Review Note

1. Initial Site Investigations

• Desktop Study: This stage includes reviewing historical geological data, regional geological maps, existing site reports, and satellite imagery to establish a preliminary understanding of ground conditions. It identifies known geological hazards, faults, and groundwater concerns.
• Purpose: A desktop study helps guide subsequent, more detailed investigations by highlighting areas of concern and establishing a general baseline of subsurface conditions​​.

2. Field Investigations

• Borehole Drilling: Boreholes are drilled at strategic locations to obtain soil and rock samples, providing direct information about subsurface stratigraphy, rock quality, groundwater levels, and soil composition. The depth and spacing of boreholes depend on project requirements and expected geological complexity.
• Test Pits and Trenches: Shallow test pits or trenches may be excavated to expose the subsurface layers and allow visual inspection and sampling, especially in the upper soil layers.
• Geophysical Surveys: Techniques like seismic reflection, ground-penetrating radar (GPR), and electrical resistivity tomography (ERT) help map geological structures and identify variations in material properties without direct excavation. These are particularly useful in identifying faults, groundwater, and rock layer continuity.
• Purpose: Field investigations provide detailed, localized data that serve as the foundation of the geotechnical model, revealing essential features like soil layering, rock quality, and hydrogeological characteristics​​.

3. In-Situ Testing

• Standard Penetration Test (SPT): Conducted within boreholes, SPT provides data on soil density and strength, particularly in sands and clays. The test measures soil resistance to penetration and gives an indication of soil compactness and bearing capacity.
• Cone Penetration Test (CPT): The CPT pushes a cone into the ground to measure resistance, providing continuous data on soil stratigraphy, density, and shear strength. It is highly effective for assessing softer soils and fine-grained sediments.
• Pressuremeter and Dilatometer Tests: These tests evaluate the in-situ stress-strain behavior of soils and rock, which is critical for understanding how the ground will deform under load and for calibrating models of ground-support interaction.
• Purpose: In-situ testing provides direct measurements of ground strength, stiffness, and other parameters essential for calibrating geotechnical models and predicting ground behavior​​.

4. Laboratory Testing of Samples

• Soil Testing: Samples from boreholes undergo laboratory tests such as grain size analysis, Atterberg limits, shear strength tests, and consolidation tests. These tests provide data on soil properties like permeability, compressibility, and cohesion, which are critical for understanding how soils will respond to excavation.
• Rock Testing: Rock samples are tested for unconfined compressive strength, triaxial strength, and durability. The tests yield values for rock modulus, tensile strength, and potential for fracturing, which are essential for tunnelling in hard rock environments.
• Purpose: Laboratory tests deliver precise geotechnical parameters for different soil and rock layers, which form the basis for material classification and inform decisions on suitable excavation and support methods​​.

5. Geological and Geotechnical Mapping

• Creation of Cross-Sections and Profiles: Based on field data, geological cross-sections and profiles illustrate the stratigraphy, fault lines, and groundwater levels along the tunnel route. This visual representation helps engineers understand spatial relationships and variations in ground conditions.
• Identification of Hazard Zones: Mapping highlights areas of potential hazards, such as fault zones, high groundwater areas, and zones with weak soil or fractured rock, which may require special design considerations.
• Purpose: Mapping organizes complex geological data into a clear visual framework, making it easier to analyze and apply in the geotechnical model​​.

6. Development of a Conceptual Geotechnical Model

• Synthesizing Data: Information from investigations and testing is integrated into a conceptual model, defining basic geological units (e.g., soil, rock) and their properties. The model considers both vertical and horizontal variations in material properties.
• Defining Ground Behavior Zones: The conceptual model categorizes zones based on their expected behavior under load, such as stable rock zones, soft ground areas, and faulted zones, which influence support and excavation planning.
• Purpose: This model serves as a preliminary framework to guide tunnel design, providing general expectations for ground-support interaction​​.

7. Numerical and Analytical Modelling

• Finite Element Analysis (FEA): FEA subdivides the conceptual model into smaller elements, allowing for detailed analysis of stress, deformation, and ground-support interaction across varying ground conditions.
• Calibration of Ground Parameters: In this stage, ground parameters such as modulus of elasticity, cohesion, and angle of internal friction are calibrated to match observed conditions and in-situ test results. Calibration ensures that the model’s predictions are realistic.
• Purpose: Numerical modeling allows engineers to predict ground deformation, stress distribution, and potential failure zones accurately, enabling proactive support design and risk management​​.

8. Determination of Geotechnical Parameters for Design

• Selection of Design Values: Using data from laboratory tests, in-situ tests, and model calibrations, specific geotechnical parameters are chosen for design, including shear strength, Young’s modulus, Poisson’s ratio, and permeability.
• Safety Factor Adjustments: Safety factors are applied to account for uncertainties in ground conditions, ensuring conservative estimates that improve design reliability.
• Purpose: Finalized geotechnical parameters guide the engineering design of tunnel supports, excavation methods, and safety measures, providing a robust basis for construction planning​​.

Summary