Geotechnical investigation

I. Basic requirements of geotechnical engineering investigation

Geotechnical engineering investigation is the first step in the implementation of engineering construction projects, and relevant technical standards for investigation must be followed, so as to improve the economic, environmental and social benefits of the project and promote the development of investigation technology.

1. purpose of geotechnical investigation

the purpose of geotechnical investigation is to correctly reflect the geotechnical conditions of the construction site, evaluate the geotechnical problems, and put forward methods and suggestions to solve the problems. Survey should be closely integrated with design and construction, and run through the whole process of project construction to ensure the quality of the project. Therefore, geotechnical engineering investigation should complete two main tasks:

(1) Evaluate the stability and suitability of the construction site, analyze and demonstrate the geological composition of the site, groundwater conditions, adverse geological phenomena, environmental engineering geological conditions, and geotechnical engineering properties including special geotechnical conditions, and predict the existing problems in geotechnical engineering and corresponding prevention measures.

(2) provide design parameters such as strength and deformation of engineering rock and soil for various engineering construction sites. Demonstrate and analyze the foundation scheme and geotechnical engineering treatment measures, and predict the problems and protective measures that should be paid attention to in the construction stage and after the completion of the project.

2. site complexity and geotechnical investigation grade

the complexity and investigation grade of the construction site are the basis for determining the workload and schedule of geotechnical investigation. Generally, the following conditions should be considered when dividing the complexity and grade:

(1) According to the type of the project and the severity of the possible damage consequences, the safety grade of the project can be divided into three levels: the first level is an important project, and once it is damaged, it will have very serious consequences; The second level is a general project, and the damage of the project will cause serious consequences; The third level is a minor project, and its damage will not cause serious consequences.

(2) the site is classified according to the landform, unfavorable geological phenomena and engineering geological environment (table 9-1).

table 9-1 classification of site

(3) classification of foundation according to the geotechnical types of foundation and the influence of groundwater, as shown in table 9-2.

table 9-2 classification of foundation

(4) the safety grade, site grade and foundation grade determined according to (1) ~ (3) can be combined into geotechnical engineering investigation grade, as shown in table 9-3.

Table 9-3 Classification of Geotechnical Engineering Survey Grades

3. Classification of Geotechnical Engineering Survey Stages and Their Tasks

The classification of survey stages is suitable for the design stage, and can be divided into site selection or feasibility study survey, preliminary survey, detailed survey or construction drawing design survey and construction survey. The work content and task requirements of each stage of investigation should be determined in combination with the geotechnical investigation grade and engineering characteristics, as shown in Table 9-4.

table 9-4 investigation contents and tasks in each stage

for areas with small site area, simple geotechnical engineering conditions or single geotechnical engineering with architectural experience, one-stage investigation can be carried out, but the arrangement of investigation workload should meet the requirements of detailed investigation. For the geotechnical engineering problems of site stability and particularity, corresponding exploration and testing or special research, demonstration and evaluation should be arranged according to the characteristics and engineering properties of geotechnical engineering. For specialized projects, such as dams and nuclear power plants, special research and investigation should be carried out according to the requirements of the nature of the project.

II. Basic procedures of geotechnical engineering investigation

Geotechnical engineering investigation requires to be carried out in stages, and the investigation procedures in each stage can be divided into undertaking investigation projects, preparing for exploration work, compiling investigation outline, conducting on-site investigation, indoor soil and water test, sorting out investigation data and writing reports, etc.

1. undertaking survey projects

usually, the construction unit and the design unit (i.e., the entrusting party, abbreviated as Party A) entrust the survey unit (i.e., the contractor, abbreviated as Party B). When signing the contract, Party A shall provide relevant documents and materials to Party B and be responsible for its reliability. Relevant documents include: project approval; Land use approval (copy with red line range); Power of attorney for geotechnical investigation and its technical requirements (including special technical requirements); Topographic map of the survey site (its scale should be suitable for the survey stage); One copy of the survey scope and one copy of the general layout plan of the building (a plan with relative position can be used in special circumstances); Existing survey and measurement data.

2. Preparation for exploration

is an important step to ensure the smooth progress of exploration.

3. compiling the survey outline

the complexity of the construction site and its geotechnical properties should be analyzed and estimated according to the requirements of the contract tasks and the results of the survey, and the corresponding survey workload should be arranged according to the requirements of the survey stage, and the survey methods and test methods should be selected. When making the plan, it is also necessary to consider the problems that may not be expected during the survey, leaving room for changing the survey plan.

4. engineering geological mapping and investigation

is carried out in the stage of site selection-feasibility study or preliminary investigation. Engineering geological mapping should also be considered for complex sites in the detailed investigation stage. Before surveying and mapping, the interpretation data of aerial photos or satellite photos should be used as much as possible, and the scale of surveying and mapping should be 1: 5 ~ 1: 5; The preliminary survey is 1 ∶ 2 ~ 1 ∶ 1; Detailed survey is 1: 5 ~ 1: 2, or larger. When the geological conditions on the spot are simple, only investigation will be made. According to the surveying and mapping results, the engineering geological conditions of the construction site can be partitioned, and the stability and construction suitability of the site can be preliminarily judged.

