(1) Soft soil has brought many problems to engineering design because of its unstable soil structure, which has been puzzling people for a long time. With the continuous solution of soft soil problems in engineering practice, people's understanding is further deepened. Analyzing the particularity of bridge design in soft soil area from the perspective of bridge disease prevention is helpful to improve the reliability of bridge design under soft soil geological conditions and prevent such diseases.
(2) Soft soil geology is generally deposited in still water or slow flowing water environment, with low strength and high compressibility, which will deform and flow under the action of gravity or external force and has obvious thixotropy. At the same time, soft soil geology is also creep, that is, under constant load, deformation develops with time. Generally speaking, this creep is very slow, but its duration is quite long, and the creep phenomenon is very common, which directly affects the service life of the structure.
(3) These characteristics of soft soil geology make it difficult to accurately simulate its actual physical characteristics in theory. At the same time, under the influence of construction disturbance and surrounding environment, there are different degrees of deviation between design and implementation, which makes it difficult for the load of the structure to be consistent with the assumed conditions of design, which is the complexity of bridge design under soft soil geological conditions. Taking a bridge disease on Minjiang River in Fujian as an example, based on the analysis of the causes of the disease, this paper discusses the particularity of bridge design under soft soil geological conditions and the corresponding disease prevention countermeasures.
Second, the symptoms of disease
(1) The disease symptoms of bridges on soft soil foundation are different due to different geological conditions and bridge structure types. However, structural diseases caused by foundation settlement and horizontal deformation of soil account for a certain proportion of bridge diseases, and most of them are diseases of nearby abutments and piers.
(2) For example, a national highway bridge located on the Minjiang River in Fujian was built at 1990. The bridge consists of prestressed concrete triangular truss, continuous beam and simply supported beam, with a total length of 1850 m and a design load of steam -20. The Minjiang River, where the bridge is located, has obvious characteristics of sediment erosion and deposition, and most of the banks have deposited thick silt layers, and the silt is buried very shallowly.
(3) In 2000, the inspection of the whole bridge found that the cone slope in front of the column abutment in deep soft foundation section was cracked, the abutment side wall was damaged, the double-column pier of 1# pier near the abutment had annular cracks from bottom to top, and there were cracks in pier 2 with the maximum crack width of 0.5 mm and the maximum crack width of 0.32 mm. The pier body showed signs of deviation. According to the construction data, the height of the fill behind the abutment is 9 m, and there is a pull rod behind the abutment. The geological condition at the abutment is the silt layer of nearly 20 m in the upper layer, which is below the rock layer, and the rock faces the river center, and the abutment and subgrade are not treated with soft foundation. The field investigation shows that there is a lot of soil on the downstream side of the bridge head, and the phenomenon of vehicle overload is serious.
Third, the cause analysis of the disease
This kind of bridge disease under soft soil geological conditions is objectively related to soft soil deformation. However, the occurrence of diseases involves human factors such as design, construction and operation.
1, design impact
(1) Due to the special geological conditions of soft soil, there is a certain deviation between the theoretical design of the structure and the actual situation. As far as design is concerned, poor consideration of geological conditions, construction interference and surrounding environment may lead to diseases.
(2) For many years, the bridge pile foundation has been calculated by "M" method. However, in soft soil foundation, the horizontal displacement of pile under the action of horizontal force is large, and the stress mode of pile is quite different from the calculation mode of "M" method.
(3) First of all, the "M" method assumes that the soil is completely elastic, the coefficient of foundation reaction is zero on the ground, and the ratio below the ground changes with depth by m: 1, and the difference of soil quality is only a matter of coefficient. These assumptions are basically consistent with the geological conditions of normal consolidated cohesive soil and sandy soil, but for soft soil geology, the application range of foundation soil resistance coefficient M is limited. According to the code, the value range of m for cast-in-place pile is 6 ~ 12 mm corresponding to the horizontal displacement of single pile on the ground, but the displacement of pile foundation in soft soil geology often exceeds its limit range, especially for high-filled abutment in soft soil geology, its horizontal deformation often exceeds the limit range of "m" method under the action of filling pressure and dynamic load behind abutment, and soil deformation is no longer elastic deformation. Does "M" method calculate the hypothesis of elastic resistance of intermediate soil? A large number of engineering measurements show that when the displacement of pile foundation is large, the calculated results are quite different from the engineering measured values.
