Before the design and construction of deep foundation pit support for geotechnical engineering, it is necessary to strengthen the understanding and understanding of the geological conditions of the site and the surrounding environment, fully collect the data, determine the economical and reasonable deep foundation pit support form and construction scheme according to the local support experience, comprehensively solve the problems existing in the construction, and strengthen the technical support and management, Thus, the design and construction of deep foundation pit support in geotechnical engineering can be comprehensively improved, meet the needs of engineering construction, and achieve significant economic and social benefits.
1 Overview of deep foundation pit support
In recent years, the foundation pit support technology in construction projects has become a systematic work mode. The more common construction technologies are row pile support technology, mixing wall support technology and soil nailing support technology. Soil nailing is widely used because of its simple construction, short period and low cost. In general, due to the different functions of the foundation pit support system, the corresponding construction schemes are also quite different. In actual support, important maintenance structures need to be selected according to the surrounding natural conditions and geological conditions, and some displacement problems of the retaining structure in subsequent use need to be considered. In order to ensure the design and construction of foundation pit support fully, geotechnical engineering survey work needs to be done well to improve the coordination between design and construction and the surrounding environment.
In terms of the selection of foundation pit support form, it should be determined comprehensively according to the geological conditions, surrounding environment and site construction conditions, and a single support structure or a combination of multiple support structures should be selected to improve the effect and level of support. In the design of foundation pit support, if cantilever or soil nailing wall support structure is adopted, the influence of horizontal displacement on the surrounding environment should be considered in addition to stability; If bolt support structure is adopted, coordination with surrounding environment shall be considered; If the cut-off curtain is used for water stop, in addition to considering the impact on the surrounding environment, appropriate construction methods should be adopted to achieve the desired effect.
When the construction adopts the normal method of excavation, the principle of "from top to bottom" shall be followed, and the excavation shall be carried out hierarchically according to the design conditions to ensure the safety of the foundation pit. During the construction, it is necessary to strengthen the overall supervision and management of the whole support process and conduct all-round deformation monitoring to achieve the effect of safe use. After the foundation pit support construction is completed, the bottom of the foundation trench shall be cleaned immediately, and the plain concrete cushion shall be laid in time to prevent the sudden rainstorm from soaking the foundation soil.
During the construction of the main underground structure, the waterproof and drainage work around the foundation trench shall be well done to prevent the water around the foundation trench from flowing into the foundation trench. When there is accumulated water in the foundation trench, the water shall be drained out of the foundation trench in time to prevent the main structure from floating up and causing cracks in beams, slabs, columns and walls, resulting in unnecessary losses.
2. Problems in design and construction of deep foundation pit support
There are many successful cases in the design and construction of deep foundation pit support in geotechnical engineering. However, from the implementation effect, some foundation pit support works do not meet the relevant technical standards and quality requirements, and there are some problems. Therefore, it is necessary to analyze the problems and shortcomings exposed in previous projects to improve the design and construction level of deep foundation pit support, So as to promote the steady development of foundation pit support industry in our construction projects. On the whole, in the deep foundation pit project, the existing problems mainly include the following aspects.
2.1 Insufficient attention is paid to the investigation work on foundation pit support influence
In order to ensure the stable performance of the supporting structure, it is necessary to fully understand the engineering geological conditions and hydrogeological conditions of the site, and fully understand the physical and mechanical properties of the rock and soil mass of the foundation pit slope. Therefore, scientific, standardized and comprehensive survey means should be used in the survey phase, so as to lay a solid foundation for the subsequent design and construction. In the survey stage, there are many problems in obtaining the authenticity and reliability of rock and soil properties. For example, the survey means are single, and only to meet the requirements of relevant specifications and drawing review, a variety of test means are not used to comprehensively analyze and evaluate to determine the design parameters; The on-site sampling is not standardized, non-standard, and arbitrary, which cannot comprehensively reflect the properties of rock and soil mass; The data of in-situ test and geotechnical test were falsified, and the technicians prepared them randomly to meet the quantity requirements, without any reference value. Hydrogeological conditions are the key and difficult points of the investigation work, especially for the site with complex topography, not only the structural characteristics of rock and soil mass are complex, but also the hydrological conditions vary with seasons, resulting in uneven spatial distribution. At this time, special hydrogeological investigation should be carried out to meet the engineering needs. If the geotechnical engineering investigation work is not fully paid attention to before the foundation pit support design and construction, and the authenticity, reliability and comprehensiveness of the investigation results cannot be guaranteed, the subsequent design and construction cannot be smoothly implemented.
2.2 The design does not fully consider the overall role of the support system and the support environment
In the design and construction of deep foundation pit support, the entire excavation conditions are not considered, which will inevitably have a certain impact on the subsequent construction. In engineering construction, the deformation of foundation pit support structure and surrounding soil will develop to a certain extent with the passage of time, and it is also closely related to the shape, size and depth of foundation pit excavation. In the design of deep foundation pit support, the designers failed to fully consider the time effect and space effect, and could not fully understand the ability of the support structure to resist deformation, the coordination of the deformation of the entire support system, and the impact of the surrounding environment, which made the support system unable to effectively integrate into the entire engineering environment and play a full role. These problems were not considered in the design, It makes the actual construction effect unsatisfactory, fails to achieve the normal use function, and affects the follow-up construction project.
2.3 The construction management and quality of foundation pit support do not meet the requirements
In the process of foundation pit excavation and support, there are often serious problems due to the lack of management or construction personnel, or the lack of technical disclosure. In addition, the construction process is affected by many factors, such as excavation and support not in accordance with the design conditions, over excavation, untimely support, and the next step of construction if the previous process does not meet the design requirements Jerry building and other issues, affecting the stability and safety of the foundation pit.