5. The exploration workload

is comprehensively determined according to engineering geological mapping, engineering properties and survey methods, with the purpose of identifying geotechnical properties and dividing strata. Exploration methods include drilling, well exploration, trench exploration and geophysical exploration, etc., and can be combined with in-situ testing and indoor soil-water test analysis by taking undisturbed soil samples and water samples. After the completion of exploration, it is also required to backfill the exploration well hole to avoid affecting the stability of the site foundation.

6. Geotechnical testing

Its purpose is to provide geotechnical technical parameters for foundation design, which are divided into indoor geotechnical tests and in-situ tests. The test items are usually determined according to geotechnical characteristics and engineering properties. In addition to geotechnical physical and mechanical tests, indoor tests sometimes simulate rebound recompression tests of deep foundation pit excavation, shear strength tests of slope stability, dynamic characteristics tests of vibrating foundation and compressive strength and tensile strength of rock mass.

7. arrangement of geotechnical engineering investigation results

this work is the last step of exploration work. The survey result is a summary of the whole survey process and is presented in the form of a report. The preparation of the report is based on the original data such as investigation, exploration and testing. Through the analysis and research of the original data, the data can be refined and the correct conclusions can be made. The report should clarify the source, purpose and requirements of the survey project; Overview of the proposed project; Survey methods and survey work layout; Elaboration and evaluation of site geotechnical engineering conditions, etc. Comprehensively analyze and demonstrate the stability and suitability of site foundation, provide geometric parameters of site stratum structure and groundwater spatial analysis, design parameters of geotechnical engineering properties for geotechnical engineering design, and put forward suggestions on foundation design scheme; Predict the impact of the proposed project on the existing project, the environmental changes caused by the project construction and the impact of environmental changes on the project, and choose the best scheme for the renovation, transformation and utilization of rock and soil; Predict the geotechnical problems that may occur during the construction and operation of geotechnical engineering, and put forward the corresponding monitoring, prevention measures and reasonable construction scheme. The report should also be accompanied by corresponding engineering geotechnical maps, such as the layout of exploration points, engineering geological histogram, engineering geological profile, in-situ test, indoor test results chart, relevant charts of geotechnical utilization, renovation or transformation design scheme, sketches and photos of relevant geological phenomena, etc.

iii. theoretical basis and related technologies of geotechnical engineering investigation

in order to complete the contents stipulated in the technical standards, basic requirements and basic procedures of geotechnical engineering investigation, geotechnical engineering investigation needs to establish a solid theoretical basis and strong technical support. The basic theories of geotechnical engineering investigation include basic geology, engineering geology, hydrogeology, engineering seismology, geotechnical mechanics, basic engineering, etc. The required technical support includes engineering geological mapping, remote sensing interpretation, engineering exploration, engineering geophysical exploration, indoor geotechnical testing, in-situ geotechnical testing, etc.

1. theoretical basis of geotechnical engineering investigation

the main task of geotechnical engineering investigation is to evaluate the stability and suitability of the construction site and provide design parameters such as strength and deformation of foundation rock and soil. Therefore, the theoretical basis of geotechnical engineering investigation has two pillars, one is geology, including basic geology, engineering geology and hydrogeology; The second is geotechnical mechanics, including soil mechanics, rock mechanics, rock dynamics and so on.

Geology studies the material composition, internal structure and surface characteristics of the earth, various geological processes in the history of the earth's development, and the forms and evolution of life that once lived on it. Engineering construction site is a part of the earth's surface medium. In order to clarify the stability and suitability of engineering construction site, it is natural to rely on geological theory. The study of geological history and Quaternary geology can reveal the origin and age of rock and soil in construction sites. The study of petrology and soil science can understand the material composition and types of rock and soil in the site; Structural geology can determine the geological structure position and structural stability of the site; The study of seismic geology can understand the seismic activity of the area where the project site is located; Hydrogeological research can reveal the occurrence and water quality of groundwater in engineering sites; Engineering geological research can identify the unfavorable geological problems and engineering geological conditions of the site. In a word, the theoretical foundation of geology enables geotechnical engineering investigation to understand the engineering construction site from the aspects of genetic history, material composition, structural stability and engineering geological conditions, so as to make a correct evaluation of the engineering suitability of the site, and then make reasonable suggestions on the engineering utilization or transformation of the site.

the engineering site is regarded as the product of geological bodies and geological processes, and the structural stability and architectural suitability of the engineering site can be understood through geological research. However, geotechnical engineering still needs to regard the geotechnical body of the engineering site as engineering materials and study its engineering characteristics and mechanical properties, which requires the application of geotechnical mechanics theory. As a natural material, the performance of rock and soil medium is more complex than that of artificial materials such as concrete, which is mainly manifested in the heterogeneity and anisotropy of material composition and internal structure. Engineering rock and soil can be divided into two categories: loose soil and consolidated rock mass. Soil mechanics studies the material composition characteristics and physical and mechanical properties of soil, including the particle size composition, mineral composition, classification of soil, the relationship between the basic physical state (composition, density, water content, porosity, saturation, consistency, etc.) and the physical and mechanical properties (permeability, compactness, collapsibility, compressibility, shear resistance, etc.) of soil, the settlement and bearing capacity of foundation soil, soil slopes and deep foundation slopes. Rock mechanics studies the physical and mechanical properties of rock mass, including the material composition and internal structure of rock mass, physical property indexes, permeability, water-rock interaction characteristics, rock mass structure and engineering rock mass classification, strength and deformation of rock mass, stress and stability analysis of surrounding rock of cavern, stress and stability analysis of rock foundation, stability analysis of rock slope, etc.