(4) Secondly, the "M" method determines the "effective pile length" according to the condition of ah=4, that is, the pile length exceeds this value, which has no influence on the calculation of the displacement and maximum bending moment of the pile top. The calculation is to determine the maximum bending moment and zero bending moment of pile foundation on the premise of assuming the length of pile, which ignores the overall interaction between pile and soil and external force to some extent and is arbitrary. In fact, in soft soil foundation, the horizontal displacement of piles under the action of horizontal force is very large. As far as the influence on the calculation of pile top displacement and maximum bending moment is concerned, the value of pile length is not limited to the characteristics of pile soil itself, but also related to the external force it bears. Considering the characteristics and stress conditions of pile and soil, the calculated pile length can be determined by the stress balance conditions of pile, which makes the calculation closer to the actual situation. Relevant data show that in soft soil geological conditions, if the range of ah=4 is taken as the calculation pile length to consider reinforcement in design, it will often lead to errors in the determination of the maximum bending moment and the zero position of bending moment, resulting in insufficient reinforcement length and insufficient or less reinforcement in the range where reinforcement is still needed.
(5) Except for the calculation method, the additional load caused by soft soil geology on the structure is not considered in the design, which is also one of the reasons for the disease. Due to the deformation of soft soil geology, soft soil geological structure bears more additional loads than non-soft soil geological structure. For example, the fill behind the abutment can actually be regarded as a local post-load on the soil. The post-load in soft soil geology will not only cause negative friction when the soil around the pile sinks, but also cause lateral deformation of the soil and plastic extrusion of the foundation soil on the steep slope, thus adding horizontal earth pressure to the pile foundation. For the abutment with high fill and deep soft foundation, if it is not treated in a certain range near the abutment, the abutment and the pile foundation adjacent to the abutment may bear the horizontal additional force caused by soil deformation. If this horizontal additional force is not considered in the design, the structural foundation may be destroyed.
(6) From the point of view of structural design, if the soft soil in a certain range near the abutment is not treated, the deformation of soft soil under unbalanced load may not only lead to the destruction of structures, but also lead to other diseases such as subgrade collapse, soft soil slip and creep behind the abutment.
2, the construction influence
(1) The construction technology and sequence arrangement of bridges in soft soil geology have great influence on the stress of pile foundation, mainly in the additional force and bearing capacity of pile foundation.
(2) The construction sequence directly affects the magnitude of additional force borne by the pile foundation, mainly in the sequence of soft foundation treatment and structural construction. In the process of soft foundation treatment, the compressibility of soft soil geology leads to large horizontal and vertical compression deformation of soil, which forms a force on the structures in it. Vertical compression produces negative friction and horizontal deformation produces horizontal thrust. If the soft foundation is treated after the pile foundation is formed, it may not only cause negative friction to the pile foundation, but also cause horizontal pressure to the pile foundation. When this horizontal force is large enough, it is very likely to cause pile breakage. In practical engineering, due to the treatment of the structure before the construction of soft foundation, structural damage sometimes occurs.
(3) The influence of construction on the bearing capacity of pile foundation is mainly due to the thixotropy of soft soil, which intensifies the influence of construction disturbance on the bearing capacity of pile foundation. Bored piles destroy the natural structure of soil in the process of hole formation, and at the same time, the pore water pressure of soil on the hole wall increases, which reduces the strength around and at the pile end. Due to the disturbance of construction soil, the bearing capacity of pile foundation is obviously reduced, which is difficult to consider in conventional pile foundation design. Although the bearing capacity of pile foundation will increase with time, due to the thixotropy of soft soil, the structural recovery and the dissipation of pore water pressure around the pile are slow, and the process of improving the bearing capacity of pile foundation is long. In projects with short construction period, the increase of upper load and running load in a short period is not conducive to the improvement and utilization of bearing capacity.
3. Late environmental impact
(1) The bad influence of the surrounding environment of the bridge site on the soft soil geology is also the cause of the bridge disease. Due to the low geological strength, compressibility and creep of soft soil, the load around the abutment will exert pressure on the underlying soft soil and the soil at the bridge site, which may not only increase the deformation of the longitudinal soil of the abutment, but also cause the lateral deformation of the soil due to the unbalanced loading on both sides of the abutment or unilateral loading, thus generating lateral thrust on the bridge pile foundation and causing pile foundation deviation. In addition, any overload may also lead to soil creep. The bridge 1# pier listed in this paper has certain longitudinal and lateral displacement of the bridge. Through the analysis, the displacement is related to a large number of late soil overloading on the side of the bridge head, and vehicle overloading also intensifies the development of soil deformation to a certain extent.
(2) From the above analysis, it can be seen that the objective factors of soft soil geology and human factors such as design, construction and operation environment constitute the comprehensive factors of bridge diseases, and the causes of bridge diseases on soft soil foundation are the result of interaction of many factors.
Fourth, the disease control plan.
(1) Bridge diseases under soft soil geological conditions are mainly reflected in the destruction of structures due to the adverse effects of geological environment. Generally, it can be considered to improve the external environment of the structure, including geological conditions and surrounding environment, and strengthen the ability of the structure to resist adverse external influences. The specific treatment plan should be determined according to the actual situation and considering the structural safety, economic benefits, social impact and other factors.
