The geotechnical investigation is an essential element of successful water resources projects and foundation recommendation report. Whether in the stages of planning, design, construction and monitoring and maintenance, each stage has specific design requirements.
Each stage of the geotechnical investigation includes the collection of existing data, site survey, preliminary survey, and supplementary survey. Figure 1 shows the flow chart of the geotechnical investigation.
The preliminary geotechnical investigation phase includes:
Data collection and data analysis → geological survey → ( geophysical prospecting ←→ Drilling engineering ←→ Excavation and survey ←→ in-situ testing ) → laboratory testing → produce a report on preliminary investigation
The supplemental investigation phase includes:
drilling → laboratory testing → in-situ testing → produce a report on supplementary survey results
When we start the geotechnical investigation, the project proposal must create first to facilitate the implementation of the work.
The purposes of the entire workflows for geotechnical investigation are:
- To understand and collect relevant data.
- To design follow-up plans based on engineering characteristics and needs.
- Conduct geological surveys and other surveys (geophysical surveys, drilling projects, excavation & survey, in-situ testing, and laboratory tests) to further obtain more information.
- Establish a preliminary qualitative or semi-quantitative assessment of site characteristics.
- Extend the investigation scope as needed to address geological issues or increase the investigated area for more detailed supplementary surveys.
- To provide more data for quantitative evaluation of the geotechnical condition.
All geotechnical investigations should begin with an existing data collection.
It may include a number of the following items (as shown in Figure 1):
Start → Data collection and data analysis → Geophysical exploration→Geological survey → Drilling engineering → Excavation survey → In-situ testing → Laboratory test
Figure.1 Geotechnical investigation procedure
Data collection and analysis:
Collect and analyze the topography associated with the project, the geological structure, the mechanical properties of the soil and rock and the engineering properties, the geological environment (such as faults, groundwater, colluvial soil, subsidence potential areas) and their interaction with existing structures.
Geological survey
The purpose of this step is to learn the topographical and geological characteristics of the site to assist in clarifying issues related to engineering planning, design, and construction. The survey plan should be drawn up according to the complexity of the site, the stage of the project plan, and the prior knowledge assessment. During the construction period, it is necessary to cooperate with the geotechnical investigation and consistent with the design requirements, continuously collect, evaluate and review the geological data, and provide the survey results promptly as a reference in the subsequent stage.
Geophysical exploration
Rocks have different physical properties (electrical, mechanical, magnetic) influenced by differential factors (such as composition, temperature, humidity). Geophysical exploration applies the principles of physics to determine the distribution and variation of physical properties combined with known geological information and evaluation of the subsurface for analysis. The most commonly used methods are seismic refraction method, electrical resistivity tomography(ERT), and ground penetrating radar(GPR).
The geological information and characteristics obtained from the above methods determine the project size, scope.
Also, the results of the geophysical exploration should get verified or corrected by drilling or direct exploration data.
Drilling engineering
Drilling is a direct approach to verify geological or geophysical exploration data. Determine the test equipment and methods according to the operational phase, the types of rock, depth of investigation, design requirements, and particular purposes, to get high-quality samples and meet data verification requirements.
The drilling engineering supervision should include the following items to satisfy the designed requirements:
- Check the drilling and sampling quality;
- Check the construction daily progress report;
- Effective communication between the investigator and the design section to adjust the drilling work on site ;
- Quality control from sample collection to transportation.
Excavation survey
The excavation survey is to gather more earth information to supplement the limitation of geological surveys and exploration data. The results of the study can be used to (1) verify the geological survey result; (2) check the characteristics within rock formations, and (3) evaluate the characteristics and quantity of engineering materials (aggregates). The excavation survey can be used in conjunction with in-situ testing and sampling for laboratory tests to learn the mechanical properties of the rock.
The excavation techniques include stripping, trench, test pit, and horizontal pit. Most rock bed foundation surveys take stripping, the trench, and the open pit. The soil foundation or material survey often uses trial pit excavation methods.
In-situ testing
The human and equipment resources needs for in-situ testing are quite enormous. Therefore, the site should be well understood before in-situ testing. The appropriate test methods, specifications, and several trials should also be selected based on the characteristics of the bedrock formation and the particular issues of the project. The location of the in-situ testing (and soil specimens) should be representative and avoid to disturb the specimens during preparation.
There are many kinds of in-situ testing, such as rock hole deformation tests, vertical and horizontal plate loading test, field direct shear test, field permeability tests, lugeon test, field density test, and pumping test. Required tests depend on the purpose of the survey and the engineering characteristics. Different tests should follow the specifications and test results and analysis.
Laboratory test
The purpose of the laboratory test is to obtain the geological characteristics of the site and the engineering properties of the earth materials. Samples were collected primarily through onsite drilling or excavation surveys.
Pay attention to the following criteria to meet the accuracy requirements:
- Onsite drilling and sampling must meet the work specifications and requirements;
- Be sure the instrument accuracy, test procedures, and all the personnel of the laboratory comply with relevant regulations.
Various types of laboratory tests:
- For Soil:
General physical properties (water content, specific weight, Atterberg limits, specific gravity, particle size analysis), triaxial compression test, one-dimensional consolidation test, water permeability test, Proctor compaction test, direct shear test, dynamic triaxial testing.
- For Rock:
General physical properties (water content, specific weight, absorption, specific gravity, porosity), uniaxial compressive strength(UCS) test, triaxial compression test, Brazilian test, direct shear test, static elastic modulus test, slake durability test, soundness test, Los Angeles abrasion test.
Depend on the purpose of the survey and the engineering characteristics to choose the required tests. The tests should follow the specifications, test results, and analysis.
Final Project Report
At the time generate the final project report by various investigation method (Data collection and data analysis, Geological survey, Geophysical exploration, Drilling engineering, Excavation survey, In-situ testing, Laboratory test), to present the results follow the relevant regulations, and evaluate the impact of the results of the investigation on construction.
The final project report also recommends which investigation methods and tests are required in the follow-up investigation phase to clarify engineering issues.