1 geotechnical problems and causes of construction engineering

1. 1 foundation settlement

1. 1. 1 The load pressure of the building is too high.

The excessive load pressure of buildings on the foundation is the main cause of foundation settlement. This is mainly caused by the consolidation process of underconsolidated soil under the action of building load and dead weight pressure. The natural soil layer in the foundation bears effective pressure during the consolidation process, that is, the pre-consolidation pressure. Sometimes, when the self-weight pressure of the existing foundation covering soil layer is greater than the pre-consolidation pressure, the soil layer is under-consolidated, which needs attention. For example: artificial fill and newly deposited cohesive soil. The main reason is that the time required for sedimentation is short and the consolidation is not completed due to the weight pressure of the soil itself. The settlement of underconsolidated soil is not only due to the additional stress of building foundation, but also due to unstable consolidation.

1. 1.2 uneven foundation

In building construction, uneven settlement will also occur due to uneven foundation. The foundation building on the poor building foundation such as expansive soil, collapsible loess, soft soil, etc., once the foundation is denatured and exceeds the specified limit, it is easy to cause foundation settlement and even cracking. In order to reduce the harm caused by settlement, we can make full use of deep foundation such as pile foundation for foundation reinforcement or special foundation treatment for building foundation, and take appropriate structural measures, structural measures and building measures in design and construction.

1. 1.3 Influence of groundwater

In some construction areas, due to excessive pumping of groundwater, the foundation settlement is caused by the drop of water level. When the groundwater level decreases, the natural soil layer within the decreasing depth will cause the change of foundation stress and the consolidation of soil layer, which will cause the ground subsidence and have a certain negative impact on the surrounding underground pipelines, buildings and other facilities. Therefore, in the process of dewatering construction, necessary environmental monitoring should be carried out, and reasonable solutions should be formulated in advance to ensure the safety of building construction.

1. 1.4 Impact of new adjacent buildings

Building a new building next to the original building may also cause foundation settlement. The load of new buildings will cause stress superposition on the original foundation and uneven additional settlement of the foundation, which will deform both old and new buildings. This deflection and deformation will seriously affect the safety of buildings and lead to cracks in buildings. Therefore, it is necessary to make a detailed pre-load analysis before the new building construction, so as to design a reasonable building structure and construction procedure.

1. 1.5 Influence of adding stories to buildings

Adding stories to an existing building increases the original load of the building and leads to settlement. Before building construction, the actual compressive capacity and bearing capacity of foundation, such as ultimate load and plastic load, should be accurately calculated. So as to determine the relevant safety factor between the bearing capacity of the added story and the original bearing capacity. At the same time, we should also consider the basic situation of pile foundation. For example, for excess pore water pressure, the bearing capacity of single pile will increase with time. In addition, when pore water pressure in soft soil foundation weakens or disappears, it will produce negative friction on pile foundation. For the settlement of the building after adding stories, especially the uneven settlement, it is necessary to make a careful prediction to ensure that the inclination and deflection are within the allowable range.

1.2 foundation pit instability

1.2. 1 foundation uplift

In the process of foundation pit excavation, due to stress release, the foundation will float up under the action of rebound. Deep foundation pit excavation is actually a process of gradually unloading load. With the increase of excavation depth, the pressure of foundation will be greatly weakened, and it is easy to cause the pit bottom to rebound due to the expansion of its soil layer. Relevant indoor compression tests should be done well in advance, and the critical point of pressure increase should be summarized, that is, when the pressure reaches a certain value, the pressure should be released, and then the rebound of the soil layer can be found, and the corresponding rebound curve can be drawn according to the test. The foundation uplift caused by the rebound of foundation pit will have a great impact on the foundation pit and will also cause serious damage to the surrounding buildings.

1.2.2 Slope soil displacement.

When the foundation pit is shallow, the soil condition is good, there are no buildings around, and the excavation surface is sufficient, slope excavation can be carried out. However, when deep excavation is carried out in the vertical direction of foundation pit, supporting facilities must be set up. In the absence of supporting conditions, the displacement of slope soil is mainly due to the excessive load of building materials stored at the top of the slope and the vibration influence of vehicle driving and piling, which breaks the original balance. When the water content in soil increases, the shear strength will decrease. In the process of deep foundation pit excavation, unreasonable supporting structure, loose backfilling, poor contact of supporting components and untimely supporting will all cause slope displacement.