(2) Taking the bridge diseases mentioned in this paper as an example, the more thorough way to solve this kind of bridge diseases is to increase the bridge holes, make the abutment far away from the free surface of the river bank, reduce the filling height of the abutment, and treat the soft foundation in necessary sections. However, the above treatment must block traffic. For the bridge on the main road, no matter from the economic benefit or social impact, the treatment scheme of blocking traffic is not the best policy. The bridge treatment scheme is mainly based on the design idea of not blocking traffic.
(3) In the choice of bridge treatment scheme, the high-pressure jet grouting technology is considered, and the soft soil before and after the abutment is grouted, and a rigid consolidation body is formed in the abutment area to resist the earth pressure formed behind the abutment and reduce the pressure of the earth pressure on the abutment and adjacent piers. However, considering the strong structure of silt, there is no flow phenomenon in appearance when the structure is not destroyed, but once it is disturbed and destroyed, its strength decreases obviously. The bridge site is a silt layer of nearly 20 m, which is located under the rock layer, and the rock faces the center of the river and the stratum is inclined greatly. For thick soft soil with inclined substratum, the disturbance during grouting will adversely affect the soil layer and structure in this area. At the same time, the increase of soil bulk density after soft soil grouting may cause the loose underlying soft soil to slide. In order to avoid the above situation, this scheme was eliminated.
(4) Further geological investigation shows that the physical and mechanical indexes of silty soil near the abutment are improved compared with those before the bridge is built, and tend to be stable under the condition of unchanged boundary conditions. Therefore, it is more practical to give priority to structural reinforcement and try to avoid soil disturbance. In view of the difficulty of abutment detection, it is not appropriate to blindly reinforce abutment when the abutment damage is unknown. At the same time, in order to avoid traffic jam, it is decided not to deal with the abutment structure itself for the time being, but to share the stress on the abutment by strengthening pier 1# to reduce the influence of the pressure behind the abutment on the structure. Considering that 1# pier bears horizontal force from itself and abutment, the single-row pile foundation of 1# pier is reinforced into a group pile foundation, and the pier body is wrapped with concrete, and the column diameter is increased to connect it into a whole. A grouting conduit is set on the tapered slope between pier 1# and abutment, and the tapered slope is partially grouted and solidified, so that a bearing body similar to a supporting beam is formed between pier 1# and abutment, and * * * bears horizontal pressure.
(5) After the disease treatment of this project, after several years of operation, no new disease symptoms were found in the steel members. However, according to the current observation, due to the temporary vehicle passage formed by the newly filled soil between pier No.2 and pier No.3, pier No.3 also has similar diseases to pier No.2. This further shows that the soil without soft foundation treatment has adverse effects on the bridge under external load. Pier 3 has also been reinforced recently. In order to further understand the geological changes of the bridge site, the observation of soil displacement is still going on.
Discussion on preventive measures of verb (abbreviation of verb)
From the analysis of the causes of diseases, it can be seen that besides objective reasons, human factors can play a leading role in preventing diseases. On the premise of fully understanding the distribution, physical and mechanical characteristics and possible deformation of soft soil at the bridge site, through perfect design, the consequences of construction and operation environment on soil and the influence of soil changes on the service life of structures can be predicted, and preventive measures can be taken as soon as possible to reduce the probability of diseases. In terms of design, construction and post-maintenance, the following countermeasures can be considered.
1, determination of bridge length
(1) Determining the route and bridge length through soft soil area plays an important role in saving costs and reducing the probability of diseases. The length of the bridge should be determined after technical and economic comparison according to the actual situation such as the depth of soft foundation and the height of filled soil.
(2) When the route passes through the deep soft foundation section, the bridge span often has certain advantages. The cost of soft foundation treatment in deep soft foundation section is high, and the effect of soft foundation treatment is not necessarily ideal due to the constraints of construction technology and construction period. Moreover, many unforeseeable factors should be considered when building abutment on deep soft foundation, especially high-filled abutment, which has high structural cost. Generally, the bridge length of expressway in Fujian Province, which was built in the early days, will not be increased because of soft soil, and the abutment is often built on deep soft foundation. The original intention of the design is to reduce the length of the bridge to save costs. In fact, not only the treatment of soft foundation is expensive, structural diseases occur from time to time, but also the problem of bump at the bridge head is more prominent, which has not achieved the ideal effect.