it can be seen that the two pillars of geotechnical engineering investigation theory, geology and geotechnical mechanics, respectively study and analyze the engineering geotechnical body from the perspectives of geological structure and engineering characteristics, which provides strong theoretical support for the engineering stability and suitability evaluation of engineering construction sites and engineering geotechnical bodies.

2. Related technologies of geotechnical engineering investigation

In order to complete the two tasks of evaluating the stability and suitability of engineering construction sites and providing design parameters such as strength and deformation of engineering rock and soil for engineering design, besides a strong theoretical foundation, geotechnical engineering investigation must also obtain first-hand information about engineering construction sites and engineering rock and soil through a series of technical work to provide a basis for theoretical analysis of geotechnical engineering investigation. The technologies related to geotechnical engineering investigation mainly include engineering geological mapping, remote sensing interpretation, engineering geophysical exploration, engineering exploration, indoor test, in-situ test and long-term observation.

engineering geological surveying and mapping is the most important and pioneering survey method in exploration work, and its essence is to observe and describe the geological bodies and geological phenomena on the ground by applying geological theory knowledge to understand the laws of geological changes. Engineering geological surveying and mapping is to reflect the engineering geological conditions and various geological elements of the site on a set of engineering geological maps and their corresponding tables and instructions according to a certain accuracy. The contents of engineering geological surveying and mapping should mainly include seven aspects: ① Strata, lithology, lithofacies changes, stratigraphic genetic types and mutual contact relations in the surveying and mapping area; (2) Geological structure, such as layered combination of soil, structural characteristics of rock mass and fault properties, etc. (3) Topography and its genetic types, and its relationship with geological structures; ④ Groundwater, knowing the groundwater level, aquifer, aquifuge, groundwater type, water inflow and water quality, etc. ⑤ Distribution, scale, degree of development, morphological and structural characteristics, activity, harmfulness and formation conditions of various physical and geological phenomena; ⑥ Deformation and damage of existing buildings; ⑦ Natural building materials. If the surveying and mapping area has the same or larger scale surveying and mapping results of geology and geomorphology, it is only necessary to make some special supplementary surveying and mapping required by engineering geology on this basis. The selection of surveying and mapping scale should be considered according to the specific situation, which should not only meet the design requirements, but also not waste the workload. Generally, three factors should be considered, namely, the stage of engineering geological exploration; Type and scale of the building; The complexity of engineering geological conditions and the degree of regional research. From reconnaissance to detailed survey, the scale of surveying and mapping generally varies from 1: 5, to 1: 1,.

Remote sensing technology (RS) is an auxiliary means of engineering geological mapping. Aerial photos and satellite photos contain a wide range of information such as stratigraphic lithology, geological structure, geomorphology and physical and geological phenomena, and a detailed interpretation can quickly get a comprehensive understanding of the surveying and mapping area. Especially in remote areas where traffic is inconvenient, it is of special significance to make full use of aerial photos and satellite photos. The GPS technology developed in recent years has a high degree of flexibility in positioning and incomparable high precision and efficiency with conventional surveying technology. It has been widely used in subway control network, highway and bridge control network, long-distance tunnel through survey control network, cadastral survey control network and so on in Beijing and Shanghai, and achieved remarkable benefits. Another new technology in engineering surveying and mapping is geographic information system (GIS), which provides useful information for the planning, management and decision-making of earth science, environmental science and urban construction, engineering design and enterprise management through a series of spatial operations and analysis of distributed data. The combination of RS, GPS and GIS forms a "3S" technical system for quickly acquiring, updating, storing, managing and analyzing geographic and spatial information, which provides a powerful technical means for engineering surveying and mapping.

engineering geophysical exploration is an indirect method. according to the physical properties such as density, magnetism, conductivity and elastic wave propagation speed of the geological medium to be measured, as well as the physical state such as water content, fracture rate and fracture degree of the rock stratum, the physical parameters of the rock stratum, especially the mechanical indexes of the rock and soil mass, are measured with special instruments and equipment, so as to divide the rock stratum, determine the geological structure, groundwater depth and karst distribution. Compared with the engineering exploration, the engineering geophysical prospecting method is economical and rapid, and it can put forward the problems that are difficult to infer and need to be solved in time. Therefore, the appropriate cooperation of geophysical prospecting is often required in the process of engineering geological surveying and mapping, especially in solving the thickness of overburden, the fluctuation of bedrock surface and tracing faults, and the effect is particularly remarkable.