3. Design and construction strategy of deep foundation pit support in geotechnical engineering
3.1 Design
(1) Select the correct support mode of deep foundation pit
Row piles and double row piles are common in deep foundation pit support engineering. This type of support mostly adopts reinforced concrete cast-in-place piles or prestressed pipe piles. During design, it is necessary to strengthen the comprehensive understanding and analysis of site conditions, and select reasonable pile types and pile parameters. When the hydrogeological conditions are complex and there is surface water or shallow underground water, waterproof, water stop and drainage design are also required, especially in areas with poor geological conditions and many surrounding surface objects, the diaphragm wall should be used to play a good supporting role, so that the deep foundation pit structure can have strong stability, Lay a solid foundation for subsequent construction (Figure 1).
Figure 1?? Double row pile support
(2) Soil nailing wall support
Soil nailing wall support is a relatively traditional construction technology, which was earlier applied to foundation pit support engineering, and a large number of engineering experiences and lessons were summarized. In fact, as one of the most widely used supporting systems, the design theory of the soil nailing wall is not perfect, and there is no complete use of structural analysis methods, among which the calculation of the tension of the soil nailing, the force on the surface layer, and the displacement under working conditions has not been solved. Through some structural measures and empirical methods, the safety and stability of the foundation pit can be satisfied. The soil nailing wall support is generally applied to the 12? The foundation pit below m shall be used as temporary support, and its service life shall not exceed 1 year. Soil nailing wall support can be used independently in the support system, or combined with other support structures, such as prestressed anchor rod, cement soil wall, and micro pile. Select one or two of them to work together to improve the actual support effect and meet the project needs (Figure 2).
Figure 2?? Soil nailing wall support (computer screenshot)
(3) Bolt support
This support method can effectively strengthen the rock and soil mass, control the deformation of the foundation pit, and reduce the probability of foundation pit collapse. In practical projects, the composite soil nailing wall or pile anchor support structure system is formed by combining the soil nailing wall and row of piles. Anchor bolt support needs more open construction space. When the geological conditions are poor and the surrounding environment is complex, such as high sensitivity soft soil, rich and uneven distribution of groundwater, and surrounding buildings using pile foundation or composite foundation, its applicability should be considered. The anchor rod can effectively control deformation by applying prestress. During construction, the cement slurry can permeate and diffuse in the soil around the anchorage section to form cement soil through secondary grouting or multiple grouting, which improves the shear strength of the soil and the friction with the soil. The design theory of friction distribution is assumed to be evenly distributed, so the spacing of the anchor rod should be controlled to avoid anchor group effect, Once unavoidable, the anchoring force shall be reduced. The test data of anchor bolt obtained from tension test and creep test in clay and soft soil shall be fed back to the designer in time. When it does not meet the design requirements, the designer shall adjust it according to the actual situation.
3.2 Selection of physical and mechanical parameters of rock and soil mass
Scientific and reasonable selection of physical and mechanical parameters is particularly important in the design, and it is also the key to ensure the safety of the support structure itself. Before design, the investigation report shall be carefully studied and analyzed. When determining the parameters, the stratum conditions shall be fully understood, the geotechnical test and in-situ test data shall be analyzed, and the design parameters applicable to the project shall be comprehensively determined in combination with the recommended values of foundation pit support design parameters provided by the investigation unit.
3.3 Construction
(1) Pay attention to the monitoring of foundation pit deformation
During the construction of deep foundation pit support, strengthen the monitoring of foundation pit deformation to avoid the impact of excessive deformation on subsequent construction. The monitoring data shall be analyzed in many aspects, including the influence of local deformation and the overall deformation coordination of the foundation pit. When the deformation in the process of excavation and support has a large development trend, stop work in time, analyze the factors of survey and design, analyze some problems and deviations in the process of excavation and support construction, propose effective improvement measures, take appropriate procedures and processes in time in the subsequent construction, and strengthen construction management to achieve ideal results.
(2) Control the construction quality of foundation pit support
For example, there are serious construction quality problems in deep foundation pit support engineering, which not only affect the orderly progress of subsequent construction work, but also endanger the life safety of construction personnel. Therefore, it is necessary to strengthen the whole process control of foundation pit support construction. Before construction, it is necessary to fully understand the geological conditions of the site and its vicinity and the surrounding buildings (structures) to reduce the impact on the surrounding environment. The construction shall be carried out according to the drawings, adhere to the principle of layered excavation and layered support, and try to shorten the slope exposure time after foundation pit excavation. At the same time, the principle of information construction shall be followed. When abnormal conditions are found, such as the stratum is inconsistent with the survey report, and the upper pipeline of the foundation pit leaks, the construction unit shall feed back to the designer in time, and the designer shall adjust the design parameters and recalculate according to the actual situation, Effective measures shall be taken to ensure the safety and normal use of the foundation pit. Corresponding drainage system shall be set during construction to prevent domestic water and rainwater from eroding the slope. Anti freezing measures shall be taken for winter construction. Construction process control is a very important link, which is mainly reflected in that if there are some problems in the construction, remedial measures can be taken later, but it is difficult to achieve the expected effect, so comprehensive quality supervision needs to be strengthened.
4 Conclusion
During the design and construction of deep foundation pit support, it is necessary to fully understand the engineering geological conditions, hydrogeological conditions and surrounding environmental factors of the site, select an economical and reasonable form of deep foundation pit support and construction scheme in combination with engineering experience, strengthen the overall supervision of the overall construction process, adopt information design and construction, and timely feedback and communication when problems are found, Optimize and improve the design and construction scheme to improve the construction quality and foundation pit safety.