(3) For the river-crossing bridge under soft soil geological conditions, the abutment should be as far away from the steep bank as possible. Although this will increase the length of the bridge, it can reduce the adverse impact of the earth pressure difference between the free surface near the river bank and the back of the abutment on the pile foundation. Compared with the short bridge on the steep slope, the earth pressure in front of the long bridge is higher, which can protect the backfill behind the bridge, avoid the additional force of soil deformation on the pile foundation and reduce the possibility of creep deformation. In terms of reliability, the long bridge scheme near the river bank is better than the short bridge scheme. From the cost point of view, although the superstructure cost of long bridge is higher than that of short bridge scheme, the cost of soft foundation treatment and substructure may be lower than that of short bridge scheme, and the two schemes are of comparative value.
2. Relevant structural measures
When dealing with (1) soft foundation, it is an effective measure to fundamentally eliminate the adverse effects of soft soil on the structure and avoid the occurrence of bridge diseases under soft soil geological conditions. For areas that may have adverse effects on the structure, such as abutment, steep bank or near the bridge adjacent to abutment, corresponding soft foundation treatment should be carried out according to the physical and mechanical indexes and thickness of soft soil, so as to improve the external environment of the structure, avoid the horizontal deformation of soft soil from generating additional horizontal force on the pile foundation, and eliminate the hidden danger of creep.
(2) In the choice of abutment structure, abutment with strong horizontal load resistance should be adopted. Column abutment has low horizontal thrust stiffness and large deformation under horizontal force, so it is not suitable for deep soft soil. Even if the soft soil near the abutment is treated with soft foundation to improve the characteristics of soil, it is difficult to ensure that the pile foundation will not bear the additional force of soil due to the reliability and aging of soft foundation treatment, so the use of column abutment should be cautious. The ribbed platform with pile group foundation has high horizontal thrust stiffness and strong ability to resist horizontal load, which is widely used in soft soil geology and has good results.
(3) Due to the low strength and easy deformation of soft soil foundation, various unforeseeable unfavorable factors have a prominent influence on pile foundation, and the maximum bending moment point and bending moment zero point of pile foundation may also change due to the influence of various unforeseeable factors. In addition to bearing capacity, the design of pile foundation should also consider leaving more room for the bending and shearing range of pile foundation. Generally, the reinforcement of pile foundation should not be reduced in soft soil, and plain concrete piles should not be set in this section.
3. Talking about structural design and calculation.
(1) The calculation of bridge pile foundation under soft soil geological conditions cannot simply adopt conventional calculation methods, but should be analyzed according to the actual stress characteristics.
(2) As far as the calculation method is concerned, the conventional method of calculating the maximum bending moment and the bending moment zero point of the pile under the assumption of' effective pile length' should be carefully adopted in soft soil geological conditions, so as to avoid mistakes in determining the positions of the maximum bending moment and the bending moment zero point, resulting in insufficient reinforcement length. In the case of large deformation of pile foundation, the pile-soil characteristics and stress conditions should be considered at the same time in calculation, and the stress mode of pile should be analyzed as a whole system to make the calculation result close to the actual situation. At the same time, when the horizontal deformation of pile foundation exceeds the limit range of "M" method, the M value of resistance coefficient of foundation soil should be determined. Because of the deviation between the basic assumption of "M" method and the stress mode of large deformation pile foundation, other calculation methods closer to the stress mode of this kind of pile foundation can be considered for comparative calculation, which is a problem that needs further discussion.
(3) When the soft soil geology may be deformed by some external force, such as the soft foundation treatment is not carried out near the abutment, or the final effect of soft foundation treatment cannot be achieved before the structure is implemented, additional forces acting on the pile foundation, including vertical and lateral deformation of soft soil, should also be considered.
4. Coordination between design and construction
The deviation between design and construction is inevitable. Within the scope of human control, the close cooperation between design and construction can make the implementation conform to the envisaged situation and reduce the possibility of diseases caused by human factors. As mentioned in the analysis of the causes of diseases, the construction technology and sequence arrangement of geological bridges in soft soil have great influence on the stress of pile foundation. The sequence of soft foundation treatment and pile foundation construction and the realization of pile foundation bearing capacity are closely related to construction control, which should be paid full attention to.
Conclusion of intransitive verbs
Principle of bridge reinforcement. Before adopting the reinforcement scheme, we should first consider the requirements of low cost, quick effect, uninterrupted traffic, feasible technology and good durability. Strengthening is a measure to improve the bearing capacity of some or all bridges by expanding or repairing bridge members. Therefore, bridge reinforcement work is generally based on the principle of not changing the original structural form, and only in more complicated circumstances can we consider changing the structural form. If the reinforcement method still cannot meet the transportation requirements, it is necessary to consider rebuilding part or all of the bridge. When choosing the bridge reinforcement method, we must consider the present situation of the old bridge, the degree of weakening bearing capacity and the future traffic volume, and it is best to refer to the construction situation of the bridge that has been successfully reinforced. Before strengthening by increasing or increasing the cross section of bridge members, the combined effect of the increased part and the original part should be considered.
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