2 Analysis of key points of geotechnical geological investigation

Before the design and construction of all kinds of construction projects, geotechnical engineering investigation must be carried out according to the basic construction procedures. That is to say, according to the requirements of each investigation stage of engineering construction, that is, the main points of investigation, correctly reflect the engineering geological conditions, find out the adverse geological effects and geological disasters, that is, geotechnical geological problems, and provide a geotechnical investigation report with complete information and correct evaluation of the proposed site through field exploration sampling test and indoor geotechnical test.

2. 1 feasibility study stage

Before the construction project starts, the feasibility investigation of geotechnical geological environment shall be conducted to evaluate the stability and suitability of the proposed site. During the feasibility study, we mainly collect geological data of the proposed site, such as topography, minerals and earthquakes. In addition, it is necessary to investigate the engineering geological conditions and architectural experience nearby. After investigating enough geological data, it is necessary to analyze and study these data in detail to fully understand the geotechnical engineering properties, adverse geological effects, strata, structures and groundwater in the survey area.

2.2 Preliminary investigation stage

The preliminary investigation of geotechnical engineering geological conditions is mainly to evaluate the stability of different engineering sections of the construction site and the geotechnical engineering technical conditions of the foundation, and to obtain a scientific and reasonable evaluation. At the same time, it provides relevant data support and reference for the general layout planning of buildings, the selection of main building basic schemes and the solutions to problems. In addition, the possible basic scheme design of building engineering foundation is scientifically demonstrated. The main contents of the preliminary investigation of geological conditions basically include the preliminary identification of geological structure, stratigraphic structure, mechanical properties of soil layer and rock; For the site with seismic fortification intensity equal to or greater than 6 degrees, the seismic effect of the site and foundation is preliminarily evaluated; At the same time, the freezing depth and hydrological related geological conditions of seasonal frozen soil are preliminarily identified; Preliminary analysis and evaluation of foundation types of high-rise buildings, foundation pit excavation and support, and engineering dewatering scheme; Investigate the scope, causes, distribution and development trend of unfavorable geological problems in construction sites, and also fully consider the influence of geotechnical geological problems on the stability and safety of buildings.

2.3 Detailed investigation stage

Detailed exploration is a detailed and serious analysis and study of relevant geological aspects on the basis of preliminary exploration, which makes a scientific evaluation of relevant engineering geological aspects of building foundation and provides scientific and accurate data information reference for construction drawing design. Collect the general construction plan, including coordinates and topography, site leveling elevation, scale nature, load, structural characteristics, foundation form, buried depth, allowable deformation of foundation, etc. ; Find out the type, depth, distribution and engineering characteristics of the geotechnical layer in the survey area, and analyze and evaluate the stability, uniformity and bearing capacity of the foundation; Find out whether there are obstacles on the ground and underground in the survey area that are unfavorable to the project; For buildings that need settlement calculation, the calculation parameters of foundation deformation are provided to predict the deformation characteristics of buildings; For the site with seismic fortification intensity equal to or greater than 6 degrees, the site category shall be determined according to the ground motion parameter zoning map approved by the state and relevant specifications, and special research suggestions shall be put forward; Find out the buried conditions of groundwater and demonstrate the influence of groundwater and soil on engineering and environment. At the same time, effective suggestions are put forward for deep foundation pit excavation, engineering support and dewatering design, prevention and control of adverse geological phenomena, foundation reinforcement and foundation design.

2.4 Construction survey stage

Construction survey is an important stage before building construction, which requires the survey department or personnel to cooperate closely with the construction unit and design unit to inspect the foundation trench of the construction foundation pit, and to conduct serious quality inspection on the treatment of foundation and the effect of pile foundation engineering. When the geotechnical conditions are inconsistent with the survey data or the abnormal situation must be identified, the construction survey should be carried out; In the process of engineering construction or use, it is also necessary to monitor geotechnical engineering, such as foundation soil, slope and groundwater. , and analyze and evaluate the impact of engineering and environment, at the same time ensure the proper solution of geotechnical geological problems, and provide accurate geological data for the change of foundation design scheme during construction.

3 Conclusion

In a word, the geotechnical problems of the construction site need to be investigated and studied in detail, and the feasibility of the construction should be fully considered. For sites whose geological conditions cannot meet the construction requirements, construction is absolutely not allowed. Doing a good job in feasibility study, preliminary investigation, detailed investigation and construction investigation can provide effective data and information for the design of construction scheme and ensure the scientificity of construction scheme, thus ensuring the overall quality of buildings, improving the stability and safety period of buildings and effectively promoting the stable, healthy, normal and sustainable development of construction industry.

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