ratio not greater than 1.3 and liquid index not greater than 1, K 1 (table omitted) can be found.
K2 is the ratio of the characteristic value of bearing capacity when the settlement after compaction is 10 mm to the bearing capacity of foundation before compaction when the settlement under compression is 10mm, or K2= 1.0.
(2) If the characteristic values of bearing capacity Fpk and deformation modulus Eop of piles, bearing capacity characteristic values Fsk and deformation modulus Eos of soil between piles (generally based on in-situ foundation) and the replacement rate m of piles in treated foundation are known, the characteristic values of bearing capacity of composite foundation can be calculated by the following formula:
Fspk=m*Fpk+( 1-m)Fsk
E0sp=m*Eop+( 1-m)Eos
Generally speaking, the calculation result of the above formula is safe. Except for a few items, the design value is higher than the measured value.
(3) If the pile-soil stress ratio is known, the characteristic value of composite foundation bearing capacity can also be calculated by the following formula:
fspk = m * n * Fsk+( 1-m)Fsk =[ 1+m(n- 1)]Fsk = Fsk/Us
Where: n-pile-soil stress ratio
Us- stress diffusion coefficient, us =1[1+m (n-1)]
(4), the bearing capacity of composite foundation can also be calculated according to the stiffness:
Fspk*A=Fpk*Ap+Fsk*As
The symbols in the formula have the same meanings as above.
Construction: The pipe sinking method (vibration, hammering) or impact should be selected according to the design requirements, pore-forming equipment, site soil quality and surrounding environment.
Quality inspection: when lime-soil compaction pile and soil compaction pile foundation are completed and accepted, the bearing capacity of composite foundation load test should be adopted.
Generally speaking, compaction piles can be arranged in equilateral triangles, which can achieve the effect of uniform compaction. Each pile has a certain compaction effect on the surrounding soil, even if a small part of the soil between piles is not compacted, because there is a stable boundary around it that will not collapse. The pile and the compacted soil around it form a composite foundation to jointly bear the upper load. It can be said that the collapsibility of soil has been completely eliminated within the length of compaction pile. The treated foundation is integrated with the superstructure, even if the soil below the pile bottom has settlement deformation, it is small and uniform, and will not pose a threat to the superstructure. The pile spacing directly affects the soil squeezing effect and is closely related to the economy of engineering construction.
Fourth, the pile foundation
Pile foundation is neither natural foundation nor artificial foundation, which belongs to the category of foundation. It is a pile that transfers the upper load to the soil (or rock) layer below the pile side and bottom, and adopts non-extrusion methods such as digging and drilling. The properties of soil around the pile hole are not improved when the soil is discharged from the hole during the hole-forming process. However, after the soil around the pile is soaked in water, the lateral resistance of the pile is greatly reduced or even disappeared. When the soil around the pile collapses under the action of self-weight, the positive friction on the pile side is quickly transformed into negative friction. Therefore, friction piles are not allowed to be used in collapsible loess sites. In addition to considering the strength of the pile, the pile foundation design should also adopt end-bearing piles (including end-bearing piles and friction end-bearing piles) passing through the collapsible loess layer according to the engineering geological conditions of the site. Stress layer below the pile bottom: in the non-gravity collapsible loess site, it must be a non-collapsible soil (rock) layer with low compressibility; In the self-weight collapsible loess site, it must be a reliable bearing layer. In this way, when the soil around the pile is soaked in water, once the positive friction on the pile side is converted into negative friction, it can be borne by the non-collapsible soil (rock) layer at the lower part of the end-bearing pile, which can meet the design requirements and ensure the safety and normal use of the building.
Five, chemical reinforcement method
It is widely used in foundation treatment in collapsible loess areas in China, and the chemical reinforcement methods with practical experience include silicification reinforcement method and alkali solution reinforcement method. The strengthening mechanism is as follows:
The physical and chemical process of strengthening collapsible loess by silicification is based on the smooth infiltration of low-concentration and low-viscosity sodium silicate solution into loess pores, and the solution and soil coagulate each other to play the role of coagulant.
Alkaline solution reinforcement: Our country began to use sodium hydroxide solution to reinforce collapsible loess foundation in 1960s. The principle of reinforcement is that after sodium hydroxide solution is injected into loess, it first reacts with soluble and exchangeable alkaline earth metal cations in soil, and the result is reflected in the formation of alkaline earth metal hydroxide on the surface of soil particles.
Pre-soaking of intransitive verbs
Pre-soaking method is to soak a large area of collapsible loess site in advance before building, so that the soil will collapse under the action of saturated dead weight stress, thus eliminating the dead weight collapsibility of all loess layers and the external load collapsibility of deep soil layers. Pre-soaking method is generally suitable for self-weight collapsible loess site with large thickness and strong collapsibility. Because the ground around the site will sink and crack when it is flooded, it is easy to cause "running water" to penetrate holes and affect the safety of buildings, so the open new area is more suitable.